Nanographene and Graphene Nanoribbon Synthesis Via Alkyne Benzannulations

molecules

Review Nanographene and Graphene Nanoribbon Synthesis via Alkyne Benzannulations

Amber D. Senese and Wesley A. Chalifoux *

Department of Chemistry, University of Nevada, Reno, 1664 N. Virginia St., Reno, NV 89557, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-775-784-6661

Academic Editor: Igor V. Alabugin Received: 8 December 2018; Accepted: 25 December 2018; Published: 30 December 2018

Abstract: The extension of π-conjugation of polycyclic aromatic hydrocarbons (PAHs) via alkyne benzannulation reactions has become an increasingly utilized tool over the past few years. This short review will highlight recent work of alkyne benzannulations in the context of large nanographene as well as graphene nanoribbon synthesis along with a brief discussion of the interesting physical properties these molecules display.

Keywords: alkyne; benzannulation; nanographene; polycyclic aromatic hydrocarbon; graphene nanoribbon

1. Introduction The term “nanographenes (NGs)” has recently become a popular term used to describe relatively large polycyclic aromatic hydrocarbons (PAHs) [1]. These molecules represent discrete sections of graphene, a material with its own interesting chemical and physical properties [2,3]. There has been increasing interest in the synthesis of NGs due to their use in applications such as organic light-emitting diodes (OLEDs), organic ﬁeld-effect transistors (OFETs), and in solar-cell applications [4–12]. The attraction to NGs is that these “soft” materials have useful physical properties that can be easily tuned through structurally changes and/or substitution. A related class of compounds made of long, narrow strips or ribbons of graphene, known as “graphene nanoribbons (GNRs),” with large aspect ratios, have shown ideal semiconducting properties for potential use in OFETs [13–15]. The development of bottom-up synthetic tools for the production of GNRs has been an area of recent interest for the purpose of imparting solubility and tuning of the material properties. This short review will discuss the state of the art for NG and GNR synthesis with a focus on alkyne benzannulations [16] as the key chemical transformation.

2. Catalyst- and Reagent-Free Alkyne Benzannulations

2.1. Alkyne Benzannulations via Pyrolysis In 1969, Hopf and Musso showed that pyrolysis of cis-1,3-hexadien-5-yne 1 leads to the formation of benzene 2 (Scheme1a) [ 17]. Pyrolysis of analogous molecules containing the 1,3-hexadien-5-yne substructure have led to small NGs such as naphthalenes [18,19]. Scott and coworkers utilized ﬂash vacuum pyrolysis (FVP) for the conversion of 3 to corannulene 4, a bowl-shaped NG, which involves a two-fold alkyne benzannulation reaction (Scheme1b) [ 20]. Shortly after this, others adopted this procedure to produce other non-planar NGs such as 6 and 8 (Scheme1c,d) [21,22].

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Scheme 1.1. ((a)) Benzene Benzene 2 fromfrom thethe pyrolysispyrolysis ofof 1,3-hexadien-5-yne1,3-hexadien-5-yne1,3-hexadien-5-yne 1 [[17].[17].17]. (( bb)) FVPFVP of of 3 toto produceproduce corannulene 4 [[20].[20].20]. Two-fold Two-fold [[21][21]21] andand four-foldfour-fold alkynealkynealkyne benzannulationsbebenzannulations [22] [22] to affordafford bowl-shapedbowl-shaped NGs (c)) 6 and ((d)) 88,, respectively.respectively. 2.2. Alkyne Benzannulatinos via Photocyclizations 2.2. Alkyne Benzannulatinos via Photocyclizations Photochemical alkyne benzannulations to afford large NGs are rare but there have been a Photochemical alkyne benzannulations to afford largelarge NGsNGs areare rarerare butbut therethere havehave beenbeen aa fewfew few examples reported of smaller NGs being formed this way. For example, photocyclization of examples reported of smaller NGs being formed thisthis way.way. ForFor example,example, photocyclizationphotocyclization ofof 2-2- 2-ethynylbiphenyls such a 9 (Scheme2a) [ 23] and 1,4-diaryl-1-buten-3-ynes 11 (Scheme2b) [ 24,25] ethynylbiphenyls such a 9 (Scheme(Scheme 2a)2a) [23][23] andand 1,4-diaryl-1-buten-3-ynes1,4-diaryl-1-buten-3-ynes 11 (Scheme(Scheme 2b)2b) [24,25][24,25] isis is known to afford phenanthrene products 10 and 12. With the success of photochemical alkyne known to afford phenanthrene products 10 andand 12.. WithWith thethe successsuccess ofof photochemicalphotochemical alkynealkyne benzannulations of smaller NG systems, the relatively underexplored area of larger NGs synthesized benzannulations of smaller NG systems, the relativelyly underexploredunderexplored areaarea ofof largerlarger NGsNGs synthesizedsynthesized in this manner appears to be ripe with low-lying fruit. inin thisthis mannermanner appearsappears toto bebe riperipe withwith low-lyinglow-lying fruit.fruit.

Scheme 2. Alkyne benzannulation of (a) 2-ethynylbiphenyl derivatives 9 [23] and (b) 1,4-diaryl-1-buten- Scheme 2. AlkyneAlkyne benzannulationbenzannulation ofof ((a)) 2-ethynylbiphenyl2-ethynylbiphenyl derivativesderivatives 9 [23][23] andand ((b)) 1,4-diaryl-1-1,4-diaryl-1- 3-ynes 11 [24,25] via a photocyclization reaction to afford phenanthrene products 10 and 12. buten-3-ynes 11 [24,25][24,25] viavia aa photocyclizationphotocyclization reactionreaction toto affordafford phenanthrenephenanthrene productsproducts 10 andand 12.. 3. Alkyne Benzannulations Promoted by Electrophilic Reagents 3. Alkyne Benzannulations Promoted by Electrophilic Reagents Iodonium Salt- or Iodine Monochloride (ICl)-Induced Alkyne Benzannulations 3.1.Iodonium Salt- or Iodine Monochloride (ICl)-induced(ICl)-induced AlkyneAlkyne BenzannulationsBenzannulations Electrophilic iodine reagents are an excellent way of cyclizing alkynes with neighboring aryl groupsElectrophilic to produce iodine a new benzenereagents ringare an [26 ].excellent This methodology way of cyclizing has the alkynes added advantagewith neighboring of providing aryl agroups functional to produce handle a in new the benzene product forring further [26]. This chemical method elaboration.ology has Thisthe added chemistry advantage was ﬁrst of developedproviding ina functional 1988 by Barluenga handle andin the coworkers product infor which further they chemical used I(py) elaboration.2BF4 as an electrophilicThis chemistry iodine was source first indeveloped the presence in 1988 of by TfOH Barluenga to produce and coworkers iodocyclohexene in which they products used I(py) from22BF 1,4-diphenyl-1-butyne44 asas anan electrophilicelectrophilic iodineiodine [27]. Swagersource in and the coworkers presence utilizedof TfOH this to reagentproduce system iodocyclohexene to cyclize terphenyl products derivative from 1,4-diphenyl-1-butyne13 to give a mixture of[27]. halogenated Swager and and coworkers non-halogenated utilized NGthisproducts reagent system14 and 15to (Schemecyclize terphenyl3)[ 28]. If derivative the incorporation 13 toto givegive of an aa iodidemixture group of halogenated in the ﬁnal product and non-halogenated is desired, using NG equal products equivalents 14 ofandand TfOH 15 and(Scheme(Scheme I(py) 2BF3)3) 4[28].[28].resulted IfIf thethe in goodincorporationincorporation yields of ofof the anan halogenated iodideiodide groupgroup product inin thethe finalfinal15. productproduct isis desired,desired, usingusing equalequal equivalentsequivalents ofof TfOHTfOH andand I(py)22BF44 resultedresulted inin goodgood yieldsyields ofof thethe halogenatedhalogenated productproduct 15..

