Hydrosilylation Reactions Catalyzed by Rhenium

molecules

ReviewReview HydrosilylationHydrosilylation ReactionsReactions CatalyzedCatalyzed byby RheniumRhenium

DuoDuo WeiWei1,2 1,2,, Ruqaya Ruqaya Buhaibeh Buhaibeh 22, ,Yves Yves Canac Canac 22 andand Jean-Baptiste Jean-Baptiste Sortais Sortais 2,3,*2,3, *

11 Univ.University Rennes, Rennes, CNRS, CNRS, ISCR - ISCR-UMR UMR 6226, 6226, F-35000 35000 Rennes, Rennes, France; France; Duo.Wei@catalysis.de [email protected] 22 LCC-CNRS,LCC-CNRS, Université Université dede Toulouse, Toulouse, UPS, UPS, 31400 31400 Toulouse, Toulouse, France; [email protected] (R.B.); [email protected]@lcc-toulouse.fr (Y.C.) (Y.C.) 33 InstitutInstitut Universitaire Universitaire de de France France 1 1 rue rue Descartes, Descartes, 75231 CEDEX Paris 05, Cedex 75231 Paris,05, France France ** Correspondence:Correspondence: [email protected] [email protected]

Abstract:Abstract: Hydrosilylation isis anan important process,process, notnot onlyonly inin thethe siliconsilicon industryindustry toto produceproduce siliconsilicon polymers,polymers, butbut alsoalso in ﬁnefine chemistry. InIn this review,review, thethe developmentdevelopment ofof rhenium-basedrhenium-based catalystscatalysts forfor thethe hydrosilylationhydrosilylation of of unsaturated unsaturated bonds bonds in in carbonyl-, carbonyl-, cyano-, cyano-, nitro-, nitro-, carboxylic carboxylic acid acid derivatives derivatives and alkenesand alkenes is summarized. is summarized. Mechanisms Mechanisms of rhenium-catalyzed of rhenium-catalyzed hydrosilylation hydrosilylation are discussed. are discussed.

Keywords: rhenium; hydrosilylation;hydrosilylation; unsaturatedunsaturated bonds;bonds; carbonyls;carbonyls; alkenesalkenes

1. Introduction Hydrosilylation is aa veryvery versatileversatile transformationtransformation consistingconsisting of thethe additionaddition ofof aa hydrosilane (H-SiR(H-SiR33) toto anan unsaturatedunsaturated bond.bond. OrganosiliconOrganosilicon compoundscompounds havehave foundfound widespreadwidespread applicationsapplications inin our our daily daily lives lives in silicon-basedin silicon-based materials materials such such as silicon as silicon rubbers,rubbers, adhesives, adhesives, paper paper release release coating, coating, and and so so forth. forth. In In addition, addition, hydrosilylation hydrosilylation isis anan atom economiceconomic reactionreaction toto accessaccess valuablevaluable organosilaneorganosilane intermediatesintermediates forfor ﬁnefine chemicalchemical Citation: Wei, D.; Buhaibeh, R.; synthesissynthesis [[1,2].1,2]. For many years, platinum has been the metalmetal ofof choicechoice forfor designingdesigning Canac, Y.; Sortais, J.-B. Citation: Wei, D.; Buhaibeh, R.; hydrosilylationhydrosilylation catalysts,catalysts, Speier’s, Speier’s, Karstedt’s Karstedt’s or Markor Markó’só’s catalysts catalysts being being the most the repre-most Hydrosilylation Reactions Canac, Y.; Sortais, J.-B.; sentative examples (Figure1). Other noble transition metals, such as rhodium, ruthenium, Catalyzed by Rhenium. Molecules representative examples (Figure 1). Other noble transition metals, such as rhodium, Hydrosilylation Reactions Catalyzed iridium or palladium, have also been used in this transformation [3]. However, the limited 2021, 26, 2598. https://doi.org/ ruthenium, iridium or palladium, have also been used in this transformation [3]. by Rhenium. Molecules 2021, 26, x. 10.3390/molecules26092598 availabilityHowever, the of these limited precious availability transition of metalsthese precious has prompted transition researchers metals tohas explore prompted alter- https://doi.org/10.3390/xxxxx nativesresearchers metals to inexplore the periodic alternatives table, inmetals particular in the ﬁrst periodic row transition table, in metalsparticular such first as iron,row Academic Editor: Vincent Ritleng cobalttransition and metals nickel [such4–6]. as iron, cobalt and nickel [4–6]. Academic Editor: Vincent Ritleng

Received: 17 March 2021 O Received: 17 March 2021 O Si Si Cy Accepted: 15 April 2021 Si Si Si N Accepted: 15 April 2021 H PtCl O Pt Published: 29 April 2021 2 6 Pt Pt Published: 29 April 2021 N O Si Si Si Cy Publisher’s Note: MDPI stays neutral Publisher’s Note: MDPI stays with regard to jurisdictional claims in neutral with regard to jurisdictional Speier's catalyst Karstedt's catalyst Marko's catalyst published maps and institutional afﬁl- claims in published maps and iations. institutional affiliations. FigureFigure 1.1. RepresentativeRepresentative platinum-basedplatinum-based hydrosilylationhydrosilylation catalysts.catalysts.

InIn thisthis regard,regard, groupgroup 77 transitiontransition metals,metals, manganesemanganese [[7]7] andand rheniumrhenium [[8],8], werewere notnot anan obviousobvious choicechoice atat firstfirst glance.glance. Indeed,Indeed, rheniumrhenium complexes,complexes, inin particularparticular oxo-rheniumoxo-rhenium Copyright:Copyright: © 2021 2021 by by the the authors. authors. compounds,compounds, havehave been been mainly mainly recognized recognized as efficientas efficient catalysts catalysts in oxidations, in oxidations, such assuch epoxi- as LicenseeLicensee MDPI, MDPI, Basel, Basel, Switzerland. Switzerland. dationepoxidation or oxygen or oxygen atom transfer atom reactionstransfer reactions [9–14]. This [9–14]. is highlighted This is highlighted by organo-rhenium(VII) by organo- ThisThis article is an an open open access access article article trioxide,rhenium(VII) particularly trioxide, methyltrioxorhenium particularly methyltrioxorhenium (MeReO3, abbreviated (MeReO3 as, abbreviated MTO), arguably as MTO), one distributeddistributed under under the the terms terms and and ofarguably the most one versatile of the transitionmost versatile metal transition catalysts knownmetal catalysts to date.Rhenium known to is date. also well-knownRhenium is conditionsconditions ofof the Creative Creative Commons Commons toalso promote well-known olefin metathesisto promote [ 15olefin] or aldehydemetathesis olefinations [15] or aldehyde [16]. In complement,olefinations [16]. the co-In 186/188 AttributionAttribution (CC(CC BY) license (https:// ordinationcomplement, chemistry the coordination of rhenium chemistry was explored of rhenium for thepotential was explored application for the of potentialRe creativecommons.org/licenses/by/(http://creativecommons.org/licenses radioisotopes in nuclear medicine and bio-medical chemistry [17,18]. 4.0/)./by/4.0/).

Molecules 2021, 26, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules Molecules 2021, 26, 2598. https://doi.org/10.3390/molecules26092598 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2598 2 of 15

The application of rhenium derivatives in hydrosilylation [19,20] was not unveiled until 2003 [21,22]. Since the application of the lighter congener of rhenium, namely manganese, has been increasing lately in hydrosilylation [23,24], we report here the evolution of rhenium in this catalytic transformation.

2. Hydrosilylation of C=O, C=N, C≡N and NO2 Bonds 2.1. Hydrosilylation of Carbonyl Derivatives The first rhenium-catalyzed hydrosilylation was described in 2003 by Toste et al. (Scheme1a). This seminal contribution represents the first example of a hydrosilylation catalyst with a high-valent metal bearing two terminal oxo ligands, reversing the traditional use of metal-oxo complexes in catalysis. Indeed, the hydrosilylation of aldehydes and ketones was promoted by the readily available iododioxo(bistriphenylphosphine)rhenium(V) complex [(PPh3)2Re(O)2I] (C1)[21,22]. The scope of this reaction included aromatic or aliphatic ketones and aldehydes with a good tolerance of functional groups (amino, nitro, halo, ester, cyano, cyclopropyl and alkene groups remained untouched). This air and moisture tolerant reaction provides silyl-protected alcohols in a straightforward reduction- protection protocol as bulky hydrosilanes were used (HSiMe2Ph, HSiEt3, HSiMe2Ph and HSitBuMe2). A detailed mechanism, supported by experimental evidence, was proposed by the same group [21,25] and confirmed by a computational study by Wu et al. in 2006 [26] (Scheme1b). The first step involves the formal [2 + 2] addition of silane to the Re=O bond in C1 to produce metal hydride I-1, which was isolated and characterized by X-ray diffraction. Alkoxy-metal intermediate I-3 is produced by the addition of rhenium hydride Molecules 2021, 26, x to the carbonyl group. Transfer of the silyl group to the alkoxy ligand, formally a3 retroof 15-[2 +

2] reaction, forms the silyl ether product and regenerates the dioxo catalyst C1.

SchemeScheme 1. ReI(O)ReI(O)2(PPh2(PPh3)2 (3C1)2 )( C1catalyzed) catalyzed hydrosilylation hydrosilylation of carbonyl of carbonyl compounds compounds with related with related mechanismmechanism studies. studies. (a ()a Scope) Scope of ofthe the reaction; reaction; (b) (Proposedb) Proposed [2 + [22] addition + 2] addition mechanistic mechanistic pathway; pathway; (c) Proposed ionic outer-sphere mechanistic pathway. (c) Proposed ionic outer-sphere mechanistic pathway.

However,Intrigued by the the computational work of Toste, studyRoyo et by al. Wei explored [27] showed in 2005 thatthe ca thetalytic ionic activity outer-sphere of a family of oxo-rhenium complexes (C2–C7, Scheme 2) [28]. All these complexes showed mechanistic pathway (Scheme1c) is energetically more favorable than the [2 + 2] addition activity in the reduction of aliphatic and aromatic aldehydes (six examples) and ketones mechanism supported by Wu for the C1. In this alternative mechanism, the activation of (four examples) with dimethylphenylsilane in C6D6 solution (Scheme 2), demonstrating Si−H goes through η1-bonding of silane to the metal center followed by a nucleophilic the general ability of oxo-rhenium complexes to promote hydrosilylation. Notably, C2 catalyzes the hydrosilylation of aldehydes at room temperature, within 30 min., affording the corresponding silyl ethers in a good yield, but is ineffective as a ketone hydrosilylation catalyst (even at 80 °C). C5–C7 (5.0 mol%) are active for the hydrosilylation of 4- trifluoromethylbenzaldehyde at 80 °C, giving 40% yield (C5, 20 h), >95% yield (C6, 10 h) and >95% yield (C7, 10 h), respectively.

