Takahiro Sasamori

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

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.

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.

Synthesis and structure of stable 1,2-diaryldisilyne

A novel 1,2-diaryldisilyne, BbtSi≡SiBbt (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl), was synthesized as a stable compound by reduction of the corresponding 1,2-dibromodisilene, Bbt(Br)Si=Si(Br)Bbt. The spectral and structural features of this first stable 1,2-diaryldisilyne are revealed, and the Si≡Si triple-bond character is evaluated with the aid of detailed theoretical calculations. The triple-bond characters of BbtSi≡SiBbt and BbtGe≡GeBbt are compared based on experimental and theoretical results.

Synthesis and characterization of an extremely hindered tetraaryl-substituted digermene and its unique properties in the solid state and in solution

Abstract An extremely hindered digermene ( E )-Tbt(Mes)GeGe(Mes)Tbt ( 1 ; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Mes=mesityl) was synthesized. X-ray crystallographic analysis of the hexane solvated single crystal [ 1 ·0.5hexane] revealed that 1 has an extremely long germanium–germanium double bond [2.416(2) A] as that of a carbon-substituted digermene. The temperature-dependent change of UV–Vis absorption of digermene 1 in solution indicated the quantitative interconversion between 1 and the corresponding germylene Tbt(Mes)Ge: ( 3 ). The thermodynamic parameters (Δ H =14.7±0.2 kcal mol −1 and Δ S =42.4±0.8 cal mol −1 deg −1 ) for the dissociation of digermene 1 to germylene 3 were obtained from temperature dependence of the absorption of 1 . Since the reactivity of germylene 3 is much higher than that of digermene 1 , almost all the intermolecular reactions of 1 in solution proceeded via dissociated 3 . For instance, the reaction of 1 with oxygen in solution resulted in a non-stereospecific formation of the cis - and trans -1,3,2,4-dioxadigermetanes 11 and 7 via the initial formation of germanone 12 derived from oxygenation of the dissociated germylene 3 . In case of the oxidation in the solid state, however, digermene 1 reacted with O 2 without dissociation to give the corresponding trans -substituted 1,3,2,4-dioxadigermetane stereospecifically. The reaction of digermene 1 with W(CO) 5 (THF) was also examined to give the corresponding terminal tungsten complex of the dissociated germylene 3 , i.e. Tbt(Mes)GeW(CO) 5 ( 23 ), as a marginally stable orange yellow paste.

Reactions of Diaryldibromodisilenes with N-Heterocyclic Carbenes: Formation of Formal Bis-NHC Adducts of Silyliumylidene Cations

Reactions of stable 1,2-dibromodisilenes ((E)-Ar(Br)Si=Si(Br)Ar) with N-heterocyclic carbenes (NHC) afforded NHC-arylbromosilylene adducts or bromide salts of the corresponding bis-NHC adducts of the formal arylsilyliumylidene cations ([ArSi:](+)). In some cases, an NHC was able to replace a bromide anion in the coordination sphere of the arylbromosilylene-NHC adduct.

Synthesis, structures, and electronic properties of [8Fe-7S] cluster complexes modeling the nitrogenase P-cluster.

High-yield synthesis of the iron-sulfur cluster [{N(SiMe(3))(2)}{SC(NMe(2))(2)}Fe(4)S(3)](2)(mu(6)-S) {mu-N(SiMe(3))(2)}(2) (1), which reproduces the [8Fe-7S] core structure of the nitrogenase P(N)-cluster, has been achieved via two pathways: (1) Fe{N(SiMe(3))(2)}(2) + HSTip (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) + tetramethylthiourea (SC(NMe(2))(2)) + elemental sulfur (S(8)); and (2) Fe(3){N(SiMe(3))(2)}(2)(mu-STip)(4) (2) + HSTip + SC(NMe(2))(2) + S(8). The thiourea and terminal amide ligands of 1 were found to be replaceable by thiolate ligands upon treatment with thiolate anions and thiols at -40 degrees C, respectively, and a series of [8Fe-7S] clusters bearing two to four thiolate ligands have been synthesized and their structures were determined by X-ray analysis. The structures of these model [8Fe-7S] clusters all closely resemble that of the reduced form of P-cluster (P(N)) having 8Fe(II) centers, while their 6Fe(II)-2Fe(III) oxidation states correspond to the oxidized form of P-cluster (P(OX)). The cyclic voltammograms of the [8Fe-7S] clusters reveal two quasi-reversible one-electron reduction processes, leading to the 8Fe(II) state that is the same as the P(N)-cluster, and the synthetic models demonstrate the redox behavior between the two major oxidation states of the native P-cluster. Replacement of the SC(NMe(2))(2) ligands in 1 with thiolate anions led to more negative reduction potentials, while a slight positive shift occurred upon replacement of the terminal amide ligands with thiolates. The clusters 1, (NEt(4))(2)[{N(SiMe(3))(2)}(SC(6)H(4)-4-Me)Fe(4)S(3)](2)(mu(6)-S){mu-N(SiMe(3))(2)}(2) (3a), and [(SBtp){SC(NMe(2))(2)}Fe(4)S(3)](2)(mu(6)-S){mu-N(SiMe(3))(2)}(2) (5; Btp = 2,6-(SiMe(3))(2)C(6)H(3)) are EPR silent at 4-100 K, and their temperature-dependent magnetic moments indicate a singlet ground state with antiferromagnetic couplings among the iron centers. The (57)Fe Mössbauer spectra of these clusters are consistent with the 6Fe(II)-2Fe(III) oxidation state, each exhibiting two doublets with an intensity ratio of ca. 1:3, which are assignable to Fe(III) and Fe(II), respectively. Comparison of the quadrupole splittings for 1, 3a, and 5 has led to the conclusion that two Fe(III) sites of the clusters are the peripheral iron atoms.