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Scheme 3. Electrophilic benzannulation with either an iodonium salt or a Brønsted acid [28]. Scheme 3. Electrophilic benzannulation with either an iodonium salt or a Brønsted acid [[28].28]. Liu and coworkers took advantage of an ICl-induced alkyne benzannulation of Liu and coworkers took advantage of an ICl-induced alkyne benzannulation of bis(biaryl)acetylenes bis(biaryl)acetylenesLiu and coworkers 16 to arrive took at iodide-functionalizedadvantage of an ICl-inducedphenanthrene alkyne intermediates benzannulation 17 (Scheme of4) 16 to arrive at iodide-functionalized phenanthrene intermediates 17 (Scheme4) [ 29]. These intermediates [29].bis(biaryl)acetylenes These intermediates 16 to arriveare poised at iodide-functionalized for a palladium-catalyzed phenanthrene intramolecular intermediates direct 17 arylation (Scheme to4) are poised for a palladium-catalyzed intramolecular direct arylation to arrive at dibenzo[g,p]chrysenes in arrive[29]. These at dibenzo[ intermediatesg,p]chrysenes are poised in good for to excellenta palladium-catalyzed overall yield. This intramolecular route allowed direct for the arylation synthesis to good to excellent overall yield. This route allowed for the synthesis of this class of NGs bearing both ofarrive this at class dibenzo[ of NGsg,p]chrysenes bearing both in good electron-rich to excellent (18 overall) and electron-poor yield. This route (19 allowed) substituents. for the Thissynthesis was electron-rich (18) and electron-poor (19) substituents. This was important as Liu and coworkers were importantof this class as ofLiu NGs and bearingcoworkers both were electron-rich able to demonstrate (18) and electron-poorthat dibenzo[g,p (19]chrysenes) substituents. functionalized This was able to demonstrate that dibenzo[g,p]chrysenes functionalized with electron-withdrawing groups have withimportant electron-withdrawing as Liu and coworkers groups were have able larger to demonstrate HOMO/LUMO that dibenzo[energy gapsg,p]chrysenes (3.10–3.18 functionalizedeV) and lower larger HOMO/LUMO energy gaps (3.10–3.18 eV) and lower quantum yields (Φ = 12.4–16.8%) than quantumwith electron-withdrawing yields (Φ = 12.4–16.8%) groups than have derivatives larger HOMO/LUMO with electron-donating energy gaps groups (3.10–3.18 (2.84–2.91 eV) and eV, lower Φ = derivatives with electron-donating groups (2.84–2.91 eV, Φ = 26.0–48.7%) [30]. They were also able to 26.0–48.7%)quantum yields [30]. ( TheyΦ = 12.4–16.8%) were also able than to derivatives convert 18 towith a liquid electron-donating crystalline derivative groups (2.84–2.91 20 in 10% eV,overall Φ = convert 18 to a liquid crystalline derivative 20 in 10% overall yield and this material displayed reasonable yield26.0–48.7%) and this [30]. material They were displayed also able reasonable to convert hole 18 to and a liquid electron crystalline transport derivative in thin films.20 in 10% This overall work hole and electron transport in thin films. This work nicely demonstrates that physical properties of nicelyyield and demonstrates this material that displayed physical reasonable properties hole of anddibenzo[ electrong,p ]chrysenetransport incan thin be films.tuned This through work dibenzo[g,p]chrysene can be tuned through substituent effects. substituentnicely demonstrates effects. that physical properties of dibenzo[g,p]chrysene can be tuned through substituent effects. 2 2 1 R2 R1 R R1 R R 2 1 2 2 1 R R R R1 R R 18, R1 = R2 = OMe, 85% I 19, R1 = F, 2R 2 = H, 50% ICl PdCl2(PPh3)2 18, R = R = OMe, 85% 1 2 I 2019, R1 = RF, R=2 = H, 50% ICl PdCl2(PPh3)2 O 20, R1 = R2 = OC H O O 8 17 1 1 2 OC H R1 R2 R R2 R R O 8 17 16 17 OC8H17 1 2 1 1 2 10% R R R R2 R R 16 17 OC8H17 10% Scheme 4. ICl-induced benzannulation used in the synthesis towards dibenzo[g,p]chrysene Scheme 4. ICl-induced benzannulation used in the synthesis towards dibenzo[g,p]chrysene derivatives [29]. derivativesScheme 4. [29].ICl-induced benzannulation used in the synthesis towards dibenzo[g,p]chrysene derivatives [29]. The Müllen group recently used an ICl-induced two-fold alkyne benzannulation of compound 21 The Müllen group recently used an ICl-induced two-fold alkyne benzannulation of compound to generate two new iodinated phenanthrene units in product 22 (Scheme5) [ 31]. They demonstrated 21 toThe generate Müllen twogroup new recently iodinated used anphenanthrene ICl-induced two-foldunits in alkyneproduct benzannulation 22 (Scheme 5)of compound[31]. They the crosscoupling utility of these intermediates by generating compound 23, which was further oxidized demonstrated21 to generate the two crosscoupling new iodinated utility phenanthrene of these intermediates units in productby generating 22 (Scheme compound 5) [31].23, whichThey under Scholl conditions [32] to afford zigzag nanographenes 24. The formation of NG intermediates wasdemonstrated further oxidized the crosscoupling under Scholl utility conditions of th ese[32] intermediates to afford zigzag by nanographenesgenerating compound 24. The formation23, which with useful coupling handles has also been used towards the formation of conjugated polymers [33]. ofwas NG further intermediates oxidized underwith usefulScholl conditionscoupling handles [32] to ahasfford also zigzag been nanographenes used towards 24the. The formation formation of conjugatedof NG intermediates polymers [33].with useful coupling handles has also been used towards the formation of conjugated polymers [33].

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Scheme 5. Müllen and coworkers synthesis of zigzag NGs [31]. Scheme 5. Müllen and coworkers synthesis of zigzag NGs [31]. Scheme 5. Müllen and coworkers synthesis of zigzag NGs [31]. Molecular structure can greatly affect the properties exhibited by the bulk material, hence, it is Molecular structure can greatly affect the properties exhibited by the bulk material, hence, it is important toto understand understand how how subtle subtle changes changes to the to molecularthe molecular structure structure affect theaffect HOMO/LUMO the HOMO/LUMO energy important to understand how subtle changes to the molecular structure affect the HOMO/LUMO gaps,energy crystal gaps, packing,crystal packing, or UV-vis or absorption UV-vis absorption and emission and emission [34]. The way[34]. inThe which way thein which twisted the molecular twisted energy gaps, crystal packing, or UV-vis absorption and emission [34]. The way in which the twisted structuremolecular of structure helicenes of pack helicenes was of pack interest was to of the intere Alabuginst to group,the Alabugin who made group, [5]helicene-like who made [5]helicene- compounds molecular structure of helicenes pack was of interest to the Alabugin group, who made [5]helicene- underlike compounds metal-free conditionsunder metal-free [35]. The conditions reaction proceeds [35]. The via reaction an ICl-promoted proceeds via alkyne an ICl-promoted benzannulation alkyne of 25 like compounds under metal-free conditions [35]. The reaction proceeds via an ICl-promoted alkyne tobenzannulation afford either iodophenanthrene of 25 to afford either26, iodochrysene iodophenanthrene27, or a mixture 26, iodochrysene of both (Scheme 27, or6). a This mixture mixture of both was benzannulation of 25 to afford either iodophenanthrene 26, iodochrysene 27, or a mixture of both subjected(Scheme 6). to aThis cyclodehydroiodination mixture was subjected reaction to a cyclod to affordehydroiodination [5]helicene derivatives reaction28 toand afford29, which[5]helicene were (Scheme 6). This mixture was subjected to a cyclodehydroiodination reaction to afford [5]helicene thederivatives two derivatives 28 and 29 that, which afforded were single the two crystals derivatives suitable that for afforded X-ray crystallography. single crystals suitable It was found for X-ray that derivatives 28 and 29, which were the two derivatives that afforded single crystals suitable for X-ray thecrystallography. 3D crystal architecture It was found significantly that the 3D varied crystal from architecture the placement significantly of the functional varied from groups the aroundplacement the crystallography. It was found that the 3D crystal architecture significantly varied from the placement [5]heliceneof the functional skeleton, groups but the around calculated the [5]helicen HOMO/LUMOe skeleton, energy but gaps the calculated remained quiteHOMO/LUMO similar in value energy for of the functional groups around the [5]helicene skeleton, but the calculated HOMO/LUMO energy mostgaps remained of the [5]helicenes quite similar synthesized in value in for the most study of (2.51–2.68 the [5]helicenes eV). synthesized in the study (2.51–2.68 gapseV). remained quite similar in value for most of the [5]helicenes synthesized in the study (2.51–2.68 eV).

Scheme 6. Synthetic route to [5]helicene-like compounds [[35].35]. Scheme 6. Synthetic route to [5]helicene-like compounds [35]. Electrophilic iodine-mediated alkyne benzannulations have proven to be an excellent tool for Electrophilic iodine-mediated alkyne benzannulations have proven to be an excellent tool for the synthesis of functionalized nanographenes [29,31,36–38]. The products are often achieved in high the synthesisElectrophilic of functionalized iodine-mediated nanographenes alkyne benzannulations [29,31,36–38]. haveThe prod provenucts toare be often an excellent achieved tool in high for yields and provide a useful handle for further chemical transformations. theyields synthesis and provide of functionalized a useful handle nanographenes for further chemical[29,31,36–38]. transformations. The products are often achieved in high yields and provide a useful handle for further chemical transformations. 4. Radical-Mediated Alkyne Benzannulations 4. Radical-mediated Alkyne Benzannulations 4. Radical-mediatedThe Alabugin group Alkyne has Benzannulations done signiﬁcant work over the past few years using high-energy The Alabugin group has done significant work over the past few years using high-energy alkyne-containing substrates to produce a host of NGs through thermodynamically “downhill” alkyne alkyne-containingThe Alabugin substratesgroup has to done produce significant a host work of NGs over through the past thermodynamically few years using high-energy“downhill” alkyne-containing substrates to produce a host of NGs through thermodynamically “downhill”

Molecules 2019, 24, 118 5 of 22 Molecules 2018, 23, x FOR PEER REVIEW 5 of 22 alkyne benzannulations [35,39–44]. Their recent work stems from their initial discovery that tin- benzannulations [35,39–44]. Their recent work stems from their initial discovery that tin-mediated mediated radical cascade cyclizations of alkynes can be used to afford substituted radical cascade cyclizations of alkynes can be used to afford substituted benzo[a]indeno[2,1-c]fluorenes benzo[a]indeno[2,1-c]fluorenes and other products [45]. Alabugin and Byers later used this strategy and other products [45]. Alabugin and Byers later used this strategy to cleverly stitch together larger to cleverly stitch together larger NGs 32 (Scheme 7a) [40]. In this single reaction, the authors NGs 32 (Scheme7a) [ 40]. In this single reaction, the authors impressively generate five new rings impressively generate five new rings and five new carbon-carbon bonds in excellent yield (ca. 93% and five new carbon-carbon bonds in excellent yield (ca. 93% yield per step), which demonstrates yield per step), which demonstrates the exceptional chemoselectivity of radical formation and the exceptional chemoselectivity of radical formation and regioselectivity of the initial cyclization on regioselectivity of the initial cyclization on the internal alkyne, as depicted in Scheme 7a. The the internal alkyne, as depicted in Scheme7a. The Alabugin group later came up with a “traceless” Alabugin group later came up with a “traceless” version of this reaction to get a range of NG products version of this reaction to get a range of NG products including the conversion of 33 to arrive at a including the conversion of 33 to arrive at a mixture of helical systems such as 34 and 35 (Scheme 7b) mixture of helical systems such as 34 and 35 (Scheme7b) [ 42]. The chemistry leaves the product with [42]. The chemistry leaves the product with a useful SnBu3 handle that their group demonstrated a useful SnBu3 handle that their group demonstrated could be further reacted with electrophiles or could be further reacted with electrophiles or used in Stille crosscoupling reactions. In 2018, and used in Stille crosscoupling reactions. In 2018, and perhaps fortuitously timed with the 2018 Olympic perhaps fortuitously timed with the 2018 Olympic Winter Games, the Alabugin group extended their Winter Games, the Alabugin group extended their methodology to peri-cyclizations, allowing them to methodology to peri-cyclizations, allowing them to synthesize a number of NGs including synthesize a number of NGs including olympicene derivative 37 (Scheme7c) [44]. olympicene derivative 37 (Scheme 7c) [44].

