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Article Cite This: Acc. Chem. Res. 2017, 50, 2776-2788

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Alkyne Benzannulation Reactions for the Synthesis of Novel Aromatic Architectures Samuel J. Hein,‡,† Dan Lehnherr,† Hasan Arslan,‡,†,§ Fernando J. Uribe-Romo,†,⊥ and William R. Dichtel*,‡,† ‡

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States

†

CONSPECTUS: Aromatic compounds and polymers are integrated into organic field effect transistors, light-emitting diodes, photovoltaic devices, and redox-flow batteries. These compounds and materials feature increasingly complex designs, and substituents influence energy levels, bandgaps, solution conformation, and crystal packing, all of which impact performance. However, many polycyclic aromatic hydrocarbons of interest are difficult to prepare because their substitution patterns lie outside the scope of current synthetic methods, as strategies for functionalizing benzene are often unselective when applied to naphthalene or larger systems. For example, cross-coupling and nucleophilic aromatic substitution reactions rely on prefunctionalized arenes, and even directed metalation methods most often modify positions near Lewis basic sites. Similarly, electrophilic aromatic substitutions access single regioisomers under substrate control. Cycloadditions provide a convergent route to densely functionalized aromatic compounds that compliment the above methods. After surveying cycloaddition reactions that might be used to modify the conjugated backbone of poly(phenylene ethynylene)s, we discovered that the Asao−Yamamoto benzannulation reaction is notably efficient. Although this reaction had been reported a decade earlier, its scope and usefulness for synthesizing complex aromatic systems had been under-recognized. This benzannulation reaction combines substituted 2-(phenylethynyl)benzaldehydes and substituted alkynes to form 2,3-substituted naphthalenes. The reaction tolerates a variety of sterically congested alkynes, making it well-suited for accessing poly- and oligo(ortho-arylene)s and contorted hexabenzocoronenes. In many cases in which asymmetric benzaldehyde and alkyne cycloaddition partners are used, the reaction is regiospecific based on the electronic character of the alkyne substrate. Recognizing these desirable features, we broadened the substrate scope to include silyl- and halogen-substituted alkynes. Through a combined experimental and computational approach, we have elucidated mechanistic insight and key principles that govern the regioselectivity outcome of the benzannulation of structurally diverse alkynes. We have applied these methods to prepare sterically hindered, shape-persistent aromatic systems, heterocyclic aromatic compounds, functionalized 2-aryne precursors, polyheterohalogenated naphthalenes, ortho-arylene foldamers, and graphene nanoribbons. As a result of these new synthetic avenues, aromatic structures with interesting properties were uncovered such as ambipolar charge transport in field effect transistors based on our graphene nanoribbons, conformational aspects of ortho-arylene architectures resulting from intramolecular π-stacking, and modulation of frontier molecular orbitals via protonation of heteroatom containing aromatic systems. Given the availability of many substituted 2-(phenylethynyl)benzaldehydes and the regioselectivity of the benzannulation reaction, naphthalenes can be prepared with control of the substitution pattern at seven of the eight substitutable positions. Researchers in a range of fields are likely to benefit directly from newly accessible molecular and polymeric systems derived from polyfunctionalized naphthalenes.

1. INTRODUCTION Substituted polycyclic aromatic hydrocarbons (PAHs) have electronic and optical properties that are desirable for organic photovoltaic devices,1 field effect transistors,2−4 redox flow batteries,5−7 light-emitting diodes,8−10 and other inexpensive devices. Methods to prepare complex, highly substituted PAHs or other novel aromatic architectures will offer new insights into © 2017 American Chemical Society

photophysical and redox processes and in turn lead to enhanced device performance. The most common approaches include transition metal cross-coupling reactions between aryl halides and organometallic reagents. Although heavily used, many of Received: August 2, 2017 Published: November 7, 2017 2776