Scheme 2. Reduction of carbonyl groups by high-valent rhenium oxides C2–C7.

In the same year, Abu-Omar and co-workers reported a new system for the hydrosilylation of aldehydes and ketones using a mono-oxorhenium(V) catalyst (C8) containing two oxazoline ligands (Scheme 3) [29,30]. The reaction proceeds efficiently under ambient temperatures with low catalyst loading (0.1 mol%), 1.5 equiv. HSiEt3 using CH2Cl2 as a solvent or without a solvent. Interestingly, the catalyst preserves its activity after being recycled (for 2-butanone, 80% NMR yield (1st cycle) and 50% NMR yield (2nd cycle)).

Molecules 2021, 26, x 3 of 15

Molecules 2021, 26, 2598 3 of 15

additionScheme 1. of ReI(O) the organic2(PPh3)2 substrate(C1) catalyzed onthe hydrosilylation Si center in ofII-1 carbonylto cleave compounds the Si−H with bond related forming themechanism anionic studies. rhenium (a) hydride Scope ofII-2 the reaction;and the activated(b) Proposed organic [2 + 2] substrate addition mechanisticII-3. Finally, pathway; hydride transfer(c) Proposed from ionic the outer-sphere rhenium metal mechanisticII-2 to pathway. the carbon atom of the organic substrate II-3 produces the desired product and regenerates complex C1. IntriguedIntrigued byby thethe workwork ofof Toste,Toste, RoyoRoyo etet al.al. exploredexplored inin 20052005 thethe catalyticcatalytic activityactivity ofof aa familyfamily ofof oxo-rheniumoxo-rhenium complexescomplexes ((C2C2––C7C7,, SchemeScheme2 )[2) 28[28].]. AllAll thesethese complexescomplexes showedshowed activityactivity inin thethe reductionreduction ofof aliphaticaliphatic andand aromaticaromatic aldehydesaldehydes (six(six examples)examples) andand ketonesketones (four(four examples)examples) withwith dimethylphenylsilanedimethylphenylsilane inin C C66DD66 solutionsolution (Scheme(Scheme2 ),2), demonstrating demonstrating thethe generalgeneral abilityability ofof oxo-rheniumoxo-rhenium complexescomplexes toto promotepromote hydrosilylation.hydrosilylation. Notably,Notably, C2C2 catalyzescatalyzes thethe hydrosilylationhydrosilylation ofof aldehydesaldehydes atat room temperature, within 30 min,min., affordingaffording thethe corresponding silyl silyl ethers ethers in in a agood good yield, yield, but but is ineffective is ineffective as a asketone a ketone hydrosilylation hydrosily- lationcatalyst catalyst (even (evenat 80 at°C). 80 ◦C5C).–C7C5 –(5.0C7 (5.0mol%) mol%) are areactive active for forthe the hydrosilylation hydrosilylation of of4- 4-triﬂuoromethylbenzaldehydetrifluoromethylbenzaldehyde at at 80 80 °C,◦C, giving giving 40% 40% yield yield ( C5,, 20 h), >95% yield ((C6,, 1010 h)h) andand >95%>95% yield ((C7C7,, 1010 h),h), respectively.respectively.

SchemeScheme 2.2. ReductionReduction ofof carbonylcarbonyl groupsgroups byby high-valenthigh-valent rheniumrhenium oxidesoxidesC2 C2––C7C7..

InIn thethe same same year, year, Abu-Omar Abu-Omar and and co-workers co-workers reported reported a new a system new forsystem the hydrosi-for the lylationhydrosilylation of aldehydes of aldehydes and ketones and using ketones a mono-oxorhenium(V) using a mono-oxorhenium(V) catalyst (C8 catalyst) containing (C8) twocontaining oxazoline two ligands oxazoline (Scheme ligands3)[ 29 (Scheme,30]. The 3) reaction [29,30]. proceeds The reaction efﬁciently proceeds under efficiently ambient temperaturesunder ambient with temperatures low catalyst with loading low catalyst (0.1 mol%), loading 1.5 (0.1 equiv. mol%), HSiEt 1.53 equiv.using CHHSiEt2Cl3 2usingas a Molecules 2021, 26, x solventCH2Cl2 oras withouta solvent a or solvent. without Interestingly, a solvent. Interest the catalystingly, preservesthe catalyst its preserves activity after its activity being4 of 15

recycledafter being (for recycled 2-butanone, (for 2-butanone, 80% NMR yield 80% (1stNMR cycle) yield and (1st 50% cycle) NMR and yield 50% (2ndNMR cycle)). yield (2nd cycle)).

SchemeScheme 3. 3.Reduction Reduction ofof carbonylcarbonyl groupsgroups by by oxazoline oxazoline containing containing rhenium rhenium oxide oxideC8 C8. .

Then,Then, Abu-Omar Abu-Omar and and co-workers co-workers prepared prepared a number a number of cationic of cationic oxorhenium(V) oxorhenium(V) salen basedsalen based complexes complexes such assuch [Re(O)(salpn)(Solv)][B(C as [Re(O)(salpn)(Solv)][B(C6F5)4]6FC95)4][ 31C9,32 [31,32]] in 2006 in 2006 (Scheme (Scheme4a) and4a) [ReO(saldach)(Hand [ReO(saldach)(H2O)][B(C2O)][B(C6F5)4] C106F5)4][ 33C10] in 2008[33] incorporatingin 2008 incorporating a chiral environment, a chiral (Schemeenvironment,4b). C9 (Schemeand C10 4b).serve C9 as goodand C10 catalysts serve for as hydrosilylation good catalysts of for carbonyl hydrosilylation compounds of (1.0carbonyl mol% compounds cat. loading, (1.0 1.5 mol% equiv. cat. silane, loading, room 1.5 temperature equiv. silane, (r.t.), room although temperature asymmetric (r.t.), versionsalthough ( C10asymmetric) of these versions reactions (C10 afford) of these poor reactions enantioselectivity, afford poor even enantioselectivity, in the presence even of bulkyin the silanepresence such of as bulky HSitBuMe silane2 such. as HSitBuMe2.

Scheme 4. Hydrosilylation of carbonyl compounds with cationic oxorhenium(V) non chiral salen (a) and chiral salen (b) based complexes.

The mechanism of action of these high-valent mono-oxo-rhenium(V) catalysts was investigated, providing new insights [34] including via a theoretical study [35–37]. The addition of silane across the Re-O multiple bond was not observed in mono-oxorhenium(V) complex Re(O)Cl3(PPh3)2 (C4) catalyzing the hydrosilylation of carbonyls. Furthermore, they observed that silanes slowly reacted with C4 to give the isolated hydride intermediate Re(O)Cl2H(PPh3)2 (III-1). However, this isolated hydride intermediate is not sufficiently reactive to account for the catalytic turnover, as only 20% yield of the silyl ether product is observed (Scheme 5 left). These observations lead the authors to propose that the reaction pathway involves the formation of a η2–silane complex (IV-1), followed by the heterolytic cleavage of the Si-H bond at the rhenium center after elimination of one PPh3 ligand (Scheme 5, right). This ionic mechanism involves the sequential transfer of a silyl ion (R3Si+) and then a hydride to a polar double bond to furnish the reduction reactions. In this proposed catalytic cycle, neither the coordination of unsaturated substrates to the metal center nor its insertion into the M-H bond is involved.

Molecules 2021, 26, x 4 of 15

Scheme 3. Reduction of carbonyl groups by oxazoline containing rhenium oxide C8.

Then, Abu-Omar and co-workers prepared a number of cationic oxorhenium(V) salen based complexes such as [Re(O)(salpn)(Solv)][B(C6F5)4] C9 [31,32] in 2006 (Scheme 4a) and [ReO(saldach)(H2O)][B(C6F5)4] C10 [33] in 2008 incorporating a chiral environment, (Scheme 4b). C9 and C10 serve as good catalysts for hydrosilylation of Molecules 2021, 26, 2598 carbonyl compounds (1.0 mol% cat. loading, 1.5 equiv. silane, room temperature4 (r.t.), of 15 although asymmetric versions (C10) of these reactions afford poor enantioselectivity, even in the presence of bulky silane such as HSitBuMe2.

SchemeScheme 4.4. HydrosilylationHydrosilylation ofof carbonylcarbonyl compoundscompounds with with cationic cationic oxorhenium(V) oxorhenium(V) non non chiral chiral salen salen (a ) and(a) and chiral chiral salen salen (b) based(b) based complexes. complexes.

TheThe mechanismmechanism ofof actionaction ofof thesethese high-valenthigh-valent mono-oxo-rhenium(V)mono-oxo-rhenium(V) catalystscatalysts waswas investigated,investigated, providingproviding newnew insightsinsights [[34]34] includingincluding viavia aa theoreticaltheoretical studystudy [[35–37].35–37]. TheThe additionaddition ofof silane silane across across the Re-Othe Re-O multiple multiple bond wasbond not was observed not inobserved mono-oxo-rhenium(V) in mono-oxo- complexrhenium(V) Re(O)Cl complex3(PPh Re(O)Cl3)2 (C4)3 catalyzing(PPh3)2 (C4 the) catalyzing hydrosilylation the hydrosilylation of carbonyls. Furthermore,of carbonyls. theyFurthermore, observed thatthey silanes observed slowly that reacted silanes with slowlyC4 to reacted give the with isolated C4 hydrideto give intermediatethe isolated Re(O)Clhydride2 H(PPhintermediate3)2 (III-1 ).Re(O)Cl However,2H(PPh this3 isolated)2 (III-1 hydride). However, intermediate this isolated is not sufﬁciently hydride reactiveintermediate to account is not for sufficiently the catalytic reactive turnover, to account as only for 20% the yield catalytic of the turnover, silyl ether as product only 20% is observedyield of the (Scheme silyl ether5 left). product These observations is observed lead(Scheme the authors 5 left). toThese propose observations that the reactionlead the pathwayauthors to involves propose the that formation the reaction of a η2–silane pathway complex involves (IV-1 the), followed formation by theof a heterolytic η2–silane cleavagecomplex of(IV-1 the), Si-Hfollowed bond by at the heterolytic rhenium center cleava afterge of elimination the Si-H bond of one at PPhthe 3rheniumligand (Schemecenter after5, right). elimination This ionic of one mechanism PPh3 ligand involves (Scheme thesequential 5, right). transferThis ionic of mechanism a silyl ion + (Rinvolves3Si ) and the then sequential a hydride transfer to a polar of a doublesilyl ion bond (R3Si to+) furnishand then the a reduction hydride to reactions. a polar double In this Molecules 2021, 26, x 5 of 15 proposedbond to furnish catalytic the cycle, reduction neither reactions. the coordination In this ofproposed unsaturated catalytic substrates cycle,to neither the metal the centercoordination nor its insertionof unsaturated into the substrates M-H bond to isth involved.e metal center nor its insertion into the M-H bond is involved.