Scheme 7. Radical mediated cascade alkyne benzannulations to arrive at (a) NG 32 [40], (b) helical Scheme 7. Radical mediated cascade alkyne benzannulations to arrive at (a) NG 32 [40], (b) helical NGs 34/35 [42], and (c) olympicene 37 [44]. NGs 34/35 [42], and (c) olympicene 37 [44]. 5. Acid-Mediated Alkyne Benzannulations 5. Acid-mediated Alkyne Benzannulations As demonstrated in Section3, electrophilic reagents can be an effective way to promote 6- endo-dig cyclizationsAs demonstrated of alkynes to in afford section a new 3, electrophilic benzene ring re andagents provide can largerbe an NGeffective structures. way to Other promote electrophiles, 6-endo- suchdig cyclizations as acids, have of alsoalkynes been to effective afford a in new alkyne benzen benzannulationse ring and provide over the larger past 30 NG years. structures. Other electrophiles, such as acids, have also been effective in alkyne benzannulations over the past 30 years. 5.1. Brønsted Acid Catalyzed Alkyne Benzannulations 5.1. Brønsted Acid Catalyzed Alkyne Benzannulations In Section3 we highlighted how iodonium reagents can be used to promote alkyne benzannulations.In section 3 Inwe many highlighted cases, the howproto iodonium(R = H, Scheme reagents3) productcan be isused desired to andpromote thus wouldalkyne requirebenzannulations. a second step In ofmany lithium-halogen cases, the proto exchange (R = H, and Scheme protonation 3) product to arrive is atdesired the desired and productthus would [46]. Perhapsrequire a the second simplest step electrophileof lithium-halogen for effecting exchange an alkyne and protonation benzannulations to arrive reaction at the todesired arrive product directly at[46]. the Perhapsproto product the simplest would beelectrophile a proton. for Some effect of theing earliestan alkyne work benzannulations of alkyne benzannulations reaction to arrive came fromdirectly the at Swager the proto group product and would during be an a early proton. study Some of electrophile-inducedof the earliest work of alkyne alkyne cyclizations benzannulations using iodoniumcame from (Scheme the Swager3) they group noted and that during the use an ofearly TfOH study or TFA of electrophile-induced alone very effectively alkyne promoted cyclizations alkyne benzannulationsusing iodonium (Scheme to arrive 3) at they NG noted products that [the28]. use This of BrønstedTfOH or TFA acid-promoted alone very effectively benzannulation promoted was hypothesizedalkyne benzannulations to occur through to arrive a 6-endo-dig at NG productspathway [28]. due This to the Brønsted carbocation acid-promoted stability through benzannulation resonance withwas hypothesized an electron-rich to areneoccur moietythrough attached a 6-endo-dig to the pathway acetylene due [46 ].to Having the carbocation an electron-rich stability aryl through group resonance with an electron-rich arene moiety attached to the acetylene [46]. Having an electron-rich aryl group proved to be essential for the success of this type of alkyne benzannulation in other examples of NG syntheses [47–52].

Molecules 2019, 24, 118 6 of 22 proved to be essential for the success of this type of alkyne benzannulation in other examples of NG Moleculessyntheses 2018 [47, 23–,52 x FOR]. PEER REVIEW 6 of 22 Swager and coworkers later explored their two-fold alkyne benzannulation reaction by looking Swager and coworkers later explored their two-fold alkyne benzannulation reaction by looking at various substitution patterns of diethynylterphenyl systems in which the two aryl (Ar) groups at various substitution patterns of diethynylterphenyl systems in which the two aryl (Ar) groups participating in the alkyne benzannulation reaction were oriented para- (38), meta- (40) and ortho- (42) participating in the alkyne benzannulation reaction were oriented para- (38), meta- (40) and ortho- (42) to each other to arrive at NGs 39, 41, and 43, respectively (Scheme8)[ 46]. Attempting to cyclize to each other to arrive at NGs 39, 41, and 43, respectively (Scheme 8) [46]. Attempting to cyclize systems with more steric strain, as the case with the ortho-terphenyl system 42, using Brønsted acids systems with more steric strain, as the case with the ortho-terphenyl system 42, using Brønsted acids only provided trace amounts of desired products. The authors found that conducting the reaction only provided trace amounts of desired products. The authors found that conducting the reaction in in the presence of silver triﬂate and iodine resulted in the desired product in 20% yield. What was the presence of silver triflate and iodine resulted in the desired product in 20% yield. What was surprising is that there was no iodine detected in the NG product under these conditions. surprising is that there was no iodine detected in the NG product under these conditions.

Scheme 8. Brønsted acid-induced alkyne benzannulations of (a) para-(38), (b) meta-(40) and (c) Scheme 8. Brønsted acid-induced alkyne benzannulations of (a) para- (38), (b) meta- (40) and (c) ortho- ortho-substituted (42) terphenyl systems to afford NGs [46]. substituted (42) terphenyl systems to afford NGs [46]. The Chalifoux group has recently adapted Swager’s alkyne benzannulation conditions to afford a The Chalifoux group has recently adapted Swager’s alkyne benzannulation conditions to afford host of nanographenes [47–50]. In one study, they are able to generate diynes 44 and 45 or tetrayne a host of nanographenes [47–50]. In one study, they are able to generate diynes 44 and 45 or tetrayne 46 intermediates via Suzuki crosscoupling of boronic ester 43 and various brominated aromatics 46 intermediates via Suzuki crosscoupling of boronic ester 43 and various brominated aromatics (Scheme9)[ 48]. The idea here was to invoke two benzannulation reactions on both ortho positions (Scheme 9) [48]. The idea here was to invoke two benzannulation reactions on both ortho positions of of the same aryl moiety, whereas in Swager’s examples they invoke only one alkyne benzannulation the same aryl moiety, whereas in Swager’s examples they invoke only one alkyne benzannulation on on one ortho position of the neighboring aryl group [28,46]. The authors thought this would allow one ortho position of the neighboring aryl group [28,46]. The authors thought this would allow them them to rapidly access larger fused NG systems with greater lateral conjugation. However, they found to rapidly access larger fused NG systems with greater lateral conjugation. However, they found that that under Swager conditions (TFA), cyclization of compound 44 only produced monocyclized under Swager conditions (TFA), cyclization of compound 44 only produced monocyclized phenanthrene product 47, albeit quantitatively. The explanation for incomplete cyclization in this case, phenanthrene product 47, albeit quantitatively. The explanation for incomplete cyclization in this as compared to systems reported by Swager and coworkers [28,46], is that the first alkyne cyclization case, as compared to systems reported by Swager and coworkers [28,46], is that the first alkyne causes planarization to form rigid phenanthrene intermediate 47. This leads to poor orbital interaction cyclization causes planarization to form rigid phenanthrene intermediate 47. This leads to poor between the second (remaining) alkyne and the remaining ortho position, resulting in a much higher orbital interaction between the second (remaining) alkyne and the remaining ortho position, resulting barrier for the second benzannulation. Adding excess amounts of TFA along with refluxing the in a much higher barrier for the second benzannulation. Adding excess amounts of TFA along with reaction mixture did lead to pyrene product 48, albeit in low yield. It was found that a stronger acid, refluxing the reaction mixture did lead to pyrene product 48, albeit in low yield. It was found that a such as TfOH, could be added to induce the second alkyne benzannulation to afford good yield of stronger acid, such as TfOH, could be added to induce the second alkyne benzannulation to afford pyrene product 48. This protocol proved to be an effective method for synthesizing highly soluble good yield of pyrene product 48. This protocol proved to be an effective method for synthesizing peropyrene and teropyrene products 49 and 50, respectively. The two-step protocol of adding TFA highly soluble peropyrene and teropyrene products 49 and 50, respectively. The two-step protocol of followed by TfOH was useful for characterizing and studying the partially cyclized intermediates adding TFA followed by TfOH was useful for characterizing and studying the partially cyclized but the final nanographene products could be cleanly formed directly using just TfOH. The alkyne intermediates but the final nanographene products could be cleanly formed directly using just TfOH. benzannulation reaction using Brønsted acid requires the presence of electron-rich ethynylaryl groups The alkyne benzannulation reaction using Brønsted acid requires the presence of electron-rich in the substrate and thus thiophene substituents are also tolerated [49]. The photophysical properties ethynylaryl groups in the substrate and thus thiophene substituents are also tolerated [49]. The photophysical properties of these compounds were studied and, as expected, there is a red-shift in both the absorbance and emission in going from 48, to 49, to 50. Peropyrene products 49 display high extinction coefficients of 5.6 × 104 M–1·cm–1 at 467 nm and display green emission (λem = 486 nm) with quantum yields up to 64%. The teropyrenes 50 were significantly red-shifted with extinction

Molecules 2019, 24, 118 7 of 22 ofMolecules these 2018 compounds, 23, x FOR PEER were REVIEW studied and, as expected, there is a red-shift in both the absorbance7 of 22 and emission in going from 48, to 49, to 50. Peropyrene products 49 display high extinction Molecules 2018, 23, x FOR PEER REVIEW 7 of 22 × 45 –1−1 –1 −1 λem coefficientscoefficients of of5.6 2.8× 1010 Mm cm·cm at 572at 467 nm nm and and have display red emission green emission ( = 590 (λem nm)= 486 with nm) quantum with quantum yields yieldsup to 61%. up to 64%. The teropyrenes 50 were significantly red-shifted with extinction coefficients of coefficients of 2.8 × 105 m–1cm–1 at 572 nm and have red emission (λem = 590 nm) with quantum yields 5 −1 −1 2.8 × 10 m cm at 572 nm and have redAr emission (λem = 590 nm) with quantum yields up to 61%. up to 61%. Ar Ar

