Masashi Yamakawa

CH/π Attraction: The Origin of Enantioselectivity in Transfer Hydrogenation of Aromatic Carbonyl Compounds Catalyzed by Chiralη6-Arene-Ruthenium(II) Complexes

The through-space CH/π attraction between the η6 -arene ligand on Ru and the carbonyl aryl substituent (see transition states in picture) plays a key role in the enantioselective transfer hydrogenation of aromatic carbonyl compounds with 2-propanol or formic acid, catalyzed by chiral η6 -arene-RuII complexes.

Pressure-dependent enantioselectivity in the organozinc addition to aldehydes in supercritical fluids

Abstract Amino alcohol-catalyzed asymmetric alkylation of benzaldehydes by dialkylzincs can be achieved in supercritical fluoroform (scCHF 3 ) and supercritical ethane (scC 2 H 6 ) with high conversion and enantioselectivity. The higher enantioselectivity in ethane compared to CHF 3 suggests that solvent polarity affects the reaction. The enantioselectivity in scCHF 3 is strongly pressure dependent.

Synthesis and characterization of 2-[di(cyclopentadienyl)zircona]-1-oxacyclopentanes. X-Ray crystal structure of [(η-C5H5)2ZrOCH2CH2CHMe]2

2-[Di(cyclopentadienyl)zircona]-1oxacyclopentanes have been synthesized, isolated, and characterized by spectroscopin and X-ray analyses which show that they are dimeric with five co-ordinated bent metallocene structures; the mechanisms of their thermal decomposition have also been studied.

Nickel(0) Catalyzed Reaction of Bicyclo[1.1.0]Butanes with Electron-Deficient Olefins

Among interesting bicyclic hydrocarbons, bicyclo[l.1.0]butanes are of particular interest since they are the most fundamental and probably the most highly strained. The first authentic bicyclo[1.1.0]butane derivative was reported by Wiberg in 1959.2 Since then a number of synthesis of this class of compounds have appeared and the abundance of the hydrocarbons allowed extensive studies of their physical and chemical properties.3 In spite of its excessively high strain energy (66 kcal/mol4), bicyclo[l.1.0]butane (1), the parent hydrocarbon, can exist rather stable and heating to around 200 °C is necessary for a considerable rate of isomerization. The thermal rearrangements of bicyclo.[l.1.0]butanes mostly proceed in such a fashion to preserve the central bond giving 1,3-butadiene derivatives. These products are readily rationalized in terms of an orbital-symmetry allowed [σ2S + σ2a] process. In contrast to such thermal rearrangement, many reactions of bicyclobutanes with electrophilic reagents such as electrondeficient olefins, alcohols, carbonyl compounds, iodine, etc. involve cleavage of the central bond.5 Another reaction which breaks the central bond is the radical polymerization.6 The extraordinary ability of transition metal ions and complexes to effect isomerizations is also worth noting.7 Historically, transition metal-promoted reaction of bicyclo[1.1.0]butanes appears to come from the hydrogenolysis in the presence of platinum or palladium catalysts.8 The common feature of the reaction is the cleavage of the central and one of the four neighboring a bonds and concurrent absorption of two moles of hydrogen, giving linear C4 products (eq 1). Now various transition metals such as Cu(I),9a Ag(I),9b Pd(II),9c Pt(IV),9d Rh(I),9e Ir(I),9a Fe(0),9f and Ru(II)9d are known to be active catalysts for the isomerization. In most of the reactions, the geminal two-bond breakage is occurring as had been observed for their hydrogenolysis to give 1,3-dienes (eq 2).

Preparation, characterization and thermal reactions of 1-oxa-2-ziconacyclopentanes and 1-oxa-2-zirconacyclohexane: crystal structures of [{Zr(C5H5)2(OCH2CH2CHMe)}2] and [Zr(C5Me5)2{OCH2(CH2)2CH2}]

1-Oxa-2-zirconacyclopentanes [{[graphic omitted]H2)}2](R = H, 3a; or Me, 3b) and [{[graphic omitted]HMe)}2]3c(cp =η-C5H5) formed from the intramolecular hydrozirconation of [Zr(cp)2H(Cl)] with potassium allyl or homoallyl alcoholates have been characterized as dimeric five-co-ordinated metallocene structures. Complex 3c crystallizes in space group P21/n(Z= 2) with a= 17.090(4), b= 8.345(3), c= 8.841(3)A, β= 102.90(2)°, and R= 0.039 for 2030 reflections. Monomeric [[graphic omitted]H2)]9 and [[graphic omitted]H2}]12 have been prepared by the reaction of appropriate Grignard reagents with [Zr(C5Me5)2Cl2]. Complex 12 crystallizes in space group Pnn2 (Z= 4) with a= 15.634(2), b= 15.024(3), c= 9.571(1)A, and R= 0.074 for 1336 reflections. Remarkable differences in the thermal stability and modes of decomposition have been observed among these complexes depending on the co-ordination numbers of the zirconium atoms and the size of the metallacycles. The mechanisms of the thermal reaction of 3 are discussed on the basis of the product analysis and by use of the deuteriated derivatives.

A molecular orbital study of carbomethoxycarbene and dicarboxycarbene

Abstract The MINDO / 3 calculations on the title carbenes suggest that the nonclassical, closed structures are favored in gas phase or under externally unperturbed conditions.