DOI: 10.1021/acs.accounts.7b00385 Acc. Chem. Res. 2017, 50, 2776−2788

Article

Accounts of Chemical Research

(Scheme 1c). Asao and Yamamoto reported a Cu(OTf)2-catalyzed benzannulation between an ortho-(phenylethynyl)benzaldehyde and an alkyne that produces a 2,3-disubstituted naphthalene (Scheme 1d).35 However, the remarkable efficiency, tolerance of steric hindrance, and versatility of this reaction, which make it useful for preparing functionalized naphthalenes and functional aromatic architectures, were not immediately recognized.36−38 This Account describes our studies of the reactivity, regioselectivity, and mechanism of the Asao−Yamamoto benzannulation for various alkynes and demonstrates how these variants may be used to prepare highly substituted naphthalene rings, novel PAHs, and congested oligomers and polymers containing ortho linkages.

these transformations are less efficient for heavily substituted or sterically hindered substrates11,12 and require the installation of halides or other functional groups. The direct functionalization of aryl C−H bonds has also emerged as an efficient method for the polymerization and derivatization of aromatic systems, and the regioselectivity of such methods tends to be directed by neighboring functional groups or the innate reactivity of the arene.13,14 Cycloaddition reactions complement these direct, single-site functionalization methods and offer a convergent strategy for accessing highly substituted aromatic compounds. The most useful of these reactions are chemoselective, regioselective, and tolerant of many functional groups and provide substitution patterns not easily obtained using electrophilic aromatic substitution or directed C−H activation strategies.15−17 Cycloaddition reactions have achieved broad use in polymer and materials chemistry. The [4 + 2] cycloaddition between a terminal alkyne and a substituted cyclopentadienone and the cyclotrimerization of alkynes are both versatile methods for preparing penta- and hexa-substituted benzenes,18 which have been elaborated to polyphenylene dendrimers,19,20 nanographenes,21 and graphene nanoribbons (GNRs).22−26 Despite the remarkable versatility of this transformation, many compounds derived from naphthalene or larger aromatic cores remain outside of its scope.27,28 A formal palladium-catalyzed [2+2+2] cycloaddition of a benzyne intermediate with excess alkynes (Scheme 1a) can provide substituted naphthalene systems, although mixtures of regioisomers are generally obtained when targeting naphthalenes with asymmetric substitution patterns.29 Transition-metal-mediated annulations of vinyl or aryl iodides and triflates developed by Larock and co-workers produce substituted naphthalenes (Scheme 1b) in which the regioselectivity is determined by the steric demands of the acetylene substituents.30−32 A related transformation reported by Li33 and Balamurugan34 uses phenylacetaldehyde reactants

2. BENZANNULATION OF DIARYLACETYLENES Yamamoto and co-workers reported a Lewis acid catalyzed benzannulation of substituted acetylenes that selectively yields two classes of substituted naphthalenes. The benzannulation product is catalyst-dependent: AuCl3 mediates the formation of naphthyl ketones,35,39 and Cu(OTf)2 forms 2,3-disubstituted naphthalenes.35 These results provided a new method for accessing substituted naphthalenes. For the Cu(OTf)2-catalyzed reaction, the authors proposed a mechanism in which copper coordinates to the alkyne of the benzaldehyde reagent (B), facilitating the nucleophilic attack by the aldehyde oxygen to form copper-bound benzopyrylium intermediate C (Figure 1). This intermediate undergoes a formal [4 + 2] cycloaddition with the diarylacetylene to give copper-bound intermediate D. Under acidic conditions, the protodemetalation of D and retro [4 + 2] reaction initiated by the conjugate base yields 2,3-diarylnaphthalene E, whereas naphthyl ketone derivative F is known to form in the absence of a Brønsted acid. Despite the high efficiency of this reaction, the potential to form mixtures of regioisomers from asymmetric alkyne and substituted benzaldehyde substrates may render it irrelevant for constructing complex aromatic compounds. Although a related AuCl3-catalyzed benzannulation of alkynes provides naphthyl

Scheme 1. Cycloaddition Reactions That Provide Highly Substituted Naphthalenes

Figure 1. Mechanism proposed by Yamamoto and co-workers35 for the Cu(OTf)2-catalyzed benzannulation of diarylacetylenes to provide a 2,3-diarylnaphthalene. A noted side reaction that forms a naphthyl ketone is also shown. 2777


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Accounts of Chemical Research Table 1. Regioselective Synthesis of 2,3-Diarylnaphthalenes from Electronically Polarized Diarylacetylenes