Br Ar TFA TfOH Ar Ar Suzuki coupling Br 54-66% TFA TfOH Ar Ar Suzuki coupling Ar 44 Ar 47 48, 49-61% 54-66% Ar Ar O Ar Ar 44 Ar 47 B Ar 48, 49-61% O Br O Ar TFA/TfOH B Ar ArO Br or Suzuki coupling TfOH only 43 49-66% TFA/TfOH Ar or Ar Ar Suzuki coupling 45 TfOH only 49, 57-72% 43 49-66% Ar Ar Ar Ar 45 49, 57-72% Ar Ar Br Br Ar Ar TFA/TfOH Ar Ar Br Suzuki coupling Br or TfOH only 28-53% TFA/TfOH Ar Ar or Suzuki coupling Ar 46 Ar 28-53% TfOH only 50, 61-67% Ar Ar Ar = OCH3 Ar 46 OCArH OC H , 6 13 , 10 21 , S C H 50, 61-67% 6 13 Ar = OCH OC H OC H 3 , 6 13 , 10 21 , S C6H13 Scheme 9. Brønsted acid-promoted two-fold and four-fold alkyne benzannulations to afford pyrenes Scheme48, peropyrenes 9. Brønsted 49, and acid-promoted teropyrenes two-fold 50 [48,49]. and four-fold alkyne benzannulations to afford pyrenes Scheme48, peropyrenes 9. Brønsted49, andacid-promoted teropyrenes two-fold50 [48,49 and]. four-fold alkyne benzannulations to afford pyrenes 48Single, peropyrenes crystals 49 suitable, and teropyrenes of X-ray crystallographic50 [48,49]. analysis were obtained for compounds 49 and Single crystals suitable of X-ray crystallographic analysis were obtained for compounds 49 and 50 50 (Ar = p-hexyloxyphenyl) (Figure 1). As can be seen, the NGs adopt a non-planar structure due to (Ar =Singlep-hexyloxyphenyl) crystals suitable (Figure of X-ray1). As crystallographic can be seen, the analysis NGs adopt were a non-planar obtained for structure compounds due to 49 steric and steric hindrance within the newly formed bay regions. This feature, along with the ability to 50hindrance (Ar = p-hexyloxyphenyl) within the newly formed(Figurebay 1). regions.As can be This seen, feature, the NGs along adopt with a the non-planar ability to functionalizestructure due the to functionalize the NGs using this methodology, is likely responsible for the high solubility of these stericNGs usinghindrance this methodology, within the isnewly likely formed responsible bay forregions. the high This solubility feature, of along these compounds.with the ability What to is compounds. What is interesting is that the twist of peropyrene 49 causes the molecule to be chiral in functionalizeinteresting is thatthe theNGs twist using of peropyrenethis methodology,49 causes is li thekely molecule responsible to be chiralfor the in high the solid-statesolubility whereasof these the solid-state whereas teropyrene 50 has a pseudo plane of symmetry. In solution, it was compounds.teropyrene 50 Whathas ais pseudointeresting plane is that of symmetry. the twist of In peropyrene solution, it 49 was causes hypothesized the molecule that to the be inversion chiral in hypothesized that the inversion barrier was low for 49 making it impossible to isolate each thebarrier solid-state was low whereas for 49 making teropyrene it impossible 50 has to a isolate pseudo each plane enantiomer of symmetry. under ambient In solution, conditions. it was enantiomer under ambient conditions. hypothesized that the inversion barrier was low for 49 making it impossible to isolate each enantiomer under ambient conditions.

Figure 1. X-ray crystal structures of peropyrene 49 and teropyrene 50. Figure 1. X-ray crystal structures of peropyrene 49 and teropyrene 50. The Chalifoux group wondered if they could impart higher steric hinderance in the bay regions of a peropyreneThe Chalifoux throughFigure group substitution 1. wonderedX-ray crystal in if order structuresthey tocould increase of impartperopyrene the higher inversion 49 and steric teropyrene barrier hinderance and 50. allow in the for bay isolation regions ofof thea peropyrene enantiomeric through products. substitution To probe in theorder viability to increase of synthesizing the inversion a persistently barrier and chiralallow peropyrene,for isolation The Chalifoux group wondered if they could impart higher steric hinderance in the bay regions of the enantiomeric products. To probe the viability of synthesizing a persistently chiral peropyrene, of a peropyrene through substitution in order to increase the inversion barrier and allow for isolation the Chalifoux group synthesized tetrayne 51 and subjected it to their Brønsted acid-catalyzed alkyne of the enantiomeric products. To probe the viability of synthesizing a persistently chiral peropyrene, benzannulation conditions (Scheme 10) [50]. Interestingly, using TFA alone resulted in a two-fold the Chalifoux group synthesized tetrayne 51 and subjected it to their Brønsted acid-catalyzed alkyne alkyne benzannulation in which the alkynes cyclized on the same side to produce chiral picene benzannulation conditions (Scheme 10) [50]. Interestingly, using TFA alone resulted in a two-fold alkyne benzannulation in which the alkynes cyclized on the same side to produce chiral picene

Molecules 2019, 24, 118 8 of 22

the Chalifoux group synthesized tetrayne 51 and subjected it to their Brønsted acid-catalyzed

Moleculesalkyne benzannulation2018, 23, x FOR PEER conditions REVIEW (Scheme 10)[50]. Interestingly, using TFA alone resulted8 of in 22 a two-fold alkyne benzannulation in which the alkynes cyclized on the same side to produce chiral intermediatepicene intermediate 52, which52 ,was which unambiguously was unambiguously confirmed conﬁrmed by X-ray by crystallographic X-ray crystallographic analysis analysis(Ar = p- (MeOCAr = p6-MeOCH4). This6H cyclization4). This cyclization mode was mode supported was supported by calculations by calculations that determined that determined a lower barrier a lower of aboutbarrier 2 ofkcal/mol about 2 for kcal/mol this product. for this Treating product. 52 Treating with TfOH52 with resulted TfOH in resulted complete in completecyclization cyclization to afford theto afford first ever the ﬁrstreported ever reportedchiral peropyrene chiral peropyrene 53. The X-ray53. The structure X-ray structure shows this shows molecule this molecule to be twisted to be ◦ fromtwisted end-to-end from end-to-end by about by 28°. about The 28inversion. The inversion barrier was barrier determined was determined to be ca. 29 to kcal/mol be ca. 29 making kcal/mol it making it possible to separate the enantiomers by chiral HPLC. An optical rotationଶହ [α] 25 = value of possible to separate the enantiomers by chiral HPLC. An optical rotation [α]ହ଼ଽ = value 589of + 1438 was +determined 1438 was determinedfor the pure for enantiomer the pure enantiomer of (P,P)-53 of. Circular (P,P)-53. dichroism Circular dichroism of 53 showed of 53 showedstrong Cotton strong ε ± −1· −1 effectsCotton effects(Δε = (±100∆ = M100−1·cm M−1 atcm 300 atnm) 300 and nm) andthe thecompound compound also also displayed displayed circularly circularly polarized polarized luminescent properties, properties, also also observed observed in in other helical NGs [53,54], [53,54], making it potentially useful in chiroptic applications.

Scheme 10. Four-fold alkyne benzannulation towards chiral peropyrenes 53 [50]. Scheme 10. Four-fold alkyne benzannulation towards chiral peropyrenes 53 [50]. Bottom-up synthesis of graphene nanoribbons (GNRs) has been attempted long before the discoveryBottom-up of graphene synthesis itself of andgraphene research nanoribbons towards the (GNRs) synthesis has ofbeen GNRs attempted is still beinglong conductedbefore the discoverytoday [14, 55of– graphene64]. In 1970, itself Stille and etresearch al. reported towards the the ﬁrst synthesis bottom-up of GNRs synthesis is still of abeing GNR, conducted however, todaythese carbon-based[14,55–64]. In ladder1970, Stille polymers et al. reported suffered the from first considerable bottom-up insolubility synthesis of [ 65a GNR,]. It wasn’t however, until these 1994 carbon-basedthat the Swager ladder group polymers realized thesuffered potential from of consider using a Brønstedable insolubility acid-promoted [65]. It alkynewasn’t benzannulationuntil 1994 that theof polymer Swager 54groupas a meansrealized of the creating potential functionalized, of using a solubleBrønsted GNRs acid-promoted55 (Scheme alkyne11a) [28 benzannulation]. Their strategy ofinvolved polymer the 54 use as of a thoughtfully means of creating designed functionalized, monomers that soluble included GNRs both 55 electron-donating (Scheme 11a) [28]. groups Their to strategyensure clean involved cyclization the use and of thoughtfully alkyl substitution designed (R) alongmonomers the backbone that included to ensure both goodelectron-donating regiocontrol. groupsThe polymers to ensure had clean high molecularcyclization weights and alkyl of Mn subs = 45,000–55,000titution (R) along g/mol the and backbone displayed to anensure absorption good regiocontrol.edge at 468 nm. The About polymers two decades had high later, molecular Wu, Zhao weights and coworkers of Mn = used45000–55000 a similar g/mol strategy and to synthesizedisplayed anmore absorption conjugated edge and at rigid468 nm. pyrene-based About two GNRs decades57 (Schemelater, Wu, 11 Zhaob) [66 ].and As coworkers expected, used they observeda similar strategyroughly ato 100 synthesize nm red-shift more in theconjug absorptionated and edge rigid to pyrene-based ca. 550 nm relative GNRs to 57 GNR (Scheme55. 11b) [66]. As expected, they observed roughly a 100 nm red-shift in the absorption edge to ca. 550 nm relative to GNR 55.

MoleculesMolecules2019 2018, 24, 23, 118, x FOR PEER REVIEW 99 of of 22 22 Molecules 2018, 23, x FOR PEER REVIEW 9 of 22

Scheme 11. (a) Synthesis of GNRs 55 by Swager and coworker [28]. (b) Synthesis of expanded GNRs Scheme 11. (a) Synthesis of GNRs 55 by Swager and coworker [28]. (b) Synthesis of expanded GNRs Scheme57 by Wu, 11. (Zhaoa) Synthesis and coworkers of GNRs [66] 55 by. Swager and coworker [28]. (b) Synthesis of expanded GNRs 57 by Wu, Zhao and coworkers [66]. 57 by Wu, Zhao and coworkers [66]. Taking a similar approach as with their nanographene synthesis (vide supra), Chalifoux and Taking a similar approach as with their nanographene synthesis (vide supra), Chalifoux and coworkersTaking aused similar a Brønstedapproach acid-catalyzedas with their nanographene alkyne benzannulation synthesis (videstrategy supra), to cyclizeChalifoux poly(2,6- and coworkers used a Brønsted acid-catalyzed alkyne benzannulation strategy to cyclize poly(2,6-dialkynyl- coworkersdialkynyl- usedp-phenylene) a Brønsted 58 intoacid-catalyzed a functionalized alkyn GNRe benzannulation 59 (Scheme 12) strategy [67]. This to cyclizeflexible poly(2,6-GNR was p-phenylene) 58 into a functionalized GNR 59 (Scheme 12)[67]. This ﬂexible GNR was reported to dialkynyl-reportedp to-phenylene) have average 58 intolengths a functionalized of 8 nm when GNRthe Suzuki 59 (Scheme polymerization 12) [67]. Thiswas carriedflexible outGNR in waterwas have average lengths of 8 nm when the Suzuki polymerization was carried out in water and toluene reportedand toluene to have as solvent average and lengths 35 nm of when 8 nm thewhen Suzuki the Suzuki polymerization polymerization was carried was carriedout in water out in and water THF as solvent and 35 nm when the Suzuki polymerization was carried out in water and THF as solvent. andas toluenesolvent. as GNR solvent 59 proved and 35 nmto be when highly the soluble Suzuki inpolymerization a number of commonwas carried organic out in solvents water and allowing THF GNR 59 proved to be highly soluble in a number of common organic solvents allowing for its study asfor solvent. its study GNR by 59IR, proved Raman, to NMR be highly and UV/Vis/NIR soluble in aspectroscopy. number of common The GNR organic showed solvents broad absorptionallowing by IR, Raman, NMR and UV/Vis/NIR spectroscopy. The GNR showed broad absorption in the forin its the study visible by IR,and Raman, near-infrared NMR and spectrum UV/Vis/NIR with spectroscopy.a large red-shift The as GNR compared showed to broad GNRs absorption 55 and 57 visible and near-infrared spectrum with a large red-shift as compared to GNRs 55 and 57 (Scheme 11), in(Scheme the visible 11), and displaying near-infrared an absorption spectrum edge wi atth 1a200 large nm, red-shift which corresponds as compared to anto GNRsoptical 55bandgap and 57 of displaying an absorption edge at 1200 nm, which corresponds to an optical bandgap of 1.03 eV. (Scheme1.03 eV. 11), displaying an absorption edge at 1200 nm, which corresponds to an optical bandgap of