SchemeScheme 5. TheThe σ-bondσ-bond metathesis metathesis pathway pathway involving involving the isolated the isolatedhydride intermediate hydride intermediate Re(O)Cl2H(PPh3)2 (left). The pathway involving a η22–silane complex (right). Re(O)Cl2H(PPh3)2 (left). The pathway involving a η –silane complex (right).

TheThe enantioselectiveenantioselective reductionreduction ofof prochiralprochiral ketonesketones [[38]38] andand iminesimines [[39]39] withwith 22 equiv.equiv. HSiMeHSiMe22PhPh waswas reportedreported byby TosteToste (Scheme (Scheme6 )6) using using chiral chiral (CN-box)Re(V)–oxo (CN-box)Re(V)–oxo complexes complexes (3.0(3.0 mol%,mol%, CN-BoxCN-Box = cyanobis(oxazoline)). cyanobis(oxazoline)). These These reduction reduction reactions reactions proceed proceed under under an anambient ambient air airatmosphere atmosphere with with a highly a highly function functionalal group group tolerant tolerant with withees of ees up ofto up>99%. to >99%.The (CNbox)Re(V)–oxo The (CNbox)Re(V)–oxo complexes complexes (C12) (canC12 be) can prepared be prepared by simply by simply stirring stirring L1 withL1 withRe(O)Cl Re(O)Cl3(OPPh3(OPPh3)(SMe3)(SMe2) (C112)() inC11 CH) in2Cl CH2 at 2r.t.Cl 2andat r.t. isolated and isolated as a green as astable green solid stable (Scheme solid (Scheme6a) or generated6a) or generated directly directlyin situ. in situ.

Scheme 6. Enantioselective reduction of ketones (b) and imines (c) catalyzed by (CN-Box)Re(V)– oxo complexes C12 (a).

Similarly, a theoretical investigation of the mechanism for the high-valent mono-oxorhenium(V) hydride Re(O)HCl2(PPh3)2 (III-1) catalyzed hydrosilylation of C=N functionalities was performed by Wei and co-workers [40]. These results suggest that an ionic SN2-Si outer-sphere pathway proceeding via the heterolytic cleavage of the Si-H bond competes with the hydride pathway featuring the C=N bond inserted into the Re-H bond. Besides these oxorhenium complexes, a series of novel low-valent Re(III) complexes were synthesized by Ison et al., such as C13 [41] in 2012 and C14 [42] in 2017 (Scheme 7a). C14 (0.03 mol%, r.t.) is more reactive than C13 (0.1 mol%, 80 °C) for the hydrosilylation of aldehydes [42]. Excellent NMR yields (65–100%, 13 examples) were achieved at ambient

Molecules 2021, 26, x 5 of 15

Scheme 5. The σ-bond metathesis pathway involving the isolated hydride intermediate Re(O)Cl2H(PPh3)2 (left). The pathway involving a η2–silane complex (right).

The enantioselective reduction of prochiral ketones [38] and imines [39] with 2 equiv. HSiMe2Ph was reported by Toste (Scheme 6) using chiral (CN-box)Re(V)–oxo complexes (3.0 mol%, CN-Box = cyanobis(oxazoline)). These reduction reactions proceed under an ambient air atmosphere with a highly functional group tolerant with ees of up to >99%. Molecules 2021, 26, 2598 The (CNbox)Re(V)–oxo complexes (C12) can be prepared by simply stirring L15 ofwith 15 Re(O)Cl3(OPPh3)(SMe2) (C11) in CH2Cl2 at r.t. and isolated as a green stable solid (Scheme 6a) or generated directly in situ.

SchemeScheme 6. 6.Enantioselective Enantioselective reduction reduction of of ketones ketones (b ()b and) and imines imines (c )(c catalyzed) catalyzed by by (CN-Box)Re(V)–oxo (CN-Box)Re(V)– complexesoxo complexesC12 ( C12a). (a).

Similarly,Similarly, a a theoretical theoretical investigation investigation of of the the mechanism mechanism for for thethe high-valenthigh-valent mono-oxo-mono-oxorhenium(V)rhenium(V) hydride hydride Re(O)HCl Re(O)HCl2(PPh2(PPh3)2 (III-13)2 )(III-1 catalyzed) catalyzed hydrosilylation hydrosilylation of C=N functional- of C=N itiesfunctionalities was performed was byperformed Wei and by co-workers Wei and [co-w40]. Theseorkers results [40]. These suggest results that ansuggest ionic SthatN2-Si an outer-sphereionic SN2-Si pathwayouter-sphere proceeding pathway via proceeding the heterolytic via cleavagethe heterolytic of the Si-Hcleavage bond of competes the Si-H withbond the competes hydride with pathway the hydride featuring pathway the C=N featuring bond inserted the C=N into bond the inserted Re-H bond. into the Re-H bond.Besides these oxorhenium complexes, a series of novel low-valent Re(III) complexes were synthesizedBesides these by oxorhenium Ison et al., such complexes, as C13 [a41 seri] ines 2012 of novel and C14 low-valent[42] in 2017Re(III) (Scheme complexes7a). ◦ C14were(0.03 synthesized mol%, r.t.) by is Ison more et reactiveal., such than as C13C13 [41](0.1 in mol%, 2012 and 80 C14C) for [42] the in hydrosilylation 2017 (Scheme 7a). of aldehydesC14 (0.03 mol%, [42]. Excellent r.t.) is more NMR reactive yields than (65–100%, C13 (0.1 13 mol%, examples) 80 °C) were for the achieved hydrosilylation at ambient of temperaturealdehydes [42]. under Excellent neat conditions NMR yields using (65–100%, dimethylphenylsilane 13 examples) were with achievedC14. The at reaction ambient affords TONs of up to 9200 and a TOF of up to 126 h−1 (Scheme7a). Based on kinetic and mechanistic studies, an ionic outer-sphere mechanism for the catalytic hydrosilylation of benzaldehyde by C14 has been proposed (Scheme7b). First, 1 2 0 silane is activated through η (V-2) or η (V-2 ) coordination to the rhenium center. Then, the aldehyde substrate attacks the Si center in (V-2) or (V-20) to prompt the heterolytic cleavage of the Si-H bond yielding an ion pair, the hydride complex V-3 and a silylated aldehyde ion. Finally, hydride transfer from rhenium to the carbonyl carbon of the activated substrate completes the catalytic cycle [43]. Berke and co-workers reported several easily available nitrosyl-rhenium complexes, for 2 example, Re(H)(PiPr3)2(NO)(NOB(C6F5)3)(C15)[44] in 2005, Re(H)2(η -C2H4)3(NO)(PR3)2 (C16, R = iPr, C17, R = Cy) [45] in 2008, Re(CH3CN)3Br2(NO) (C18)[46] in 2009, Re(CH3CN)3Cl2(NO) (C19)[47] and Re(CH3CN)Cl2(NO)(PCy3)2 (C20)[47] in 2011. Those nitrosyl rhenium complexes were employed in the catalytic hydrosilylation of a variety of carbonyl compounds (Scheme8a). Hydrosilylations of carbonyl compounds were carried ◦ out with the catalyst C15, HSiR3 (1 equiv., R = Et or Ph) from r.t. to 70 C either in toluene or under solvent-free conditions. TONs up to 9000 and TOFs up to 22,500 h−1 were observed. C16 and C17 exhibited similar activity in hydrosilylation of acetophenone (0.5 mol% cat. ◦ −1 loading at 70 C in toluene-d8, giving full conv. and TOF up to 800 h ). Molecules 2021, 26, x 6 of 15

Molecules 2021, 26, 2598 6 of 15 temperature under neat conditions using dimethylphenylsilane with C14. The reaction affords TONs of up to 9200 and a TOF of up to 126 h−1 (Scheme 7a).

SchemeScheme 7.7. Cationic rhenium rhenium complex complex catalyzed catalyzed hydrosilylation hydrosilylation of ofaldehydes. aldehydes. (a) Scope (a) Scope of the of the reaction;reaction; ( b(b)) Proposed Proposed mechanism. mechanism.

