Advances in Visible-Light-Mediated Carbonylative Reactions Via Carbon Monoxide (CO) Incorporation

catalysts

Review Advances in Visible-Light-Mediated Carbonylative Reactions via Carbon Monoxide (CO) Incorporation

Vinayak Botla 1, Aleksandr Voronov 1, Elena Motti 1, Carla Carfagna 2, Raffaella Mancuso 3 , Bartolo Gabriele 3 and Nicola Della Ca’ 1,*

1 Department of Chemistry, Life Sciences and Environmental Sustainability (SCVSA), University of Parma, Parco Area delle Scienze, 17/A, 43124 Parma, Italy; [email protected] (V.B.); [email protected] (A.V.); [email protected] (E.M.) 2 Department of Industrial Chemistry “T. Montanari”, University of Bologna, 40136 Bologna, Italy; [email protected] 3 Laboratory of Industrial and Synthetic Organic Chemistry (LISOC), Department of Chemistry and Chemical Technologies, University of Calabria, Via P. Bucci 12/C, 87036 Arcavacata di Rende Cosenza, Italy; [email protected] (R.M.); [email protected] (B.G.) * Correspondence: [email protected]; Tel.:+39-0521-905676

Abstract: The abundant and inexpensive carbon monoxide (CO) is widely exploited as a C1 source for the synthesis of both ﬁne and bulk chemicals. In this context, photochemical carbonylation reactions have emerged as a powerful tool for the sustainable synthesis of carbonyl-containing compounds (esters, amides, ketones, etc.). This review aims at giving a general overview on visible light-promoted

carbonylation reactions in the presence of metal (Palladium, Iridium, Cobalt, Ruthenium, Copper) and organocatalysts as well, highlighting the main features of the presented protocols and providing

Citation: Botla, V.; Voronov, A.; useful insights on the reaction mechanisms. Motti, E.; Carfagna, C.; Mancuso, R.; Gabriele, B.; Della Ca’, N. Advances Keywords: carbon monoxide; carbonylation; photochemical reactions; visible light; carbonyl- in Visible-Light-Mediated containing compounds Carbonylative Reactions via Carbon Monoxide (CO) Incorporation. Catalysts 2021, 11, 918. https:// doi.org/10.3390/catal11080918 1. Introduction Carbon monoxide (CO) is largely used in the chemical industry for manufacturing Academic Editor: Ekaterina bulk chemicals (i.e., methanol, acetic acid) and ﬁne chemicals (i.e., ibuprofen) and is fre- A. Kozlova quently employed as an inexpensive and readily available C1 source in a wide range of carbonylative transformations for the synthesis of high-value-added carbonyl-containing Received: 26 June 2021 compounds, such as acids, esters, amides and ketones [1–6]. The carbonyl unit is ubiqui- Accepted: 27 July 2021 Published: 29 July 2021 tous in a myriad of bioactive molecules, such as natural products, pharmaceuticals and agrochemicals, as well as materials. It can be also manipulated and transformed into a

Publisher’s Note: MDPI stays neutral series of other functional groups, including amines, alcohols and olefins. In the era of with regard to jurisdictional claims in sustainability, the development of economically improved and environmentally friendly published maps and institutional affil- catalytic protocols is highly recommended. In recent years, visible-light photocatalysis has iations. received much attention from the synthetic chemists’ community since, unlike traditional thermal and catalytic reactions, it enables unprecedented reaction pathways, high selec- tivities and mild reaction conditions [7–10]. The combination of carbon monoxide-based carbonylation strategy with the advantages that come from the application of photocatalysis can potentially lead to a highly sustainable process [11–15]. However, the first reports Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. displayed problems connected with a limited generality, poor selectivity and/or efficiency, This article is an open access article high pressure of carbon monoxide or high energy of light irradiation [16–23]. Over the distributed under the terms and last two decades, new and more performing catalytic systems allowed to solve in part conditions of the Creative Commons these issues. The present review will cover the major advances in the area of visible light- Attribution (CC BY) license (https:// mediated catalyzed carbonylations from 2000, with a focus on CO-based carbonylation creativecommons.org/licenses/by/ methodologies. Sometimes, for a better and more general overview, less recent reports 4.0/). will be mentioned. The review is organized into different sections; each one is related to a

Catalysts 2021, 11, 918. https://doi.org/10.3390/catal11080918 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 34

methodologies. Sometimes, for a better and more general overview, less recent reports will be mentioned. The review is organized into different sections; each one is related to a Catalysts 2021, 11, 918 2 of 35 different metal, and one section is dedicated to metal-free protocols, which include the use of organic photocatalysts.

different1.1. Visible-Light-Promoted metal, and one section Palladium-Catalyzed is dedicated to metal-free Carbonylations protocols, which include the use of organic photocatalysts. The major achievements in the area of visible-light-driven photocatalysis have been 1.1.reached Visible-Light-Promoted with palladium, Palladium-Catalyzed thanks to its superior Carbonylations versatility. One of the first reports was relatedThe to major the achievementssynthesis of unsymmetrical in the area of visible-light-driven ketones, which photocatalysiswere readily accessed have been by means reachedof a palladium-catalyzed with palladium, thanks cross-coupling to its superior reaction versatility. of Oneiodoalkanes of the ﬁrst and reports 9-alkyl-9-borabicy- was re- latedclo[3.3.l]nonane to the synthesis derivatives of unsymmetrical (9-alkyl-9-BBN) ketones, which under were atmospheric readily accessed pressure by means of CO of a (Scheme palladium-catalyzed1) [24]. The presence cross-coupling of K3PO4 was reaction essential, of iodoalkanes while the and reaction 9-alkyl-9-borabicyclo[3.3.l] was significantly acceler- nonaneated by derivatives the irradiation (9-alkyl-9-BBN) of light. underAlkyl atmospherichalides bearing pressure β hydrogens of CO (Scheme often1)[ lead24]. The to competi- presence of K PO was essential, while the reaction was signiﬁcantly accelerated by the tive pathways3 in4 traditional cross-coupling reactions, such as the formation of alkenes by irradiation of light. Alkyl halides bearing β hydrogens often lead to competitive pathways inβ–hydride traditional elimination. cross-coupling In this reactions, case, various such as theprimary, formation secondary of alkenes and by tertiaryβ–hydride alkyl iodides elimination.afforded unsymmetrical In this case, various ketones primary, in moderate secondary to andgood tertiary yields. alkyl Different iodides functional afforded groups unsymmetrical(i.e., acetal, nitrile, ketones carbomethoxy in moderate to groups) good yields. were Different well tolerated functional on groups both iodoalkanes (i.e., acetal, and 9- nitrile,alkyl-9-BBN carbomethoxy under groups)the optimized were well reaction tolerated co onnditions. both iodoalkanes Selected and examples 9-alkyl-9-BBN are shown in underScheme the 1. optimized reaction conditions. Selected examples are shown in Scheme1.

R2 B O hv, 3% Pd(PPh3)4 1 R -I CO R1 R2 (1atm) K3PO4,C6H6 r.t., 24h 50–76% 9 examples R1 = primary, secondary and tertiary alkyl groups Features: 1991 Suzuki R2 = primary alkyl groups -hv: 100 W tungsten light - Alkyl iodides with hydrogens Selected examples: -1atmofCO O O O CO2Me H CO2Me 5 7 10 H 67% 76% O 65% O OMe O O O 73% AcO 50% OMe

Proposed mechanism: CO hv Pd(PPh3)4 Pd(CO)(PPh3)3 Pd(CO)(PPh3) - PPh 3 I - 2PPh3 II

CO L SET 9-R2-9-BBN R1 I +RII 1PdX R1COPdX base L III L IV L R1COPdR2 R1COR2 + Pd(0)L 2 L = CO, PPh3 L V SchemeScheme 1. 1.The The synthesis synthesis of of ketones ketones from from iodoalkanes, iodoalkanes, 9-alkyl-9-BBN 9-alkyl-9-BBN derivatives derivatives and and CO viaCO avia a light- light-acceleratedaccelerated Pd-catalyzed Pd-catalyzed reaction. reaction. The reaction is supposed to proceed through a free radical mechanism at the oxidative The reaction is supposed to proceed through a free radical mechanism at the oxida- addition step [25]. Firstly, Pd(PPh3)4 likely converts to the low reactive Pd(CO)(PPh3)3 complextive additionI under step CO [25]. atmosphere. Firstly, InPd(PPh the presence3)4 likely of lightconverts irradiation, to the complexlow reactiveI undergoes Pd(CO)(PPh3)3

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Catalysts 2021, 11, 918 3 of 35 complex I under CO atmosphere. In the presence of light irradiation, complex I undergoes dissociation of ligands to give the unsaturated palladium species II. The latter enables an electron transfer to the alkyl iodide with the formation of a radical pair (Pd(I)X + R˙) that dissociation of ligands to give the unsaturated palladium species II. The latter enables could provide the oxidative intermediate RPdX (III). The migration of the alkyl group to an electron transfer to the alkyl iodide with the formation of a radical pair (Pd(I)X + R˙) thata coordinated could provide CO the molecule oxidative intermediateaffords complex RPdX (RCOPdXIII). The migration IV. Then, of thethe alkyl base group facilitates the totransfer a coordinated of the R CO2 group molecule from affords 9-R2-9-BBN complex to RCOPdXpalladiumIV .providing Then, the V base, which facilitates yields the ke- thetone transfer and a of palladium(0) the R2 group species from 9-R through2-9-BBN a toreductive palladium elimination providing Vstep, which (Scheme yields 1, proposed themechanism ketone and). a palladium(0) species through a reductive elimination step (Scheme1, proposedBased mechanism on their). previous studies [25], Miyaura and co-workers developed a palladium-catalyzedBased on their three-component previous studies [25 reaction], Miyaura between and co-workers iodoalkenes, developed carbon a palladium- monoxide and 9- catalyzedalkyl- or three-component 9-aryl-9-BBN derivatives reaction between to obta iodoalkenes,in cyclized carbon unsymmetrical monoxide andketones 9-alkyl- under pho- or 9-aryl-9-BBN derivatives to obtain cyclized unsymmetrical ketones under photoirra- toirradiation conditions (Scheme 2) [26]. Analogously to the previously reported mecha- diation conditions (Scheme2)[ 26]. Analogously to the previously reported mechanism (Schemenism (Scheme1), the iodoalkenes 1), the iodoalkenes provided provided cyclized ketonescyclized via ketones radical via cyclization, radical cyclization, carbon carbon monoxidemonoxide insertion insertion and and coupling coupling with with 9-R 29-R-9-BBN2-9-BBN derivatives. derivatives. Iodoalkynes Iodoalkynes led to led the to the cor- correspondingresponding cyclized cyclized unsaturatedunsaturate ketonesd ketones as aas 1:1 a mixture1:1 mixture of E and of ZE isomers.and Z isomers. Interest- Interest- ingly,ingly, iodocycloalkenes iodocycloalkenes resulted resulted in the formationin the formation of cis-fused of bicyclic cis-fused alkanes bicyclic starting alkanes from starting transfromiodides trans iodides as well (Scheme as well2 ).(Scheme This, together 2). This, with together the predominance with the predominance of 5-membered of 5-mem- overbered the over 6-membered the 6-membered annulation annulation mode, can mode, further can conﬁrm further the confirm radical nature the radical of the nature of reactionthe reaction pathway. pathway.

SchemeScheme 2. The2. The synthesis synthesis of ketones of ketones from from iodoalkenes, iodoalkenes, 9-R2-9-BBN 9-R2-9-BBN derivatives derivatives and CO viaand a light-CO via a light- acceleratedaccelerated Pd-catalyzed Pd-catalyzed reaction. reaction.

Palladium-catalyzedPalladium-catalyzed photochemical photochemical carbonylation carbonylation reactions reactions aimed ataimed the synthesis at the synthesis of ketones, α,β-unsaturated ketones, esters, lactones and carbamoylacetates have been of ketones, α,β-unsaturated ketones, esters, lactones and carbamoylacetates have been ex- extensively studied by lhyong Ryu and co-workers [15,27,28]. The initial idea was to inserttensively carbon studied monoxide by lhyong on an alkyliodide Ryu and co-workers by homolytic [15,27,28]. cleavage ofThe the initial C–I bond. idea Thewas to insert ideacarbon has beenmonoxide successfully on an realized alkyliodide under by irradiation homoly andtic cleavage metal-free of conditions the C–I bond. (vide infra The, idea has Sectionbeen successfully1.6)[29]. realized under irradiation and metal-free conditions (vide infra, Section 1.6)The [29]. combination of palladium catalysis with irradiation conditions was crucial for the carbonylation-cyclization-carbonylationThe combination of palladium catalysis sequence with reported irradiation in 2002 conditions by Ryu and was co- crucial for workersthe carbonylation-cyclization-carbonylation [30]. A variety of 4-alkenyl iodides were sequence efﬁciently reported converted in to2002 the desiredby Ryu and co- ketonesworkers in the[30]. presence A variety of alcohol, of 4-alkenyl Pd(PPh iodides3)4 (5 mol we %),re Etefficiently3N, DMAP converted (5–10 mol %)to the under desired ke- irradiation (500 W xenon lamp, 185–2600 nm, Pyrex) and 40 atm of CO pressure (Scheme3). tones in the presence of alcohol, Pd(PPh3)4 (5 mol %), Et3N, DMAP (5–10 mol %) under When the transformation was performed with diethyl amine in place of alcohol, the triply carbonylatedirradiation (500α,δ-diketo W xenon amides lamp, were 185–2600 obtained nm, as thePyrex) major and product. 40 atm Alkylof CO bromidespressure (Scheme 3). When the transformation was performed with diethyl amine in place of alcohol, the triply carbonylated α,δ-diketo amides were obtained as the major product. Alkyl bromides gave promising but lower yields if compared with the corresponding iodides. This

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gave promising but lower yields if compared with the corresponding iodides. This free- radical-basedfree-radical-basedfree-radical-based methodology methodology methodology complements complements the complements traditional palladium-catalyzedthe thetraditional traditional palladium-catalyzed carbonylative palladium-catalyzed car- car- reactionsbonylative that reactions arereactions extensively that that employedare are extensively extensively for aromatic employed employed and vinylic for aromatic halidesfor aromatic [31 and]. vinylic and vinylic halides halides [31]. [31].