1.03 eV. OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR OR

1) TFA 2) TfOH n 1) TFA 2) TfOH79-87% n n 79-87% n OR OR OR OR OR OR OR OR OR OR OR OR OR OR 59 58 R = OR OR OR OR OR OR OR OR OR OR OR OR OR OR 59 58 R = Scheme 12. Soluble GNR 59 reported by Chalifoux and coworkers [67]. Scheme 12. Soluble GNR 59 reported by Chalifoux and coworkers [67]. The use of BrønstedScheme acid-promoted12. Soluble GNR alkyne59 reported benzannulations by Chalifoux and has coworkers proven to [67] be a. convenient route The use of Brønsted acid-promoted alkyne benzannulations has proven to be a convenient route towards the synthesis of NGs and GNRs. However, the limitation that electron-rich ethynylaryl groups towards the synthesis of NGs and GNRs. However, the limitation that electron-rich ethynylaryl are neededThe use for of aBrønsted successful acid-promoted 6-endo-dig cyclization alkyne benzan undernulations these conditions has proven makes to be it aa challengeconvenient to route tune towardsgroups theare neededsynthesis for of a successfulNGs and 6-GNRs.endo- digHowever, cyclization the under limitation these that conditions electron-rich makes ethynylarylit a challenge the electronic and optical properties through substituent effects. groupsto tune are the needed electronic for a and successful optical 6-prendooperties-dig cyclization through substituent under these effects. conditions makes it a challenge to5.2. tuneπ-Lewis the electronic Acid Catalyzed and optical Alkyne pr Benzannulationsoperties through substituent effects. 5.2. π-Lewis Acid Catalyzed Alkyne Benzannulations 5.2. πOver-Lewis the Acid last Catalyzed ca. 30 years, Alkyne signiﬁcant Benzannulations optimization of the alkyne benzannulation reaction has led to the discoveryOver the last of aca. variety 30 years, of reagentssignificant that optimization promote cyclization of the alkyne in moderate benzannulation to good reaction yields underhas led to theOver discovery the last ca.of a30 variety years, significantof reagents optimization that promote of cyclization the alkyne inbenzannulation moderate to good reaction yields has under led torelatively the discovery mild ofreaction a variety conditions. of reagents There that have prom beenote numerouscyclization examples in moderate where to good π-Lewis yields acids under have relatively mild reaction conditions. There have been numerous examples where π-Lewis acids have

Molecules 2019, 24, 118 10 of 22 Molecules 2018, 23, x FOR PEER REVIEW 10 of 22 Molecules 2018, 23, x FOR PEER REVIEW 10 of 22 relatively mild reaction conditions. There have been numerous examples where π-Lewis acids have been used to activate an alkyne towards cyclization [68–75]. The η2-metal complex formed with the been used to activate an alkyne towards cyclization [68–75]. The η2-metal complex formed with the alkynebeen used makes to activate it susceptible an alkyn to enucleophilic towards cyclization attack from [68–75]. the adjacent The η2-metal aromatic complex ring leading formed towith a new the alkyne makes it susceptible to nucleophilic attack from the adjacent aromatic ring leading to a new C-C C-Calkyne bond. makes In most it susceptible cases, strongly to nucleophilic deactivating attack gr oups,from thesuch adjacent as esters, aromatic attached ring to leadingthe alkyne to a were new bond. In most cases, strongly deactivating groups, such as esters, attached to the alkyne were found foundC-C bond. to favor In most the 5-exo-digcases, strongly pathway, deactivating whereas activatinggroups, such groups as esters, almost attached exclusivity to the formed alkyne the were 6- to favor the 5-exo-dig pathway, whereas activating groups almost exclusivity formed the 6-endo-dig endo-digfound to product favor the [76]. 5-exo-dig With somepathway, understanding whereas activating to mechanism groups of almost this reaction, exclusivity researchers formed thehave 6- product [76]. With some understanding to mechanism of this reaction, researchers have begun taking begunendo-dig taking product advantage [76]. With for thesome purpose understanding of synthesizing to me largechanism NG ofsystems. this reaction, researchers have advantage for the purpose of synthesizing large NG systems. begun taking advantage for the purpose of synthesizing large NG systems. 5.2.1. Transition Metal-Catalyzed Cyclizations 5.2.1. Transition Metal-Catalyzed Cyclizations 5.2.1. Transition Metal-Catalyzed Cyclizations Ruthenium catalysts have been shown to be effective at effecting alkyne benzannulations to Ruthenium catalysts have been shown to be effective at effecting alkyne benzannulations to afford affordRuthenium NGs [68,75–81]. catalysts NGs have doped been with shown boron to beand effe siliconctive haveat effecting also been alkyne produced benzannulations using Ru(II) to NGs [68,75–81]. NGs doped with boron and silicon have also been produced using Ru(II) catalysts [82]. catalystsafford NGs [82]. [68,75–81]. Scott and coworkerNGs doped demonstrated with boron that and two- silicon and haveeven four-folalso beend alkyne produced benz usingannulations Ru(II) Scott and coworker demonstrated that two- and even four-fold alkyne benzannulations reactions using reactionscatalysts [82].using Scott (Ph 3andP)Ru(cymene)Cl coworker demonstrated2 could produce that two- a host and of even NGs, four-fol includingd alkyne coronene benz annulations61 (Scheme (Ph3P)Ru(cymene)Cl2 could produce a host of NGs, including coronene 61 (Scheme 13)[77]. Liu and 13)reactions [77]. Liu using and (Ph coworkers3P)Ru(cymene)Cl were able2 tocould improve produce the yielda host of of coronene NGs, including and extend coronene the scope 61 (Scheme of NGs coworkers were able to improve the yield of coronene and extend the scope of NGs synthesized in this synthesized13) [77]. Liu inand this coworkers way using were a more able re toactive improve Ru(II) the catalyst yield of system coronene [78]. and extend the scope of NGs way using a more reactive Ru(II) catalyst system [78]. synthesized in this way using a more reactive Ru(II) catalyst system [78].

Scheme 13. First synthesis of coronene 61 using a Ru(II)-catalyzed four-fold alkyne benzannulation Scheme 13. First synthesis of coronene 61 using a Ru(II)-catalyzed four-fold alkyne benzannulation [77]. [77]Scheme. 13. First synthesis of coronene 61 using a Ru(II)-catalyzed four-fold alkyne benzannulation The[77]. Liu group was able to utilize Ru-catalysis to synthesize a series of NG ribbons of various The Liu group was able to utilize Ru-catalysis to synthesize a series of NG ribbons of various lengths (Scheme 14)[79]. They later showed that substrates containing Ph2N groups were also tolerated lengthsin thisThe reaction (Scheme Liu group [80 14)]. Thesewas [79]. able second-generationThey to utilizelater sh Ru-catalysisowed ribbons that tosubstrates were synthesize reported containing a toseries have of excellentPh NG2N ribbonsgroups quantum wereof various yields also tolerateduplengths to 96.3% (Scheme in withthis largerreaction14) [79]. HOMO/LUMO [80].They These later second-gensh energyowed gapsthateration substrates vs compounds ribbons containing were65–67 reported. Ph2N groupsto have wereexcellent also quantumtolerated yieldsin this up reaction to 96.3% [80]. with These larger second-gen HOMO/LUMOeration energy ribbons gaps were vs compounds reported to 65 have–67. excellent quantum yields up to 96.3% with larger HOMO/LUMO energy gaps vs compounds 65–67.

Scheme 14. Synthesis of GNRs 65–67 via a Ru(II)-catalyzed two-fold alkyne benzannulation by Liu andScheme coworkers 14. Synthesis [79] [79].. [Tp[Tp of ==GNRs tris(1-pyrazolyl)borate)] 65–67 via a Ru(II)-catalyzed two-fold alkyne benzannulation by Liu and coworkers [79]. [Tp = tris(1-pyrazolyl)borate)] Though Ru(II) catalysis is useful for the formation of NGs and GNRs, it is limited to cyclizing Though Ru(II) catalysis is useful for the formation of NGs and GNRs, it is limited to cyclizing compounds that contain terminal acetylenes. As one might imagine, planarization of the system leads compoundsThough that Ru(II) contain catalysis terminal is useful acetylenes. for the As formation one might of imagine, NGs and planarization GNRs, it is limited of the system to cyclizing leads to decreased solubility and thus incorporation of appropriate solubilizing groups elsewhere in the tocompounds decreased that solubility contain and terminal thus incorporationacetylenes. As of one appropriate might imagine, solubilizing planarization groups of elsewhere the system in leads the substrate needs to be done if using this methodology. substrateto decreased needs solubility to be done and if thus using incorporation this methodology. of appropriate solubilizing groups elsewhere in the Alkyne benzannulations using Pt(II) catalysis has also become a popular method to arrive at substrateAlkyne needs benzannulations to be done if using using this Pt(II) methodology. catalysis has also become a popular method to arrive at various NGs [79,80,83–85]. For example, Liu and coworkers developed a four-step synthesis of variousAlkyne NGs benzannulations[79,80,83–85]. For using example, Pt(II) catalysisLiu and hascoworkers also become developed a popular a four-step method synthesis to arrive ofat chrysene 69 from cheap, commercially available starting materials with the key step involving a chrysenevarious NGs 69 from [79,80,83–85]. cheap, commercially For example, available Liu and starting coworkers materials developed with thea four-step key step synthesisinvolving ofa Pt(II)-catalyzed alkyne benzannulation of intermediate 68 (Scheme 15)[83]. Chrysene itself is a small Pt(II)-catalyzedchrysene 69 from alkyne cheap, benzannulation commercially of available intermediate starting 68 (Scheme materials 15) with [83]. theChrysene key step itself involving is a small a NGPt(II)-catalyzed that emits blue alkyne light benzannulation at 383 nm with of intermediatea quantum yield 68 (Scheme of 16% 15)making [83]. Chrysenederivatives itself of ischrysene a small promisingNG that emits in OLED blue applicationslight at 383 nm[86]. with Studies a quantum on the effects yield thatof 16% substituents making derivativeshave on the of chrysene’s chrysene promising in OLED applications [86]. Studies on the effects that substituents have on the chrysene’s