Scheme 2. Models for Predicting the Regiochemical Outcome of Alkyne Benzannulation via the Relative Stability of Regioisomeric Carbocations Derived from the Acetylene Used in the Benzannulation

ketone derivatives (F) with high regioselectivity,39 it was unknown whether similarly high regioselectivity would be observed for naphthalene products (E) under Cu(OTf)2-catalyzed conditions. We observed that similar electronic effects govern the regioselectivity of the copper-catalyzed reaction.40 We first assessed the regioselectivity of the benzannulation of diarylalkyne 2a to determine the role of a π-electron-donating methoxy group (Table 1). Using fluorinated and 13C-labeled benzaldehydes 1a and 1b, respectively, we obtained products 3aa and 3ba, respectively, as single regioisomers. By contrast, diarylalkyne 2b, which bears electronically similar 4-methyl and 4′-t-butyl substituents, provides a nearly 1:1 mixture of the naphthalene regioisomers 3ab/4ab and 3bb/4bb. Finally, substrate 2c was evaluated to determine whether steric factors also influence the regiochemical outcome. We benzannulated 2c regioselectively to provide 3ac and 3bc, consistent with the products obtained from 2a based on the electron-donating effects of the two methyl groups. The steric demands of the methyl groups ortho to the alkyne neither detract from the efficient formation of the product

nor influence regioselectivity. The tolerance of the benzannulation reaction to alkynes bearing ortho-substituted aryl groups is remarkable, as noted in greater detail below. These studies suggest that the [4 + 2] cycloaddition proposed by Yamamoto35 proceeds either asynchronously or through sequential bond formation and results in one of the two alkyne carbons becoming electron deficient. The regioselectivity is therefore determined by how the alkyne substituents best stabilize this developing positive charge (Scheme 2).40 Although density functional theory (DFT) calculations of the transition states were later shown to be effective in predicting regiochemical outcomes, a DFT model relying on ground-state calculations of truncated structures allowed for rapid, low-computational-cost predictions. The regiochemical outcome is predicted by comparing the relative ground-state energies of cation-1 and cation-2 (truncated surrogates of transition states TS-1 and TS-2, respectively). This model effectively predicted the regiochemical outcome for the benzannulation of diarylacetylenes (see Table 1) as well as haloalkyne substrates (vide inf ra). Groups shown to stabilize the 2778


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Oligomers of ortho(phenylene ethynylene)s and related structures containing diyne groups provide all ortho-linked oligoarylenes upon benzannulation (Figure 2A).45 By modulating the sequence of mono- and diynes, we obtained specific sequences of phenylene and naphthalene units, such as 7 and 8. Single-crystal X-ray diffraction indicated that these compounds adopt a helical conformation (Figure 2C) in the solid state. This conformation is also dominant in solution, as one- and two-dimensional NMR spectroscopy confirmed that 91% of the population of 8-H adopts a completely folded conformation and the remaining population was partially folded. The ortho-arylenes adopt a higher percentage of the folded conformer in CDCl3 solutions at 0 °C when the naphthalenes are halogenated (e.g., 8-Cl and 8-F in Figure 2B), an outcome we attribute to more favorable π-stacking interactions between halogenated and nonhalogenated arenes. The foldamer length, substitution, and sequence dictate these intramolecular interactions, which in turn influence their optical and electrochemical properties. This method provides a direct synthetic approach to ortho-arylene oligomers through a modular benzannulation of ortho(phenylene ethynylene)s. However, decreased reaction efficiencies at longer oligomer lengths prevent the benzannulation of higher order oligo- and poly(ortho-phenylene ethynylene)s to make poly(ortho-phenylene)s, which is in contrast to the highly efficient benzannulation of poly(para-phenylene ethynylene) isomers. This inefficiency might arise from the acid sensitivity of oligo(ortho-phenylene ethynylene)s. ortho-Arylene polymers and oligomers are of additional interest as precursors to well-defined nanographenes and GNRs. These structures are obtained through an oxidative cyclodehydrogenation reaction that fuses neighboring aromatic rings to yield a graphitized structure (Figure 3A). This bottom-up synthetic approach enables exceptional control of the width, length, and edge structures of these materials. Thermal oxidation of an oligo(ortho-arylene) was attained through the evaporation of 9 under ultrahigh vacuum onto Au(111). Scanning tunneling microscopy images of single molecules of 9 revealed their nonplanarity (Figure 3B), and heating the substrate to 286 °C planarizes the molecules to form PAH 10 (Figure 3C).45 These experiments highlight the promise for using benzannulated aromatic structures as precursors for PAHs or designed graphitic materials. Scholl-type oxidation reactions of these polymers and oligomers also provide GNRs and PAHs, respectively. The oxidation of PPE 6b using 2,3-dichloro-5,6-dicyanobenzoquinone in the presence of CH3SO3H yields GNR 11 (Figure 3D). The cyclodehydrogenation reaction was characterized using Raman spectroscopy, IR spectroscopy, and cross-polarization magic angle spinning 13C NMR spectroscopy of two 13C-enriched derivatives. The bandgaps of isolated GNRs were estimated with DFT models to be 1.06 eV, which agreed reasonably well with the energy of their photoemission. Devices made from these materials were fabricated via aerosol-assisted chemical vapor deposition onto a copper foil substrate and transferred to SiO2 using traditional graphene transfer techniques. Field-effect transistors fabricated using this technique exhibited ambipolar charge transport.46 Notable examples of GNRs prepared using bottom-up approaches have been reported by Müllen,25 Rubin,47 Swager,48 Chalifoux,49 and Nuckolls,50,51 and identifying reliable strategies to prepare and incorporate these materials into devices remains a key frontier. Benzannulation reactions followed by Scholl-type oxidations can be used to form twisted and contorted PAHs, which typically offer outstanding solubility in organic solvents and optical

