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Nanographene and Graphene Nanoribbon Synthesis Via Alkyne Benzannulations

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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]. Molecules 2019, 24, 118; doi:10.3390/molecules24010118 www.mdpi.com/journal/molecules Molecules 2019, 24, 118 2 of 22 Molecules 2018,, 23,, xx FORFOR PEERPEER REVIEWREVIEW 2 of 22 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 ofof 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.. Molecules 20192018, 2423, 118x FOR PEER REVIEW 33 of of 22 Molecules 2018, 23, x FOR PEER REVIEW 3 of 22 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
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