Molecules 2019, 24, 118 11 of 22

Molecules 2018, 23, x FOR PEER REVIEW 11 of 22 NG that emits blue light at 383 nm with a quantum yield of 16% making derivatives of chrysene Moleculespromising 2018 in, 23 OLED, x FOR applicationsPEER REVIEW [ 86]. Studies on the effects that substituents have on the chrysene’s11 of 22 emission and quantum yield show that tetra- and disubstituted molecules still emit at blue emission and quantum yield show that tetra- and disubstituted molecules still emit at blue wavelengths wavelengthsemission and and quantum the quantum yield showyield thatcan betetra- increased and disubstituted to 49% [87]. moleculesChrysene still69 can emit be atfurther blue and the quantum yield can be increased to 49% [87]. Chrysene 69 can be further functionalized though functionalizedwavelengths and though the quantumbromination yield at thecan 3-,be 6-, increased 9-, 12-postions to 49% to [87]. afford Chrysene tetrabrominated 69 can be chrysene further bromination at the 3-, 6-, 9-, 12-postions to afford tetrabrominated chrysene which can undergo whichfunctionalized can undergo though Pd-catalyzed bromination atcrosscou the 3-,pling 6-, 9- , to12-postions afford chrysene to afford derivatives tetrabrominated 70 [83,87]. chrysene An Pd-catalyzed crosscoupling to afford chrysene derivatives 70 [83,87]. An analogous version of this analogouswhich can versionundergo of thisPd-catalyzed reaction hascrosscou also beenpling reported to afford using chrysene microwave derivatives irradiation 70 [83,87].to arrive An at reaction has also been reported using microwave irradiation to arrive at other substitution patterns of otheranalogous substitution version patterns of this reaction of chrysene has [88].also been reported using microwave irradiation to arrive at chrysene [88]. other substitution patterns of chrysene [88].

Scheme 15. Pt-catalyzed alkyne benzannulation to afford chrysene 69 [83,87]. Scheme 15. Pt-catalyzed alkyne benzannulation to afford chrysene 69 [83,87]. Scheme 15. Pt-catalyzed alkyne benzannulation to afford chrysene 69 [83,87]. In an attempt to create cove-edged GNRs using solution-phase chemistry, Müllen and coworkers In an attempt to create cove-edged GNRs using solution-phase chemistry, Müllen and coworkers employed a Pt(II)-catalyzed alkyne benzannulation of 71 to synthesize chrysene-based dimer 72 employedIn an attempt a Pt(II)-catalyzed to create cove-edged alkyne benzannulation GNRs using solution-phase of 71 to synthesize chemistry, chrysene-based Müllen and coworkers dimer 72 (Scheme 16) [85]. Dimer 72 was subjected to Ullmann coupling followed by a Scholl oxidation to form (Schemeemployed 16 )[a 85Pt(II)-catalyzed]. Dimer 72 was alkyne subjected benzannulation to Ullmann coupling of 71 to followed synthesize by achrysene-based Scholl oxidation dimer to form 72 a a mixture of NGs 73–76. The authors noted that longer NGs 75 and 76 suffered from incomplete (Schememixture of 16) NGs [85].73–76 Dimer. The 72 authorswas subjected noted that to Ullmann longer NGs coupling75 and followed76 suffered by froma Scholl incomplete oxidation oxidative to form oxidative aryl-aryl coupling. X-ray crystallographic analysis of NG 74 showed that the carbon aaryl-aryl mixture coupling. of NGs 73–76 X-ray. crystallographicThe authors noted analysis that longer of NG 74NGsshowed 75 and that 76 thesuffered carbon from framework incomplete was framework was nonplanar due to steric hindrance in the cove regions, adopting an “up-down” oxidativenonplanar aryl-aryl due to steric coupling. hindrance X-ray in thecrystallographic cove regions, adoptinganalysis anof “up-down”NG 74 showed pattern that that the resulted carbon in pattern that resulted in 74 being chiral in the solid-state. Not surprisingly, the optical bandgap in framework74 being chiral was in nonplanar the solid-state. due Notto steric surprisingly, hindrance the in optical the cove bandgap regions, in going adopting from 73 anto “up-down”74 shows a going from 73 to 74 shows a decrease as a function of length, with values 2.36 and 1.90 eV, patterndecrease that as aresulted function in of 74 length, being with chiral values in the 2.36 solid-state. and 1.90 eV,No respectively.t surprisingly, the optical bandgap in respectively.going from 73 to 74 shows a decrease as a function of length, with values 2.36 and 1.90 eV, respectively.

Scheme 16. Cove-edge NGs produced from Pt(II)-catalyzed alkyne benzannulations [85]. Scheme 16. Cove-edge NGs produced from Pt(II)-catalyzed alkyne benzannulations [85]. DueScheme to their 16. disk Cove-edge like shape NGs produced and perfect fromπ Pt(I-conjugationI)-catalyzed onalkyne the benzannulations outer most rings, [85]. coronene derivativesDue to have their become disk like of greatshape interest and perfect for applications π-conjugation such ason discotic the outer liquid most crystals rings, and coronene organic derivativeselectronicsDue to [have89 their–92 become]. disk Some like of of great theshape earliest interest and syntheticperfect for applications π routes-conjugation tosuch make as on discotic coronenes the outer liquid have most crystals been rings, riddledand coronene organic with electronicsderivativeslong procedures [89–92].have andbecome Some low of yieldsof great the earliest [ 93interest,94]. synthetic Recently, for applications anroutes effort to such tomake optimize as discoticcoronenes this liquid synthesis have crystals been has riddled and been organic made with longelectronicsincluding procedures the[89–92]. use and of Some transition-metallow yieldsof the [93,94].earliest assisted Recently,synthetic benzannulation anroutes effort to tomake optimize [77 coronenes,78]. this Müllen synthesis have and been coworkers has riddled been made havewith includinglong procedures the use and of lowtransition-metal yields [93,94]. assisted Recently, benzannulation an effort to optimize [77,78]. thisMüllen synthesis and coworkers has been madehave recentlyincluding used the Pt(II)-catalyzeduse of transition-metal alkyne benzannulation assisted benzannulation of 77 with [77,78]. concomitant Müllen desi andlylation coworkers to produce have coronenerecently used tetracarboxdiimide Pt(II)-catalyzed 78alkyne (Scheme benzannulation 17a) [95]. The of 77synthetic with concomitant approach towardsdesilylation functionalized to produce coronene tetracarboxdiimide 78 (Scheme 17a) [95]. The synthetic approach towards functionalized

Molecules 2019, 24, 118 12 of 22 recentlyMolecules 2018 used, 23 Pt(II)-catalyzed, x FOR PEER REVIEW alkyne benzannulation of 77 with concomitant desilylation to produce12 of 22 coronene tetracarboxdiimide 78 (Scheme 17a) [95]. The synthetic approach towards functionalized coronenes was recently improved by Liu, Yin and coworkers by simplifying the starting material to coronenes was recently improved by Liu, Yin and coworkerscoworkers by simplifying the starting material to a functionalized triphenylene moiety 79, which was converted to their key intermediate 80 in 3 steps a functionalized triphenylene moiety 79,, whichwhich was converted to their key intermediate 80 in 3 steps (Scheme 17b) [96]. The key step was an alkyne benzannulation step using Pt(II)-catalysis to product (Scheme 1717b)b) [[96].96]. The key step was an alkyne alkyne benzannulation step using Pt(II)-catalysis to product functionalized coronenes 81. These functionalized coronene derivatives have good solubility in functionalized coronenescoronenes81 81. These. These functionalized functionalized coronene coronene derivatives derivatives have good have solubility good solubility in common in common organic solvents due to the presence of alkyl groups around the periphery of the molecule. organiccommon solvents organic due solvents to the due presence to the ofpresence alkyl groups of alkyl around groups the around periphery the ofperiphery the molecule. of the Theymolecule. have They have optical energy gaps of 3.02–3.07 eV, showing no photodegradation when irradiated with opticalThey have energy optical gaps energy of 3.02–3.07 gaps of eV, 3.02–3.07 showing eV, no showing photodegradation no photodeg whenradation irradiated when with irradiated white lightwith white light (100 W) for 8 hours and only minimal degradation after irradiation with UV light (6 W) (100white W) light for 8(100 h and W) only for 8 minimal hours and degradation only minimal after irradiationdegradation with after UV irradiation light (6 W) with after UV 8 h. light (6 W) after 8 hours.

Scheme 17. 1Pt(II)-catalyzed benzannulation towards functionalized coronenes 79 [96]. Scheme 17. 1Pt(II)-catalyzed benzannulation towards functionalized coronenes 79 [96]. Various helicenes [97,98], and derivatives thereof, have been produced by Pt(II)-catalyzed alkyne benzannulationsVarious helicenes [99–103 [97,98],]. For example,and derivatives the Storch thereof, group have was been able toproduced synthesize by [6]helicene Pt(II)-catalyzed83 in alkyne benzannulations [99–103]. For example, the Storch group was able to synthesize [6]helicene only 6 steps using PtCl2 (Scheme 18a) [99]. Interestingly, the synthesis of aza[6]helicene 84 required 83 in only 6 steps using PtCl2 (Scheme 18a) [99]. Interestingly, the synthesis of aza[6]helicene 84 a mixture of InCl3 and PtCl4 to effectively promote the alkyne benzannulation [100]. Wanting to extendrequired the a mixture substrate ofscope InCl3 and of aza[6]helicenes, PtCl4 to effectively Fuchter promote and coworkersthe alkyne benzannulation sought to make [100]. functionalized Wanting aza[6]heliceneto extend the derivativessubstrate usingscope theof sameaza[6]helic benzannulationenes, Fuchter conditions and coworkers as outlined sought by Storch to make [101]. Theyfunctionalized found that aza[6]helicene the reaction conditions,derivatives asusing reported, the same yielded benzannulation very little product, conditions even as whenoutlined up by to halfStorch an [101]. equivalent They offound catalyst that wasthe used.reaction Optimizing conditions, of as the reported, reaction conditionsyielded very by little changing product, solvents, even when up to half an equivalent of catalyst was used. Optimizing of the reaction conditions by changing increasing the temperature, and switching to PtCl4 gave better yields for the conversion of 85 to productssolvents, 86increasingand 87 (Scheme the temperature, 18b). The and authors switching noted to that PtCl the4 gave pre-cyclized better yields atropisomers for the conversion of 85 (R = Hof) could85 to products be separated 86 and by 87 semipreparative (Scheme 18b). The chiral authors HPLC noted and that cyclized the pre-cyclized to give enantioenriched atropisomers of (M 85)- 86(R (90%= H) could ee) and beP separated-86 (92% ee). by semipreparative chiral HPLC and cyclized to give enantioenriched (M)- 86 (90% ee) and P-86 (92% ee).