Scheme 3. Benzannulation of Poly(phenylene ethynylene)s

developing positive charge on the alkyne include aryl groups, silyl groups (through the β-silyl effect), and a lone pair of a halogen (R = I, Br, Cl). Building on this understanding of regiochemistry and the ability to benzannulate sterically hindered alkynes, we developed new methods to prepare ortho-arylene systems. Poly(phenylene)s have attracted much interest in recent years for their applications in organic light-emitting diodes, field effect transistors, and optoelectronic devices.42 Poly(ortho-phenylene)s adopt specific helical conformations in solution and in the solid state, with cofacial π-stacking of every third aromatic system along the backbone.11,41−43 However, because of their steric demands, these structures are difficult to prepare by traditional cross-coupling approaches. By contrast, many poly(para-phenylene ethynylene)s (PPEs) with high molecular weights can be synthesized from readily available starting materials. We transformed the embedded alkyne moieties along a PPE backbone into 2,3-naphthyl linkages to provide polyarylenes containing alternating para-phenylene (half of them tetrasubstituted) and ortho-naphthalene linkages. This transformation was the first reported cycloaddition along a conjugated polymer backbone (Scheme 3), which demonstrates the impressive efficiency of the benzannulation reaction.44 Owing to the congested nature of the ortho-phenylene backbone, the benzannulated poly(phenylene) (6a) was computationally predicted to adopt one of a few compact, nonplanar conformations in organic solvents. UV−visible and photoemission spectroscopy provided the first indication of efficient benzannulation. The inability of adjacent aromatic rings to planarize prevents conjugation along the polymer backbone and blue-shifts the maximum absorption of 6a by 140 nm relative to the parent 5a. A similar trend was observed in the photoemission spectrum, in which the λem of 6a was blue-shifted by 22 nm and significantly broadened compared with that of 5a, a feature common to other ortho-phenylene-linked polymers.41 More direct spectroscopic evidence for the efficient benzannulation of the polymer backbone was provided by 13C NMR analysis of a sample of 5a containing 13 C-enriched alkynes. Upon benzannulation, signals corresponding to 13C-labeled alkynes were shifted to the aromatic region, and no residual alkyne resonances were observed. These results demonstrated a new approach to the synthesis of high-molecular-weight polyphenylenes containing ortho-naphthyl linkages. 2779


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Figure 2. (A) Synthetic approach to oligo(ortho-arylene)s from oligo(ortho-phenylene ethynylene)s. (B) Sequences of ortho-arylene oligomers derived from the benzannulation of ortho-arylethynyl oligomers. (C) X-ray crystallographic structure of 8-H illustrating folding and selected interatomic distances in Å. Adapted with permission from ref 45. Copyright 2016 The Royal Society of Chemistry.

