K+ -Mediated vs Pd-Catalyzed Cyclotrimerization of 9,10-Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)- and (α,α,β)-Fragments of Cubic Graphite

Organic Materials K+-Mediated vs Pd-Catalyzed Cyclotrimerization of 9,10-Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)- and (α,α,β)-Fragments of Cubic Graphite Jesús Bello-García, Jesús A. Varela,* and Carlos Saá* Dedicated to Professor Luis Castedo on the occasion of his 85th birthday. Angewandte Chemie Communication www.angewandte.org How to cite: Angew. Chem. Int. Ed. 2024,63, e202414017 doi.org/10.1002/anie.202414017 Angew. Chem. Int. Ed. 2024,63, e202414017 (1 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Chemie Abstract: A remarkable stereodivergent cyclotrimerization of 9,10-didehydrotribenzo[8]annulene (tribenzoCOTyne) to the corresponding (α,α,α)- and (α,α,β)-benzofused derivatives has been developed by controlling the reaction conditions. While the K+-mediated cyclotrimerization afforded the (α,α,α) stereoisomer, using Pd as a catalyst resulted in the (α,α,β)-derivative. Both stereoisomers were evidenced by spectroscopic data and crystal X-Ray analysis. The (α,α,α) stereoisomer is a fragment of cubic graphite (CG), an elusive 3D carbon allotrope that contains carbon cages, since all of its sixty carbons are part of the structure of CG, and 36 constitute a part of the C48 molecular cage of CG. Experimental and computational mechanistic studies revealed that the potassium ion would play a key role as a template to favor the formation of the (α,α,α) stereoisomer. sp2-Hybridized carbon allotropes,[1] such as fullerenes (0D),[7] carbon nanotubes (1D),[10] graphenes, nanographenes and the combined sp2sp hybridized γ-graphyne (2D),[13] has attracted great attention since these versatile nanomaterials have a wide range of technological applications.[12,17,19] The three-dimensional (3D) carbon allotropes, diamond (Csp3)[23] and layered graphite (Csp2),[26] are well-known carbon phases with extraordinary properties but the hypothetical Riley’s cubic graphite[28] remains elusive despite the thermal and mechanical stability between other expected relevant physical properties.[29] In cubic graphite, each benzene ring is connected to six other different rings and each benzene ring is part of three equivalent poly-pphenylene (PPP) chains. Four neighboring benzene rings are arranged in an eight-membered tetraphenylene[32] ring with a strong mutual twist between them (Scheme 1a),[33] rendering all carbon atoms equivalent, and converting this material into the perfect three-dimensional structure of polyphenylene.[36] Indeed, the presence of suitably sized diffusion channels for metal ions (e.g. Li+) should qualify cubic graphite as active material for rechargeable batteries and supercapacitors.[29] Several synthetic strategies have been developed to model cubic graphite subunits based on cross-couplings[39] and cycloadditions[40] that had targeted three-dimensional dendritic oligophenylenes[41] to delineate the crowded nature of oligophenylene but neglecting the very important tetraphenylene moiety.[32] More recently, an attractive tridimensional approach to benzo-fused tri[8]annulenes as molecular models of cubic graphite has been disclosed based on selective transition-metal-catalyzed (Ru, Pd) cyclotrimerizations of cyclooctatrienynes containing phenanthrenyl units.[42] A closer symmetry-driven look to cubic graphite structure, as observed by Baughman,[29] shows that it contains carbon cages which are centered at 43 m sites (Scheme 1a). Each carbon cage is formed by 48 carbon atoms (C48),[4,43] 24 of which are located in four benzene rings which are tetrahedrally positioned about the cage center. The remaining 24 carbons in the C48 cage belong to 12 phenyls which each contribute two carbons (one π bond) to this cage. The whole cage comprises 4 hexagons, 4 dodecagons and 6 octagons featuring 3 channels of 8 carbon atoms crossing the cage center. The cubic symmetry and three-dimensional electronic structure make this material an interesting candidate for superconductivity.