Scheme 18. a)( a[6]Helicene) [6]Helicene 83 83andand aza[6]helicene aza[6]helicene 84 84synthesissynthesis by byStorch Storch and and coworkers coworkers [99] [.99 b)]. (Aza[6]helicenesb) Aza[6]helicenes 86 and86 and 87 synthesized87 synthesized by Fuchter by Fuchter and and coworkers coworkers [101] [101. ].

Molecules 2019, 24, 118 13 of 22 Molecules 2018, 23, x FOR PEER REVIEW 13 of 22 Molecules 2018, 23, x FOR PEER REVIEW 13 of 22 Surprisingly, the synthesis of large NGs using Au-catalysis is rare but has been successfully Surprisingly, the synthesis of large NGs using Au-catalysis is rare but has been successfully employed for alkyne benzannulations to afford various small NGs, including chiral helicene-like employed for alkyne benzannulations to afford various small NGs, including chiral helicene-like compounds [104], pyrenes [71,72,84], and phenanthrenes [69,75,76]. compounds [104], pyrenes [71,72,84], and phenanthrenes [69,75,76]. The Dichtel group has recently popularized the Asao-Yamamoto benzannulation [105]—a type of The Dichtel group has recently popularized the Asao-Yamamoto benzannulation [105] – a type alkyne benzannulation catalyzed by Cu(II) and Brønsted acid that proceeds through a benzopyrylium of alkyne benzannulation catalyzed by Cu(II) and Brønsted acid that proceeds through a intermediate—to afford a number of NG precursors that could be oxidized via the Scholl reaction benzopyrylium intermediate – to afford a number of NG precursors that could be oxidized via the to afford various NGs [106–113]. For example, treatment of diyne 88 under the reaction conditions Scholl reaction to afford various NGs [106–113]. For example, treatment of diyne 88 under the reaction resulted in a two-fold alkyne benzannulation to afford NG precursors 89 and 90 (Scheme 19)[111]. conditions resulted in a two-fold alkyne benzannulation to afford NG precursors 89 and 90 (Scheme Compounds 89 and 90 were oxidized under Scholl conditions to afford NG derivatives 91 and 92. 19) [111]. Compounds 89 and 90 were oxidized under Scholl conditions to afford NG derivatives 91 The Dichtel group has also utilized this strategy in a polymeric system to afford GNRs after Scholl and 92. The Dichtel group has also utilized this strategy in a polymeric system to afford GNRs after oxidation [114]. Scholl oxidation [114].

Scheme 19. Asao-Yamamoto alkynealkyne benzannulationbenzannulation reportedreportedby by Dichtel Dichtel and and coworkers coworkers towards towards NGs Scheme 19. Asao-Yamamoto alkyne benzannulation reported by Dichtel and coworkers towards 91NGsand 9192 and[111 92]. . NGs 91 and 92 [111]. π 5.2.2. Main Groupπ -Lewis Acid-Catalyzed Cyclizations 5.2.2. Main group π-Lewis Acid-catalyzed Cyclizations Other metal catalysts have successfully cyclized alkynes to afford a benzene ring, including Other metal catalysts have successfully cyclized alkynes to afford a benzene ring, including GaClOther3 [75,76 metal], SbCl catalysts5 [115], andhave InCl successfully3 [75,76]. In cyclized many cases, alkynes InCl to3 hasafford proven a benzene to be superior ring, including catalyst GaCl3 [75,76], SbCl5 [115], and InCl3 [75,76]. In many cases, InCl3 has proven to be superior catalyst in inGaCl terms3 [75,76], of yield SbCl for5 [115], 6-endo and-dig InClcyclization3 [75,76]. of In alkynes many cases, onto heteroaromaticsInCl3 has proven and to be biphenyls superior containingcatalyst in terms of yield for 6-endo-dig cyclization of alkynes onto heteroaromatics and biphenyls containing haloalkyneterms of yield substituents for 6-endo- [dig76]. cyclization Gryko and of coworkersalkynes onto took heteroaromatics advantage of and this biphenyls by utilizing containing InCl3 to haloalkyne substituents [76]. Gryko and coworkers took advantage of this by utilizing InCl3 to cyclize cyclizehaloalkyne both substituents electron-rich [76]. and Gryko electron-poor and coworker ethynylaryls took groups advantage onto pyrrolo[3,2-b]pyrrolesof this by utilizing InCl933 toto cyclize afford both electron-rich and electron-poor ethynylaryl groups onto pyrrolo[3,2-b]pyrroles 93 to afford π- πboth-expanded electron-rich indolo[3,2-b]indole and electron-poor products ethynylaryl94 (Scheme groups 20)[ onto70]. pyrrolo[3,2-b]pyrroles 93 to afford π- expanded indolo[3,2-b]indole products 94 (Scheme 20) [70].

Scheme 20. Synthesis of compounds 94 by a two-fold alkyne benzannulation using InCl3 [70]. Scheme 20. Synthesis of compounds 94 by a two-fold alkyne benzannulation using InCl3 [70]. Scheme 20. Synthesis of compounds 94 by a two-fold alkyne benzannulation using InCl3 [70]. The Chalifoux group had shown that only electron-rich ethynylaryl substituents successfully The Chalifoux group had shown that only electron-rich ethynylaryl substituents successfully cyclizeThe using Chalifoux strong group Brønsted had acids shown to give that NGs only (vide electr supra).on-rich Turning ethynylaryl to the subs worktituents of Fürstner successfully [75,76] cyclize using strong Brønsted acids to give NGs (vide supra). Turning to the work of Fürstner [75,76] andcyclize Gryko using [70 strong] as inspiration, Brønsted theacids Chalifoux to give NGs group (vid explorede supra). the Turning use of InCl to the3 in work their of two- Fürstner and four-fold [75,76] and Gryko [70] as inspiration, the Chalifoux group explored the use of InCl3 in their two- and four- alkyneand Gryko benzannulation [70] as inspiration, towards the the Chalifoux synthesis group of peropyrenes explored the and use teropyrenes of InCl3 in their [116 ].two- This and catalyst four- fold alkyne benzannulation towards the synthesis of peropyrenes and teropyrenes [116]. This catalyst systemfold alkyne not onlybenzannulation proved to be towards superior the in synthesis terms of yieldof peropyrenes for the cyclization and teropyrenes of electron-rich [116]. This ethynylaryl catalyst system not only proved to be superior in terms of yield for the cyclization of electron-rich ethynylaryl substituentssystem not only to produce proved to peropyrenes be superior96 inand terms97 ofbut yield allowed for the for cyclization the cyclization of electron-rich of electron-neutral ethynylaryl98, substituents to produce peropyrenes 96 and 97 but allowed for the cyclization of electron-neutral 98, electron-poorsubstituents to99 produce, and even peropyrenes alkyl-substituted 96 and ethynylaryl 97 but allowed groups for 100the (Schemecyclization 21a). of Thiselectron-neutral catalyst system 98, electron-poor 99, and even alkyl-substituted ethynylaryl groups 100 (Scheme 21a). This catalyst waselectron-poor also able to99 promote, and even a four-fold alkyl-substitu alkyneted benzannulation ethynylaryl groups to afford 100 a broader(Schemescope 21a). ofThis teropyrene catalyst system was also able to promote a four-fold alkyne benzannulation to afford a broader scope of productssystem was102 also–105 ,able including to promote the differentially a four-fold substitutedalkyne benzannulation “push-pull” to derivative afford a 106broader(Scheme scope 21 b).of teropyrene products 102–105, including the differentially substituted “push-pull” derivative 106 Thisteropyrene broader products scope of 102 NGs–105 allowed, including them tothe study differentially the substituent substituted effects “push-pull” on crystal packing, derivative optical, 106 (Scheme 21b). This broader scope of NGs allowed them to study the substituent effects on crystal and(Scheme electrochemical 21b). This broader properties. scope of NGs allowed them to study the substituent effects on crystal packing, optical, and electrochemical properties.

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Scheme 21. InClInCl3-catalyzed alkyne benzannulation towardstowards aa broadbroad scopescope ofof (a)a) peropyrenes 96–100 Scheme 21. InCl3-catalyzed alkyne benzannulation towards a broad scope of a) peropyrenes 96–100 and (b) teropyrenes 102–106 [116]. and andb) teropyrenes b) teropyrenes 102– 102106– [116]106 [116]. . The use of indium(III) catalysts have not only opened the door to a broader scope of NG products TheThe use useof indium(III) of indium(III) catalysts catalysts have have not only not only open opened theed door the door to a broader to a broader scope scope of NG of products NG products available, but also for the cyclization of challenging substrates. Chalifoux and coworkers were able to available,available, but butalso also for thefor cyclizationthe cyclization of challenging of challenging substrates. substrates. Chalifoux Chalifoux and coworkersand coworkers were wereable able use InCl3 to invoke a four-fold cyclization to afford highly contorted chiral peropyrenes, including the to useto useInCl InCl3 to 3invoke to invoke a four-fold a four-fold cyclization cyclization to afford to afford highly highly contorted contorted chiral chiral peropyrenes, peropyrenes, including including extremely sterically hindered derivative 107 (Figure2a) [ 116]. This method proved to be more mild the theextremely extremely sterically sterically hindered hindered derivative derivative 107 (Figure107 (Figure 2a) [116]. 2a) [116]. This Thismethod method proved proved to be tomore be more and efﬁcient than what was reported using strong Brønsted acids such as TfOH [48]. The same group mildmild and and efficient efficient than than what what was wasreported reported using using strong strong Brønsted Brønsted acids acids such suchas TfOH as TfOH [48]. The[48]. same The same recently showed that other NG scaffolds are also accessible using indium(III) catalysis. A four-fold groupgroup recently recently showed showed that thatother other NG scaffoldsNG scaffolds are also are accessiblealso accessible using us indium(III)ing indium(III) catalysis. catalysis. A four- A four- alkyne benzannulation was recently used in the synthesis of benzodipyrene (BDP) derivatives 108 and foldfold alkyne alkyne benzannulation benzannulation was wasrecently recently used used in the in synthesisthe synthesis of benzodipyrene of benzodipyrene (BDP) (BDP) derivatives derivatives 109 (Figure2b) [ 117]. This reaction required a stronger catalyst system that consisted of InCl /AgNTf 108 108and and 109 109(Figure (Figure 2b) 2b)[117]. [117]. This This reaction reaction required required a stronger a stronger catalyst catalyst system system that consistedthat consisted3 of of2 (1:1). The BDP molecules show close stacking of ca. 4 Å in the solid-state, as determined by X-ray InCl3 3/AgNTf2 2 (1:1). The BDP molecules show close stacking of ca. 4 Å in the solid-state, as determined InCl /AgNTf (1:1). The BDP molecules show close stacking of ca. 4 Å in the solid-state, as determined1 crystallographic analysis. The BDPs also show excellent photostability and behave as O2 sensitizers1 by X-ray crystallographic analysis. The BDPs also show excellent photostability and behave1 2as O2 by X-ray crystallographic analysis. The BDPs1 also show excellent photostability and behave as O with up to 70% quantum efﬁciency of O2 generation. sensitizerssensitizers with with up toup 70% to 70% quantum quantum efficiency efficiency of 1O of2 generation. 1O2 generation.