ForBasedC18 on, chlorobenzene kinetic and mechanistic was found studies, to be superioran ionic outer-sphere to all the other mechanism solvents for used. the Variouscatalytic aliphatic hydrosilylation and aromatic of benzaldehyde silanes were tested.by C14Excellent has been yields proposed were (Scheme achieved 7b). at r.t. First, in dichloromethanesilane is activated using through triethylsilane, η1 (V-2) or the η2 reaction(V-2′) coordination affording TOF to the values rhenium of up center. to 495 Then, h−1. the aldehydePhosphine-free substrate complex attacksC19 theproved Si center to be in less (V-2 effective) or (V-2 than′) to the prompt bisphosphine the heterolytic deriva- tivecleavageC20, asof thethe reactionSi-H bond of benzophenoneyielding an ion and pair, Et the3SiH hydride (1.12 equiv.) complex with V-3 1.0 and mol% a silylated of C19 wasaldehyde carried ion. out atFinally, 80 ◦C, hydride a conversion transfer of less from than rhenium 10% was to achieved the carbonyl within carbon 4 h, while of the in theactivated same conditions, substrate completesC20 gave athe 99% catalytic yield. cycle [43]. ABerke possible and mechanismco-workers reported for the hydrosilylation several easily available of ketones nitrosyl-rhenium catalyzed by C18 complexes,was pro- 2 posedfor example, (Scheme8 b).Re(H)(P The initialiPr3)2(NO)(NOB(C step consists6F of5)3 the) dissociation(C15) [44] ofin a CH2005,3CN ligandRe(H)2( toη - generateC2H4)3(NO)(PR the pre-catalyst3)2 (C16, RVI-1 = iPr,followed C17, R = by Cy) the [45] coordination in 2008, Re(CH of the3CN) silane3Br2 to(NO) the ( rheniumC18) [46] 2 centerin 2009, to Re(CH form a3CN)η –silane3Cl2(NO) complex (C19) [47]VI-2 and. Displacement Re(CH3CN)Cl of2(NO)(PCy a second3) CH2 (C203CN) [47] ligand in 2011. by theThose ketone nitrosyl at the rhenium rhenium complexes center yields were complex employVI-3ed in. The the silyl-oxycatalytic complexhydrosilylationVI-4 could of a bevariety obtained of carbonyl either from compound the transfers (Scheme of a hydride 8a). Hydrosilylatio from the silanens of to carbonyl the ketone compounds atom or fromwere thecarried oxidative out with addition the catalyst of the silaneC15, HSiR followed3 (1 equiv., by a β R-hydride = Et or Ph) transfer from process. r.t. to 70 Fi-°C nally,either the in toluene reductive or under elimination solvent-free step producesconditions. the TONs siloxy up product to 9000 and and TOFs regenerates up to 22,500 the pre-catalysth−1 were observed.VI-1. C16 and C17 exhibited similar activity in hydrosilylation of acetophenone (0.5 mol% cat. loading at 70 °C in toluene-d8, giving full conv. and TOF up to 800 h−1). For C18, chlorobenzene was found to be superior to all the other solvents used. Various aliphatic and aromatic silanes were tested. Excellent yields were achieved at r.t.

Molecules 2021, 26, x 7 of 15

in dichloromethane using triethylsilane, the reaction affording TOF values of up to 495 h−1. Phosphine-free complex C19 proved to be less effective than the bisphosphine derivative C20, as the reaction of benzophenone and Et3SiH (1.12 equiv.) with 1.0 mol% of C19 was carried out at 80 °C, a conversion of less than 10% was achieved within 4 h, while in the same conditions, C20 gave a 99% yield. A possible mechanism for the hydrosilylation of ketones catalyzed by C18 was proposed (Scheme 8b). The initial step consists of the dissociation of a CH3CN ligand to generate the pre-catalyst VI-1 followed by the coordination of the silane to the rhenium center to form a η2–silane complex VI-2. Displacement of a second CH3CN ligand by the ketone at the rhenium center yields complex VI-3. The silyl-oxy complex VI-4 could be obtained either from the transfer of a hydride from the silane to the ketone atom or from Molecules 2021, 26, 2598 the oxidative addition of the silane followed by a β-hydride transfer process. Finally,7 of the 15 reductive elimination step produces the siloxy product and regenerates the pre-catalyst VI-1

SchemeScheme 8.8. Rhenium oxides oxides catalyzed catalyzed hydrosilylation hydrosilylation of of carbonyl carbonyl compounds. compounds. (a) (Scopea) Scope of the of the reaction;reaction; ( b(b)) Proposed Proposed mechanism. mechanism.

Molecules 2021, 26, x In 2012, carbonyl rhenium(I) complexes, namely Re(CO)5Cl (C21) and Re28(CO) of 1510 In 2012, carbonyl rhenium(I) complexes, namely Re(CO)5Cl (C21) and Re2(CO)10 (C22(C22),), havehave been been found found by by Fan Fan and and co-workers co-workers [48 [48]] to beto effectivebe effective catalysts catalysts for thefor hy-the drosilylationhydrosilylation of carbonylof carbonyl substrates substrates with with various various silanes silanes and and with with TOF TOF of 20of −2025−25 h− h1−1for for aldehydesregeneratesaldehydes (Scheme (Scheme the catalyst9 a).9a). In InVII-2 this this methodology,. methodology,When either 1.0the 1.0 mol% carbonylmol% catalyst catalyst or silane and and a hasa Et Et3 3SiH:carbonylbeenSiH:carbonyl depleted, ratio ratio the of 3:1 were used. When different silanes such as Ph2SiH2 and Ph3SiH were used, a decrease ofEt 3:13SiRe(CO) were used.4 complex( WhenVII-2 different) coordinates silanes such back as Phto2 HRe(CO)SiH2 and5 Ph (VII-33SiH) were and used,becomes a decrease part of inthein the the resting corresponding corresponding state VII-1 silyl silyl. ether ether yield yield was was observed. observed. A detailed mechanism for the hydrosilylation of carbonyl compounds was proposed by the same author to account for the experimental observations (Scheme 9b). Upon photolysis of Et3SiH with Re(CO)5Cl or Re2(CO)10, a dimeric rhenium carbonyl species (VII-1) with a bridging hydride was identified in the mixture. When VII-1 was isolated and tested for aldehyde hydrosilylation, the silyl ether was generated about 2–3 times faster in comparison to Re(CO)5Cl. Upon photolysis, the dimer complex VII-1 dissociates to afford Et3SiRe(CO)4 (VII-2) and HRe(CO)5 (VII-3) which was found to be inactive in catalysis. Then, the coordination of the carbonyl substrate to the vacant site of VII-2 facilitates the silyl ligand shift onto the oxygen atom and forms an alkyl ligand bound to the Re center VII-5. Another silane coordinates to the Re center in VII-5 via a η2-silyl complex or a σ-silyl (σH) complex. Migration of a H atom from the silane to the alkyl group yields the silyl ether product and

5 SchemeScheme 9.9. HydrosilylationHydrosilylation ofof carbonylscarbonyls viavia Re(CO)Re(CO)5ClCl photolysisphotolysis ((aa)) andand proposed proposed mechanism mechanism ( b). (b).

2.2. Hydrosilylation of Nitriles The reduction of nitriles using rhenium are rare. However, in 2011, Fernandes reported a new catalytic system for the reduction of nitriles into the corresponding primary amines with silane as a reducing agent and catalyzed by oxo-rhenium complexes. Using (10 mol%) of rhenium(V)-dioxo complex C1 and 3 equiv. of phenylsilane in refluxing toluene under air atmosphere, a large range of nitriles bearing functional groups, such as halogen derivatives, OCH3, SCH3, SO2CH3 and NHTs, was reduced with high chemoselectivity (Scheme 10a) [49].

Molecules 2021, 26, 2598 8 of 15

A detailed mechanism for the hydrosilylation of carbonyl compounds was proposed by the same author to account for the experimental observations (Scheme9b). Upon photolysis of Et3SiH with Re(CO)5Cl or Re2(CO)10, a dimeric rhenium carbonyl species (VII-1) with a bridging hydride was identiﬁed in the mixture. When VII-1 was isolated and tested for aldehyde hydrosilylation, the silyl ether was generated about 2–3 times faster in comparison to Re(CO)5Cl. Upon photolysis, the dimer complex VII-1 dissociates to afford Et3SiRe(CO)4 (VII-2) and HRe(CO)5 (VII-3) which was found to be inactive in catalysis. Then, the coordination of the carbonyl substrate to the vacant site of VII-2 facilitates the silyl ligand shift onto the oxygen atom and forms an alkyl ligand bound to the Re center VII-5. Another silane 2 coordinates to the Re center in VII-5 via a η -silyl complex or a σ-silyl (σH) complex. Migration of a H atom from the silane to the alkyl group yields the silyl ether product and regenerates the catalyst VII-2. When either the carbonyl or silane has been depleted, the Et3SiRe(CO)4 complex(VII-2) coordinates back to HRe(CO)5 (VII-3) and becomes part of the resting state VII-1.

2.2. Hydrosilylation of Nitriles The reduction of nitriles using rhenium are rare. However, in 2011, Fernandes reported a new catalytic system for the reduction of nitriles into the corresponding primary amines with silane as a reducing agent and catalyzed by oxo-rhenium complexes. Using (10 mol%) of rhenium(V)-dioxo complex C1 and 3 equiv. of phenylsilane in reﬂuxing toluene under air atmosphere, a large range of nitriles bearing functional groups, such as halogen Molecules 2021, 26, x derivatives, OCH3, SCH3, SO2CH3 and NHTs, was reduced with high chemoselectivity9 of 15

(Scheme 10a) [49].

Scheme 10. Reduction of nitriles with silanes and ReIO2(PPh3)2 (a) and proposed catalytic cycle (b). Scheme 10. Reduction of nitriles with silanes and ReIO2(PPh3)2 (a) and proposed catalytic cycle (b). Then, the catalytic cycle was proposed by the same author (Scheme 10b): Replacing two phosphine ligands with two nitriles at the rhenium center in C1 produces complex ReIO2(nitrile)2 (VIII-1). The addition of the Si-H bond to one of the oxo-rhenium bond results in the formation of the hydride species (nitrile)2(O)IRe(H)OSiR3 (VIII-2). Dihydros- ilylation of the nitrile to the corresponding N,N-disilylamino-rhenium complex VIII-5, followed by hydrolysis of the N-disilylamine, affords the primary amine as product.

2.3. Reductive Amination The direct reductive amination of aldehydes with primary and secondary anilines [50], using the same oxorhenium complex C1, was achieved by the group of Fernandes. A large variety of functional groups, such as nitro, sulfone, ester, nitrile, amide, halides including iodide, were well tolerated, using 2.5% of catalyst loading under refluxing THF for 5 min to 7 h (Scheme 11) [51]. The same mechanism as in Scheme 10 was proposed by the authors. In 2013, the same group has reported a series of oxorhenium complexes containing heterocyclic ligands, which were found to also be very efficient catalysts for such transformation. For instance, similar efficiency and chemoselectivity were obtained with the oxo-rhenium complex [ReOBr2(L)(PPh3) C23 (L = 2-(2′-hydroxy-5′-methylphenyl)benzotriazole) (Scheme 11) [52].

Molecules 2021, 26, 2598 9 of 15

Then, the catalytic cycle was proposed by the same author (Scheme 10b): Replacing two phosphine ligands with two nitriles at the rhenium center in C1 produces complex ReIO2(nitrile)2 (VIII-1). The addition of the Si-H bond to one of the oxo-rhenium bond results in the formation of the hydride species (nitrile)2(O)IRe(H)OSiR3 (VIII-2). Dihy- drosilylation of the nitrile to the corresponding N,N-disilylamino-rhenium complex VIII-5, followed by hydrolysis of the N-disilylamine, affords the primary amine as product.