[29] Herein we report a stereoselective synthesis of a C60 fragment of cubic graphite (benzofused (α,α,α)-tris(tribenzo- [8]annulene), TTBA 2)[46] as a suitable building block for its assembly (Scheme 1b). It occurs on a striking K+-mediated cyclotrimerization of tribenzoCOTyne 1. The isomeric (α,α,β)-TTBA 2could be accessed in a stereodivergent manner from the same COTyne using Pd as catalyst.[47] Both stereoisomers were shown to be thermally stable and noninterconvertible. DFT studies suggested a key role of potassium ion as template for the tribenzoCOTyne cyclotrimerization to the (α,α,α)-TTBA 2. The known thermal instability of tribenzoCOTyne 1 directed us toward the search for stable and readily available precursors of 1.[48] Tribenzo[a,c,e]cyclooctanone 3was chosen as starting material since the formation of vinyl triflates under mild conditions and their easy conversion to alkynes is well established (Scheme 2).[51] Tribenzocyclooctanone 3was readily prepared on gram scale following Zhang’s procedure from Pd-catalyzed cross-coupling of 2- [*] Dr. J. Bello-García, Prof. J. A. Varela, Prof. C. Saá Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Departamento de Química Orgánica Universidade de Santiago de Compostela 15782 Santiago de Compostela, Spain E-mail: [email protected] [email protected] © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is noncommercial and no modifications or adaptations are made. Scheme 1. Hypothetical segment of cubic graphite, structural features and stereodivergent cyclotrimerization of tribenzoCOTyne 1. Angewandte Chemie Communication Angew. Chem. Int. Ed. 2024,63, e202414017 (2 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License iodobiphenyl with o-chloroacetophenone in 67% yield.[52] Subsequent treatment of 3with excess Tf2O and DIPEA provided the vinyl triflate precursor 4in 78% yield (Scheme 2).[53] In an effort to initially prepare the (α,α,α)-TTBA 2, we carried out a Ru(II)-catalyzed cyclotrimerization of tribenzoCOTyne 1(generated in situ from vinyl triflate 4with KOtBu in DCM)[54] in the presence of [Cp*Ru(CH3CN)3]PF6 at rt obtaining a mixture of (α,α,α) and (α,α,β) of TTBA 2in 1.5:1 ratio (Table 1, entry 1).[42] The C3v symmetry of (α,α,α)- TTBA 2was confirmed by 1H NMR with the appearance of only six signals (Figure 1). A similar ratio of stereoisomers was obtained using THF as a solvent (Table 1, entry 2). The alternative Pd(0)-catalyzed cyclotrimerization of tribenzoCOTyne 1gave a similar result to that of the Ru catalyst, with the (α,α,α) stereoisomer being the major product in 1.6:1 ratio (Table 1, entry 3).[42] These identical results obtained with two different catalysts (Ru, Pd) led us to carry out a control test without a transition-metal catalyst, as its role in the reaction was not clear enough. Strikingly, the cyclotrimerization reaction of tribenzoCOTyne 1at rt gave rise again to similar yield and ratio of stereoisomers of TTBA 2(Table 1, entry 4).[55] This remarkable result firmly suggests that the cyclotrimerization of tribenzoCOTyne 1 occurs at room temperature and that transition metal catalyst doesn’t play any significative role in the course of the reaction under these conditions. Having found these totally unexpected results (Table 1) we launched into a broader exploration of the reaction conditions starting with the evaluation of the transitionmetal-free conditions (Table 2). The concentration of the reaction turned out to be a variable with a significant impact on the course of the reaction. Decreasing it to 0.04 M, although it did not significatively affect to the overall yield, did increase the amount of the major (α,α,α)-TTBA 2to a 4.5:1 ratio (Table 2, entries 1 and 2).