Scheme 3. Pd-catalyzed/light-induced Cyclizative Carbonylation to esters and amides. SchemeScheme 3. Pd-catalyzed/light-induced3. Pd-catalyzed/light-induced Cyclizative Cyclizative Carbonylation Carbonylation to esters and amides. to esters and amides. The efficiency of the reaction was improved by the presence of palladium catalysts; however,The efﬁciency the stereoselectivity of the reaction was observed improved with by the cyclic presence substrates of palladium was catalysts;identical with those however,foundThe in the palladium-free efficiency stereoselectivity of the radical observed reaction carbonylation with was cyclic improved substrates sequences. by was theThe identical presenceinterplay with of thoseof organometallicpalladium catalysts; foundhowever,species in palladium-freeand the radicals stereoselectivity radicalcan be carbonylationexplained observed by sequences. the withmechanism The cyclic interplay depictedsubstrates of organometallic in Schemewas identical 4. As previ- with those speciesfoundously and observedin palladium-free radicals [25], can bethe explained palladium(0)radical by carbonylation the mechanismspecies can depictedsequences. be active in Scheme Thein the interplay4. AsC–I pre- bond of organometalliccleavage viouslyspecies observed and radicals [25], the can palladium(0) be explained species by can the be activemechanism in the C–I depicted bond cleavage in Scheme 4. As previ- providingproviding the the radical radical pair pair Pd(I)I Pd(I)I and alkyl and radical alkyl radicalI. The high I. The pressure high of pressure carbon monoxide of carbon monoxide favorsouslyfavors the observedthe formation formation [25], of the of acylthethe radicalpalladium(0)acyl radicalII, which II , undergoesspecieswhich undergoes can cyclization be active cyclization and furtherin the and CO C–I further bond CO cleavage insertionprovidinginsertion to toIV the .IV Then,. radical Then, intermediate intermediate pair Pd(I)IIV can andIV recombine can alkyl recombine radical with the I Pd(I)Iwith. The the species high Pd(I)I pressure leading species to Vof leading, carbon to monoxide V, whichfavorswhich affords affordsthe theformation the desired desired product of theproduct by acyl reaction by radical reaction with anII, alcohol.withwhich an alcohol.undergoes cyclization and further CO insertion to IV. Then, intermediate IV can recombine with the Pd(I)I species leading to V, which affords the desired product by reaction with an alcohol.

SchemeScheme 4. 4.The The proposed proposed mechanism mechanism for the for Pd-catalyzed/Light-induced the Pd-catalyzed/Light-induced Cyclizative CarbonylationCyclizative Carbonylation toto ketones ketones and and amides. amides.

BasedBased on on their their previous previous report report [30], Ryu [30], and Ryu co-workers and co-workers reported a reported general method a general method forScheme the synthesis 4. The ofproposed esters and mechanism amides [32 ]for and, the in Pd-catalyzed particular, described/Light-induced the acceleration Cyclizative Carbonylation effecttofor ketones the of light-inducedsynthesis and amides. of atom esters transfer and amides carbonylation [32] and, reactions in particular, obtained in described the presence the of acceleration Pd(0)effect complexes of light-induced and Mn2(CO) atom10. Therefore, transfer forcarbonyl example,ation when reactions 1-iodooctane obtained was caused in the to presence of Pd(0) complexes and Mn2(CO)10. Therefore, for example, when 1-iodooctane was caused Based on their previous report [30], Ryu and co-workers reported a general method to react with CO and ethanol under photoirradiation conditions (Xenon lamp, 185–2600 for the synthesis of esters and amides [32] and, in particular, described the acceleration nm), ethyl nonanoate was obtained in a moderate yield of 54% even with prolonged reac- effect of light-induced atom transfer carbonylation reactions obtained in the presence of tion time (50 h). However, the same reaction in the presence of Pd(PPh3)4 (5 mol %) af- Pd(0)forded complexes the expected and ethyl Mn ester2(CO) in10 87%. Therefore, yield after for 16 example, h (Scheme when 5a). Under 1-iodooctane similar condi- was caused totions, react the with synthesis CO and of a ethanol precursor under of the photoi (-)-Hinokininrradiation was conditions efficiently achieved(Xenon lamp, (Scheme 185–2600 nm),5b). ethyl nonanoate was obtained in a moderate yield of 54% even with prolonged reaction time (50 h). However, the same reaction in the presence of Pd(PPh3)4 (5 mol %) afforded the expected ethyl ester in 87% yield after 16 h (Scheme 5a). Under similar conditions, the synthesis of a precursor of the (-)-Hinokinin was efficiently achieved (Scheme 5b).

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react with CO and ethanol under photoirradiation conditions (Xenon lamp, 185–2600 nm), ethyl nonanoate was obtained in a moderate yield of 54% even with prolonged reaction time (50 h). However, the same reaction in the presence of Pd(PPh3)4 (5 mol %) afforded Catalysts 2021, 11, x FOR PEER REVIEW 5 of 34 the expected ethyl ester in 87% yield after 16 h (Scheme5a). Under similar conditions, the synthesis of a precursor of the (-)-Hinokinin was efﬁciently achieved (Scheme5b).

SchemeScheme 5. 5.Palladium Palladium tetrakis-accelerated tetrakis-accelerated photo-irradiated photo-irradiated atom atom transfer transfer carbonylation carbonylation of alkyl of alkyl io- iodidesdides to to esters esters ((aa),), lactoneslactones (b ()b and) and amides amides (c). (c).

TheThe atom atom transfer transfer carbonylation carbonylation of secondary of secondary and tertiary and tertiary alkyl iodides alkyl iodides proceeded proceeded smoothly, affording moderate to high yields of esters in a shorter reaction time (6.5 h). In smoothly, affording moderate to high yields of esters in a shorter reaction time (6.5 h). In this case, the use of Mn2(CO)10 in place of Pd(PPh3)4 was found to be even more effective. Thethis selective case, the synthesis use of Mn of the2(CO) corresponding10 in place of amides Pd(PPh was3)4 againwas found achieved to be in theeven presence more effective. The selective synthesis of the corresponding amides was again achieved in the presence of Mn2(CO)10 (Scheme5c), while a palladium(0) precursor led to mixtures of amides and ketoamides.of Mn2(CO) Later,10 (Scheme highly functionalized5c), while a palladium(0) linear ester and precursor lactone derivatives led to mixtures were obtained of amides and byketoamides. means of four Later, and three-componenthighly functionalized photocatalytic linear methodologies,ester and lactone respectively, derivatives under were ob- similartained reactionby means conditions of four and [33]. three-component photocatalytic methodologies, respectively, underIn 2010,similar a three-component reaction conditions approach [33]. for the synthesis of alkynyl ketones was pro- posedIn by 2010, the Ryu a researchthree-component group [34]. approach This new methodology, for the synthesis which isof an alkynyl alternative ketones to the was pro- acylation of alkynyl organometallic reagents with acid chlorides, is based on the reaction of posed by the Ryu research group [34]. This new methodology, which is an alternative to alkyl iodides, CO and terminal alkynes in the presence of a palladium catalyst and xenon the acylation of alkynyl organometallic reagents with acid chlorides, is based on the reac- light, under mild reaction condition (Scheme6). tion of alkyl iodides, CO and terminal alkynes in the presence of a palladium catalyst and xenon light, under mild reaction condition (Scheme 6).

Scheme 6. The synthesis of alkynyl ketones via a Pd-catalyzed/light-induced three-component cross-coupling reaction.

Remarkably, the source of light, the palladium catalyst (PdCl2(PPh3)2) and the aqueous medium proved to be crucial for this transformation. Various substituents on the alkyl iodide, such as chloro, methyl ester and ether (TBS) groups, were well tolerated under

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Scheme 5. Palladium tetrakis-accelerated photo-irradiated atom transfer carbonylation of alkyl iodides to esters (a), lactones (b) and amides (c).

The atom transfer carbonylation of secondary and tertiary alkyl iodides proceeded smoothly, affording moderate to high yields of esters in a shorter reaction time (6.5 h). In this case, the use of Mn2(CO)10 in place of Pd(PPh3)4 was found to be even more effective. The selective synthesis of the corresponding amides was again achieved in the presence of Mn2(CO)10 (Scheme 5c), while a palladium(0) precursor led to mixtures of amides and ketoamides. Later, highly functionalized linear ester and lactone derivatives were obtained by means of four and three-component photocatalytic methodologies, respectively, under similar reaction conditions [33]. In 2010, a three-component approach for the synthesis of alkynyl ketones was proposed by the Ryu research group [34]. This new methodology, which is an alternative to the acylation of alkynyl organometallic reagents with acid chlorides, is based on the reac- Catalysts 2021, 11, 918 tion of alkyl iodides, CO and terminal alkynes in the presence of 6a of palladium 35 catalyst and xenon light, under mild reaction condition (Scheme 6).

Catalysts 2021, 11, x FOR PEER REVIEW 6 of 34 SchemeScheme 6. The6. The synthesis synthesis of alkynyl of ketonesalkynyl via aketones Pd-catalyzed/light-induced via a Pd-catalyzed/light-induced three-component three-component cross-couplingcross-coupling reaction. reaction. standard conditions, whileRemarkably, aryl, theas sourcewell as of alkyl light, thefragments, palladium catalystcan be (PdClefficiently2(PPh3) 2employed) and the aqueous in mediumRemarkably, proved to be crucial the source for this of transformation. light, the palladium Various substituents catalyst on (PdCl the alkyl2(PPh3)2) and the aque- the alkyne partner. Once again, this methodology, which starts from a variety of alkyl iodide,ous medium such as chloro, proved methyl to be ester crucial and ether for (TBS)this tran groups,sformation. were well tolerated Various under substituents on the alkyl iodides, is complementarystandard conditions, to the transition while aryl, metal-catalyzed as well as alkyl fragments, coupling can reactions, be efﬁciently which employed are mainly restricted intoiodide, thethe alkyne use such of partner. aryl as andchloro, Once vinyl again, methyl halides this methodology, ester [31]. and which ether starts (TBS) from groups, a variety ofwere alkyl well tolerated under Later, Ryu andiodides, co-workers is complementary reported to the vers transitionatile four-component metal-catalyzed coupling coupling reactions, reactions which are mainly restricted to the use of aryl and vinyl halides [31]. leading to functionalized esters using α-substituted iodoalkanes, alkenes, CO and alco- Later, Ryu and co-workers reported versatile four-component coupling reactions lead- hols under photoirradiationing to functionalized conditions esters (500 using W αxenon-substituted lamp), iodoalkanes, 45 atm of alkenes,CO pressure CO and in alcohols the presence of PdClunder2(PPh photoirradiation3)2 (5 mol %) as conditions catalyst (500 (Scheme W xenon 7a) lamp), [35]. 45 atmSeveral of CO electron-with- pressure in the pres- drawing groups wereence of tolerated PdCl2(PPh (EWG3)2 (5 mol = COOR, %) as catalyst CN, (Scheme PhSO27)a) on [ 35 the]. Several iodoalkane electron-withdrawing coupling partner, and bothgroups terminal were and tolerated internal (EWG alkenes = COOR, gave CN, PhSOsatisfactory2) on the results. iodoalkane When coupling alkenyl partner, and both terminal and internal alkenes gave satisfactory results. When alkenyl alcohols alcohols were employedwere employed in place in place of alkene of alkene/ROH/ROH mixture, mixture, lactonelactone derivatives derivatives were were obtained ob- via tained via intramolecularintramolecular esterification esteriﬁcation (Scheme (Scheme 77b).b).

SchemeScheme 7. Pd-catalyzed/Light-induced7. Pd-catalyzed/Light-induced four and four three-component and three- couplingcomponent reactions coupling to esters reactions (a) and lactones to esters (b). (a) and lactones (b).

The same research group has also reported a novel synthetic method of carbamoylacetates from α-iodoacetate, carbon monoxide and amines under 15 W black light conditions in the presence of PdCl2(PPh3)2 as the catalyst [36]. The methodology was restricted to the use of α-iodo ethyl acetate, while a wide variety of amines, including primary and secondary amines as well as aryl, heteroaryl and aliphatic amines, were employed (Scheme 8).

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standard conditions, while aryl, as well as alkyl fragments, can be efficiently employed in the alkyne partner. Once again, this methodology, which starts from a variety of alkyl iodides, is complementary to the transition metal-catalyzed coupling reactions, which are mainly restricted to the use of aryl and vinyl halides [31]. Later, Ryu and co-workers reported versatile four-component coupling reactions leading to functionalized esters using α-substituted iodoalkanes, alkenes, CO and alcohols under photoirradiation conditions (500 W xenon lamp), 45 atm of CO pressure in the presence of PdCl2(PPh3)2 (5 mol %) as catalyst (Scheme 7a) [35]. Several electron-withdrawing groups were tolerated (EWG = COOR, CN, PhSO2) on the iodoalkane coupling partner, and both terminal and internal alkenes gave satisfactory results. When alkenyl alcohols were employed in place of alkene/ROH mixture, lactone derivatives were obtained via intramolecular esterification (Scheme 7b).

Scheme 7. Pd-catalyzed/Light-induced four and three-component coupling reactions to esters (a) Catalysts 2021, 11, 918 7 of 35 and lactones (b).

The same research group has also reported a novel synthetic method of car- bamoylacetatesThe same research from groupα-iodoacetate, has also reported carbon amonoxide novel synthetic and amines method under of carbamoylac- 15 W black light etates from α-iodoacetate, carbon monoxide and amines under 15 W black light conditions conditions in the presence of PdCl2(PPh3)2 as the catalyst [36]. The methodology was re- in the presence of PdCl (PPh ) as the catalyst [36]. The methodology was restricted stricted to the use of α2-iodo3 ethyl2 acetate, while a wide variety of amines, including pri- to the use of α-iodo ethyl acetate, while a wide variety of amines, including primary andmary secondary and secondary amines asamines well as as aryl, well heteroaryl as aryl, andheteroaryl aliphatic and amines, aliphatic were amines, employed were em- (Schemeployed 8(Scheme). 8).

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Scheme 8. Palladium-catalyzed/light-induced synthesis of carbamoyl acetates. Scheme 8. Palladium-catalyzed/light-induced synthesis of carbamoyl acetates. A mechanism based on the combined action of radical species and organometallic intermediatesA mechanism was based proposed on the combined(Scheme action9). The of acetate radical speciesradical and I is organometallicinitially formed through intermediates was proposed (Scheme9). The acetate radical I is initially formed through the cleavage of the C–I bond, which can be promoted by single-electron transfer from the the cleavage of the C–I bond, which can be promoted by single-electron transfer from thephoto-excited photo-excited Pd Pd complex. complex. The The subsequent coupling coupling between betweenI and I and Pd(I)I Pd(I)I leads leads to to an α- anpalladoα-pallado ester ester II,II which, which undergoes undergoes aminocarbonylation aminocarbonylation to carbamoyl to carbamoyl esters withesters the with the re- regenerationgeneration of Pd(0)Pd(0) (Scheme (Scheme9). 9).

Scheme 9. 9.A A plausible plausible reaction reaction mechanism. mechanism.

Inspired by the work of Suzuki and Miyaura [24,26], the same research group reported a PdCl2(PPh3)2-catalyzed light-promoted synthesis of alkyl aryl ketones via carbonylative cross-coupling reaction of alkyl iodides, CO and arylboronic acids.[37] Basic conditions (K2CO3), 45 atm of CO and the presence of water were essential for the success of the reaction (Scheme 10). Primary, secondary and tertiary alkyl iodides were successfully coupled with CO and a series of substituted arylboronic acids, bearing both electron- withdrawing and electron-donating groups. The mechanism can likely pass through the formation of an acylpalladium complex via carbonylation of the alkyl radical I, which is generated by the action of Pd(0) and light from the alkyl iodide (Scheme 11). The transmetalation of an arylboronic acid with the acylpalladium intermediate III leads to the corresponding acyl(aryl)palladium species, which undergoes reductive elimination to the desired alkyl aryl ketone and Pd(0) (Scheme 11).