Figure 3. (A) On-surface oxidation of ortho-phenylene oligomers for nanographene synthesis. (B) Scanning tunneling microscopy (STM) image of several molecules of 9, which appear nonplanar, taken on Au(111) at T = 7 K. (C) STM image of an individual molecule of 10 obtained after annealing 9 at 286 °C taken on Au(111) at T = 7 K: its DFT-optimized structure is superimposed. Adapted from ref 45. Reprinted with permission of the Royal Society of Chemistry. (D) Chemical oxidation of BPP−OHxg into solution-dispersible graphene nanoribbons. Adapted with permission from ref 46. Copyright 2016 The American Chemical Society.

and electronic properties distinct from those of their planar counterparts. As a consequence of their curvature, some of these structures form host−guest complexes with fullerenes, which makes them potentially useful in photovoltaic devices.51

Benzannulation strategies can provide rapid access to extended and contorted hexabenzocoronenes (Figure 4A).52 Diarylacetylenes 12a−d were benzannulated to afford 13a−d, which were oxidized using FeCl3. Although other ortho-phenylene 2780


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Figure 4. (A) Synthesis of partially fused hexabenzocoronene (HBC) derivatives 14a−d and fully fused HBC 15a,b using a two-step benzannulation− cyclodehydrogenation strategy. (B) DFT-optimized structures of partially and fully fused HBC compounds 14a and 15a. n-C9H19 was truncated to CH3 groups in the calculations. Adapted with permission from ref 52. Copyright 2013 The Royal Society of Chemistry.

strategy that could produce larger aromatic systems by expanding the available substrates to include silyl-protected acetylenes.57 Expanding the scope of the benzannulation reaction to silyl-protected alkynes required modification of the typical Cu(OTf)2-catalyzed conditions (Scheme 4), which use strong Brønsted acids to suppress the formation of naphthyl ketone side products (see Figure 1). Most silicon protecting groups are unstable under these conditions, and protodesilylated 2-phenylnaphthalene was isolated as the major or only benzannulation product. When the amount of Brønsted acid is reduced to 1 equiv, most of the silyl-protected phenylacetylenes (17b−e) provide a 2-silyl-3-phenylnaphthalene product (19b−e) in high yield. The only exception is the most labile trimethylsilyl phenylacetylene (17a), which does not survive the Cu(OTf)2catalyzed conditions but can be benzannulated in 75% yield when ZnCl2 is used without an additional Brønsted acid.36 The silylprotected arylacetylenes also react with high regioselectivity, as exemplified by the reaction between 17a and bromobenzaldehyde 16b, which affords 19f in 50% yield (Scheme 5) as the major regioisomer. We studied the scope of this reaction beyond phenylacetylenes by evaluating a series of aromatic compounds containing triisopropylsilyl protected acetylenes (18a−e). Impressively, the highly sterically hindered 18a is well-tolerated in the reaction, yielding 20a in 78% yield. Electron-donating moieties (18b, 18d, and 18e) yield the corresponding naphthalenes 20b, 20d, and 20e; however, electron-withdrawing substituents, such as the pyridine

structures rearranged upon cyclodehydrogenation under similar conditions, as described by King and co-workers,53 rearrangements were not observed for these compounds. Although all of the structures underwent rapid C−C bond formation to yield 14a−d, only 14a and 14d fused completely to products 15a and 15d, respectively. The steric hindrance associated with the t-butyl or naphthyl groups present in 14b and 14c precluded the formation of the final C−C bonds. DFT-optimized structures of partially and fully fused compounds 14a and 15a further supported evidence of contorted conformations (Figure 4B). This successful synthesis of extended and contorted hexabenzocoronenes demonstrates that a benzannulation strategy is an effective approach for the synthesis of increasingly complex PAHs.