2.3. Reductive Amination The direct reductive amination of aldehydes with primary and secondary anilines [50], using the same oxorhenium complex C1, was achieved by the group of Fernandes. A large variety of functional groups, such as nitro, sulfone, ester, nitrile, amide, halides including iodide, were well tolerated, using 2.5% of catalyst loading under refluxing THF for 5 min to 7 h (Scheme 11)[51]. The same mechanism as in Scheme 10 was proposed by the authors. In 2013, the same group has reported a series of oxorhenium complexes containing heterocyclic ligands, which were found to also be very efficient catalysts for such transformation. For instance, similar efficiency and chemoselectivity Molecules 2021, 26, x were obtained with the oxo-rhenium complex [ReOBr (L)(PPh ) C23 (L = 2-(20-hydroxy-510 of 015- Molecules 2021, 26, x 2 3 10 of 15 methylphenyl)benzotriazole) (Scheme 11)[52].

SchemeScheme 11. 11.Direct Direct reductive reductive amination amination of of aldehydes aldehydes using using silane/oxorhenium silane/oxorhenium system.system.

Furthermore,Furthermore, inin 2013, 2013, Ghorai Ghorai reported reported a a direct direct reductive reductive amination amination of of ketones ketones such such 2 7 asas alkanones alkanones and and cycloalkanones cycloalkanones with with electron-deﬁcient electron-deficient amines amines using using Re Re2O2O77(1.5 (1.5 mol%) mol%) and NaPF 6(20 mol%) as the catalytic system and triethylsilane (1.2 equiv.) as the reductant and NaPF66 (20 mol%) as the catalytic system and triethylsilane (1.2 equiv.) as the reductant ◦ inin dichloromethane dichloromethane atat 5050 °CC for for 12–60 12–60 h h (Sch (Schemeeme 12) 12 )[[53].53]. Excellent Excellent diastereoselective diastereoselective re- reductiveductive amination amination of of 2-alkyl 2-alkyl cyclohexanones cyclohexanones was was also also observed, observed, the the formation formation of ofcis-se- cis- selectivelective 2-alkyl 2-alkyl amines amines was was obtained obtained with with high high chemoselectivity chemoselectivity (up (up to to >1:99). >1:99).

Scheme 12. Direct reductive amination of ketones with electron-deficient amines using Scheme 12. Direct reductive amination of ketones with electron-deﬁcient amines using Re2O7/NaPF6 Re2O7/NaPF6 catalyst system. catalystRe2O7/NaPF system.6 catalyst system.

2.4.2.4. Hydrosilylation Hydrosilylation of of Nitro Nitro Derivatives Derivatives InIn 2009, 2009, the the group group of of Fernandes Fernandes also also reported reported thethe reductionreduction ofof aromaticaromatic nitronitro com-com- poundspounds to to the the corresponding corresponding amines amines with with silanes silanes catalyzed catalyzed by by high high valent valent oxo-rhenium oxo-rhenium 2 complexescomplexesC1 C1or orC4 C4in in the the presence presence of of 3.6 3.6 equiv. equiv. of of PhMe PhMe22SiHSiH inin reﬂuxingrefluxing toluenetoluene condi-condi- tionstions for for 1–48 1–48 h h (Scheme (Scheme 13 13))[ 54[54].]. Aniline Aniline derivatives derivatives were were then then isolated isolated in in moderate moderate to to goodgood yields yields (31–96%). (31–96%). It It is is noticeable noticeable that that this this catalytic catalytic transformation transformation tolerated tolerated a huge a huge va- rietyvariety of functional of functional groups groups such such as halides, as halides, esters, esters, amides, amides, sulfones sulfones and nitriles.and nitriles. By contrast, By con- thetrast, hydrosilylation the hydrosilylation of nitroalkanes, of nitroalkanes, such as 2-nitroethylbenzene,such as 2-nitroethylbenzene, led to the led corresponding to the corre- nitrilesponding in 38% nitrile yield. in 38% yield.

Scheme 13. Reduction of aromatic nitro compounds with C1 or C4.

2.5. Hydrosilylation of Carboxyl Acids Derivatives Following our study of the hydrosilylation of various carboxylic acids catalyzed with 2 10 Mn2(CO)10 [55], our group recently reported the direct reduction of carboxylic acids [56] 2 10 and esters [57] catalyzed by Re2(CO)10, under mild conditions (r.t., irradiation 350, 395, or 365 nm), with low catalyst loading (0.5 mol%) in the presence of a stoichiometric amount 3 of Et3SiH as a reducing agent (Scheme 14). A large variety of carboxylic acids and esters was reduced in moderate to good yields to the corresponding protected aldehydes without noticeable formation of silylethers arising from over-reduction. Good functional group tolerance was achieved over amino, halogeno, isolated C=C as well as heteroaromatic groups. In addition, this new protocol was applied to a range of benzoic acid derivatives that are reluctant to be reduced by traditional methods with more drastic condi- 2 10 2 tions: Re2(CO)10 (5 mol%), Ph2MeSiH (4.0 equiv.) to afford the corresponding aldehydes after acid treatment (Scheme 14a).

Molecules 2021, 26, x 10 of 15

Scheme 11. Direct reductive amination of aldehydes using silane/oxorhenium system.

Furthermore, in 2013, Ghorai reported a direct reductive amination of ketones such as alkanones and cycloalkanones with electron-deficient amines using Re2O7 (1.5 mol%) and NaPF6 (20 mol%) as the catalytic system and triethylsilane (1.2 equiv.) as the reductant in dichloromethane at 50 °C for 12–60 h (Scheme 12) [53]. Excellent diastereoselective reductive amination of 2-alkyl cyclohexanones was also observed, the formation of cis-selective 2-alkyl amines was obtained with high chemoselectivity (up to >1:99).

Scheme 12. Direct reductive amination of ketones with electron-deficient amines using Re2O7/NaPF6 catalyst system.

2.4. Hydrosilylation of Nitro Derivatives In 2009, the group of Fernandes also reported the reduction of aromatic nitro compounds to the corresponding amines with silanes catalyzed by high valent oxo-rhenium complexes C1 or C4 in the presence of 3.6 equiv. of PhMe2SiH in refluxing toluene conditions for 1–48 h (Scheme 13) [54]. Aniline derivatives were then isolated in moderate to good yields (31–96%). It is noticeable that this catalytic transformation tolerated a huge Molecules 2021, 26, 2598 variety of functional groups such as halides, esters, amides, sulfones and nitriles. By10 ofcon- 15 trast, the hydrosilylation of nitroalkanes, such as 2-nitroethylbenzene, led to the corresponding nitrile in 38% yield.

SchemeScheme 13.13. ReductionReduction ofof aromaticaromatic nitronitro compoundscompounds withwithC1 C1or orC4 C4..

2.5.2.5. HydrosilylationHydrosilylation ofof CarboxylCarboxyl AcidsAcids DerivativesDerivatives FollowingFollowing ourour studystudy ofof thethe hydrosilylationhydrosilylation ofof variousvarious carboxyliccarboxylic acidsacids catalyzedcatalyzed withwith Mn (CO) [55], our group recently reported the direct reduction of carboxylic acids [56] Mn22(CO)10 [55], our group recently reported the direct reduction of carboxylic acids [56] and esters [57] catalyzed by Re (CO) , under mild conditions (r.t., irradiation 350, 395, or and esters [57] catalyzed by Re22(CO)1010, under mild conditions (r.t., irradiation 350, 395, or 365365 nm),nm), withwith lowlow catalystcatalyst loadingloading (0.5(0.5 mol%)mol%) inin thethe presencepresence ofof aa stoichiometricstoichiometric amountamount of Et SiH as a reducing agent (Scheme 14). A large variety of carboxylic acids and esters of Et33SiH as a reducing agent (Scheme 14). A large variety of carboxylic acids and esters waswas reducedreduced in in moderate moderate to to good good yields yields to to the the corresponding corresponding protected protected aldehydes aldehydes without with- noticeable formation of silylethers arising from over-reduction. Good functional group out noticeable formation of silylethers arising from over-reduction. Good functional tolerance was achieved over amino, halogeno, isolated C=C as well as heteroaromatic group tolerance was achieved over amino, halogeno, isolated C=C as well as heteroaro- groups. In addition, this new protocol was applied to a range of benzoic acid derivatives matic groups. In addition, this new protocol was applied to a range of benzoic acid deriv- that are reluctant to be reduced by traditional methods with more drastic conditions: Molecules 2021, 26, x atives that are reluctant to be reduced by traditional methods with more drastic11 condi- of 15 Re2(CO)10 (5 mol%), Ph2MeSiH (4.0 equiv.) to afford the corresponding aldehydes after tions: Re2(CO)10 (5 mol%), Ph2MeSiH (4.0 equiv.) to afford the corresponding aldehydes acid treatment (Scheme 14a). after acid treatment (Scheme 14a).

Scheme 14. Rhenium-catalyzed chemoselective chemoselective reduction reduction of of carboxylic carboxylic acids acids (a ()a and) and esters esters (b ()b to) to aldehydesaldehydes withwith hydrosilanes.hydrosilanes.

3. Hydrosilylation of C=C bonds Bonds 3.1. Hydrosilylation of Alkenes Despite the importance of the hydrosilylation of alkenes for the production of sili- cones,cones, only limited limited examples examples have have been been report reporteded with with rhenium rhenium catalysts catalysts in contrast in contrast to the to thehydro-elementation hydro-elementation of polar of polar bonds. bonds. In 2006, In 2006,the group the group of Hua of reported Hua reported the addition the addi- of tion of hydrosilanes to styrenes catalyzed by low-valent rhenium complexes, such as hydrosilanes to styrenes catalyzed by low-valent rhenium complexes, such as Re(CO)5Br Re(CO) Br (C24), to selectively afford anti-Markovnikov adducts in good to high yields (C24), to5 selectively afford anti-Markovnikov adducts in good to high yields (Scheme 15) (Scheme 15) [58]. For the hydrosilylation of styrenes, 1.2 equiv. of HSiMePh2 was em- [58]. For the hydrosilylation of styrenes, 1.2 equiv. of HSiMePh2 was employed at 120 °C ployed at 120 ◦C in toluene (1 M) for 10 h. It is worth mentioning that other rhenium in toluene (1 M) for 10 h. It is worth mentioning that other rhenium complexes, such as complexes, such as Re(CO)5Cl (C21) and Re2(CO)10 (C22), also showed catalytic activity for Re(CO)5Cl (C21) and Re2(CO)10 (C22), also showed catalytic activity for the hydrosilylation the hydrosilylation of styrenes, although less actively than C24, to give silylated alkanes in of styrenes, although less actively than C24, to give silylated alkanes in moderate yields moderate yields as well as a slightly decreased selectivity. However, CpRe(CO)3 (C25) and as well as a slightly decreased selectivity. However, CpRe(CO)3 (C25) and NH4ReO4 (C26) NH ReO (C26) did not show any catalytic activity at all in such a transformation. Aliphatic did 4not show4 any catalytic activity at all in such a transformation. Aliphatic alkenes, such alkenes, such as 1-octene and methyl methacrylate, were reduced to their corresponding as 1-octene and methyl methacrylate, were reduced to their corresponding adducts in adducts in poor yields (30% and 28%, respectively). poor yields (30% and 28%, respectively).