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Inspired by the work of Suzuki and Miyaura [24,26], the same research group reported a PdCl2(PPh3)2-catalyzed light-promoted synthesis of alkyl aryl ketones via carbonylative cross-coupling reaction of alkyl iodides, CO and arylboronic acids [37]. Basic conditions (K2CO3), 45 atm of CO and the presence of water were essential for the success of the reaction (Scheme 10). Primary, secondary and tertiary alkyl iodides were successfully coupled with CO and a series of substituted arylboronic acids, bearing both electron- withdrawing and electron-donating groups. The mechanism can likely pass through the formation of an acylpalladium complex via carbonylation of the alkyl radical I, which is generated by the action of Pd(0) and light from the alkyl iodide (Scheme 11). The Catalysts 2021, 11, x FOR PEER REVIEW 8 of 34 transmetalation of an arylboronic acid with the acylpalladium intermediate III leads to the Catalysts 2021, 11, x FOR PEER REVIEW 8 of 34 corresponding acyl(aryl)palladium species, which undergoes reductive elimination to the desired alkyl aryl ketone and Pd(0) (Scheme 11).

SchemeScheme 10. 10.The The synthesis synthesissynthesis of alky ofof alky arylalky ketonesaryl aryl ketones ketones via a Pd-catalyzed/light-induced via via a aPd-catalyzed/light-induced Pd-catalyzed/light-induced three-component three-component three-component cross-couplingcross-couplingcross-coupling reaction. reaction.

Scheme 11. The proposed reaction pathway. Scheme 11. 11.The The proposed proposed reaction reaction pathway. pathway. Impressively, this protocol was also applied to the four-component coupling reaction betweenImpressively, ethyl iodoacetate, this protocol 1-octene, was COalso and applie phenylboronicd to the four-component acid to give the coupling correspond- reaction betweening functionalized ethyl iodoacetate, ketone derivative 1-octene, inCO 62% and yield phenylboronic under the standard acid to give optimized the correspond- condi- ingtions functionalized (Scheme 12a). ketone The analogous derivative cyclizativ in 62%e yield double-carbonylation under the standard reaction, optimized where condi- 5- tionsiodo-1-pentene (Scheme 12a).replace The the analogous couple iodo cyclizativ alkyl/alkene,e double-carbonylation took place, leading to reaction, the aryl ketonewhere 5- iodo-1-pentenewith a cyclopentanone replace moiety, the couple in 57% iodo yield alkyl/al (Schemekene, 12b). took place, leading to the aryl ketone with a cyclopentanone moiety, in 57% yield (Scheme 12b).

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Scheme 10. The synthesis of alky aryl ketones via a Pd-catalyzed/light-induced three-component cross-coupling reaction.

Catalysts 2021, 11, 918 Scheme 11. The proposed reaction pathway. 9 of 35

Impressively, this protocol was also applied to the four-component coupling reaction between ethylImpressively, iodoacetate, this protocol 1-octene, was also CO applied and phenylboronic to the four-component acid couplingto give reactionthe correspond- between ethyl iodoacetate, 1-octene, CO and phenylboronic acid to give the corresponding ing functionalized ketone derivative in 62% yield under the standard optimized condi- functionalized ketone derivative in 62% yield under the standard optimized conditions tions (Scheme(Scheme 12 a).12a). The The analogous analogous cyclizative cyclizativ double-carbonylatione double-carbonylation reaction, where reaction, 5-iodo-1- where 5- iodo-1-pentenepentene replace replace the couple the couple iodo alkyl/alkene, iodo alkyl/al tookkene, place, took leading place, to the leading aryl ketone to withthe aryl a ketone with acyclopentanone cyclopentanone moiety, moiety, in 57% in yield 57% (Scheme yield 12 (Schemeb). 12b).

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Scheme 12. The synthesis of diketones via Pd-catalyzed/light-induced cross-coupling reactions.

Scheme 12. The synthesisMore of diketonesrecently,via a Pd-catalyzed/light-inducedsimilar approach was employed cross-coupling to reactions. synthesize aromatic β-keto esters from α-iodoesters, CO and arylboronic acids under lower CO pressures (10 atm) [38]. Tak- ing Moreadvantage recently, of a this similar strategy, approach Li wasand employed co-workers to synthesize developed aromatic a protocolβ-keto to esters synthesize aryl α fromketones-iodoesters, from aryl CO iodides and arylboronic and aryl acidsboronic under acids lower by COcarbonylative pressures (10 Suzuki–Miyaura atm) [38]. cou- Taking advantage of this strategy, Li and co-workers developed a protocol to synthesize arylpling ketones using from DMF aryl as iodides CO surrogate. and aryl boronic In this acids case, by TiO carbonylative2 was found Suzuki–Miyaura essential in the in situ couplinggeneration using of DMF CO from as CO DMF surrogate. and Inalso this in case, the TiOredu2 wasction found of Pd(II) essential to inPd(0) the inspecies, situ as it was generationable to start of COthe from catalytic DMF andcycle also [39]. in the reduction of Pd(II) to Pd(0) species, as it was able toA start carbonylative the catalytic cycleMizoroki–Heck [39]. reaction was also developed under palladium catal- ysisA and carbonylative photo-induced Mizoroki–Heck conditions reaction [40]. was The also reaction developed of alkyl under iodides, palladium CO cataly- and aryl alkenes sis and photo-induced conditions [40]. The reaction of alkyl iodides, CO and aryl alkenes leading to α,β-unsaturated ketones was carried out in the presence of Pd(PPh3)4 as a cata- leading to α,β-unsaturated ketones was carried out in the presence of Pd(PPh3)4 as a catalystlyst and and DBU DBU as as a a base base under irradiation irradiation of aof xenon a xenon lamp lamp (500 W)(500 (Scheme W) (Scheme 13). 13).

SchemeScheme 13. 13.The The synthesis synthesis of ketones of ketones via the via Pd-catalyzed/light-induced the Pd-catalyzed/light-induc carbonylativeed carbonylative Mizoroki– Mizoroki– HeckHeck reaction. reaction.

In this approach, alkyl radicals (I) were formed from alkyl iodides via single-electron transfer (SET) and underwent a sequential addition to CO and alkenes to give β-keto radicals (III). It is proposed that DBU would abstract a proton in α to the carbonyl to form radical anions (IV), giving α,β-unsaturated ketones via SET (Scheme 14).

Scheme 14. The proposed reaction pathway.

Recently, Odell and co-workers have developed a Pd(PPh3)4-catalyzed carbonylative Suzuki–Miyaura coupling of unactivated alkyl halides with aryl boronic acids by means

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 34

Scheme 12. The synthesis of diketones via Pd-catalyzed/light-induced cross-coupling reactions.

More recently, a similar approach was employed to synthesize aromatic β-keto esters from α-iodoesters, CO and arylboronic acids under lower CO pressures (10 atm) [38]. Tak- ing advantage of this strategy, Li and co-workers developed a protocol to synthesize aryl ketones from aryl iodides and aryl boronic acids by carbonylative Suzuki–Miyaura coupling using DMF as CO surrogate. In this case, TiO2 was found essential in the in situ generation of CO from DMF and also in the reduction of Pd(II) to Pd(0) species, as it was able to start the catalytic cycle [39]. A carbonylative Mizoroki–Heck reaction was also developed under palladium catalysis and photo-induced conditions [40]. The reaction of alkyl iodides, CO and aryl alkenes leading to α,β-unsaturated ketones was carried out in the presence of Pd(PPh3)4 as a catalyst and DBU as a base under irradiation of a xenon lamp (500 W) (Scheme 13).

Catalysts 2021, 11, 918 Scheme 13. The synthesis of ketones via the Pd-catalyzed/light-induced carbonylative10 of 35 Mizoroki– Heck reaction.

InIn this this approach, approach, alkyl alkyl radicals radicals (I) were (I) formedwere formed from alkyl from iodides alkyl via iodides single-electron via single-electron transfertransfer (SET) (SET) and and underwent underwent a sequential a sequential addition addition to CO to and CO alkenes and alkenes to give toβ-keto give β-keto rad- radicalsicals (III (III).). It It is is proposed that that DBU DBU would would abstract abstract a proton a proton in α to the in carbonylα to the to carbonyl form to form radicalradical anions anions (IV (),IV giving), givingα,β-unsaturated α,β-unsaturated ketones ketones via SET via (Scheme SET 14(Scheme). 14).

SchemeScheme 14. 14.The The proposed proposed reaction reaction pathway. pathway. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 34

Recently, Odell and co-workers have developed a Pd(PPh ) -catalyzed carbonylative Recently, Odell and co-workers have developed a3 Pd(PPh4 3)4-catalyzed carbonylative Suzuki–Miyaura coupling of unactivated alkyl halides with aryl boronic acids by means Suzuki–Miyaura coupling of unactivated alkyl halides with aryl boronic acids by means ofof visible-lightvisible-light irradiationirradiation withwith lowlow COCO pressurepressure (2–3(2–3 atm)atm) atat roomroom temperaturetemperature forfor thethe synthesissynthesis ofof arylaryl alkylalkyl ketonesketones [[41].41]. ThisThis methodologymethodology featuresfeatures thethe useuse ofof solidsolid CO-sourceCO-source instead of gaseous CO, readily available LED lights and a double-chamber system, one instead of gaseous CO, readily available LED lights and a double-chamber system, one for the CO generation and the other for the palladium-catalyzed reaction (Scheme 15). for the CO generation and the other for the palladium-catalyzed reaction (Scheme 15). Under the optimized conditions, various alkyl iodides and bromides reacted with CO and Under the optimized conditions, various alkyl iodides and bromides reacted with CO and differently substituted aryl boronic acids to provide the desired ketones with moderate to differently substituted aryl boronic acids to provide the desired ketones with moderate to good yields. The same protocol was successfully applied for the synthesis of a precursor of good yields. The same protocol was successfully applied for the synthesis of a precursor melperone, an antipsychotic drug. of melperone, an antipsychotic drug.

SchemeScheme 15.15.The The synthesissynthesis ofof alkyl alkyl aryl aryl ketones ketones via via the th Pd-catalyzed/light-inducede Pd-catalyzed/light-induced carbonylativecarbonylative Suzuki–MiyauraSuzuki–Miyaura coupling.coupling.

Based on control experiments, the authors verified the crucial role of the visible light as well as of the palladium catalyst. Moreover, the addition of TEMPO completely prevented the product formation while, at the same time, the R-TEMPO adduct was detected. A recent report by Arndtsen’s group has brought a breakthrough in the area of palladium-catalyzed carbonylation reactions. A new palladium-catalyzed/light-induced methodology for the synthesis of a broad array of carbonylated compounds (acid chlorides, esters, amides and ketones) starting from aryl/alkyl iodides and bromides has been reported (Scheme 16) [42]. Compared to previous protocols, this method features high versatility, high functional group tolerance, an impressive reaction scope and mild reaction conditions. In particular, this protocol is compatible with aldehydes, protected amines, esters, nitriles, thioethers and heterocyclic substrates on aryl iodides and bromides. The synthesis of β-amino acid derivatives was achieved with high efficiency. In addition, electron-rich arenes and heteroarenes were successfully employed in place of nitrogen or oxygen nucleophiles. The importance of palladium catalyst and visible light in this radical reaction has been consistently demonstrated, and, in particular, mechanistic studies revealed that light acts as two different roles in this transformation, enabling, together with the palladium catalytic system, both the oxidative addition and reductive elimination steps.

Catalysts 2021, 11, 918 11 of 35

Based on control experiments, the authors veriﬁed the crucial role of the visible light as well as of the palladium catalyst. Moreover, the addition of TEMPO completely prevented the product formation while, at the same time, the R-TEMPO adduct was detected. A recent report by Arndtsen’s group has brought a breakthrough in the area of palladium-catalyzed carbonylation reactions. A new palladium-catalyzed/light-induced methodology for the synthesis of a broad array of carbonylated compounds (acid chlorides, esters, amides and ketones) starting from aryl/alkyl iodides and bromides has been reported (Scheme 16)[42]. Compared to previous protocols, this method features high versatility, high functional group tolerance, an impressive reaction scope and mild reaction conditions. In particular, this protocol is compatible with aldehydes, protected amines, esters, nitriles, thioethers and heterocyclic substrates on aryl iodides and bromides. The synthesis of β-amino acid derivatives was achieved with high efﬁciency. In addition, electron-rich arenes and heteroarenes were successfully employed in place of nitrogen or oxygen nucleophiles. The importance of palladium catalyst and visible light in this radical reaction has been consistently demonstrated, and, in particular, mechanistic studies revealed that light acts as two different roles in this transformation, enabling, Catalysts 2021, 11, x FOR PEER REVIEWtogether with the palladium catalytic system, both the oxidative addition and reductive11 of 34

elimination steps.

SchemeScheme 16.16. TheThe generalgeneral synthesissynthesis ofof carbonylated carbonylated compounds compounds via via the the palladium-catalyzed/light- palladium-catalyzed/light- inducedinduced carbonylativecarbonylative approach.approach.

Recently,Recently, thethe palladium-catalyzedpalladium-catalyzed aminocarbonylationaminocarbonylation ofof aliphaticaliphatic andand aromaticaromatic io-iodidesdides underunder visible-lightvisible-light irradiationirradiation has beenbeen reportedreported byby SardanaSardana andand co-workersco-workers [[43].43]. TheThe methodology,methodology, basedbased onon thethe employmentemployment ofof COgenCOgen (9-methyl-9H-ﬂuorene-9-carbonyl(9-methyl-9H-fluorene-9-carbonyl chloride,chloride, 1 equiv) in in a a two-chamber two-chamber system system,, can can be be extended extended to tothe the synthesis synthesis of [ of14C]-am- [14C]- amidesides starting starting from from the the corresponding corresponding 14Cogen.14Cogen. Despite Despite the themoderate moderate yields, yields, tertiary tertiary am- amidesides and and C-14-labeled C-14-labeled pharmaceutical pharmaceutical products products were were successfully successfully accessed. accessed. OxalamidesOxalamides representrepresent aa classclass ofof relevantrelevant compoundscompounds frequentlyfrequently foundfound inin bioactivebioactive andand pharmaceuticalpharmaceutical moleculesmolecules [[44]44] andand usedused asas aa ligandligand inin asymmetricasymmetric catalysiscatalysis [[45].45]. WuWu andand co-workersco-workers havehave recentlyrecently developeddeveloped aa novel,novel, highlyhighly sustainablesustainable visible-light-inducedvisible-light-induced palladium-catalyzed method for the synthesis of oxalamides from amines and carbon monoxide [46]. The versatile and green methodology relies on the use of visible light in combination with palladium/BINAP catalysts (Scheme 17) without any oxidant or base. In addition, the protocol features high selectivity (no urea byproducts were detected) and the possibility to reuse the Pd complex without loss of efficiency. Notably, in some cases, the presence of methylene blue as photosensitizer led to a significant improvement in the yield.

Catalysts 2021, 11, 918 12 of 35

palladium-catalyzed method for the synthesis of oxalamides from amines and carbon monoxide [46]. The versatile and green methodology relies on the use of visible light in combination with palladium/BINAP catalysts (Scheme 17) without any oxidant or base. In addition, the protocol features high selectivity (no urea byproducts were detected) and the Catalysts 2021,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 12 of 34 possibility to reuse the Pd complex without loss of efﬁciency. Notably, in some cases, the presence of methylene blue as photosensitizer led to a signiﬁcant improvement in the yield.