Figure 2. (a) Sterically hindered chiral peropyrene 107 [116]. (b) BDPs 108 and 109 synthesized by a FigureFigure 2. (a 2.) Sterically (a) Sterically hindered hindered chiral chiral peropyrene peropyrene 107 [116] 107 [116]. (b) BDPs. (b) BDPs 108 and 108 109 and synthesized 109 synthesized by a by a four-fold alkyne benzannulation [117]. four-foldfour-fold alkyne alkyne benzannulation benzannulation [117] [117]. . Recently, the very ﬁrst domino alkyne benzannulation of diynes to make a host of irregular NGs Recently, the very first domino alkyne benzannulation of diynes to make a host of irregular NGs was reportedRecently, by the the very Chalifoux first domino group alkyne [118]. Thisbenzannulati reaction wason of shown diynes to to be make highly a host regioselective of irregular where NGs was reported by the Chalifoux group [118]. This reaction was shown to be highly regioselective where diynewas reported110 cyclizes by the on Chalifoux carbon 1 ofgrou thep naphthyl[118]. This group reaction rather was than shown carbon to be 3, highly leading regioselective to intermediate where111 diyne 110 cyclizes on carbon 1 of the naphthyl group rather than carbon 3, leading to intermediate anddiyne a newly110 cyclizes formed onbay carbonregion 1 of (Scheme the naphthyl 22a). The group new rabaytherregion than carbon in 111 is3, poisedleading for to aintermediate subsequent 111 and a newly formed bay region (Scheme 22a). The new bay region in 111 is poised for a subsequent alkyne111 and benzannulation a newly formed to bay produce region (Scheme NG 112 in 22a). excellent The new yield. bay Regioselectiveregion in 111 is cyclization poised for a onto subsequent various alkyne benzannulation to produce NG 112 in excellent yield. Regioselective cyclization onto various otheralkyne PAH benzannulation fragments (anthryl, to produce phenanthryl, NG 112 in and excellent pyrenyl) yield. allowed Regioselective the authors cyclization to demonstrate onto thatvarious this other PAH fragments (anthryl, phenanthryl, and pyrenyl) allowed the authors to demonstrate that isother a nice PAH method fragments for arriving (anthryl, at variousphenanthryl, NGs 113 and–117 pyrenyl). The authors allowed were the alsoauthors able to to demonstrate effect a four-fold that thisthis is a isnice a nice method method for arrivingfor arriving at various at various NGs NGs 113– 113117–. The117. authorsThe authors were were also ablealso toable effect to effect a four- a four- foldcyclization cyclization of of118 118to to make make four four new new carbon-carbon carbon-carbon bonds bonds and and four four new new aryl aryl rings rings in ain single a single reaction fold cyclization of 118 to make four new carbon-carbon bonds and four new aryl rings in a single reactionto arrive to arrive at a largeat a large NG-based NG-based chiral chiral “butterﬂy” “butterfly” molecule molecule119 119that that could could serve serve as as a newa new chiral chiral ligand reaction to arrive at a large NG-based chiral “butterfly” molecule 119 that could serve as a new chiral ligandscaffold scaffold (Scheme (Scheme 22b). 22b). ligand scaffold (Scheme 22b).

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Scheme 22. ((a)) Regioselective Regioselective domino domino alkyne alkyne benzannula benzannulationtion of of diynes diynes to to form form various various NGs NGs 112112––117117.. Scheme 22. (a) Regioselective domino alkyne benzannulation of diynes to form various NGs 112–117. (b) Four-fold alkynealkyne benzannulationbenzannulation towardstowards aa chiralchiral butterﬂybutterfly ligandligand motifmotif 119119 [[118]118]. . (b) Four-fold alkyne benzannulation towards a chiral butterfly ligand motif 119 [118]. 6. Base-Mediated Alkyne Benzannulations 6. Base-mediated6. Base-mediated Alkyne Alkyne Benzannulations Benzannulations Coronene tetracarboxdiimides (CDIs) have attracted interest as solar collectors [119] and in Coronene tetracarboxdiimides (CDIs) have attracted interest as solar collectors [119] and in lasersCoronene [120] duetetracarboxdiimides to their chemical, thermal,(CDIs) have and photochemicalattracted interest stability. as solar More collectors recently, these[119] derivativesand in lasers [120] due to their chemical, thermal, and photochemical stability. More recently, these lasershave [120] been due of interestto their for chemical, their application thermal, inan biologicald photochemical systems stability. [121] as wellMore as recently, OLED and these OFET derivatives have been of interest for their application in biological systems [121] as well as OLED and derivativesmaterials have [122 been]. Syntheses of interest of for these their derivatives application are in lengthybiological and systems with poor[121] yields.as well as To OLED optimize and the OFET materials [122]. Syntheses of these derivatives are lengthy and with poor yields. To optimize OFETsynthetic materials routes [122]. to Syntheses these CDIs, of scientiststhese derivative have beguns are lengthy to use benzannulationand with poor yields. methods To optimize to grow the the synthetic routes to these CDIs, scientists have begun to use benzannulation methods to grow the the numbersynthetic aromatic routes to rings these around CDIs, scientists the perylenediimide have begun(PDI) to use core. benzannulation Müllen and methods coworkers to reportedgrow the the number aromatic rings around the perylenediimide (PDI) core. Müllen and coworkers reported the numbersynthesis aromatic of CDIs rings using around a DBU-promoted the perylenediimid alkynee (PDI) benzannulation core. Müllen reaction and coworkers of diethynyl-substituted reported the synthesis of CDIs using a DBU-promoted alkyne benzannulation reaction of diethynyl-substituted synthesisPDIs 120 of CDIsto arrive using at a alkylated DBU-promoted CDI derivatives alkyne benzannulation121 in good yieldsreaction (Scheme of diethynyl-substituted 23)[123]. Since this PDIs 120 to arrive at alkylated CDI derivatives 121 in good yields (Scheme 23) [123]. Since this report, PDIsreport, 120 to there arrive have at alkylated been many CDI examples derivatives of base-induced 121 in good yields alkyne (Scheme benzannulations 23) [123]. Since to produce this report, various there have been many examples of base-induced alkyne benzannulations to produce various NGs thereNGs have [122 been,124 ,125many], heteroatom-containing examples of base-induced NGs alkyne [126], and benzannulations polymers [127 –to129 produce]. various NGs [122,124,125],[122,124,125], heteroatom-containing heteroatom-containing NGs NGs [126], [126], and and polymers polymers [127–129]. [127–129].

Scheme 23. Base-mediated alkyne benzannulations towards CDIs 121 [123]. Scheme 23. Base-mediated alkyne benzannulations towards CDIs 121 [123]. Scheme 23. Base-mediated alkyne benzannulations towards CDIs 121 [123]. 7. Conclusions 7. Conclusions 7. ConclusionsWith the increasing interest in the use of alkyne benzannulation reactions to rapidly extend the π-conjugationWithWith the theincreasing increasing towards interest NG interest structures, in the in theuse it willuse of alkyn of be alkyn interestinge benzannulatione benzannulation to see what reactions new reactions structures to rapidly to rapidly will extend be extend presented the the π π-conjugationgoing-conjugation into thetowards towards future. NG The NG structures, current structures, toolbox it willit willhas be beenbeinteresting interesting expanded to seeto to see includewhat what new a numbernew structures structures of electrophilic will will be be presentedreagents,presented going radicalgoing into processesinto the thefuture. future. and The catalysts, The current current many tool toolbox ofbox which has has been offer been expanded advantages expanded to dependingincludeto include a number ona number what of core of electrophilicstructureelectrophilic isreagents, desired. reagents, radical This radical allows processes processes access and toand catalyst a broadercatalysts, manys, scope many of of which of NGs, which offer which offer advantages isadvantages highly desirabledepending depending as on their on whatphotophysical,what core core structure structure chemical, is desired. is desired. physical This This allows andallows access electronic access to a to broader properties a broader scope canscope of be NGs, of tuned NGs, which through which is highly is variation highly desirable desirable of their as theirsizeas their andphotophysical, photophysical, structure. With chemical, chemical, many physical of thephysical methods and and elec discussed electronictronic properties here properties being can amenable can be tunedbe tuned to through the through incorporation variation variation of of varioustheirof their size functionalsize and and structure. groupsstructure. intoWi thWi the thmany NGmany andof theof GNR themethods precursors,methods discussed discussed chemistry here here now being havebeing amenable a amenable way to alsoto theto “ﬁne the incorporationtune”incorporation the physical of variousof various properties functional functional of thesegroups groups highly into into desirablethe theNG NG and materials. and GNR GNR precursors, precursors, chemistry chemistry now now have have a a wayway to also to also “fine “fine tune” tune” the thephysical physical proper propertiesties of these of these highly highly desirable desirable materials. materials.

Molecules 2019, 24, 118 16 of 22

Author Contributions: Writing—original draft preparation, A.D.S.; funding acquisition, writing—final draft preparation, review and editing, W.A.C. Funding: This research was funded by the National Science Foundation via a CAREER award to WAC, grant number CHE-1555218. Conflicts of Interest: The authors declare no conflict of interest.

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