Scheme 15. Re(CO)5Br-catalyzed hydrosilylation of styrenes.

Interestingly, rhenium mono-nitrosyl complex C16 developed by Berke et al., can also promote the catalytic hydrosilylation of cyclohexene [45] (Scheme 16a) under similar conditions that those employed for ketones (0.5 mol% cat., 80 °C, 3 h). This process was investigated theoretically by the group of Li with ethylene and trimethylsilane as model reactants (Scheme 16b) [59]. The proposed catalytic cycle starts with the insertion of the ethylene ligand into the Re-H bond, followed by the cleavage of the agostic interaction involved in VIII-1 to afford VIII-2. Then, a silane molecule coordinates to the Re center to give a σ complex VIII-3. The next step involves an oxidative addition process to afford a seven coordinate complex VIII-4. Last step consists of a reductive elimination process to form the product. The catalytic cycle completes by coordination of an ethylene to VIII-5.

Molecules 2021, 26, x 11 of 15

Scheme 14. Rhenium-catalyzed chemoselective reduction of carboxylic acids (a) and esters (b) to aldehydes with hydrosilanes.

3. Hydrosilylation of C=C bonds 3.1. Hydrosilylation of Alkenes Despite the importance of the hydrosilylation of alkenes for the production of sili- cones, only limited examples have been reported with rhenium catalysts in contrast to the hydro-elementation of polar bonds. In 2006, the group of Hua reported the addition of hydrosilanes to styrenes catalyzed by low-valent rhenium complexes, such as Re(CO)5Br (C24), to selectively afford anti-Markovnikov adducts in good to high yields (Scheme 15) [58]. For the hydrosilylation of styrenes, 1.2 equiv. of HSiMePh2 was employed at 120 °C in toluene (1 M) for 10 h. It is worth mentioning that other rhenium complexes, such as Re(CO)5Cl (C21) and Re2(CO)10 (C22), also showed catalytic activity for the hydrosilylation of styrenes, although less actively than C24, to give silylated alkanes in moderate yields as well as a slightly decreased selectivity. However, CpRe(CO)3 (C25) and NH4ReO4 (C26) Molecules 2021, 26, 2598 did not show any catalytic activity at all in such a transformation. Aliphatic alkenes,11 such of 15 as 1-octene and methyl methacrylate, were reduced to their corresponding adducts in poor yields (30% and 28%, respectively).

SchemeScheme 15.15. Re(CO)Re(CO)55Br-catalyzed hydrosilylationhydrosilylation ofof styrenes.styrenes.

Interestingly,Interestingly, rheniumrhenium mono-nitrosylmono-nitrosyl complexcomplex C16C16 developeddeveloped byby BerkeBerke etet al.,al., cancan alsoalso promotepromote thethe catalyticcatalytic hydrosilylationhydrosilylation ofof cyclohexenecyclohexene [[45]45] (Scheme(Scheme 16 16a)a) underunder similarsimilar ◦ conditionsconditions thatthat thosethose employedemployed forfor ketonesketones (0.5(0.5 mol%mol% cat.,cat., 8080 °C,C, 33 h).h). ThisThis processprocess waswas investigatedinvestigated theoreticallytheoretically byby thethe groupgroup ofof LiLi withwith ethyleneethylene andand trimethylsilanetrimethylsilane asas modelmodel reactantsreactants (Scheme(Scheme 1616b)b) [[59].59]. TheThe proposedproposed catalyticcatalytic cyclecycle startsstarts withwith thethe insertioninsertion ofof thethe ethyleneethylene ligandligand intointo thethe Re-HRe-H bond,bond, followedfollowed byby thethe cleavagecleavage ofof thethe agosticagostic interactioninteraction involvedinvolved inin VIII-1VIII-1 toto affordaffordVIII-2 VIII-2.. Then,Then, aa silanesilane moleculemolecule coordinatescoordinates toto thethe ReRe centercenter toto givegive aa σσ complexcomplex VIII-3VIII-3.. TheThe nextnext stepstep involvesinvolves anan oxidativeoxidative additionaddition processprocess toto affordafford aa Molecules 2021, 26, x 12 of 15 sevenseven coordinatecoordinate complexcomplex VIII-4VIII-4.. LastLast stepstep consistsconsists ofof aa reductivereductive eliminationelimination processprocess toto formform thethe product.product. TheThe catalyticcatalytic cyclecycle completescompletes byby coordinationcoordination ofof anan ethyleneethylene totoVIII-5 VIII-5..

a) PiPr3 H NO Re H

PiPr3 C16 (0.5 mol%) SiEt3 + HSiEt3 o C6D6, 80 C, 3 h 1.2 equiv. conv.: 81%, TOF: 54 h−1

b) H [Re]

C16 [Re] H H [Re] VIII-5

CH2CH2SiMe3 VIII-1

SiMe3 [Re] [Re] H VIII-2 VIII-4 SiMe3 [Re] HSiMe3 VIII-3 SchemeScheme 16. 16.Rhenium Rhenium mono-nitrosyl mono-nitrosyl complex complex catalyzed catalyzed hydrosilylation hydrosilylation of of alkenes alkenes (a ()a and) and proposed pro- mechanismposed mechanism (b). (b).

InIn 2014, 2014, the the same same group group reported reported a highly a highly selective selective hydrosilylation hydrosilylation of acrylonitrile of acrylonitrile and itsand derivatives its derivatives catalyzed catalyzed by C18 by(1.5 C18 mol%) (1.5 mol%) by reaction by reaction with alkyl with or alkyl aryl silanesor aryl (1silanes equiv.) (1 atequiv.) 60–115 at◦ 60–115C in 8–48 °C hin (Scheme8–48 h (Scheme 17)[60]. 17) The [60]. regioselectivity The regioselectivity of all the of all reactions the reactions were foundwere tofound be around to be aroundα:β = 3:1. α:β Improved = 3:1. Improved regioselectivity regioselectivity was obtained was obtained with a high with ratio a high of acrylonitrile:silane.ratio of acrylonitrile:silane.

Scheme 17. Re-catalyzed hydrosilylation of nitrile substituted olefins.

3.2. Dehydrogenative Silylation of Alkenes Dehydrogenative silylation is often considered an undesired side reaction in the hydrosilylation of alkenes, but the selective production of vinylsilanes is also a valuable transformation. In this context; Berke et al. reported several rhenium mono-nitrosyl complexes (C18 [60], C19–C20 [47], C25–C26 [61], C27–C28 [62]) catalyzing the dehydrogenative silylation of alkenes with high chemoselectivity to produce silyl alkenes, along with the corresponding alkanes, with 1.0–4.0 mol% cat., at 70–110 °C in toluene-d8 (Scheme 18). Hydrosilylation products appear only rarely in very minor amounts. The complexes C19– C20 and C27–C28 gave better E/Z selectivity (96:4→99:1). Noticeably, ethylene can be transformed quantitatively into the corresponding dehydrogenative silylation product with C27–C28 as catalysts with a ratio dehydro/hydro silylation products of 79:21 and 68:32, respectively. C18 catalyzes the dehydrogenative coupling of styrene derivatives with HSiMe2Ph leading to dehydro/hydro silylation products in ratio of about 85/15.

Molecules 2021, 26, x 12 of 15

a) PiPr3 H NO Re H

PiPr3 C16 (0.5 mol%) SiEt3 + HSiEt3 o C6D6, 80 C, 3 h 1.2 equiv. conv.: 81%, TOF: 54 h−1

b) H [Re]

C16 [Re] H H [Re] VIII-5

CH2CH2SiMe3 VIII-1

SiMe3 [Re] [Re] H VIII-2 VIII-4 SiMe3 [Re] HSiMe3 VIII-3 Scheme 16. Rhenium mono-nitrosyl complex catalyzed hydrosilylation of alkenes (a) and proposed mechanism (b).

In 2014, the same group reported a highly selective hydrosilylation of acrylonitrile and its derivatives catalyzed by C18 (1.5 mol%) by reaction with alkyl or aryl silanes (1 equiv.) at 60–115 °C in 8–48 h (Scheme 17) [60]. The regioselectivity of all the reactions Molecules 2021, 26, 2598 12 of 15 were found to be around α:β = 3:1. Improved regioselectivity was obtained with a high ratio of acrylonitrile:silane.

SchemeScheme 17.17. Re-catalyzedRe-catalyzed hydrosilylationhydrosilylation ofof nitrilenitrile substitutedsubstituted oleﬁns.olefins.

3.2.3.2. Dehydrogenative SilylationSilylation ofof AlkenesAlkenes DehydrogenativeDehydrogenative silylationsilylation is often consideredconsidered an undesired side reaction in the hy- drosilylationdrosilylation ofof alkenes,alkenes, butbut thethe selectiveselective productionproduction ofof vinylsilanesvinylsilanes isis alsoalso aa valuablevaluable transformation.transformation. InIn thisthis context;context; BerkeBerke etet al.al. reported several rhenium mono-nitrosyl com-com- plexesplexes ((C18C18 [[60],60], C19C19––C20C20 [[47],47], C25C25––C26C26 [[61],61], C27C27––C28C28 [[62])62]) catalyzingcatalyzing thethe dehydrogena-dehydrogena- tivetive silylationsilylation ofof alkenesalkenes withwith highhigh chemoselectivitychemoselectivity toto produceproduce silylsilyl alkenes,alkenes, alongalong withwith the corresponding alkanes, with 1.0–4.0 mol% cat., at 70–110 ◦C in toluene-d (Scheme 18). the corresponding alkanes, with 1.0–4.0 mol% cat., at 70–110 °C in toluene-d88 (Scheme 18). C19 HydrosilylationHydrosilylation productsproducts appearappear onlyonly rarelyrarely inin veryvery minorminor amounts.amounts. The complexes C19– C20 and C27–C28 gave better E/Z selectivity (96:4→99:1). Noticeably, ethylene can be C20 and C27–C28 gave better E/Z selectivity (96:4→99:1). Noticeably, ethylene can be transformed quantitatively into the corresponding dehydrogenative silylation product transformed quantitatively into the corresponding dehydrogenative silylation product with C27–C28 as catalysts with a ratio dehydro/hydro silylation products of 79:21 and with C27–C28 as catalysts with a ratio dehydro/hydro silylation products of 79:21 and Molecules 2021, 26, x 68:32, respectively. C18 catalyzes the dehydrogenative coupling of styrene derivatives13 with of 15 68:32, respectively. C18 catalyzes the dehydrogenative coupling of styrene derivatives HSiMe2Ph leading to dehydro/hydro silylation products in ratio of about 85/15. with HSiMe2Ph leading to dehydro/hydro silylation products in ratio of about 85/15.