SchemeScheme 17.17. TheThe generalgeneral photocatalyticphotocatalytic synthesissynthesis ofof oxalamidesoxalamides viavia thethe palladium-catalyzedpalladium-catalyzed dehy-dehy- drogenativedrogenative carbonylationcarbonylation ofof amines.amines.

BasedBased onon controlcontrol andand EPREPR experiments,experiments, thethe authorsauthors proposepropose thatthat the the active active L L22Pd(0)*Pd(0)* speciesspecies II isis firstfirst ﬁrst generatedgenerated generated inin in situsitu situ fromfrom from LL L2Pd(II)Cl2Pd(II)Cl2 inin2 inthethe the presencepresence presence ofof thethe of the amineamine amine underunder under vis-vis- visible-lightible-light irradiation irradiation (Scheme (Scheme 18). 18 ).Then, Then, afte afterr a aSET SET process process with with the the amine, amine, a nitrogen-nitrogen- radicalradical andand aa Pd(I)-radicalPd(I)-radical speciesspecies IIII are areare generated. generated.generated. TheTheThe reactionreactionreaction of ofof CO COCO with withwiththe thethenitrogen nitrogennitrogen radicalradical leads to to an an acyl acyl radical, radical, which which recombines recombines with with II deliveringII delivering intermediate intermediate III.. TheTheIII. Thelatter latter undergoes undergoes a ligand a ligand exchange exchange to provide to provide complex complex IV IVandandand molecularmolecular molecular hydrogen,hydrogen, hydrogen,which which was detected was detected by gas by GC. gas The GC. insertion The insertion of another of another CO molecule CO molecule provides provides a bis(car- a bis(carbamoyl)palladiumbamoyl)palladium intermediate intermediate V,, whichwhichV, which undergoesundergoes undergoes aa reductivereductive a reductive eliminationelimination elimination stepstep steptoto givegive to givethe expected the expected oxamide oxamide and anda Pd(0) a Pd(0) species. species.

SchemeScheme 18.18. TheThe proposedproposed reactionreaction mechanism.mechanism.

In 2016, Lei et al. disclosed a new intramolecular oxidative carbonylation of enamides as a mild and environmentally friendly protocol for the synthesis of 1,3-oxazin-6-ones. Oxygen was employed as a terminal oxidant, avoiding the need for stoichiometric amounts of metal salts, such as Cu(II) salts [47]. The success of the strategy relies on the combination of palladium and photoredox catalysis and, in particular, Pd(OAc)2 andand Ru(bpy)3Cl2 werewere bothboth usedused inin catalyticcatalytic amountsamounts together with Xantphos as phosphine

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Catalysts 2021, 11, 918 13 of 35

ligand (Scheme 19). The generality of the reaction was demonstrated as both electron- In 2016, Lei et al. disclosed a new intramolecular oxidative carbonylation of enamides aswithdrawing a mild and environmentally and electron-donating friendly protocol forgroups, the synthesis including of 1,3-oxazin-6-ones. heteroaromatic rings (furan and Oxygenthiophene), was employed were as well a terminal tolerated oxidant, under avoiding the the needoptimized for stoichiometric conditions. amounts The authors suggest that ofthe metal photocatalyst salts, such as Cu(II) is saltsexclusively [47]. The success involved of the strategyin the relies reoxidation on the combination of palladium, as described in of palladium and photoredox catalysis and, in particular, Pd(OAc)2 and Ru(bpy)3Cl2 were bothScheme used in 20. catalytic Firstly, amounts taking together advantage with Xantphos of the as phosphine amide group, ligand (Scheme a Pd 19catalyzed). alkenyl C–H ac- Thetivation generality affords of the reaction a vinylpalladium was demonstrated complex as both electron-withdrawing I. Then, a molecule and electronof CO coordinates and in- donating groups, including heteroaromatic rings (furan and thiophene), were well tolerated underserts, the leading optimized to conditions. the acylpalladium The authors suggest intermediate that the photocatalyst II. Subsequently, is exclusively DABCO enables the for- involvedmation in of the complex reoxidation ofIII palladium,, and, finally, as described a reductive in Scheme 20 .elimination Firstly, taking advan- step delivers the oxazinone tageproduct of the amide and group,Pd(0) a Pdspecies. catalyzed Re-oxidation alkenyl C–H activation of palladium(0) affords a vinylpalladium can be exerted by the excited complex I. Then, a molecule of CO coordinates and inserts, leading to the acylpalladium intermediateRu(II)* leadingII. Subsequently, to Pd(II) DABCOL, which enables restart the formation a new of complexcatalyticIII ,cycle, and, ﬁnally, and a Ru(I), which, in its turn, reductivecan be eliminationoxidized stepback delivers to Ru(II) the oxazinone by molecular product and oxygen. Pd(0) species. The Re-oxidationgenerated superoxide anion may of palladium(0) can be exerted by the excited Ru(II)* leading to Pd(II)L, which restart a new catalyticalso oxidize cycle, and the Ru(I), Pd(0) which, species in its turn, by can electron be oxidized transfer. back to Ru(II) The bypresence molecular of KI was found to im- oxygen.prove Thethe generated efficiency superoxide of this anion transformation, may also oxidize thelikely Pd(0) through species by electroncoordination of palladium(II) transfer. The presence of KI was found to improve the efﬁciency of this transformation, species, while Ac2O was suggested to reduce the possible formation of inactive palla- likely through coordination of palladium(II) species, while Ac2O was suggested to reduce thedium(0) possible formationspecies ofunder inactive the palladium(0) CO atmosphere. species under the CO atmosphere.

SchemeScheme 19. 19.Visible-light-promoted Visible-light-promoted Pd/Ru-catalyzed Pd/Ru-catalyzed intramolecular oxidative intramolecular carbonylation oxidative of carbonylation of en- enamidesamides to to 1,3-oxazin-6-ones. 1,3-oxazin-6-ones.

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SchemeScheme 20. 20.The The proposed proposed reaction reaction pathway. pathway.

1.2. Visible-Light-Promoted Iridium-Catalyzed Carbonylations 1.2. Visible-Light-Promoted Iridium-Catalyzed Carbonylations Amides are ubiquitous in both natural and synthetic compounds, including pharma- ceuticalAmides products. are Odell ubiquitous and co-workers in both have natural recently and developed synthetic a compounds, visible light-mediated including pharma- facceutical-Ir(ppy) products.3 catalyzed Odell amino and carbonylation co-workers of have unactivated recently alkyl developed iodides under a visible mild light-mediated re- actionfac-Ir(ppy) conditions3 catalyzed [48]. The amino desired carbonylation amides are produced of unactivated in moderate alkyl to excellent iodides yields under mild reac- throughtion conditions a two-chamber [48]. system The desired (H-tube), amides where the are CO produced is released in ex situmoderate from Mo(CO) to excellent6) yields (Scheme 21). Secondary and tertiary iodides can be successfully aminocarbonylated with a through a two-chamber system (H-tube), where the CO is released ex situ from Mo(CO)6) wide range of amine nucleophiles, whereas primary iodides provide satisfactory results in combination(Scheme 21). with Secondary hindered amines and tertiary only. Remarkably, iodides can a dealkylative-aminocarbonylation be successfully aminocarbonylated with pathwaya wide range occurs of when amine alkyl nucleophiles, halides react withwhereas tertiary primary amines. iodides The reaction provide mechanism satisfactory results startsin combination with the reductive with dehalogenationhindered amines of an only. alkyl halideRemarkably, (R–X) to givea dealkylative-aminocarbonyla- the alkyl radical I, followedtion pathway by CO insertion occurs withwhen the alkyl formation halides of an reac acylt radicalwith tertiary intermediate amines.II. The The latter reaction is mecha- eithernism quenchedstarts with by thethe starting reductive alkyl iodidedehalogenation to produce anof acylan iodidealkyl halideIII or oxidized (R–X) to angive the alkyl acylium ion IV. The nucleophilic attack of the amine on the acyl iodide or the acylium ion radical I, followed by CO insertion with the formation of an acyl radical intermediate II. affords the final amide (Scheme 22). TheA latter variety is ofeither acetate-containing quenched by 2,3-dihydrobenzofurans the starting alkyl iodide have to been produce synthesized an acyl by iodide III or Polyzosoxidized et al.to an through acylium a visible ion IV light-mediated. The nucleophilic Ir-catalyzed attack radical of the carbonylation amine on the pro- acyl iodide or cessthe underacylium continuous ion affords flow the conditions final amide [49]. (Scheme Overcoming 22). the traditional limitations of both Pd-catalyzed alkoxycarbonylation reactions and classical radical-free carbonylation processes in terms of regioselectivity, the pressure of CO and sustainability [50], alkenyl-tethered arenediazonium salts underwent a versatile and stereoselective (exclusive 5-exo-dig cyclization) cyclization and alkoxycarbonylation cascade to a wide range of 2,3-dihydrobenzofuran derivatives in the presence of [Ir(dtbbpy)(ppy)2]PF6 as catalyst under blue LEDs (14 W) at room temperature (Scheme 23). Electron-donating and electron-withdrawing substituents on the arenediazonium salt moiety were nicely tolerated under the standard conditions. Regardless of steric bulk, alkyl alcohols were successfully employed as coupling partners to produce the desired esters in moderate to good yields. This continuous flow methodology also features a very short reaction time (200 s), moder-

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Catalysts 2021, 11, x FOR PEER REVIEW 15 of 34

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 34 ate CO pressures (25 atm) and a straightforward scale-up. Interestingly, the commercial Ru(bpy)3Cl2·6H2O was found to be slightly less efﬁcient in this transformation. Chamber 1: Ir(ppy)3, TBA,Chamber Hantzsch 1: Ir(ppy) ester3, O TBA, Hantzsch ester 1 Blue LEDs, r.t. O 3 R I HNR2R3 1 R 1 Blue LEDs, r.t. R N 3 R I HNR2R3 1 R Chamber 2: R N R2 Mo(COChamber) ,DBU,70°C 2: R2 6 up to 90% Mo(CO)6,DBU,70°C up33 to examples 90% R1 = secondary and tertiary alkyl groups, 33 examples R1 = secondary and tertiary alkyl groups, primary (limited to hindered amines) Features: 2016 Odell Features: 2016 Odell primary (limited to hindered amines) -hv:blueLEDs -hv:blueLEDs - 1–2 atm of ex situ generated CO - 1–2 atm of ex situ generated CO Selected examples: - Secondary and primary alkyl iodides Selected examples: - Secondary and primary alkyl iodides - mild reaction conditions O - mild reaction conditions O O O O O NN O O Bu N Bu N N N N Bu 72%72% Bu 62% 11 -HSD1 inhibitor 67% 48%48% (from (from TBA) TBA) 11 -HSD1 inhibitor 67%

SchemeScheme 21. 21.Visible Visible light-mediated light-mediatedlight-mediatedfac-Ir(ppy) fac fac-Ir(ppy)3-Ir(ppy)-catalyzed3-catalyzed3-catalyzed aminocarbonylation aminocarbonyla aminocarbonyla of alkyltion iodides.tion of alkylof alkyl iodides. iodides.

Scheme 22. The proposed reaction pathway. Scheme 22. 22.The The proposed proposed reaction reaction pathway. pathway. A variety of acetate-containing 2,3-dihydrobenzofurans have been synthesized by PolyzosA variety et al. through of acetate-containing a visible light-mediated 2,3-dihydr Ir-catalyzedobenzofurans radical have carbonylation been synthesized process by Polyzosunder continuous et al. through flow aconditions visible light-mediated [49]. Overcoming Ir-catalyzed the traditional radical limitations carbonylation of both process Pd- undercatalyzed continuous alkoxycarbonylation flow conditions reactions [49]. Overcomingand classical radical-free the traditional carbonylation limitations processes of both Pd- catalyzedin terms of alkoxycarbonylation regioselectivity, the reactionspressure ofand CO classical and sustaina radical-freebility [50],carbonylation alkenyl-tethered processes inarenediazonium terms of regioselectivity, salts underwent the pressure a versatile of andCO andstereoselective sustainability (exclusive [50], alkenyl-tethered 5-exo-dig cy- arenediazoniumclization) cyclization salts and underwent alkoxycarbonylation a versatile cascade and stereoselective to a wide range (exclusive of 2,3-dihydroben- 5-exo-dig cyclization)zofuran derivatives cyclization in and the alkoxycarbonylationpresence of [Ir(dtbbpy)(ppy) cascade2]PF to a6 aswide catalyst range under of 2,3-dihydroben- blue LEDs zofuran(14 W) at derivatives room temperature in the presence (Scheme of 23). [Ir(dtbbpy)(ppy) Electron-donating2]PF 6and as catalyst electron-withdrawing under blue LEDs (14substituents W) at room on the temperature arenediazonium (Scheme salt 23).moie Electron-donatingty were nicely tolerated and electron-withdrawingunder the standard substituentsconditions. Regardless on the arenediazonium of steric bulk, saltalkyl moie alcoholsty were were nicely successfully tolerated employed under the as standard cou- conditions.pling partners Regardless to produce of thesteric desired bulk, esters alkyl inalcohols moderate were to goodsuccessfully yields. This employed continuous as cou- plingflow methodology partners to produce also features the desired a very short esters re inaction moderate time (200 to goods), moderate yields. COThis pressures continuous flow(25 atm) methodology and a straightforward also features scale-up. a very short Interestingly, reaction timethe commercial (200 s), moderate Ru(bpy) CO3Cl 2pressures·6H2O was found to be slightly less efficient in this transformation. (25 atm) and a straightforward scale-up. Interestingly, the commercial Ru(bpy)3Cl2·6H2O was found to be slightly less efficient in this transformation.

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Scheme 23. Visible light-mediated [Ir(dtbbpy)(ppy)2]PF6-catalyzed cyclizative alkoxycarbonylation of alkenyl-tethered arenediazonium salts under continuous flow conditions. SchemeScheme 23.23. Visible light-mediatedlight-mediated [Ir(dtbbpy)(ppy)[Ir(dtbbpy)(ppy)22]PF]PF66-catalyzed cyclizativecyclizative alkoxycarbonylationalkoxycarbonylation ofof alkenyl-tetheredalkenyl-tetheredThe mechanism arenediazoniumarenediazonium is supposed saltssalts underunder to continuouscontinuousstart with ﬂowflow the conditions.conditions. homolytic cleavage of the C–N bond of the Theallyloxy-tetheredThe mechanism is supposed supposed arenediazonium to to start start with with saltthe the homolytic homolyticby single-electron cleavage cleavage of of the transfer the C–N C–N bond from bond of the photocata- ofthelyst the allyloxy-tethered in allyloxy-tethered its excited arenediazoniumstate arenediazonium (PC*), with salt salt bythe single-electron by generation single-electron transfer of transferan fromaryl from theradical thephotocata- photo- I and the oxidized catalystlystphotocatalyst in its in excited its excited PC state•+ state (Scheme(PC*), (PC*), with with 24). the the Angeneration generation intramolecular of of an an aryl aryl radical radical II andalkene thethe oxidizedoxidizedaddition provides the •+ photocatalystphotocatalystprimary alkyl PCPC •+radical (Scheme II ,2424). which,). An intramolecularintramolecular after CO insertion, radicalradical alkenealkene gives additionaddition the acyl providesprovides radical thethe III . Oxidation of primaryprimary alkylalkyl radicalradical IIII,, which,which, afterafter COCO insertion,insertion, givesgives thethe acylacyl radicalradical IIIIII.. OxidationOxidation ofof the acyl radical by the oxidized photocatalyst•+ PC•+ results in the formation of an acylium thethe acylacyl radicalradical byby thethe oxidizedoxidized photocatalystphotocatalyst PCPC•+ results inin thethe formationformation ofof anan acyliumacylium ionionion IVIV IV and and the the regeneration regeneration ofof the of photocatalyst the photocatalyst PC.PC. Lastly,Lastly, PC. thethe reactionLastly,reaction of theofIV IV reaction withwith alcoholalcohol of IV with alcohol leadsleadsleads toto to thethe the ﬁnalfinal final product.product. product.