3. EXPANDING BEYOND DIARYLACETYLENES Sufficiently electron-rich diarylacetylenes are effective substrates for the Asao−Yamamoto benzannulation reaction, but their use enforces 2,3-diaryl substitution in the resulting naphthalenes. Acetylenes bearing other substituents, particularly those capable of further derivatization, would greatly expand the scope and utility of this transformation. We first investigated silanes because they have structural versatility and are common alkyne protecting groups.54,55 Once incorporated into aromatic systems, silyl groups can be elaborated to form C−C bonds under Hiyama crosscoupling conditions or substituted with various electrophiles.56 We wanted to integrate the benzannulation reaction into a general 2781


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silylacetylenes suggested to us a new strategy to access highly crowded aromatic architectures. Compound 21 was cross-coupled onto 1,3,5-triethynylbenzene to afford trialkyne 22 (see Figure 5A), which was further available for another 3-fold benzannulation to give oligo(arylene) 23 in good yield. Full characterization of this compound was achieved with two-dimensional NMR, which showed two conformations in solution at room temperature, assigned as the major anti and minor syn conformations in Figure 5b, with Cs and C3v symmetry, respectively. These two conformations result from the hindered bond rotations of each arm around the central benzene ring, and their interconversion remains slow on the 1H NMR time scale even at 140 °C. The synthesis of this compound exemplifies the tolerance of the benzannulation reaction for congested substrates. The limited regioselectivity of diarylacetylenes with substituents of similar electron density, combined with the poor reactivity of diaryl- and silyl-protected acetylenes bearing electronwithdrawing groups, led us to investigate the benzannulation of halo-arylacetylenes (Table 2).58 In contrast to previous diaryl- or silylacetylenes, haloacetylenes tolerate electronically deficient aryl groups with high efficiency. Arenes substituted with electron-withdrawing methyl ester (25a), nitrile (25b), and halogen (25c−e) groups undergo benzannulation in high yields to give the expected naphthalene products (26a−e). However, electron-donating para-methoxyphenylbromoacetylene (25f) shows the low conversion typical of inactive acetylenes. This work offered a complementary method for accessing polyheterohalogentated naphthalenes derived from haloacetylenes bearing electron-deficient phenyl substituents. Expanding on our understanding of the mechanism of this reaction, we proposed two mechanistic pathways. Mechanism A features [4 + 2] addition of the acetylene before protodemetalation and was first proposed by Yamamoto and co-workers (Figure 1). In mechanism B, protodemetalation of the copper-bound benzopyrylium occurs before undergoing formal [4 + 2] addition

Scheme 4. Reaction Scope for a Variety of Silylacetylenes

containing 18c, proved to be poor substrates, yielding only benzaldehyde decomposition products and unreacted acetylene. Aryl silane 19e is further transformed to aryl iodide 21 with iodine monochloride (Figure 5A).55 The outstanding efficiency and steric tolerance of the benzannulation reaction combined with the synthetic versatility of halogenated naphthalenes derived from Scheme 5. Regioselective Benzannulation of Silylacetylenes

Figure 5. (A) Reaction scheme for the synthesis of sterically congested oligo(ortho-arylene) 23. (B) Schematic depiction of the major anti and syn conformers of 23. Adapted with permission from ref 57. Copyright 2014 American Chemical Society. 2782


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Accounts of Chemical Research Table 2. Synthesis of Heterohalogenated Naphthalenes from Halo-arylacetylenes

Scheme 6. Concomitant Regioselective Benzannulation and Halogenation Using CuX2

functionalization of this Cu−C bond.38,58 Benzannulations employing stoichiometric quantities of CuCl2 or CuBr2 in place of catalytic Cu(OTf)2 indeed provide this additional halide on the naphthalene, ultimately enabling well-controlled substitution at seven of eight positions of a naphthalene ring (Scheme 6). Using haloacetylenes, we accessed functionalized and solutionprocessable diazatetracenes in two short steps (Scheme 7).59 Dichloronaphthalenes 27, 31, and 32 were constructed through the benzannulation of substituted acetylenes 25c, 29, and 30,

with the alkyne (Table 2, bottom). To test the viability of this route, we synthesized a ketal dimer of benzaldehyde 24a that fragments into free benzopyrylium upon treatment with TFA. This copper-free approach affords a halogenated naphthalene with regioselectivity identical to that of the copper-catalyzed route. DFT calculations of the proposed transition states of both mechanisms predict the observed regioselectivity in the reaction. On the premise that the mechanism proceeds via a copper-bound benzopyrylium, the intermediate provides a means to install an additional halide via ipso2783


