Norihiro Tokitoh

Stable Heavier Carbene Analogues

In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

Recent topics in the chemistry of heavier congeners of carbenes

Abstract Several recent topics in the chemistry of divalent species of heavier Group 14 elements, i.e. the heavier congeners of carbenes, are outlined. As the main topics, the synthesis, structure, and properties of stable silylenes are described together with those of related divalent species of the other Group 14 elements. Also, some detailed descriptions are allotted to new aspects in the relationship between the divalent species and the corresponding double-bond species of heavier Group 14 elements. Both electronic stabilization by heteroatom-containing substituents and kinetic stabilization by bulky substituents play very important roles in the advances of the chemistry of these interesting low-coordinate species.

Concise Synthesis and Crystal Structure of [12]Cycloparaphenylene

bottom-up chemical synthesis of this simple molecular entity had been recognized as a Holy Grail in synthetic chemistry, three groups including our own have recently succeeded in synthesizing some [n]CPPs. Although these studies from the three research groups established the synthetic viability of the long-awaited CPPs, important issues remain unresolved (Scheme 1). For example, any synthetic route must be more concise, cost-effective, and scalable to provide CPP in useful quantities and to ensure that this interesting molecular entity is studied further. In addition, the molecular structure of CPP must be concretely validated by X-ray crystallographic analysis. We herein report a concise nickel-based synthesis of [12]CPP and the first X-ray crystal structure of [12]CPP. Some of the key features of the previous methods of making CPPs are summarized in Scheme 2. Both the group of Bertozzi and ours utilized the palladium-catalyzed Suzuki–Miyaura coupling of terphenyl-convertible bent

Synthesis and Reactions of a Stable 1,2-Diaryl-1,2-dibromodisilene: A Precursor for Substituted Disilenes and a 1,2-Diaryldisilyne

Synthesis and isolation of the stable diaryldibromodisilene, Bbt(Br)SiSi(Br)Bbt, has been accomplished for the first time. The dibromodisilene underwent substitution reactions with organometallic reagents on the low-coordinated silicon atom to afford the corresponding substituted disilenes. Furthermore, the reaction of 1 with t-BuLi afforded the corresponding 1,2-diaryldisilyne, BbtSi[triple bond]SiBbt, the characters of which were revealed by spectroscopic and crystallographic analyses.

Planar chiral tetrasubstituted [2.2]paracyclophane: optical resolution and functionalization.

We achieved optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane and subsequent transformation to planar chiral building blocks. An optically active propeller-shaped macrocyclic compound containing a planar chiral cyclophane core was synthesized, showing excellent chiroptical properties such as high fluorescence quantum efficiency and a large circularly polarized luminescence dissymmetry factor.

Regioselective Ru-catalyzed direct 2,5,8,11-alkylation of perylene bisimides.

Perylene tetracarboxylic acid bisimides (PBIs) are an important class of dyes and pigments for widespread practical use, which have been extensively investigated for a long time both in academia and industry. Recently, they have also received much attention as n-type semiconducting materials. Furthermore, owing to high fluorescence quantum efficiency and photostability, they have been a popular motif for single-molecule spectroscopy. Chemical modifications of PBIs are quite important to gain desirable photophysical and electronic properties as well as solubility. In spite of their rich material chemistry, functionalization of the perylene core of PBIs relies on halogenation of the bay area (1,6,7,12-positions) and subsequent transformations (Scheme 1). Selective functionalization at 2,5,8,11-positions remains unavailable to date. Here we wish to disclose the first selective synthesis of 2,5,8,11-substituted PBIs. We have envisioned the potential of direct functionalization of PBIs by organometallic and catalytic means: ruthenium-catalyzed C H bond activation and addition strategy, namely, the Murai–Chatani–Kakiuchi protocol (Scheme 2). This reaction can introduce alkyl substituents to the proximal position of the directing groups. Successful installation of alkyl chains at the 2,5,8,11-positions of PBIs would allow greater modification of properties in the solid state or condensed phase. The reaction procedure is quite simple. A mixture of bis(N-ethylpropyl)PBI 1a and trimethylvinylsilane was heated in mesitylene at 165 8C for 60 h in the presence of [a] Prof. Dr. H. Shinokubo Department of Applied Chemistry Graduate School of Engineering, Nagoya University Chikusa-ku, Nagoya 464-8603 (Japan) E-mail : hshino@apchem.nagoya-u.ac.jp [b] S. Nakazono, Y. Imazaki, Prof. Dr. A. Osuka Department of Chemistry, Graduate School of Science Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail : osuka@kuchem.kyoto-u.ac.jp [c] H. Yoo, J. Yang, Prof. Dr. D. Kim Department of Chemistry, Yonsei University Seoul 120-749 (Korea) E-mail : dongho@yonsei.ac.kr [d] Dr. T. Sasamori, Prof. Dr. N. Tokitoh Institute for Chemical Research Kyoto University, Kyoto 611-0011 (Japan) [e] T. C dric, Prof. Dr. H. Kageyama Department of Chemistry, Graduate School of Science Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail : kage@kuchem.kyoto-u.ac.jp Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.200901318. Scheme 1. Functionalization of perylene bisimides.