[56] Reaction performed in DCM as solvent gave a lower yield and ratio of TTBA 2stereoisomers while ketone 3became favored (Table 2, entry 3). Temperature control experiments were then analyzed. At 80°C the alkoxy derivative 5derived from a nucleophilic addition to the tribenzoCOTyne intermediate becomes favored (Table 2, entry 4). However, reaction performed at low temperature, 4°C in a cold room, gave a slight increase in the reaction yield and, more importantly, a better isomer ratio (6.5:1) favoring the (α,α,α) stereoisomer (Table 2, entry 5). Further dilution to 0.01 M led to a lower overall yield with total isomer selectivity but together with a certain amount of ketone 3(Table 2, entry 6). Slow generation of the cyclic alkyne intermediate by dropwise addition of KOtBu gave poor results (Table 2, entry 7). The nature of the alkali base was evaluated next. Interestingly, the alkoxide counterion was essential for the reaction (Table 2, Scheme 2. Synthesis of vinyl triflate 4as a precursor of tribenzoCOTyne 1. Table 1: Transition-metal-free and TM-catalyzed cyclotrimerizations of tribenzo-COTyne 1to TTBA 2.[a] Entry Catalyst (x mol%) Solvent Reaction yield 2[b] (α,α,α):(α,α,β) ratio[c] 1 [Cp*RuL3]PF6(10) DCM 34% (1.5:1 mixture) 2 [Cp*RuL3]PF6(10) THF 35% (1.6:1 mixture) 3 Pd2(dba)3(5) THF 33% (1.6:1 mixture) 4 – THF 34% (1.6:1 mixture) [a] Conditions: 4(0.2 mmol) in solvent (0.4 M) at rt, overnight (o/n), sublimated KOtBu before use. [b] Isolated yield. [c] Crude 1H NMR ratio. L=CH3CN. Figure 1. 1H NMR of (α,α,α)- and (α,α,β)-TTBA 2.(500 MHz) in CD2Cl2. Angewandte Chemie Communication Angew. Chem. Int. Ed. 2024,63, e202414017 (3 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License entry 8) and reaction in the presence of a potassium sequestering agent, 18-crown-6 ether, practically stopped it (Table 2, entry 9). Other alkali bases (LiOtBu, NaOtBu) failed to give the cyclotrimerization.[57] To our delight, it was possible to separate and purify (α,α,α)-TTBA 2from the mixture of tris(tribenzo[8]- annulenes) 2by just washing the crude mixture with hexane. X-ray analysis of a crystal, grown from a solution of (α,α,α)- TTBA 2in DCM by slow diffusion in hexane, unambiguously confirmed the formation of the (α,α,α)-stereoisomer of TTBA 2consisting of three boat-shaped COT moieties bended to the same face of central benzene ring (Figure 2, left).[53] The side-walled benzenes are oriented nearly perpendicular (78°–82°) to the benzene core made from cyclotrimerization. In the crystal, pairs head-to-head and tail-to-tail alignments were found as in a hypothetical segment of cubic graphite (Scheme 1a). According to Baughman’s predictions,[29] large ions (such as K+), which would be trapped in the void spaces of hypothetical cubic graphite, should be immobile an unable to react with the atmosphere. To probe this prediction in our open fragment of cubic graphite, a suspension of (α,α,α)- TTBA 2and potassium tetrakis(perfluorophenyl)borate (K+ [B(C6F5)4]) in DCM was stirred at rt for 24 h, yielding a quantitative yield of a new complex that shows 1H NMR signals shifted more downfield than initial (α,α,α)-TTBA 2 but keeping the C3v symmetry of starting material. The addition of 18-crown-6-ether to the solution caused recovery of the initial (α,α,α)-TTBA 2stereoisomer.[57] To our delight, it was possible to unambiguously confirm its nature as (α,α,α)-TTBA 2�K+as an X-ray crystal of this complex grew by slow evaporation of the solution in DCM (Figure 3). In this complex, the K+ion is placed at the center of the Table 2: Transition-metal-free cyclotrimerization of tribenzoCOTyne 1to TTBA 2.[a] Entry Molarity T (°C) Reaction yield 2[b] (ratio isomers)[c] 1 0.4 rt 34% (1.6:1 mixture) 2 0.04 rt 31% (4.5:1 mixture) 3[d] 0.04 rt 24% (1.3:1 mixture) 4[e] 0.04 80 32% (3.5:1 mixture) 5 0.04 4 36% (6.5:1 mixture) 6[f] 0.01 4 21%[g] 7[h] 0.04 4 15%[g] 8[i] 0.04 4 – 9[j] 0.04 4 4% [a] Conditions: 4(0.4 mmol) in THF (×M), overnight (o/n), sublimated KOtBu before use. [b] Isolated yield of chromatographic mixture. [c] Crude 1H NMR ratio. [d] DCM as solvent. Ketone 3was isolated in 23% yield as major compound. [e] The alkoxy derivative 5is the major compound observed (24%). [f] Ketone 3was also isolated in 18% yield. [g] More than 95% (α,α,α)-2by 1H NMR. [h] Slow addition of base (1 M solution) in THF during 30 min. [i] KPF6instead KOtBu. Vinyl triflate 4was recovered (89%). [j] 18-Crown-6 ether (1.5 equiv) was added. Vinyl triflate 4(43%) was