Scheme 24. A plausible reaction pathway.

In 2020, Polyzos and co-workers reported an Ir-catalyzed visible-light-promoted aminocarbonylation of alkyl and aryl halides with CO and amines under continuous flow Scheme 24. A plausible reaction pathway. conditionsScheme 24. (Scheme A plausible 25) [51]. reaction The iridium-based pathway. photocatalyst ([Ir(ppy)2(dtbbpy)]PF6) was used in combination with DIPEA (N,N-diisopropylethylamine) as a sacrificial reductant and underIn 2020, blue PolyzosLED (54 W) and irradiation. co-workers A large reported variety ofan aryl Ir-catalyzed halides (I, Br, visible-light-promoted Cl) and alkyl aminocarbonylation of alkyl and aryl halides with CO and amines under continuous flow conditions (Scheme 25) [51]. The iridium-based photocatalyst ([Ir(ppy)2(dtbbpy)]PF6) was

used in combination with DIPEA (N,N-diisopropylethylamine) as a sacrificial reductant and under blue LED (54 W) irradiation. A large variety of aryl halides (I, Br, Cl) and alkyl

Catalysts 2021, 11, 918 17 of 35

In 2020, Polyzos and co-workers reported an Ir-catalyzed visible-light-promoted Catalysts 2021, 11, x FOR PEER REVIEW aminocarbonylation of alkyl and aryl halides with CO and amines under continuous ﬂow17 of 34

conditions (Scheme 25)[51]. The iridium-based photocatalyst ([Ir(ppy)2(dtbbpy)]PF6) was used in combination with DIPEA (N,N-diisopropylethylamine) as a sacriﬁcial reductant and under blue LED (54 W) irradiation. A large variety of aryl halides (I, Br, Cl) and alkyl iodidesiodides (primary, (primary, secondary secondary and and tertiary) tertiary) reacted reacted successfully successfully with CO with and CO an array and ofan alkyl array of alkyland and aryl aryl amines amines to provide to provide the expected the expected amides in amides good to in high good yields. to high The continuous yields. The ﬂow contin- uoussystem flow wassystem assembled was assembled from commercially from comme availablercially components, available components, featuring operational featuring op- erationalsimplicity, simplicity, high applicability, high applicability, improved safetyimproved and easy safety scalability. and easy scalability.

SchemeScheme 25. Visible 25. Visible light-mediated light-mediated [Ir(dtbbpy)(ppy) [Ir(dtbbpy)(ppy)22]PF]PF66-catalyzed-catalyzed aminocarbonylationaminocarbonylation of of aryl aryl and and alkyl alkyl halides halides under under continuouscontinuous flow ﬂowconditions. conditions.

YnonesYnones represent represent an an important important motifmotif present present in in bioactive bioactive compounds compounds and areand useful are useful intermediates in the synthesis of heterocycles [52]. Alternative to the previously described intermediates in the synthesis of heterocycles [52]. Alternative to the previously described Ryu’s method [34], Lu and Xiao et al. have recently developed an efﬁcient protocol for Ryu’sthe method synthesis [34], of ynones Lu and through Xiao et a al. visible-light-induced have recently developed photocatalytic an efficient decarboxylative protocol for the synthesiscarbonylative of ynones alkynylation through of a carboxylic visible-light- acidsinduced in the presence photocatalytic of carbonmonoxide decarboxylative (CO) at car- bonylativeroom temperature alkynylation [53]. of The carboxylic decarboxylative acids alkynylationin the presence reaction of carbon was carried monoxide out using (CO) at roomethynylbenziodoxolones temperature [53]. The (EBX) decarboxylative as the alkynylating alkynylation agent, carboxylic reaction acid, was in the carried presence out of using ethynylbenziodoxolonesIr[dF(CF3)ppy]2(dtbbpy)PF (EBX)6 as aas photocatalyst the alkynylating and Cs agent,2CO3 as ca arboxylic base (Scheme acid, 26 in). the A range presence of Ir[dF(CFof aliphatic3)ppy] carboxylic2(dtbbpy)PF acids gave6 as a the photocatalyst corresponding and ynones Cs2CO in good3 as a to base excellent (Scheme yields 26). A rangeunder of mildaliphatic conditions carboxylic but extremely acids gave high the pressure corresponding of CO. Mechanistically, ynones in itgood is proposed to excellent that the excited photocatalyst Ir(III)*, generated from Ir(III) under visible-light irradiation, yields under mild conditions but extremely high pressure of CO. Mechanistically, it is is able to oxidize the substrate with the formation of an alkyl radical I (Scheme 27). Then, proposedthe latter, that after the CO excited insertion, photocatalyst gives rise toIr(III) an*, acyl generated radical II from, which Ir(III) undergoes under visible-light radical irradiation,addition tois able the alkylating to oxidize agent the substrate affording with the radical the formation intermediate of anIII alkyl. The radical subsequent I (Scheme 27). Then, the latter, after CO insertion, gives rise to an acyl radical II, which undergoes radical addition to the alkylating agent affording the radical intermediate III. The subsequent radical elimination reaction yields the desired ynone and the benziodoxolonyl radical IV. Finally, radical IV is reduced to ortho-iodobenzoate by Ir(II), which, in turn, is oxidized to Ir(III) for a new catalytic cycle.

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Catalysts 2021, 11, x FOR PEER REVIEW 18 of 34 radical elimination reaction yields the desired ynone and the benziodoxolonyl radical IV. Catalysts 2021, 11, x FOR PEER REVIEW 18 of 34 Finally, radical IV is reduced to ortho-iodobenzoate by Ir(II), which, in turn, is oxidized to Ir(III) for a new catalytic cycle.

SchemeSchemeScheme 26. 26. Visible 26. VisibleVisible light-promoted light-promoted light-promoted photo-catalyzed photo-catalyzedphoto-catalyzed decarboxylative decarboxylative decarboxylative carbonylative carbonylative carbonylative alkynylation alkynylation ofof carboxylic carboxylic of acids carboxylicacids to acids to ynones.ynones.to ynones.

Scheme 27. The proposed reaction mechanism.

SchemeMore 27.recently,SchemeThe proposed the 27. same The reaction proposed research mechanism. groupreaction de mechanism.veloped a visible-light-driven photocatalyzed synthesis of α,β-unsaturated ketones starting from alkyl Katritzky salts as a source More recently, the same research group developed a visible-light-driven photocat- of radicals [54]. In the context of a more general reactivity, a limited number of alkyl Kat- alyzedMore synthesis recently, of α,β-unsaturated the same research ketones starting group from de alkylveloped Katritzky a visible-light-driven salts as a source photocata- ritzkyoflyzed radicals salts synthesis reacted [54]. Inwith of the α 1,1-diphenylet context,β-unsaturated of a morehylene ketones general in the reactivity, starting presence afrom of limited [Ir(4-Fppy) alkyl number Katritzky2(bpy)]PF of alkyl salts6 and as a source DABCOKatritzkyof radicals in MeCN salts [54]. reacted under In withthe CO context 1,1-diphenylethylene atmosphere of a more and geblue inneral the LEDs presence reactivity, irradiation, of [Ir(4-Fppy) a limited leading2(bpy)]PF number to α6,β-un- of alkyl Kat- saturatedand DABCO ketones in MeCN (Scheme under 28). CO In atmosphere the proposed and reaction blue LEDs mechanism, irradiation, leadingthe excited to α, βphoto-- ritzky salts reacted with 1,1-diphenylethylene in the presence of [Ir(4-Fppy)2(bpy)]PF6 and unsaturated ketones (Scheme 28). In the proposed reaction mechanism, the excited pho- catalystDABCO Ir(III)* in MeCNis supposed under to CO reduced atmosphere alkyl Katritzky and blue salts LEDs through irradiation, a SET leadingprocess to α,β-un- (Schemetocatalyst 29). Ir(III)*The generated is supposed radicals to reduced I inserts alkyl one Katritzky molecule salts of CO through with the a SET formation process of an saturated ketones (Scheme 28). In the proposed reaction mechanism, the excited photo- acyl radical II, which reacts with 1,1-diphenylethylene yielding the radical species III. In- termediatecatalyst III Ir(III)* is then is reduced supposed by the to photocatalyst reduced alkyl Ir(IV) Katritzky with the formationsalts through of the acati- SET process onic(Scheme intermediate 29). TheIV, which,generated lastly, radicals undergoes I inserts a DABCO-mediated one molecule ofdeprotonation CO with the step formation to of an deliveracyl the radical final ketone.II, which reacts with 1,1-diphenylethylene yielding the radical species III. In- termediate III is then reduced by the photocatalyst Ir(IV) with the formation of the cationic intermediate IV, which, lastly, undergoes a DABCO-mediated deprotonation step to deliver the final ketone.

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Catalysts 2021, 11, x FOR PEER REVIEW 19 of 34

(Scheme 29). The generated radicals I inserts one molecule of CO with the formation of an acyl radical II, which reacts with 1,1-diphenylethylene yielding the radical species III. Intermediate III is then reduced by the photocatalyst Ir(IV) with the formation of the Catalysts 2021, 11, x FOR PEER REVIEW 19 of 34 cationic intermediate IV, which, lastly, undergoes a DABCO-mediated deprotonation step to deliver the ﬁnal ketone.

SchemeScheme 28.28. 28. VisibleVisible Visible light-promoted light-promoted light-promoted Ir-catalyzed Ir-catalyzed Ir-catalyzed carb carbonylativeonylative carb alkyl-Heck-typeonylative alkyl-Heck-type alkyl-Heck-type reactions reactions to α to,β αreactions-un-,β- to α,β-un- unsaturatedsaturatedsaturated ketones. ketones. ketones.

Scheme 29. The proposed reaction mechanism.

1.3. Visible-Light-Promoted Cobalt-Catalyzed Carbonylations SchemeCobalt 29. 29.The saltsThe proposed proposedhave reactionbeen reaction found mechanism. to mechanism. efficiently catalyze the carbonylation of alkenes, chloro and bromoalkanes under UV irradiation [55–59]. The first report on visible-light- 1.3.mediated Visible-Light-Promoted cobalt-catalyzed Cobalt-Catalyzed carbonylation reacti Carbonylationson appeared in 2012, when Jia, Yin and col- 1.3. Visible-Light-Promoted Cobalt-Catalyzed Carbonylations leagues,Cobalt based salts on have their been previous found findings to efficiently [60], catalyzedeveloped the a carbonylation Co(OAc)2-catalyzed of alkenes, alkoxycar- chloro andbonylation bromoalkanesCobalt of arylsalts underbromides have UV been irradiationunder found visible-light [55 –to59 ].efficientl Theirradiation first reporty andcatalyze onmild visible-light-mediated conditions the carbonylation (Scheme of alkenes, cobalt-catalyzedchloro30) [61]. andThe usebromoalkanes carbonylationof a strong base under reaction (NaOMe) UV appeared irradiationand an in organic 2012, [55–59]. when sensitizer Jia, The Yin (PhCOPh) andfirst colleagues, report was es- on visible-light- basedsential on to their the previousreaction’s findings success. [60 Aryl], developed bromides a with Co(OAc) a chloro2-catalyzed substituent alkoxycarbonylation behaved better ofmediatedthan aryl tolyl bromides bromides; cobalt-catalyzed under however, visible-light a carbonylation selectivity irradiation higher and reacti than mild 99%on conditions appeared was always (Scheme in observed. 2012, 30)[ 61when ]. The Jia, Yin and col- useleagues, of a strong based base on (NaOMe)their previous and an findings organic sensitizer [60], developed (PhCOPh) a was Co(OAc) essential2-catalyzed to the alkoxycar- reaction’sbonylation success. of aryl Aryl bromides bromides under with a visible-light chloro substituent irradiation behaved and better mild than conditions tolyl (Scheme bromides;30) [61]. however,The use aof selectivity a strong higher base than (NaOMe) 99% was and always an observed. organic sensitizer (PhCOPh) was essential to the reaction’s success. Aryl bromides with a chloro substituent behaved better than tolyl bromides; however, a selectivity higher than 99% was always observed.

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Scheme 30. Visible-light-mediated cobalt-catalyzed alkoxycarbonylation of aryl bromides. SchemeScheme 30. 30.Visible-light-mediated Visible-light-mediated cobalt-catalyzed cobalt-catalyzed alkoxycarbonylation alkoxycarbonylation of aryl bromides. of aryl bromides. Recently, Alexanian et al. reported the visible-light-promoted aminocarbonylation of Recently,Recently, Alexanian Alexanian et al. et reportedal. reported the visible-light-promoted the visible-light-promoted aminocarbonylation aminocarbonylation of of (hetero)(hetero) aryl/vinyl aryl/vinyl bromides bromides and and chlorides chlorides using using an inexpensive an inexpensive cobalt catalystcobalt catalyst (Co (CO) (Co) 2(CO)8) (hetero) aryl/vinyl bromides and chlorides using an inexpensive cobalt catalyst2 (Co8 2(CO)8) inin conjunctionconjunction with with tetramethylpiperidine tetramethylpiperidine (TMP) (TMP) (Scheme (Scheme 31)[62 31)]. [62]. in conjunction with tetramethylpiperidine (TMP) (Scheme 31) [62].