Recent advances in the chemistry of group 14-group 16 double bond compounds

Publisher Summary This chapter discusses the remarkable progress in the chemistry of heavy ketones leading to the systematic comparison of their structures and properties that are capable of existence as stable species if their highly reactive M=X bond is adequately protected toward dimerization. Kinetic stabilization is superior to thermodynamic stabilization to elucidate the intrinsic nature of heavy ketones. A more general account of the whole chemistry of stable double bonds between heavier Group 14 and Group 16 elements is reviewed. The structural features of their M=X bonds (M = Si, Ge, Sn; X = chalcogen atom), such as trigonal planar geometry and bond shortening compared to the corresponding M–X single bonds are similar to those of ketones, heavy ketones have much higher reactivity because of their weak π bonds. The systematic study on heavy ketones containing silicon through lead using steric protection with the Tbt group reveals that a heavy ketone containing lead, R 2 Pb═X, is unique in that it is less stable than its isomeric lumbylene RPb–XR.

Asymmetric reduction of nitroalkenes with baker's yeast

Abstract Various α,β-disubstituted and trisubstituted nitroalkenes were chemoselectively reduced with bakers yeast to the corresponding nitroalkanes. Stereoselectivities of the reduction of α,β-disubstituted nitroalkenes were modest to low, and e.e.s up to 52% were obtained. Trisubstituted nitroalkenes could be reduced to the corresponding nitroalkanes with excellent enantioselectivities, moderate diastereoselectivities and in good yield.

New aspects in the chemistry of low-coordinated inter-element compounds of heavier Group 15 elements

Abstract Recent results obtained in the studies of the author and co-workers on the synthesis and properties of doubly bonded systems between heavier Group 15 elements are described together with a brief historical survey on the chemistry of low-coordinated heavier Group 15 elements. The first stable distibene and dibismuthene were successfully synthesized by taking an advantage of kinetic stabilization using new bulky substituents and the spectroscopic studies and crystallographic analysis of them led to the systematic comparison of structural parameters and physical properties for all doubly bonded systems between heavier Group 15 elements from phosphorus to bismuth. In addition to these experimental data, theoretical calculations also revealed the intrinsic character of low-coordinated inter-element compounds containing heavier Group 15 elements, especially that of dibismuthene, i.e. the heaviest double-bond compounds of non-radioactive elements. Furthermore, a unique intermolecular crystalline-state reaction was observed in the oxidation of the overcrowded distibene and dibismuthene, the reaction process of which was successfully monitored by repeated measurements of the cell dimensions using an imaging-plate X-ray diffraction technique.

Reactivity of an aryl-substituted silicon–silicon triple bond: 1,2-disilabenzenes from the reactions of a 1,2-diaryldisilyne with alkynes

The reactivity of a diaryl-substituted disilyne, Ar-Si[triple bond, length as m-dash]Si-Ar, with alkynes was examined. Reaction of the disilyne with acetylene yielded a 1,2-disilabenzene as the sole product.