recovered and ketone 3(18%) was isolated. Figure 2. Single crystal X-ray structures illustrating cavities and dihedral angles (Φ) of three p-terphenyl units of (α,α,α)-TTBA 2(left) and (α,α,β)-TTBA 2(right). Figure 3. Single crystal X-ray structure of complex (α,α,α)-TTBA 2�K+. Angewandte Chemie Communication Angew. Chem. Int. Ed. 2024,63, e202414017 (4 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License cage, 2.9 Å from the center of the benzene rings placed on the walls and 3.0 Å from the center of the benzene ring at the bottom of the cage. Our next challenge was to find appropriate conditions to access to the (α,α,β)-TTBA 2as a major isomer. To do so, transition-metal-catalyzed cyclotrimerizations of tribenzoCOTyne 1were explored (Table 3). When using the less bulky Cp ligand on the Ru center, the (α,α,α)-TTBA 2 remains as a main (Table 3, entry 1). On heating the reaction at 60°C the ratio of (α,α,β)-TTBA 2slightly improves but still not sufficiently (Table 3, entry 2). After these unsatisfactory tests with Ru catalysts (Tables 1 and 3, entries 1 and 2),[58] Pd complexes were evaluated. By heating vinyl triflate 4with KOtBu in the presence of Pd2(dba)3in toluene at reflux, a mixture of stereoisomers 0.8:1 was obtained in 37% yield, which means that the temperature affected the evolution of the palladacycle intermediate (Tables 1 and 3, entry 3). Addition of the PPh3ligand significantly altered the isomer ratio by favoring the (α,α,β) but did not affect the overall yield (Table 3, entry 4).[42] The influence of PPh3as ligand was corroborated by using preformed Pd(PPh3)4since similar yield and ratio of TTBA 2stereoisomers were obtained at room temperature (Table 3, entry 5). To our delight, when Pd(PPh3)4was used as catalyst in refluxing toluene, a slightly lower overall yield was obtained, but the amount of the desired (α,α,β)-TTBA 2could be increased up to 1:10 ratio (Table 3, entries 6 and 7). The Cssymmetry of (α,α,β)-TTBA 2(C60H36) was confirmed by integrating the 18H signals into the 1H NMR spectrum (Figure 1). Interestingly, no variation of 1H NMR signals was observed after heating at 140°C in DMSO for 24 h, indicating no interconversion between (α,α,α) and (α,α,β) conformers even at high temperatures (calculated ΔG‡= 75.8 kcalmol1).[57] Single crystal X-Ray analysis of the (α,α,β)-TTBA 2,[53] grown from a solution of 2in DCM by slow diffusion in hexane, unambiguously confirmed its saddle-shaped structure in which one boat-shaped COT faces the opposite molecular hemisphere with respect to the central benzene ring compared to the other two (Figure 2, right). The spatial arrangement of the p-terphenyl units of both stereoisomers was compared by X-Ray analysis. While a nearly parallel arrangement for the terminal phenyl units is observed in the (α,α,α)-TTBA 2with an average dihedral angle (Φ) of 3°, a twisted arrangement (dihedral angles (Φ) of 0°(α) and 40° (β) is observed in the (α,α,β)-TTBA 2(Figure 2, right). Considering the strong influence of the potassium cation on the reaction course (Table 2) and its known ability to bind π unsaturated moieties[59] and to form the complex (α,α,α)-TTBA 2�K+(Figure 3), we performed DFT calculations to gain insight into a plausible mechanism for potassium-mediated vs non-potassium-mediated cyclotrimerization of tribenzoCOTyne 1(Figure 4).[57] Initial approach of two units of 1leads to species Iwhich readily evolves to diradical s-cis 1,3-butadiene II (ΔG‡=12.6 kcalmol1and ΔG0=19.8 kcalmol1). The same process but now mediated by potassium would initially start with complex I�K+, 15.5 kcalmol1more stable, that would evolve to the s-trans 1,3-butadiene diradical II�K+(ΔG‡=13.2 kcalmol1,ΔG0= 20.6 kcalmol1), being this pathway kinetically more favorable. Incorporation of a new COTyne 1to afford complex IIt�K+followed by a second radical coupling would give rise to the s-trans-s-cis-hexatriene diradical III�K+(ΔG‡= 4.6 kcalmol1and ΔG0=51.4 kcalmol1). Evolution of III �K+by counterclockwise rotation of the s-trans vinylic residue led directly the (α,α,α)-TTBA 2(ΔG‡= 18.7 kcalmol1,ΔG0=104.1 