Scheme 31. Visible-light-mediated cobalt-catalyzed aminocarbonylation of bromo and chloroaryls. SchemeScheme 31. 31.Visible-light-mediated Visible-light-mediated cobalt-catalyzed cobalt-catalyzed aminocarbonylation aminocarbonylation of bromo of and bromo chloroaryls. and chloroaryls. ArylAryl bromides bromides having having both both electron-rich electron-rich and and electron-deficient electron-deficient groups, groups, as well as aswell as vi- Aryl bromides having both electron-rich and electron-deficient groups, as well as vi- vinylnyl bromides, bromides, were were well tolerated.tolerated. In In addition, addition, bromo bromo heteroaryls, heteroaryls, such such as pyridines, as pyridines, fu- nyl bromides, were well tolerated. In addition, bromo heteroaryls, such as pyridines, fu- furans,rans, quinolines quinolines and and indoles, indoles, alsoalso performedperformed nicely nicely in in this this reaction. reaction. Aryl Aryl chlorides chlorides with rans, quinolines and indoles, also performed nicely in this reaction. Aryl chlorides with withelectron-deficient electron-deficient groups groups gave gave the the expected expected am amidesides inin satisfactorysatisfactory to to good good yields. yields. A A wide wideelectron-deficient variety of primary groups and gave secondary the expected aliphatic amines,amides includingin satisfactory amino to acids, good gave yields. the A wide variety of primary and secondary aliphatic amines, including amino acids, gave the cor- correspondingvariety of primary amides and in moderate secondary to highaliphatic yields. amines, Preliminary including mechanistic amino investigations acids, gave the corresponding amides in moderate to high yields. Preliminary mechanistic− investigations wereresponding consistent amides with the in firstmoderate formation to high of a cobaltate yields. Preliminary anion [Co(CO) mechanistic4] (I) as an activeinvestigations were consistent with the first formation of a cobaltate anion [Co(CO)4]− (I) as an active catalystwere consistent and the subsequent with the generationfirst formation of an electronof a cobaltate donor–acceptor anion [Co(CO) complex4] (EDA)− (I) asII an active catalyst and the subsequent generation of an electron donor–acceptor complex (EDA) II catalyst and the subsequent generation of an electron donor–acceptor complex (EDA) II (Scheme 32). The intermediacy of the visible light leads to the corresponding excited spe- (Scheme 32). The intermediacy of the visible light leads to the corresponding excited species III. Then, a single-electron transfer produces two radical species (IV), which recom- cies III. Then, a single-electron transfer produces two radical species (IV), which recombines, leading to a (hetero)aryl or vinyl cobalt species V. Finally, migratory insertion of bines, leading to a (hetero)aryl or vinyl cobalt species V. Finally, migratory insertion of

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Catalysts 2021, 11, x FOR PEER REVIEW 21 of 34

(Scheme 32). The intermediacy of the visible light leads to the corresponding excited species III. Then, a single-electron transfer produces two radical species (IV), which recombines, leadingCO affords to a (hetero)aryl intermediate or vinyl cobaltVI, which species undergoesV. Finally, migratory displacement insertion of of CO the affords amine delivering the intermediate VI, which undergoes displacement of the amine delivering the amide and the activeamide catalyst. and the active catalyst.

SchemeScheme 32. 32.The The proposed proposed reaction reaction pathway. pathway. 1.4. Visible-Light-Promoted Rutenium-Catalyzed Carbonylations 1.4. CarboxylicVisible-Light-Promoted acids are ubiquitous Rute in anium-Catalyzed wide range of bioactive Carbonylations molecules and industrially relevantCarboxylic compounds acids and represent are ubiquitous a versatile, in functionala wide range group of in bioactive organic synthesis. molecules and industri- Bousquet et al. have recently reported a ruthenium-catalyzed visible-light-promoted synthesisally relevant of benzoic compounds acids starting and from represent aryl diazonium a versat salts,ile, carbon functional monoxide (10–50group atm) in organic synthesis. andBousquet water under et al. mild have conditions recently (Scheme reported 33)[63 ].a Theruthenium-catalyzed reaction displays a good visible-light-promoted functional syn- groupthesis tolerance of benzoic and generality. acids starting Notably, satisfactoryfrom aryl results diazonium were achieved salts, directlycarbon starting monoxide (10–50 atm) fromand anilineswater under through mild the in conditions situ generation (Scheme of aryl diazonium 33) [63]. salts,The avoidingreaction in displays this way a good functional their isolation. The proposed reaction pathway starts with the generation of an aryl radical (groupI) by SET tolerance with the excited and Ru(II)*generality. (Scheme Notably, 34). Next, thesatisf insertionactory of results CO on the were aryl radical achieved directly start- givesing from rise to anilines an acyl radical through (II), which the isin oxidized situ generation by the Ru(III) of speciesaryl diazonium to an acylium salts, ion avoiding in this IIIway. The their Ru(II) isolation. is regenerated, The and proposed the acylium reaction ion reacts pathway with water starts delivering with the the desired generation of an aryl carboxylic acid. radical (I) by SET with the excited Ru(II)* (Scheme 34). Next, the insertion of CO on the aryl radical gives rise to an acyl radical (II), which is oxidized by the Ru(III) species to an acylium ion III. The Ru(II) is regenerated, and the acylium ion reacts with water delivering the desired carboxylic acid.

Scheme 33. The Ru-catalyzed synthesis of carboxylic acids via hydroxycarbonylation of aryl diazonium salts under irradiation conditions.

Catalysts 2021, 11, x FOR PEER REVIEW 21 of 34

CO affords intermediate VI, which undergoes displacement of the amine delivering the amide and the active catalyst.

Scheme 32. The proposed reaction pathway.

1.4. Visible-Light-Promoted Rutenium-Catalyzed Carbonylations Carboxylic acids are ubiquitous in a wide range of bioactive molecules and industrially relevant compounds and represent a versatile, functional group in organic synthesis. Bousquet et al. have recently reported a ruthenium-catalyzed visible-light-promoted synthesis of benzoic acids starting from aryl diazonium salts, carbon monoxide (10–50 atm) and water under mild conditions (Scheme 33) [63]. The reaction displays a good functional group tolerance and generality. Notably, satisfactory results were achieved directly starting from anilines through the in situ generation of aryl diazonium salts, avoiding in this way their isolation. The proposed reaction pathway starts with the generation of an aryl radical (I) by SET with the excited Ru(II)* (Scheme 34). Next, the insertion of CO on the Catalysts 2021, 11, 918 aryl radical gives rise to an acyl radical (II), which is oxidized by the Ru(III) species22 to of an 35 acylium ion III. The Ru(II) is regenerated, and the acylium ion reacts with water delivering the desired carboxylic acid.

Catalysts 2021, 11, x FOR PEER REVIEW 22 of 34 SchemeScheme 33.33. TheThe Ru-catalyzedRu-catalyzed synthesissynthesis ofof carboxyliccarboxylic acidsacids viavia hydroxycarbonylationhydroxycarbonylation ofof arylaryl diazo-diazo- niumnium saltssalts underunder irradiationirradiation conditions.conditions.

SchemeScheme 34. 34.The The proposed proposed reaction reaction mechanism mechanism for the hydroxycarbonylationfor the hydroxycarbonylation reaction. reaction. 1.5. Visible-Light-Promoted Copper-Catalyzed Carbonylations 1.5. Visible-Light-Promoted Copper-Catalyzed Carbonylations To the best of our knowledge, the first and only example of copper-catalyzed photoin- ducedTo carbonylation the best of reaction our knowledge, has been recently the first reported and by only Chen example and co-workers of copper-catalyzed [64] and pho- istoinduced aimed at the carbonylation synthesis of cyano-tethered reaction has amides been rece by aminocarbonylationntly reported by Chen of oxime and esters co-workers [64] withand aminesis aimed and at CO the under synthesis mild conditions of cyano-tethered (Scheme 35). amides A number by ofaminocarbonylation cycloketone oxime of oxime estersesters and with alkyl/aryl amines amines and CO were under evaluated, mild and condit a highions level (Scheme of functional 35). groupA number tolerance of cycloketone was found. Larger than four-membered rings, such as cyclopentanone and cyclohexanone oximeoxime esters, esters did and not alkyl/aryl lead to the expectedamines were products. evaluated, For this aminocarbonylationand a high level of reaction, functional group thetolerance authors was proposed found. a visible Larger light-mediated than four-membe Cu(I)/Cu(II)/Cu(III)-basedred rings, such as catalyticcyclopentanone cycle and cy- (Schemeclohexanone 36). The oxime first step esters, is a single-electrondid not lead to transfer the expected between theproducts. photoexcited For this LnCu(I)– aminocarbonyl- NHPhation complexreaction, (II the)* or, authors alternatively, proposed the ground a visibl statee Llight-mediatednCu(I)–NHPh species Cu(I)/Cu(II)/Cu(III)-based (I) and the oxime,catalytic leading cycle to (Scheme an iminyl 36). radical TheIII firstand step the is oxidized a single-electron LnCu(II)–NHPh transfer complex between (IV). the photo- Then, III undergoes a selective β–C–C bond cleavage to form the alkyl radical V, which excited LnCu(I)–NHPh complex (II)* or, alternatively, the ground state LnCu(I)–NHPh can react with Cu(II)-complex IV producing a Cu(III) organometallic species VI. The latter, afterspecies sequential (I) and coordination the oxime, and leading insertion to an of CO,iminyl leads radical to an acyl III copper and the intermediate oxidized (LVIInCu(II)–NHPh orcomplexVIII), providing (IV). Then, the finalIII undergoes amide by reductive a selective elimination β–C–C andbond regenerating cleavage to the form active the alkyl rad- Cu(I)ical V catalyst., which can react with Cu(II)-complex IV producing a Cu(III) organometallic species VI. The latter, after sequential coordination and insertion of CO, leads to an acyl copper intermediate (VII or VIII), providing the final amide by reductive elimination and regenerating the active Cu(I) catalyst.

Catalysts 2021, 11, x FOR PEER REVIEW 22 of 34

Scheme 34. The proposed reaction mechanism for the hydroxycarbonylation reaction.

1.5. Visible-Light-Promoted Copper-Catalyzed Carbonylations To the best of our knowledge, the first and only example of copper-catalyzed photoinduced carbonylation reaction has been recently reported by Chen and co-workers [64] and is aimed at the synthesis of cyano-tethered amides by aminocarbonylation of oxime esters with amines and CO under mild conditions (Scheme 35). A number of cycloketone oxime esters and alkyl/aryl amines were evaluated, and a high level of functional group tolerance was found. Larger than four-membered rings, such as cyclopentanone and cyclohexanone oxime esters, did not lead to the expected products. For this aminocarbonylation reaction, the authors proposed a visible light-mediated Cu(I)/Cu(II)/Cu(III)-based catalytic cycle (Scheme 36). The first step is a single-electron transfer between the photoexcited LnCu(I)–NHPh complex (II)* or, alternatively, the ground state LnCu(I)–NHPh species (I) and the oxime, leading to an iminyl radical III and the oxidized LnCu(II)–NHPh complex (IV). Then, III undergoes a selective β–C–C bond cleavage to form the alkyl radical V, which can react with Cu(II)-complex IV producing a Cu(III) organometallic species VI. The latter, after sequential coordination and insertion of CO, leads to an acyl copper Catalysts 2021, 11, 918 23 of 35 intermediate (VII or VIII), providing the final amide by reductive elimination and regenerating the active Cu(I) catalyst.

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SchemeScheme 35.35.The The Cu-catalyzedCu-catalyzed synthesissynthesis ofof amidesamides viavia aminocarbonylationaminocarbonylation of of oximes oximes under under visible-light visible-light irradiation. irradiation.

SchemeScheme 36.36. TheThe proposedproposed reactionreaction pathway.pathway.

1.6.1.6. Visible-Light-PromotedVisible-Light-Promoted Metal-FreeMetal-Free CarbonylationsCarbonylations TheThe majormajor achievementsachievements inin visible-light-mediatedvisible-light-mediated carbonylationcarbonylation reactionsreactions werewere ob-ob- tainedtained underunder transition-metal transition-metal catalysis. catalysis. The The requirement requirement of of transition-metal transition-metal catalysts catalysts often of- inten connection in connection with with organic organic ligands ligands is undoubtedlyis undoubtedly expensive, expensive, and and the the removal removal of of their their tracestraces fromfrom thethe ﬁnalfinal products is is necessary, necessary, pa particularlyrticularly in in the the synthesis synthesis of of pharmaceutical pharmaceuti- calcompounds. compounds. Metal-free Metal-free photocatalytic photocatalytic prot protocolsocols can can overcome thethe above-mentionedabove-mentioned drawbacksdrawbacks andand maymay representrepresent attractiveattractive greenergreener and and more more sustainable sustainable alternatives. alternatives. In 1997, the efﬁcient conversion of alkyl iodides to the corresponding esters was In 1997, the efficient conversion of alkyl iodides to the corresponding esters was achieved under photoirradiation conditions in the absence of catalysts (Scheme 37)[29]. achieved under photoirradiation conditions in the absence of catalysts (Scheme 37) [29]. The presence of hydrogens in the β position on the alkyl iodides results in the formation of positional isomers via the β-elimination pathway under conventional transition metal- catalyzed carbonylation conditions [65]. This radical protocol allows the selective alkoxycarbonylation at the carbon attached to the halogen, and, for this reason, a wide range of aliphatic iodides, including tertiary ones, can be efficiently employed (Scheme 37, selected examples). High pressure of CO (from 20 to 55 atm) and the presence of an organic (triethylamine) or inorganic base (K2CO3, KOH) were essential to the reaction outcome. Mechanistically, by irradiation, the homolytic cleavage of the C–I bond produces an alkyl radical species that reacts with the carbon monoxide, providing an acyl radical intermediate. In the presence of another molecule of alkyl iodide, an acyl iodide is then generated, and, after reaction with R2OH, the corresponding ester is produced (Scheme 37, proposed mechanism). Remarkably, the reaction can be extended to the synthesis of amides using amines in place of alcohols [28].

Catalysts 2021, 11, 918 24 of 35

The presence of hydrogens in the β position on the alkyl iodides results in the formation of positional isomers via the β-elimination pathway under conventional transition metal-catalyzed carbonylation conditions [65]. This radical protocol allows the selective alkoxycarbonylation at the carbon attached to the halogen, and, for this reason, a wide range of aliphatic iodides, including tertiary ones, can be efﬁciently employed (Scheme 37, selected examples). High pressure of CO (from 20 to 55 atm) and the presence of an organic (triethylamine) or inorganic base (K2CO3, KOH) were essential to the reaction outcome. Mechanistically, by irradiation, the homolytic cleavage of the C–I bond produces an alkyl radical species that reacts with the carbon monoxide, providing an acyl radical intermediate. In the presence of another molecule of alkyl iodide, an acyl iodide is then generated, 2 Catalysts 2021, 11, x FOR PEER REVIEWand, after reaction with R OH, the corresponding ester is produced (Scheme 37, proposed 24 of 34 mechanism). Remarkably, the reaction can be extended to the synthesis of amides using amines in place of alcohols [28].

SchemeScheme 37. 37.The The catalyst-free catalyst-free synthesis synthesis of esters of esters via radical via radical alkoxycarbonylation alkoxycarbonylation of alkyl iodides of alkyl iodides un- underder irradiation irradiation conditions.

TheThe ﬁrst first example example of radical of radical alkoxycarboxylation alkoxycarboxylation of aryldiazonium of aryldiazonium salts using COsalts gas using CO gas through visible-light-induced photoredox catalysis has been reported in 2015 by the Xiao researchthrough group visible-light-induced (Scheme 38)[66]. A widephotoredox variety of cataly arylcarboxylicsis has been acid esters,reported bearing in 2015 both by the Xiao electron-donatingresearch group (Scheme and electron-withdrawing 38) [66]. A wide groups, variety were of arylcarboxylic obtained in good acid to excellentesters, bearing both yieldselectron-donating at room temperature. and electron-withdrawing The generality of the alcoholgroups, coupling were obtained partner, includingin good to excellent theyields use ofat chiralroom alcohols,temperature. was also The demonstrated. generality of Notably, the alcohol terminal coupling alkynes, partner, which are including the knownuse of to chiral be good alcohols, acceptors inwas radical also reactions, demonstrated. remained untouched.Notably, terminal Sensitive functionalalkynes, which are groups (such as iodo, bromo) in traditional transition metal-catalyzed alkoxycarbonylation known to be good acceptors in radical reactions, remained untouched. Sensitive func- reactions were compatible in this process, thus allowing further synthetic manipulations. tional groups (such as iodo, bromo) in traditional transition metal-catalyzed alkoxycarbonylation reactions were compatible in this process, thus allowing further synthetic manipulations. Remarkably, a low loading of an organic dye (fluorescein) is employed as a photocatalyst, and a low energy visible light (16 W blue LEDs) is enough to promote the reaction. However, high pressure of CO (80 atm) proved to be crucial for the success of the reaction.