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Accounts of Chemical Research Scheme 7. Synthesis of Diazatetracenes

Table 3. Synthesis of 2-Naphthyne Precursors from Various Substituted Benzaldehydes

Unfortunately, haloacetylenes substituted with n-butyl, triisopropylsilyl, or methyl ester groups resist Cu(OTf)2 catalyzed benzannulation. By using a ZnCl2 catalyst, we demonstrated that halo-silylacetylenes undergo benzannulation with high efficiency and regioselectivity (Table 3).60 This transformation shows a preference for iodo-silylacetylene (37a) over bromo-silylacetylene (37b), and the yield decreases from 93% to 18% for 38b and 38c, respectively. Although this reaction occurs with complete regioselectivity when halo-silylacetylenes (37a,b) are used, a 1:1 mixture of regioisomers (38f,g) is observed when trimethylsilylacetylene (37c) is used (see Table 3). Notably, the regioselectivity for this substrate class was the reverse of that observed for haloarylacetylenes (Figure 6A). We hypothesized that this reversal occurs because the developing positive charge on the acetylenic carbon is best stabilized by

respectively, using dichlorobenzaldeyde 24b. A Buchwald− Hartwig amination of dichloronaphthalene using 1,2-diaminobenzene generates the diazatetracene core, which aromatizes to compounds 33−35 under the reaction conditions. The electronic properties of the diazatetracenes can be tuned by modifying the alkyne substrate. This outcome is most clear in the UV− visible absorption spectra of 33 and 34, in which the extended conjugation of 33 red-shifts due to the extended conjugation provided by the additional ethynyl and aryl groups. The protonation of the embedded nitrogens dramatically affects the optical and electronic properties of these substrates as observed in their cyclic voltammograms in which a >1 V shift in the lowest unoccupied molecular orbital was observed, accompanied by a significant red shift in the UV−vis absorption and quenching of the emission band. 2784


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Figure 6. (A) Haloarylacetylenes and halosilylacetylenes provide opposite regioselectivity in benzannulation reactions, as demonstrated with X-ray crystallography. Ellipsoids set to 50% probability level for 38c. (B) Rationale for the regioselectivity of each reaction. Adapted with permission from ref 60. Copyright 2017 The Royal Society of Chemistry.

Scheme 8. 2-Naphthyne Generation and Trapping with Furan (left) and Oligomerization and Cyclization of a 2-Naphthyne Intermediate in the Presence of CuCN and n-BuLi (right)a

a

The major product(s) shifts as a function of reaction temperature and CuCN loading.

being β to the silicon for halosilylacetylene substrates, whereas the carbon α to the aryl group is preferred for haloarylacetylenes (Figure 6B). A DFT model of regioisomeric transition states predicted a 2.0 kcal/mol preference for the formation of the anti regioisomer in the case of 38b compared with 5.6 kcal/mol in favor of syn regioisomer formation for a bromo-phenylacetylene. Structural comparisons can be made among 38a−e and popular aryne precursors. Arynes compose a reactive class of intermediates that are most commonly generated through the desilylation/elimination of ortho-silylaryl triflates/halides.61−63 Transformations involving arynes have made these compounds highly useful building blocks for the synthesis of natural products and PAHs.64−70 Although arynes derived from fused aromatic rings enable the synthesis of more complex structures, making these structures commercially and synthetically available remains challenging. Substituted 2-naphthyne precursors have been reported by both Wong71 and Maly,72 but the seven-step syntheses limits the general adaptation of these methods. We proposed to use the naphthalene products afforded by the benzannulation of halo-silylacetylenes to access previously

underrepresented 2-naphthynes.60 Compounds 38a, 38b, 38d, and 38e were sealed in a vial with cesium fluoride (2 equiv), 18-crown-6 (4 equiv), and furan (15 equiv). After 12 h, [2.2.1]oxabicyclic alkenes (39a−d) were isolated in high yields (68−86%), which confirmed that these compounds are aryne precursors (Scheme 7). Endoxides similar to 39a−d can be reductively aromatized to afford anthracene derivatives,73 highlighting the usefulness of these results to access larger PAHs with interesting substitution patterns. The oligomerization of a 2-naphthyne intermediate (38e) to either a discrete dimer (40), trimer (41), or longer oligo(ortho-naphthylenes) (42) is achieved by modifying the reaction temperature and catalyst loadings of a Lipshutz cuprate catalytic system (Scheme 8). Given the availability of many substituted benzaldehyde cycloaddition partners, these findings demonstrate that benzannulation chemistry provides highly substituted naphthalenes with rapid entry to diazatetracenes and 2-naphthyne intermediates.