kcalmol1) through a 5-exotrig or to the (α,α,β)-TTBA 2(ΔG‡=18.2 kcalmol1,ΔG0= 110.6 kcalmol1) through a 6-endo-trig transition states. On the other hand, the counterpart clockwise rotation led to the intermediate benzvalene[62] V�K+(ΔG‡= 17.1 kcalmol1,ΔG0=45.1 kcalmol1) through a 5-exo-trig transition state from intermediate IV�K+that evolved to the (α,α,α)-TTBA 2(ΔG‡=15.0 kcalmol1,ΔG0= 59.0 kcalmol1). Note that the potassium template effect would play two fundamental roles: a) facilitate the formation of the s-trans-s-cis-hexatriene diradical III�K+and b) Table 3: Metal-catalyzed cyclotrimerization of tribenzoCOTyne 1to TTBA 2.[a] Entry Catalyst (x mol%) Solvent T (°C) Reaction yield 2[b] (ratio isomers)[c] 1 [CpRuL3]PF6(10) DCM rt 30% (2.1:1 mixture) 2 [CpRuL3]PF6(10) DCM 60 36% (1.4:1 mixture) 3 Pd2(dba)3(5) Tol 120 37% (0.8:1 mixture) 4 Pd2(dba)3(10)+PPh3(20) Tol 120 36% (1:2.5 mixture) 5 Pd(PPh3)4(10) THF rt 33% (1:2.5 mixture) 6 Pd(PPh3)4(10) Tol 120 25% (1:10 mixture) 7[d] Pd(PPh3)4(10) Tol 120 28% (1:10 mixture) [a] Conditions: 4(0.4 mmol) in solvent (0.04 M), overnight (o/n), sublimated KOtBu before use. [b] Isolated yield of chromatographic mixture. [c] Crude 1H NMR ratio. [d] 1 mmolscale. L=CH3CN. Angewandte Chemie Communication Angew. Chem. Int. Ed. 2024,63, e202414017 (5 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License kinetically favor the formation of (α,α,α)-TTBA 2vs (α,α,β)- TTBA 2(ΔΔG‡=1.1 kcalmol1). The alternative non-potassium-mediated cyclotrimerization of tribenzoCOTyne 1was also analyzed by DFT calculations based on previously described mechanisms for thermal cyclotrimerizations of linear acetylenes that would involve tandem processes of both stepwise [2+2] and subsequent [4+2] cycloadditions.[64] Not unexpectedly, formation of the kinetic (α,α,α)-TTBA 2could not be explained on the basis of this non-potassium-mediated pathway (Figure S13).[57] In conclusion, we have synthesized the two noninterconvertible conformers (α,α,α) and (α,α,β) of TTBA 2 in a remarkably stereodivergent cyclotrimerization of tribenzoCOTyne by controlling reaction conditions (K+-mediated vs Pd-catalyzed). The (α,α,α)-TTBA 2is a fragment of cubic graphite since all of its sixty carbons are part of the structure of CG, and 36 constitute a bit of the C48 molecular cage of CG. Experimental and computational mechanistic studies suggest that potassium ion plays a key role as a template to favor the formation of the (α,α,α)–TTBA 2. Functionalizations and further applications of TTBA are currently in progress in our lab. Supporting Information Further details for optimization of the reactions, experimental procedures, compound characterization data, details for DFT calculations, cartesian coordinates, imaginary frequencies, and absolute energies in hartrees for all optimized geometries. The authors have cited additional references within the Supporting Information.[65–69,71, 75–77] Acknowledgements We acknowledge financial support from MICIN (projects PID2020-118048GBI00/AEI/10.13039/501100011033, PID2023-151279NBI00 and ORFEO-CINQA network RED2022-134287-T), the Xunta de Galicia (project ED431 C 2022/27 and Centro singular de investigación de Figure 4. Free energy profile for cyclotrimerizations of tribenzoCOTyne 1with and without mediation of K+. Energies are relative to tribenzoCOTyne 1combined with those of the relevant substrates. Angewandte Chemie Communication Angew. Chem. Int. Ed. 2024,63, e202414017 (6 of 8) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Galicia accreditation 2023–2027, ED431G 2023/03) and the European Union (European Regional Development Fund – ERDF). We are also grateful to the CESGA (Xunta de Galicia) for computational time. J. BG. thanks MICIN for a predoctoral contract. Conflict of Interest The authors declare no conflict of interest. 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Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2024, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202414017 by Uni Santiago Compostela, Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License