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Scheme 37. The catalyst-free synthesis of esters via radical alkoxycarbonylation of alkyl iodides under irradiation conditions.

The first example of radical alkoxycarboxylation of aryldiazonium salts using CO gas through visible-light-induced photoredox catalysis has been reported in 2015 by the Xiao research group (Scheme 38) [66]. A wide variety of arylcarboxylic acid esters, bearing both electron-donating and electron-withdrawing groups, were obtained in good to excellent yields at room temperature. The generality of the alcohol coupling partner, including the use of chiral alcohols, was also demonstrated. Notably, terminal alkynes, which are known to be good acceptors in radical reactions, remained untouched. Sensitive functional groups (such as iodo, bromo) in traditional transition metal-catalyzed alkoxycar- Catalysts 2021bonylation, 11, 918 reactions were compatible in this process, thus allowing further synthetic25 ofma- 35 nipulations. Remarkably, a low loading of an organic dye (fluorescein) is employed as a photocatalyst, and a low energy visible light (16 W blue LEDs) is enough to promote the Remarkably, a low loading of an organic dye (ﬂuorescein) is employed as a photocatalyst, reaction. However,and high a low energypressure visible of light CO (16 (80 W blueatm) LEDs) proved is enough to be to promotecrucial the for reaction. the success However, of the reaction. high pressure of CO (80 atm) proved to be crucial for the success of the reaction.

Catalysts 2021, 11, x FOR PEER REVIEW 25 of 34

Scheme 38. Visible light-induced fluorescein-catalyzed alkoxycarboxylation of aryldiazonium salts.

This radical alkoxycarboxylation protocol was applicable to other radical carboxyla- Scheme 38. Visibletion light-induced cascade ﬂuorescein-catalyzedreactions. For example, alkoxycarboxylation when benzenediazonium of aryldiazonium salts. salts, bearing the allyl or the Thispropargyl radical function alkoxycarboxylation at the orthoprotocol position, was were applicable causedto to other react radical under carboxyla- standard reaction tionconditions, cascade reactions. the corresponding For example, whenmethyl benzenediazonium 2-(2,3-dihydrobenzofuran-3-yl)acetate salts, bearing the allyl or the (Scheme propargyl39a) and function methyl at2-(benzofuran-3-yl)acetate the ortho position, were caused (Scheme to react 39b) under were standard obtained reaction in con- satisfactory ditions,yields. theMechanistic corresponding studies methyl suggested 2-(2,3-dihydrobenzofuran-3-yl)acetate that the reaction might proceed(Scheme via 39 a)carbonand radical methylintermediates 2-(benzofuran-3-yl)acetate I formed by single-electron (Scheme 39b) werereduction obtained of inthe satisfactory substrate yields.with the Mech- fluorescein anisticphotocatalyst studies suggested in its excited that the state. reaction The mightreactive proceed intermediate via carbon I might radical be intermediates able to trap the CO I formed by single-electron reduction of the substrate with the ﬂuorescein photocatalyst molecule delivering the benzoyl radical II, which is oxidized by the dye radical cation in its excited state. The reactive intermediate I might be able to trap the CO molecule + delivering(Dye· ) with thebenzoyl concomitant radical restoringII, which is of oxidized the acti byve the photocatalyst. dye radical cation The (Dye resulting·+) with cation III concomitantcould be directly restoring trapped of the active by various photocatalyst. alcohols The leading resulting to cation a wideIII rangecould beof directlyalkyl benzoates trapped(Scheme by 40). various alcohols leading to a wide range of alkyl benzoates (Scheme 40).

SchemeScheme 39. 39.Fluorescein-catalyzed Fluorescein-catalyzed radical radical addition/alkoxycarboxylation addition/alkoxycarboxylation reaction reaction sequences sequences from from alkenealkene (a ()a and) and alkyne alkyne (b) ( derivatives.b) derivatives.

Scheme 40. The proposed mechanism for the fluorescein-catalyzed visible light-induced alkoxycarboxylation of aryldiazonium salts.

A very similar approach was developed in the same period by Jacobi von Wangelin’s group. Alkyl benzoates were efficiently obtained from arene diazonium salts, carbon monoxide and alcohols under visible-light irradiation and in the presence of eosin Y as the catalyst (Scheme 41) [67]. Under metal and base-free conditions, a variety of function- alities were well tolerated both on the substrates and on various functionalized additives. Interestingly, tertiary esters, which are difficult to obtain by conventional esterification

Catalysts 2021, 11, x FOR PEER REVIEW 25 of 34

Scheme 38. Visible light-induced fluorescein-catalyzed alkoxycarboxylation of aryldiazonium salts.

This radical alkoxycarboxylation protocol was applicable to other radical carboxylation cascade reactions. For example, when benzenediazonium salts, bearing the allyl or the propargyl function at the ortho position, were caused to react under standard reaction conditions, the corresponding methyl 2-(2,3-dihydrobenzofuran-3-yl)acetate (Scheme 39a) and methyl 2-(benzofuran-3-yl)acetate (Scheme 39b) were obtained in satisfactory yields. Mechanistic studies suggested that the reaction might proceed via carbon radical intermediates I formed by single-electron reduction of the substrate with the fluorescein photocatalyst in its excited state. The reactive intermediate I might be able to trap the CO molecule delivering the benzoyl radical II, which is oxidized by the dye radical cation (Dye·+) with concomitant restoring of the active photocatalyst. The resulting cation III could be directly trapped by various alcohols leading to a wide range of alkyl benzoates (Scheme 40).

Catalysts 2021, 11, 918 Scheme 39. Fluorescein-catalyzed radical addition/alkoxycarboxylation reaction26 of 35 sequences from alkene (a) and alkyne (b) derivatives.

SchemeScheme 40. 40.The The proposed proposed mechanism mechanism for the fluorescein-catalyzed for the fluorescein- visiblecatalyzed light-induced visible alkoxycar- light-induced alkoxycar- boxylationboxylation of aryldiazoniumof aryldiazonium salts. salts. A very similar approach was developed in the same period by Jacobi von Wangelin’s group.A Alkyl very benzoates similar approach were efficiently was obtaineddeveloped from in arene the diazoniumsame period salts, by carbon Jacobi von Wangelin’s monoxide and alcohols under visible-light irradiation and in the presence of eosin Y as the Catalysts 2021, 11, x FOR PEER REVIEWgroup. Alkyl benzoates were efficiently obtained from arene diaz26 ofonium 34 salts, carbon catalyst (Scheme 41)[67]. Under metal and base-free conditions, a variety of functional- itiesmonoxide were well and tolerated alcohols both onunder the substrates visible-light and on irra variousdiation functionalized and in the additives. presence of eosin Y as Interestingly,the catalyst tertiary (Scheme esters, 41) which [67]. areUnder difficult metal to obtain and bybase-free conventional conditions, esterification a variety of function- proceduresproceduresalities were due well to their tolerated steric hindrance, both on can can the also also substrat be be prepared preparedes and in in excellent excellenton various yields. yields. func Moreo- More-tionalized additives. over,ver,Interestingly, the the industrially industrially tertiary relevant relevant esters, intermediate, intermediate, which 2-ethylhexyl 2-ethylhexyl are diffic ultbenzoate, to obtain has been by obtainedobtainedconventional inin esterification 58% yield fromfrom PhNPhN22BF4,, 2-ethylhexanol 2-ethylhexanol and and CO CO under under standard standard conditions.

Scheme 41.41. TheThe synthesissynthesis of of benzoates benzoates through through visible visibl light-drivene light-driven eosin eosin Y-catalyzed Y-catalyzed alkoxycarbony- alkoxycar- lationbonylation of arene of arene diazonium diazonium salts. salts.

The mechanisticmechanistic investigations, investigations, including including the the observation observation of benzoyl-TEMPOof benzoyl-TEMPO adduct, ad- supportduct, support the sequential the sequential reduction reduction (SET), (SET), carbonylation, carbonylation, and oxidationand oxidation (SET) (SET) to aroylium to aroy- cations,lium cations, which which undergo undergo rapid rapid addition addition to alcohols to alcohols (Scheme (Scheme 40, Dye 40, = Dye Eosin = Eosin Y). Y). From thethe pioneeringpioneering workwork ofof HeckHeck onon palladium-catalyzedpalladium-catalyzed aminocarbonylationaminocarbonylation ofof aryl iodides [68], [68], several several efforts efforts have have been been accomplished accomplished for for the the synthesis synthesis of aromatic of aromatic am- amidesides [69,70] [69,70 through] through radical radical carbonylative carbonylative pathways pathways as as well well [66,67,71]. [66,67,71]. Recently, Recently, the ﬁrstfirst example ofof catalyst-freecatalyst-free photo-inducedphoto-induced aminocarbonylationaminocarbonylation ofof arylaryl iodidesiodides withwith COCO andand amines hashas beenbeen reportedreported by by Ryu Ryu and and co-workers co-workers [72 [72].]. A A wide wide variety variety of of amides, amides, including including hetero aromatic amides, has been obtained in good yields under mild reaction conditions (Scheme 42).

Scheme 42. Photo-induced aminocarbonylation of aryl iodides with primary and secondary amines.

The aryl radical I, generated by a photo-induced cleavage of the starting aryl iodide, might react with CO leading to the corresponding acyl radical II (Scheme 43). The nucleophilic addition of an amine to the latter gives a zwitterionic radical intermediate III. Fi- nally, electron transfer to the aryl iodide would provide the aryl radical I and the expected amide.

Catalysts 2021, 11, x FOR PEER REVIEW 26 of 34

procedures due to their steric hindrance, can also be prepared in excellent yields. Moreo- ver, the industrially relevant intermediate, 2-ethylhexyl benzoate, has been obtained in 58% yield from PhN2BF4, 2-ethylhexanol and CO under standard conditions.

Scheme 41. The synthesis of benzoates through visible light-driven eosin Y-catalyzed alkoxycarbonylation of arene diazonium salts.

The mechanistic investigations, including the observation of benzoyl-TEMPO adduct, support the sequential reduction (SET), carbonylation, and oxidation (SET) to aroylium cations, which undergo rapid addition to alcohols (Scheme 40, Dye = Eosin Y). From the pioneering work of Heck on palladium-catalyzed aminocarbonylation of aryl iodides [68], several efforts have been accomplished for the synthesis of aromatic am-

Catalysts 2021, 11, 918 ides [69,70] through radical carbonylative pathways as well [66,67,71]. Recently,27 of 35 the first example of catalyst-free photo-induced aminocarbonylation of aryl iodides with CO and amines has been reported by Ryu and co-workers [72]. A wide variety of amides, includ- heteroing hetero aromatic aromatic amides, amides, has been has obtained been inobtained good yields in good under yields mild reaction under conditions mild reaction condi- (Schemetions (Scheme 42). 42).

Scheme 42. 42.Photo-induced Photo-induced aminocarbonylation aminocarbonylation of aryl iodidesof aryl withiodides primary with and primary secondary and amines. secondary amines.

TheThe aryl aryl radical radicalI, generatedI, generated by aby photo-induced a photo-induced cleavage cleavage of the of starting the starting aryl io- aryl iodide, dide, might react with CO leading to the corresponding acyl radical II (Scheme 43). The might react with CO leading to the corresponding acyl radical II (Scheme 43). The nucle- Catalysts 2021, 11, x FOR PEER REVIEWnucleophilic addition of an amine to the latter gives a zwitterionic radical intermediate 27 of 34 IIIophilic. Finally, addition electron of transfer an amine to the to aryl the iodidelatter wouldgives a provide zwitterionic the aryl radical radical Iintermediateand the III. Fi- expectednally, electron amide. transfer to the aryl iodide would provide the aryl radical I and the expected amide.

SchemeScheme 43. 43.The The proposed proposed mechanistic mechanistic pathway for pathway the catalyst-free for the photo-induced catalyst-free aminocarbonyla- photo-induced aminocarbonyl- tionation of arylof aryl iodides. iodides.

Inspired by Xiao [66] and Wangelin [67] independent reports, the Eosin-Y-catalyzed synthesis of indol-3-yl aryl ketones from indoles, CO and aryldiazonium salts has been recently reported by Gu and Li (Scheme 44) [73]. This protocol features high versatility, high functional group tolerance and mild reaction conditions except for the high pressure of CO required (70 atm).

Scheme 44. The photo-induced synthesis of indol-3-yl aryl ketones from indoles, CO and aryldiazonium salts.

The synthesis of indol-3-yl aryl ketones was achieved also by Li, Liang and colleagues, starting from arylsulfonyl chlorides in place of aryldiazonium salts under very similar photocatalytic conditions (Scheme 45) [74]. Notably, the methodology demonstrated wide applicability with both electron-rich and electron-deficient functional groups at reduced reaction time if compared with Gu and Li’s protocol [73]. The proposed pathway starts with the reduction of the arylsulfonyl chloride by a single-electron transfer process (SET) to produce an aryl radical I and the Eosin radical cation [Eosin˙+] (Scheme 46). Then, the aryl radical I reacts with CO, delivering an acyl radical II, which is oxidized

Catalysts 2021, 11, x FOR PEER REVIEW 27 of 34

Catalysts 2021, 11, 918 Scheme 43. The proposed mechanistic pathway for the catalyst-free photo-induced 28aminocarbonyl- of 35 ation of aryl iodides.

InspiredInspired by by Xiao Xiao [66 [66]] and and Wangelin Wangelin [67] independent[67] independent reports, reports, the Eosin-Y-catalyzed the Eosin-Y-catalyzed synthesissynthesis of of indol-3-yl indol-3-yl aryl aryl ketones ketones from from indoles, indoles, CO and CO aryldiazonium and aryldiazonium salts has salts been has been recentlyrecently reported reported by by Gu Gu and and Li (Scheme Li (Scheme 44)[ 7344)]. This[73]. protocolThis protocol features featur highes versatility, high versatility, highhigh functional functional group group tolerance tolerance and and mild mild reaction reaction conditions conditions except forexcept the highfor the pressure high pressure ofof CO CO required required (70 (70 atm). atm).

SchemeScheme 44. 44.The The photo-induced photo-induced synthesis synthesis of indol-3-yl of indol-3-yl aryl ketones aryl ketones from indoles, from CO indoles, and aryldiazo- CO and aryldia- niumzonium salts. salts.