4. CONCLUSION This Account reviews a general method for accessing novel aromatic systems via the transition-metal-catalyzed 2785


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Accounts of Chemical Research benzannulation of alkynes. Through our investigations of diarylacetylenes, we have established that benzannulation exhibits high regioselectivity determined by the electronic character of the substituted alkyne. By expanding the synthetic scope, we found that silyl- and halo-substituted acetylenes open new pathways for the synthesis of highly crowded aromatic architectures, diazatetracenes, and 2-naphthyne precursors. The steric tolerance and regioselectivity of this reaction make it an indespensible synthetic tool for the construction of highly substituted PAHs.

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and functionalized interfaces, especially those capable of dynamic covalent or noncovalent processes.

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ACKNOWLEDGMENTS This work was supported by the Beckman Young Investigator Program of the Arnold and Mabel Beckman Foundation, the National Science Foundation (NSF; CHE-1124754), the Doctoral New Investigator Program of the ACS Petroleum Research Fund (52019-DN17), the Cottrell Scholar Program of the Research Corporation for Science Advancement, and a Sloan Research Fellowship from the Alfred P. Sloan Foundation.

AUTHOR INFORMATION

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Corresponding Author

*E-mail: [email protected].

REFERENCES

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ORCID

William R. Dichtel: 0000-0002-3635-6119 Present Addresses §

Hasan Arslan: Department of Chemistry, Bucknell University, Lewisburg, PA 17837, U.S.A. ⊥ Fernando J. Uribe-Romo: Department of Chemistry, University of Central Florida, Orlando, FL 32816, U.S.A. Notes

The authors declare no competing financial interest. Biographies Samuel J. Hein obtained his B.Sc. from the University of Wisconsin− Eau Claire in 2012 and recently received his Ph.D. from Cornell University with William Dichtel. He is interested in the design of conjugated organic materials for semiconductor devices and energy storage. Dan Lehnherr obtained his B.Sc. from the University of Victoria and his Ph.D. from the University of Alberta. He conducted postdoctoral research at Harvard University with Eric Jacobsen and at Cornell University with William Dichtel. His research has spanned topics in organic materials, semiconductor devices, computational chemistry, catalysis, and mechanistic studies of reactions. Hasan Arslan obtained his B.S. from the Middle East Technical University and his Ph.D. from Cornell University. He completed postdoctoral research with Sir Fraser Stoddart at Northwestern prior to becoming an Assistant professor at Bucknell University in 2017 where his research group is interested in the synthesis and supramolecular properties of macrocycles and polymers. Fernando J. Uribe-Romo received a B.Sc. in Chemistry from ITESM in Mexico in 2006. He performed his graduate studies in inorganic chemistry at UCLA with Omar Yaghi. He was a visiting scholar at Newcastle University and postdoctoral associate at Cornell University with William Dichtel. He joined the University of Central Florida in 2013. His research focuses on the synthesis of framework materials, in particular crystalline metal−organic and covalent−organic frameworks for their application in energy conversion, photoredox catalysis, charge transport, and optical activity. William R. Dichtel earned his B.S. at MIT and Ph.D. under the supervision of Jean M. J. Fréchet at UC-Berkeley. He next held a joint postdoctoral appointment with Fraser Stoddart, then at UCLA, and James Heath at Caltech. He began his independent career in the Department of Chemistry and Chemical Biology at Cornell University in 2008 and was promoted to the rank of Associate Professor in 2014. He moved to Northwestern University in the summer of 2016 as the Robert L. Letsinger Professor of Chemistry. His research group focuses on the design, synthesis, and applications of organic molecules, materials, 2786


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on November 7, 2017, with an error to Figure 4. The corrected version was reposted November 10, 2017.

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