SchemeScheme 18. 18.Rhenium Rhenium mono-nitrosyl mono-nitrosyl complexes complexes catalyzed catalyzed dehydrogenative dehydrogenative silylation silylation of of alkenes. alkenes.

4.4. ConclusionsConclusions InIn conclusion, conclusion, we we have have highlighted highlighted in in this this review review the the diversity diversity of of rhenium rhenium complexes complexes thatthat can can be be encountered encountered as as pre-catalysts pre-catalysts for for hydrosilylation hydrosilylation reactions. reactions. The The oxidation oxidation state state ofof thethe reported catalysts spans spans from from +VII +VII to to+I +Ileading leading to an to extraordinary an extraordinary variety variety of coor- of coordinationdination complexes, complexes, but butalso also diverse diverse mechanis mechanisms.ms. It is It now is now evident evident that that rhenium rhenium can can be bean analternative alternative to tonoble noble metals metals for forhydrosilylat hydrosilylationion reactions, reactions, but butwe westrongly strongly believe believe that thatthis thisless-studied less-studied metal metal also has also great has greatpotent potentialial to discover to discover new or new complementary or complementary reactiv- reactivityity and selectivity and selectivity in this infield, this in ﬁeld, particular in particular for alkyne for alkynederivatives derivatives and in other and incatalytic other catalyticprocesses. processes.

Author Contributions: Collecting of the data and original draft preparation, D.W and R.B.; con- ceptualization, methodology of this review, writing—review and editing, funding acquisition, Y.C. and J.-B.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Centre National de la Recherche Scientifique (CNRS), the Université de Rennes 1, the Université Toulouse III, Paul Sabatier, and the Institut Universi- taire de France (IUF) (general support). R.B. is grateful to the Embassy of Yemen in Paris, and the program Pause, Collège de France, for financial support. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Marciniec, B., Catalysis of hydrosilylation of carbon-carbon multiple bonds: Recent progress. Silicon Chem. 2002, 1, 155–174, doi:10.1023/A:1021235922742. 2. Roy, A.K., A Review of Recent Progress in Catalyzed Homogeneous Hydrosilation (Hydrosilylation). Adv. Organomet. Chem. 2007, 55, 1–59, doi:10.1016/S0065-3055(07)55001-X. 3. Nakajima, Y.; Shimada, S., Hydrosilylation reaction of olefins: Recent advances and perspectives. RSC Adv. 2015, 5, 20603–20616, doi:10.1039/C4RA17281G. 4. Tamang, S.R.; Findlater, M. Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation. Mol. 2019, 24, 3194, doi:10.3390/molecules24173194. 5. Du, X.; Huang, Z. Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catal. 2017, 7, 1227–1243, doi:10.1021/acscatal.6b02990. 6. Obligacion, J.V.; Chirik, P.J. Earth-abundant transition metal catalysts for alkene hydrosilylation and hydroboration. Nat. Rev. Chem. 2018, 2, 15–34, doi:10.1038/s41570-018-0001-2. 7. Valyaev, D.A.; Lavigne, G.; Lugan, N. Manganese organometallic compounds in homogeneous catalysis: Past, present, and prospects. Co-ord. Chem. Rev. 2016, 308, 191–235, doi:10.1016/j.ccr.2015.06.015. 8. Kuninobu, Y.; Takai, K., Organic Reactions Catalyzed by Rhenium Carbonyl Complexes. Chem. Rev. 2011, 111, 1938–1953, doi:10.1021/cr100241u.

Molecules 2021, 26, 2598 13 of 15

Author Contributions: Collecting of the data and original draft preparation, D.W. and R.B.; concep- tualization, methodology of this review, writing—review and editing, funding acquisition, Y.C. and J.-B.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Centre National de la Recherche Scientifique (CNRS), the Université de Rennes 1, the Université Toulouse III, Paul Sabatier, and the Institut Universitaire de France (IUF) (general support). R.B. is grateful to the Embassy of Yemen in Paris, and the program Pause, Collège de France, for financial support. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Marciniec, B. Catalysis of hydrosilylation of carbon-carbon multiple bonds: Recent progress. Silicon Chem. 2002, 1, 155–174. [CrossRef] 2. Roy, A.K. A Review of Recent Progress in Catalyzed Homogeneous Hydrosilation (Hydrosilylation). Adv. Organomet. Chem. 2007, 55, 1–59. [CrossRef] 3. Nakajima, Y.; Shimada, S. Hydrosilylation reaction of olefins: Recent advances and perspectives. RSC Adv. 2015, 5, 20603–20616. [CrossRef] 4. Tamang, S.R.; Findlater, M. Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation. Molecules 2019, 24, 3194. [CrossRef] 5. Du, X.; Huang, Z. Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catal. 2017, 7, 1227–1243. [CrossRef] 6. Obligacion, J.V.; Chirik, P.J. Earth-abundant transition metal catalysts for alkene hydrosilylation and hydroboration. Nat. Rev. Chem. 2018, 2, 15–34. [CrossRef] 7. Valyaev, D.A.; Lavigne, G.; Lugan, N. Manganese organometallic compounds in homogeneous catalysis: Past, present, and prospects. Coord. Chem. Rev. 2016, 308, 191–235. [CrossRef] 8. Kuninobu, Y.; Takai, K. Organic Reactions Catalyzed by Rhenium Carbonyl Complexes. Chem. Rev. 2011, 111, 1938–1953. [CrossRef][PubMed] 9. Harms, R.G.; Herrmann, W.A.; Kühn, F.E. Organorhenium dioxides as oxygen transfer systems: Synthesis, reactivity, and applications. Coord. Chem. Rev. 2015, 296, 1–23. [CrossRef] 10. Owens, G.S.; Arias, J.; Abu-Omar, M.M. ChemInform Abstract: Rhenium Oxo Complexes in Catalytic Oxidations. ChemInform 2010, 31, 317–363. [CrossRef] 11. Romão, C.C.; Kühn, F.E.; Herrmann, W.A. Rhenium(VII) Oxo and Imido Complexes: Synthesis, Structures, and Applications. Chem. Rev. 1997, 97, 3197–3246. [CrossRef] 12. Espenson, J.H. Atom-transfer reactions catalyzed by methyltrioxorhenium(VII)-mechanisms and applications. Chem. Commun. 1999, 6, 479–488. [CrossRef] 13. Fuchs, P.L. Catalytic Oxidation Reagents; John Wiley & Sons: Chichester, UK, 2013. 14. Yudin, A.K.; Sharpless, K.B. Bis(trimethylsilyl) Peroxide Extends the Range of Oxorhenium Catalysts for Olefin Epoxidation. J. Am. Chem. Soc. 1997, 119, 11536–11537. [CrossRef] 15. Herrmann, W.A.; Kuchler, J.G.; Felixberger, J.K.; Herdtweck, E.; Wagner, W. Methylrhenium Oxides: Synthesis from R2O7 and Catalytic Activity in Olefin Metathesis. Angew. Chem. Int. Ed. 1988, 27, 394–396. [CrossRef] 16. Herrmann, W.A.; Roesky, P.W.; Wang, M.; Scherer, W. Multiple Bonds between Main-Group Elements and Transition Metals. 135. Oxorhenium(V) Catalysts for the Olefination of Aldehydes. Organometallics 1994, 13, 4531–4535. [CrossRef] 17. Volkert, W.A.; Hoffman, T.J. Therapeutic radiopharmaceuticals. Chem. Rev. 1999, 99, 2269–2292. [CrossRef] 18. Dilworth, J.R.; Parrott, S.J. The biomedical chemistry of technetium and rhenium. Chem. Soc. Rev. 1998, 27, 43–55. [CrossRef] 19. Du, G.; Abu-Omar, M.M. ChemInform Abstract: Oxo and Imido Complexes of Rhenium and Molybdenum in Catalytic Reductions. Chemistry 2009, 40, 1185–1198. [CrossRef] 20. Kobayashi, Y. Reduction with Hydrosilanes Catalyzed by Metal-oxo Complexes. J. Synth. Org. Chem. Jpn. 2010, 68, 866–867. [CrossRef] 21. Kennedy-Smith, J.J.; Nolin, K.A.; Gunterman, H.P.; Toste, F.D. Reversing the Role of the Metal-Oxygen π-Bond. Chemose-lective Catalytic Reductions with a Rhenium(V)-Dioxo Complex. J. Am. Chem. Soc. 2003, 125, 4056–4057. [CrossRef] 22. Thiel, W.R. On the Way to a New Class of Catalysts—High-Valent Transition-Metal Complexes That Catalyze Reductions. Angew. Chem. Int. Ed. 2003, 42, 5390–5392. [CrossRef] 23. Yang, X.; Wang, C. Manganese-Catalyzed Hydrosilylation Reactions. Chem. Asian J. 2018, 13, 2307–2315. [CrossRef] 24. Royo, B. Recent advances in catalytic hydrosilylation of carbonyl groups mediated by well-defined first-row late transition metals. Adv. Organomet. Chem. 2019, 72, 59–102. 25. Nolin, K.A.; Krumper, J.R.; Pluth, M.D.; Bergman, A.R.G.; Toste, F.D. Analysis of an Unprecedented Mechanism for the Catalytic Hydrosilylation of Carbonyl Compounds. J. Am. Chem. Soc. 2007, 129, 14684–14696. [CrossRef][PubMed] 26. Chung, L.W.; Lee, H.G.; Lin, Z.; Wu, Y.-D. Computational Study on the Reaction Mechanism of Hydrosilylation of Carbonyls Catalyzed by High-Valent Rhenium(V)−Di-oxo Complexes. J. Org. Chem. 2006, 71, 6000–6009. [CrossRef] Molecules 2021, 26, 2598 14 of 15

