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Trimethylsumanene: Enantioselective Synthesis, Substituent Effect on Bowl Structure, Inversion Energy, and Electron Conductivity

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Trimethylsumanene: Enantioselective Synthesis, Substituent Effect on Bowl Structure, Inversion Energy, and Electron Conductivity 450 Bull. Chem. Soc. Jpn. Vol. 85, No. 4, 450­467 (2012) © 2012 The Chemical Society of Japan Selected Papers Trimethylsumanene: Enantioselective Synthesis, Substituent Effect on Bowl Structure, Inversion Energy, and Electron Conductivity Shuhei Higashibayashi,1 Ryoji Tsuruoka,1 Yarasi Soujanya,2 Uppula Purushotham,2 G. Narahari Sastry,2 Shu Seki,3 Takeharu Ishikawa,4 Shinji Toyota,4 and Hidehiro Sakurai*1 1Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki,Aichi 444-8787 2Molecular Modeling Group, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500607, India 3Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 4Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama 700-0005 Received October 3, 2011; E-mail: [email protected] C3 symmetricchiral trimethylsumanene was enantioselectively synthesized through Pd-catalyzed syn-selective cyclotrimerization of an enantiomerically pure iodonorbornenone, ring-opening/closing olefin metathesis, and oxidative aromatization where the sp3 stereogenic center was transmitted to the bowl chirality. Chiral HPLC analysis/resolution of the derivatives were also achieved. Based on theoretical calculations, the columnar crystal packing structure of sumanene and trimethylsumanene was interpreted as due to attractive electrostaticorCH­³ interaction. According to the experimental and theoretical studies, the bowl depth and inversion energy were found to increase on methylation for sumanene in contrast to corannulene. Dissimilarities of the effect of methylation on the bowl structure and inversion energy of sumanene and corannulene were ascribed to differences in steric repulsion. A double-well potential model was fitted to the bowl structure­inversion energy correlation of substituted sumanenes, with a small deviation. The effects of various substituents on the sumanene structure and bowl-inversion energy were analyzed by density functional theory calculations, and it was shown that the bowl rigidity is controlled by a combination of electronic and stericeffects of the substituents. The electron conductivity of trimethylsumanene was investigated by time-resolved microwave conductivity method, compared with that of sumanene. In the wake of the discovery offullerene, the chemical and in 2003. Since then, the chemical and physical properties of physical properties of buckybowls have attracted a great deal of sumanenes have been studied, including its bowl structure,6a interest because of theirunique bowl-shaped ³-conjugated face selectivity,6a bowl-inversion energy,4,6b,6c crystal pack- structure.1­3 The science of buckybowls has grown as a result ing,6a electron conductivity,6d metal complexes,6e­6h and optical of the development of practical routes for the synthesis properties.6i Sumanene derivatives, such as heterasumanenes,7 of compounds such as sumanene (1)4 and corannulene (2)5 have also been studied. With this background knowledge, we (Figure 1). Sumanene (1), which is the smallest C3v symmetric studied five aspects of C3 symmetrictrimethylsumanene (3), buckybowl, was first synthesized by solution-phase method as listed below. Direct Selective Synthesisof C3 Symmetric Substituted Sumanenes. One of the major difficulties in studying the properties of sumanene (1) is that of selective synthesisofits derivatives. Because sumanene is C3v symmetric, the selective synthesisof C3 symmetrictrisubstituted derivatives through functionalization at either methylene or benzene isdifficult to achieve. Functionalization of the parent compound 1 results inmixtures of regioisomers, with the desired C3 symmetric derivatives as minor products that are difficult to separate.6c,6i Because of the importance of C3 symmetrictrisubstituted derivatives for studies on the physical properties of sumanene (1) and its derivatives or for further transformation into ³- Figure 1. Sumanenes and corannulene. conjugated derivatives, functionalized sumanenes have to be Published on the web March 28, 2012; doi:10.1246/bcsj.20110286 S. Higashibayashi et al.Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012) 451 prepared by a direct route, rather than by derivatization of 1. of 1 and 3 are analyzed by theoretical calculations. Substituent Because gas-phase methods (flash vacuum pyrolysis) cannot effects on the sumanene skeleton are studied with experimental be used in the case of substituted buckybowls, a solution- data, and the effects of various substituents are predicted by phase method is the only available route.1 We will report means of calculations from first principles. full detailsof adirect selective synthesisof C3 symmetric Electron Conductivity. Sumanene (1) has a columnar trimethylsumanene (3)byasolution-phase route.8 crystal packing structure inwhich the molecules are stacked in Chirality Control of Chiral Buckybowls. Because of a convex-to-concave fashion with all the columns oriented in their three-dimensional bowl structure, buckybowls can possess the same direction. Because ofits stacking structure, sumanene chirality, bowl chirality, inasimilar manner to chiralfullerenes has a high electron mobility (0.75 cm2 V¹1 s¹1)with a large or carbon nanotubes. Bowl chirality can be induced by the ³- anisotropy (9.2 times) along the column axis, as measured by conjugated skeleton itself (as, for example, in hemifullerene), time-resolved microwave conductivity (TRMC).6d Although by the introduction of substituents, or by the replacement of the columnar crystal packing structures of buckybowls must be skeletal carbons by heteroatoms (as in heterabuckybowls). correlated with theirelectronic properties,1d these correlations Although many buckybowls possess this bowl chirality, no have not been studied until now. Transfer integralsof LUMO enantioselective synthesis or even a resolution of a racemate orbitals in the contacting buckybowls are predicted to play has been reported to date.9,10 Needless to say, the primary a crucial role in the effective transport of electrons along problem of enantioselective synthesisof buckybowls is that of the columnar stacking axes, and unidirectional stacking may introducing homochirality; a secondary problem that occurs provide a ratchet-type potentialfor electrons because of the with buckybowlswith relatively low bowl-inversion energies asymmetric lobes of the LUMO orbitals in the convex-to- is that of racemization through bowlinversion. These problems concave sides of the buckybowls. Because trimethylsumanene must be overcome if we are to achieve an enantioselective (3) was found to possess a columnar crystal structure with synthesisof atrisubstituted sumanene. In addition to the direct neighboring columns aligned in opposite directions, in contrast selective synthesisof trimethylsumanene (3), we have suc- to 1, the effect of column alignment on the electron transport ceeded in controlling the bowl chirality of 3 by means of properties of 3 was studied by the TRMC method. achiral transmission method. Racemization was inhibited l i i by performing operations and derivatizations at low temper- Resu ts and D scuss on atures.8a We also succeeded in resolving racemictrimethyl- Enantioselective Synthesisof Trimethylsumanene (3) and sumanenetrione (4) by means of chiral HPLC.8b With the Determination of Its Bowl-Inversion Energy. Our strategy enantiomerically pure or enantiomerically enriched chiral for the enantioselective synthesisof C3 symmetrictrimethyl- buckybowlin hand, the bowl-inversion energy could be easily sumanene (3) is based on a conversion involving a syn-selec- determined by measuring the racemization kinetics by means of tive cyclotrimerization of an enantiomerically pure norbornene circular dichroism spectroscopy, rather than by NMR methods, derivative to give a C3 symmetric syn-tris(norbornadieno)- which can only be used to determine the bowl-inversion energy benzene derivative, ring-opening/closing metathesis reactions of buckybowlswith diastereotopic protons. to form the nonconjugated bowl structure, and aromatization Substituent Effects on the Bowl Structure and the Bowl- to the conjugated bowl structure inwhich the chirality of Inversion Energy. Bowlinversion is one of the unique the sp3 stereogenic center is transformed into bowl chirality properties of buckybowls, and it has been extensively studied (Scheme 1).8a In the final chirality-transmission process, the since the determination of the bowl-inversion energy of a bowl-inversion energy of (C)-3 must be taken into account, corannulene derivative by Scott and co-workers.5b The effects because inversion from (C)-3 to (A)-3 corresponds to a race- of substituents on the bowl structure and the correlation mization process for the chiral buckybowl.12 The bowl- between the bowl structure and the inversion energy of inversion energy of 3 was estimated to be ca. 21 kcal mol¹1 corannulene (2)with that of many substituted corannulenes by comparison between the experimental value (¦G‡ = 20.3 have been studied by Siegel and co-workers.11 However, the kcal mol¹1 (303 K)6b)of 1 and the calculated values (B3LYP/ corresponding properties of substituted sumanenes have not 6-311+G(d,p): 18.3 and 19.2 kcal mol¹1)of 1 and 3, respec- been studied, mainly because oflimitations on the derivatiza- tively. Because this energy corresponds to a half-lifetime for tion of these compounds. On the basisof the experimental bowl racemization of about 2 h at 0 °C, the aromatization step must structures of
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