TheThe synthesis synthesis of indol-3-yl of indol-3-yl aryl ketones aryl ketones was achieved was alsoachieved by Li, Liangalso andby Li, colleagues, Liang and col- startingleagues, from starting arylsulfonyl from arylsulfonyl chlorides in placechlorides of aryldiazonium in place of aryldiazonium salts under very salts similar under very photocatalytic conditions (Scheme 45)[74]. Notably, the methodology demonstrated wide similar photocatalytic conditions (Scheme 45) [74]. Notably, the methodology demon- applicability with both electron-rich and electron-deficient functional groups at reduced reactionstrated timewide if applicability compared with with Gu both and electron-rich Li’s protocol [ 73and]. Theelectron-deficient proposed pathway functional starts groups withat reduced the reduction reaction of the time arylsulfonyl if compared chloride with by Gu a single-electronand Li’s protocol transfer [73]. process The proposed (SET) path- toway produce starts an with aryl radicalthe reductionI and the of Eosin the radicalarylsulf cationonyl [Eosin˙chloride+] (Schemeby a single-electron 46). Then, the transfer arylprocess radical (SET)I reacts to produce with CO, deliveringan aryl radical an acyl I radicaland theII ,Eosin which radical is oxidized cation to an [Eosin acylium˙+] (Scheme intermediate46). Then, theIII arylby the radical Eosin I radical reacts cationwith CO, [Eosin delivering·+]. The active an acyl photocatalytic radical II, which species is is oxidized indeed regenerated while the acylium ion undergoes nucleophilic attack by the indolyl species, leading to the final product. It has been demonstrated that not only indoles but also (hetero)arenes might serve as a platform to trap benzylidyneoxonium cations [75]. Under very similar reaction conditions [67,73], a wide variety of unsymmetrical aryl and heteroaryl ketones were successfully obtained from (hetero)arenes, CO and aryldiazonium salts in the presence of Eosin Y and green LEDs (Scheme 47). Interestingly, the electronic nature of the diazonium salt has little influence on the reaction, while electron-rich (hetero)arenes gave the best results. Based on control experiments and in agreement with the previous reports [67], the proposed mechanism is shown in Scheme 48. Under irradiation conditions, the excited photocatalyst (Eosin Y*) is generated. Then, the electron-deficient phenyl diazonium salt is reduced to I by Eosin Y* through a single-electron transfer (SET). The aryl radical species I react with CO, providing the acyl radical II, which is oxidized by Eosin Y·+ to cationic intermediate III. Finally, benzylidyneoxonium III is trapped by the arene (or heteroarene) derivative, delivering the desired ketone. Catalysts 2021, 11, x FOR PEER REVIEW 28 of 34

to an acylium intermediate III by the Eosin radical cation [Eosin·+]. The active photocata- Catalysts 2021, 11, x FOR PEER REVIEWlytic species is indeed regenerated while the acylium ion undergoes28 nucleophilic of 34 attack

by the indolyl species, leading to the final product.

to an acylium intermediate III by the Eosin radical cation [Eosin·+]. The active photocata- Catalysts 2021, 11, 918 29 of 35 lytic species is indeed regenerated while the acylium ion undergoes nucleophilic attack by the indolyl species, leading to the final product.

Scheme 45. The photo-induced synthesis of indol-3-yl aryl ketones from indoles, CO and aryl sul- SchemeScheme 45. 45.The The photo-inducedphoto-induced synthesis synthesis of of indol-3-yl indol-3-yl aryl aryl ketones ketones from from indoles, indoles, CO CO and and aryl aryl sul- sulfonylfonylfonyl chlorides. chlorides. chlorides.

Scheme 46. The proposed mechanism for the Eosin-catalyzed visible-light-promoted indolylcarbox- ylation of aryl sulfonyl chlorides.

SchemeSchemeIt 46.has 46.The been The proposed demonstrated proposed mechanism mechanism that for the not Eosin-catalyzed only for indoles the Eosin-cata visible-light-promoted but also lyzed(hetero)arenes visible-light-promoted indolylcarboxy- might serve as indolylcarbox- lationylationa platform of aryl of sulfonylarylto trap sulfonyl chlorides. benzylidyneoxonium chlorides. cations [75]. Under very similar reaction conditions [67,73], a wide variety of unsymmetrical aryl and heteroaryl ketones were successfully obtained from (hetero)arenes, CO and aryldiazonium salts in the presence of Eosin It has been demonstrated that not only indoles but also (hetero)arenes might serve as Y and green LEDs (Scheme 47). Interestingly, the electronic nature of the diazonium salt ahas platform little influence to trap on thebenzylidyneoxonium reaction, while electron-rich cations (hetero)arenes [75]. Under gave very the best similar results. reaction condi- tionsBased [67,73],on control a experimentswide variety and of in unsymmetrica agreement with lthe aryl previous and heteroaryl reports [67], ketonesthe pro- were success- fullyposed obtainedmechanism from is shown (hetero)arenes, in Scheme 48. UnderCO and irradiation aryldiazonium conditions, salts the excited in the pho- presence of Eosin Ytocatalyst and green (Eosin LEDs Y*) is (Scheme generated. 47). Then, Interestingly, the electron-deficient the electronic phenyl diazonium nature of salt the is diazonium salt reduced to I by Eosin Y* through a single-electron transfer (SET). The aryl radical species has little influence on the reaction, while electron-rich (hetero)arenes gave the best results. I react with CO, providing the acyl radical II, which is oxidized by Eosin Y·+ to cationic Based on control experiments and in agreement with the previous reports [67], the pro- intermediate III. Finally, benzylidyneoxonium III is trapped by the arene (or heteroarene) posedderivative, mechanism delivering theis shown desired inketone. Scheme 48. Under irradiation conditions, the excited photocatalyst (Eosin Y*) is generated. Then, the electron-deficient phenyl diazonium salt is reduced to I by Eosin Y* through a single-electron transfer (SET). The aryl radical species

I react with CO, providing the acyl radical II, which is oxidized by Eosin Y·+ to cationic intermediate III. Finally, benzylidyneoxonium III is trapped by the arene (or heteroarene) derivative, delivering the desired ketone.

CatalystsCatalystsCatalysts 2021 20212021, ,11 11, ,11 ,x x, FOR 918FOR PEER PEER REVIEW REVIEW 30 of 35 2929 of of 34 34

SchemeScheme 47. 47.47.The TheThe photo-induced photo-inducedphoto-induced synthesis synthesissynthesis of unsymmetrical ofof unsymmetricaunsymmetrica aryl ketonesll arylaryl from ketonesketones (hetero)arenes, fromfrom (hetero)arenes,(hetero)arenes, CO COCO andand aryldiazonium aryldiazoniumaryldiazonium salts. salts.salts.

SchemeScheme 48. 48.48.The TheThe proposed proposedproposed mechanism mechanismmechanism for the forfor photo-induced thethe photo-inducephoto-induce synthesisdd synthesis ofsynthesis indol-3-yl ofof aryl indol-3-ylindol-3-yl ketones arylaryl ketonesketones fromfrom indoles, indoles,indoles, CO COCO and andand aryldiazonium aryldiazoniumaryldiazonium salts. salts.salts. Recently, the first example of photocatalyzed oxidative carbonylation of organosili- Recently, the first example of photocatalyzed oxidative carbonylation of organosilicates toRecently, unsymmetrical the first ketones example has been of reportedphotocatalyz by Fukuyama,ed oxidative Ollivier, carbonyl Ryu, Fensterbankation of organosili- andcatescates co-workers toto unsymmetricalunsymmetrical [76]. This metal-free ketoneketoness procedure hashas beenbeen is basedreportedreported on the byby use Fukuyama,Fukuyama, of an inexpensive Ollivier,Ollivier, or- Ryu,Ryu, Fen-Fen- ganicsterbanksterbank dye, 4CzIPN,andand co-workersco-workers which, in combination [76].[76]. ThisThis metal-free withmetal-free the blue procedureprocedure led, is able toisis catalyze basedbased on theon the reactionthe useuse ofof anan inex-inex- ofpensivepensive alkyl bis(catecholato)silicates, organicorganic dye,dye, 4CzIPN,4CzIPN, CO which,which, (80 atm) inin and combinationcombination activated olefins withwith to the producethe blueblue a led, wideled, isis range ableable toto catalyzecatalyze ofthethe ketones reaction reaction (Scheme of of alkyl alkyl 49). bis(catecholato)silicates, bis(catecholato)silicates, Notably, primary, secondary CO CO and (80 (80 tertiary atm) atm) alkyl and and radicalsactivated activated generated olefins olefins to to produce produce by the photocatalyzed oxidation of organosilicates underwent efficient carbonylation with aa widewide rangerange ofof ketonesketones (Scheme(Scheme 49).49). Notably,Notably, primary,primary, secondarysecondary andand tertiarytertiary alkylalkyl rad-rad- CO under standard conditions. Interestingly, 1,4-dicarbonyl compounds were efficiently accessedicalsicals generatedgenerated through this byby protocol, thethe photocatalyzedphotocatalyzed while allyl sulfones oxidatoxidat wereionion successfully ofof organosilicatesorganosilicates employed asunderwentunderwent radical efficientefficient acceptorscarbonylationcarbonylation in this three-componentwithwith COCO underunder reaction. standardstandard condconditions.itions. Interestingly,Interestingly, 1,4-dicarbonyl1,4-dicarbonyl com-com- poundspounds werewere efficientlyefficiently accessedaccessed throughthrough thisthis protocol,protocol, whilewhile allylallyl sulfonessulfones werewere success-success- fullyfully employedemployed asas radicalradical acceptorsacceptors inin thisthis three-componentthree-component reaction.reaction.

Catalysts 2021, 11, x FOR PEER REVIEW 30 of 34

CatalystsCatalysts2021 2021, 11, 11, 918, x FOR PEER REVIEW 3130 of of 35 34

Scheme 49. The synthesis of unsymmetrical ketones via 4CzIPN-catalyzed radical carbonylations. SchemeScheme 49. 49.The The synthesis synthesis of of unsymmetrical unsymmetrical ketones ketones via via 4CzIPN-catalyzed 4CzIPN-catalyzed radical radical carbonylations. carbonylations. Mechanistic studies reveal that, initially, the photocatalyst is excited under visible- lightMechanisticMechanistic irradiation. studiesstudies Then, revealreveal the that,alkylthat, initially, initially,bis(catecholato)silicate thethe photocatalyst photocatalyst is isundergoes excitedexcited underunder oxidation visible-visible- by the ex- lightcitedlight irradiation. irradiation.4CzIPN* Then, throughThen, the the alkyl a alkyl single-electron-transfer bis(catecholato)silicate bis(catecholato)silicate undergoes (SET)undergoes process, oxidation oxidation leading by the by excited the to ex-the formation 4CzIPN*ofcited the 4CzIPN* alkyl through radical through a single-electron-transfer species a single-electron-transfer I and the reduced (SET) process,(SET) organocatalyst process, leading leading to the [4CzIPN] formation to the formationˉ. ofThe the alkyl radical alkylof the radical alkyl radical species speciesI and theI and reduced the reduced organocatalyst organocatalyst [4CzIPN] [4CzIPN]−. Theˉ. The alkyl alkyl radical radicalI I reacts with CO to generate the acyl radical II, which, in turn, adds to the activated alkene reactsI reacts with with CO CO to to generate generate the the acyl acyl radical radicalII II, which,, which, in in turn, turn, adds adds to to the the activated activated alkene alkene affordingaffordingaffording the the the radical radical radical intermediate intermediate intermediateIII III. Catalyst. Catalyst III. Catalyst [4CzIPN] [4CzIPN] [4CzIPN]− ˉconverts convertsˉ IIIconverts IIIinto into the the III carbanionic carbanionic into the carbanionic speciesspeciesspeciesIV IV IVby by by reduction reduction reduction (SET), (SET), (SET), and, and, inand, in this this in way, way, this the theway, photocatalyst photocatalyst the photocatalyst is is able able to to restart restartis able a a newto new restart a new catalyticcatalyticcatalytic cycle. cycle. cycle. Finally, Finally, Finally, the the base thebase (KH base (KH2PO 2(KHPO4) provides4) 2providesPO4) theprovides the proton proton forthe the forproton formation the formation for ofthe the formationof ﬁnal the of the productfinalfinal productproduct (Scheme (Scheme (Scheme 50). 50). 50).

Scheme 50. Proposed mechanism for the 4CzIPN-catalyzed carbonylative synthesis of ketones from organosilicates, CO and activated alkenes.

SchemeSchemeMore 50. 50.Proposed recently, Proposed mechanism the mechanism same forauthors the for 4CzIPN-catalyzed thehave 4CzIPN-catalyzed developed carbonylative a visible-light-driven carbonylative synthesis of synthesis ketones photocata- from of ketones from organosilicates,organosilicates,lyzed aminocarbonylation CO CO and and activated activated of alkyl alkenes. bis(catecholato)silicatesalkenes. with amines under similar reaction conditions [77]. In this approach, the CCl4 was used as a mediator in order to pro- moteMore Morethe formation recently, recently, theof acylthe same chloride,same authors authors which have is developedhave intermediate developed a visible-light-driven in the a formationvisible-light-driven of photocat-the corre- photocata- alyzed aminocarbonylation of alkyl bis(catecholato)silicates with amines under similar lyzedsponding aminocarbonylation amides. of alkyl bis(catecholato)silicates with amines under similar re- reaction conditions [77]. In this approach, the CCl4 was used as a mediator in order to promoteaction2. Conclusions conditions the formation [77]. of In acyl this chloride, approach, which the is intermediateCCl4 was used in the as formationa mediator of in the order to pro- correspondingmote the formation amides. of acyl chloride, which is intermediate in the formation of the corre- In this review, the major advances in the field of visible-light-induced catalyzed and spondingun-catalyzed amides. carbon monoxide-based carbonylation reactions have been described. The use of visible light in carbonylation chemistry has led to valuable and more sustainable 2. Conclusions In this review, the major advances in the field of visible-light-induced catalyzed and un-catalyzed carbon monoxide-based carbonylation reactions have been described. The

use of visible light in carbonylation chemistry has led to valuable and more sustainable

Catalysts 2021, 11, 918 32 of 35

2. Conclusions In this review, the major advances in the field of visible-light-induced catalyzed and un-catalyzed carbon monoxide-based carbonylation reactions have been described. The use of visible light in carbonylation chemistry has led to valuable and more sustainable alternatives for the synthesis of carbonyl-containing compounds. Mild reaction conditions, such as room temperature, are routinely applied. However, the use of low CO pressure is less frequent, and many improvements are expected from this point of view. The employment of greener reaction media and the application of continuous flow conditions can be beneficial to this chemistry. The use of CO surrogates, so attractive in conventional carbonylation reactions, can lead to several advantages in photocatalytic carbonylation reactions. The research in this field should be focused on discovering more efficient photocatalysts, possibly in combination with co-catalysts or additives, in order to increase both turnover number and turnover frequency (TON and TOF) and to improve the industrial attractiveness of these carbonylative methodologies. Heterogeneous systems, al- though sometimes hardly compatible, should be considered to improve the sustainability of these processes. Despite the increasing number of high-impact publications in the field, this is, undoubtedly, an underdeveloped area, and the real potential of this highly sustainable carbonylation strategy still needs to be discovered. We hope that this contribution might help other researchers in developing even more attractive visible-light-driven carbonylation protocols in the coming years.

Author Contributions: Conceptualization, N.D.C.; writing—original draft preparation, V.B., R.M., E.M. and A.V.; writing—review and editing, N.D.C., B.G., C.C.; supervision, N.D.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: V.B. and N.D. acknowledge European Union for Marie Skłodowska-Curie Individual Fellowship grant No. 894026. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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