27. Huang, L.; Wang, W.; Wei, X.; Wei, H. New Insights into Hydrosilylation of Unsaturated Carbon-Heteroatom (C=O, C=N) Bonds by Rhenium(V)-Dioxo Complexes. J. Phys. Chem. A 2015, 119, 3789–3799. [CrossRef] 28. Royo, B.; Romão, C.C. Reduction of carbonyl groups by high-valent rhenium oxides. J. Mol. Catal. A Chem. 2005, 236, 107–112. [CrossRef] 29. Ison, E.A.; Trivedi, E.R.; Corbin, R.A.; Abu-Omar, M.M. Mechanism for Reduction Catalysis by Metal Oxo: Hydrosilation of Organic Carbonyl Groups Catalyzed by a Rhenium(V) Oxo Complex. J. Am. Chem. Soc. 2005, 127, 15374–15375. [CrossRef] [PubMed] 30. Ison, E.A.; Corbin, R.A.; Abu-Omar, M.M. Hydrogen Production from Hydrolytic Oxidation of Organosilanes Using a Cationic Oxorhenium Catalyst. J. Am. Chem. Soc. 2005, 127, 11938–11939. [CrossRef][PubMed] 31. Du, G.; Abu-Omar, M.M. Catalytic Hydrosilylation of Carbonyl Compounds with Cationic Oxorhenium(V) Salen. Organometallics 2006, 25, 4920–4923. [CrossRef] 32. Ison, E.A.; Cessarich, J.E.; Du, G.; Fanwick, A.P.E.; Abu-Omar, M.M. Synthesis of Cationic Oxorhenium Salen Complexes via µ-Oxo Abstraction and Their Activity in Catalytic Reductions. Inorg. Chem. 2006, 45, 2385–2387. [CrossRef][PubMed] 33. Du, G.; Fanwick, P.E.; Abu-Omar, M.M. Cationic oxorhenium chiral salen complexes for asymmetric hydrosilylation and kinetic resolution of alcohols. Inorg. Chim. Acta 2008, 361, 3184–3192. [CrossRef] . 34. Du, G.; Fanwick, P.E.; Abu-Omar, M.M. Mechanistic Insight into Hydrosilylation Reactions Catalyzed by High Valent Re.X (X = O, NAr, or N) Complexes: The Silane (SiH) Does Not Add across the Metal−Ligand Multiple Bond. J. Am. Chem. Soc. 2007, 129, 5180–5187. [CrossRef][PubMed] 35. Gu, P.; Wang, W.; Wang, Y.; Wei, H. Hydrosilylation of Carbonyls Catalyzed by the Rhenium(V) Oxo Complex [Re(O)(hoz)2]+—A Non-Hydride Pathway. Organometallics 2012, 32, 47–51. [CrossRef] 36. Huang, L.; Zhang, Y.; Wei, H. Role of the Isolable Hydride Intermediate in the Hydrosilylation of Carbonyl Compounds Catalyzed by the High-Valent Mono-Oxido-Rhenium(V) Complex. Eur. J. Inorg. Chem. 2014, 2014, 5714–5723. [CrossRef] 37. Huang, L.; Wang, W.; Wei, H. A computational study on high-valent mono-oxo-rhenium(V) complex-catalyzed hydrosilyla-tion of carbonyls: What a difference an oxo ligand makes. J. Mol. Catal. A Chem. 2015, 400, 31–41. [CrossRef] 38. Nolin, K.A.; Ahn, R.W.; Kobayashi, Y.; Kennedy-Smith, J.J.; Toste, F.D. Enantioselective Reduction of Ketones and Imines Catalyzed by (CN-Box)ReV-Oxo Complexes. Chem. Eur. J. 2010, 16, 9555–9562. [CrossRef] 39. Nolin, K.A.; Ahn, R.W.; Toste, F.D. Enantioselective reduction of imines catalyzed by a Rhenium (V)− Oxo complex. J. Am. Chem. Soc. 2005, 127, 12462–12463. [CrossRef] 40. Wang, J.; Wang, W.; Huang, L.; Yang, X.; Wei, H. The Unexpected Mechanism Underlying the High-Valent Mono-Oxo-Rhenium(V) Hydride Catalyzed Hydrosilylation of C=N Functionalities: Insights from a DFT Study. ChemPhysChem 2015, 16, 1052–1060. [CrossRef] 41. Smeltz, J.L.; Boyle, P.D.; Ison, E.A. Role of Low-Valent Rhenium Species in Catalytic Hydrosilylation Reactions with Ox-orhenium Catalysts. Organometallics 2012, 31, 5994–5997. [CrossRef] 42. Pérez, D.E.; Smeltz, J.L.; Sommer, R.D.; Boyle, P.D.; Ison, E.A. Cationic rhenium(iii) complexes: Synthesis, characterization, and reactivity for hydrosilylation of aldehydes. Dalton Trans. 2017, 46, 4609–4616. [CrossRef] 43. Brown, C.A.; Abrahamse, M.; Ison, E.A. Re–Silane complexes as frustrated lewis pairs for catalytic hydrosilylation. Dalton Trans. 2020, 49, 11403–11411. [CrossRef] 44. Huang, W.; Berke, H. Rhenium Complexes as Highly Active Catalysts for the Hydrosilylation of Carbonyl Compounds. Chim. Int. J. Chem. 2005, 59, 113–115. [CrossRef] 45. Choualeb, A.; Maccaroni, E.; Blacque, O.; Schmalle, H.W.; Berke, H. Rhenium Nitrosyl Complexes for Hydrogenations and Hydrosilylations. Organometallics 2008, 27, 3474–3481. [CrossRef] 46. Dong, H.; Berke, H. A Convenient and Efficient Rhenium-Catalyzed Hydrosilylation of Ketones and Aldehydes. Adv. Synth. Catal. 2009, 351, 1783–1788. [CrossRef] 47. Jiang, Y.; Blacque, O.; Berke, H. Probing the catalytic potential of chloro nitrosyl rhenium(i) complexes. Dalton Trans. 2011, 40, 2578–2587. [CrossRef][PubMed] 48. Toh, C.K.; Sum, Y.N.; Fong, W.K.; Ang, S.G.; Fan, W.Y. Catalytic Hydrosilylation of Carbonyls via Re(CO)5Cl Photolysis. Organometallics 2012, 31, 3880–3887. [CrossRef] 49. Cabrita, I.; Fernandes, A.C. A novel efficient and chemoselective method for the reduction of nitriles using the system silane/oxorhenium complexes. Tetrahedron 2011, 67, 8183–8186. [CrossRef] 50. Li, B.; Sortais, J.-B.; Darcel, C. Amine synthesis via transition metal homogeneous catalysed hydrosilylation. RSC Adv. 2016, 6, 57603–57625. [CrossRef] 51. Sousa, S.C.; Fernandes, A.C. Efficient and highly chemoselective direct reductive amination of aldehydes using the system silane/oxorhenium complexes. Adv. Synth. Catal. 2010, 352, 2218–2226. [CrossRef] 52. Bernardo, J.R.; Sousa, S.C.A.; Florindo, P.R.; Wolff, M.; Machura, B.; Fernandes, A.C. Efficient and chemoselective direct reductive amination of aromatic aldehydes catalyzed by oxo–rhenium complexes containing heterocyclic ligands. Tetrahedron 2013, 69, 9145–9154. [CrossRef] 53. Das, B.G.; Ghorai, P. Stereoselective direct reductive amination of ketones with electron-deficient amines using Re2O7/NaPF6 catalyst. Org. Biomol. Chem. 2013, 11, 4379–4382. [CrossRef][PubMed] Molecules 2021, 26, 2598 15 of 15

54. de Noronha, R.G.; Romão, C.C.; Fernandes, A.C. Highly Chemo- and Regioselective Reduction of Aromatic Nitro Compounds Using the System Silane/Oxo-Rhenium Complexes. J. Org. Chem. 2009, 74, 6960–6964. [CrossRef] 55. Zheng, J.; Chevance, S.; Darcel, C.; Sortais, J.-B. Selective reduction of carboxylic acids to aldehydes through manganese catalysed hydrosilylation. Chem. Commun. 2013, 49, 10010–10012. [CrossRef][PubMed] 56. Wei, D.; Buhaibeh, R.; Canac, Y.; Sortais, J.-B. Rhenium-Catalyzed Reduction of Carboxylic Acids with Hydrosilanes. Org. Lett. 2019, 21, 7713–7716. [CrossRef] 57. Wei, D.; Buhaibeh, R.; Canac, Y.; Sortais, J.-B. Manganese and rhenium-catalyzed selective reduction of esters to aldehydes with hydrosilanes. Chem. Commun. 2020, 56, 11617–11620. [CrossRef] 58. Zhao, W.-G.; Hua, R. Highly regioselective rhenium-catalyzed hydrosilylation of styrenes. Eur. J. Org. Chem. 2006, 5495–5498. [CrossRef] 59. Liu, L.; Bi, S.; Sun, M.; Yuan, X.; Zheng, N.; Li, P. Mechanistic investigation on hydrogenation and hydrosilylation of ethylene catalyzed by rhenium nitrosyl complex. J. Organomet. Chem. 2009, 694, 3343–3348. [CrossRef] 60. Dong, H.; Jiang, Y.; Berke, H. Rhenium-mediated dehydrogenative silylation and highly regioselective hydrosilylation of nitrile substituted oleﬁns. J. Organomet. Chem. 2014, 750, 17–22. [CrossRef] 61. Jiang, Y.; Blacque, O.; Fox, T.; Frech, C.M.; Berke, H. Highly selective dehydrogenative silylation of alkenes catalyzed by rhenium complexes. Chem. Eur. J. 2009, 15, 2121–2128. [CrossRef] 62. Jiang, Y.; Blacque, O.; Fox, T.; Frech, C.M.; Berke, H. Facile Synthetic Access to Rhenium(II) Complexes: Activation of Carbon- Bromine Bonds by Single-Electron Transfer. Chem. A Eur. J. 2010, 16, 2240–2249. [CrossRef][PubMed]