The stereochemistry (inversion vs. retention) of reactions at sp3-carbanion centers is one of the pressing problems in organic chemistry. This review describes our recent studies on the stereochemistry of carbanion reactions using enantiomerically defined α-(alkoxy) organolithiums generated from the enantio-enriched stannanes via Sn/Li exchange : (1) A new practical method for asymmetric synthesis of α-hydroxystannanes is described. (2) The [2, 3]-Wittig rearrangement was shown to proceed with complete inversion of configuration at the Li-bearing migrating terminus. (3) The carbolithiative cyclization is proved to proceed with complete retention of configuration at the Li-bearing carbon. (4) The [1, 2]-Wittig rearrangement is shown to proceed predominantly with inversion of configuration at the Li-bearing terminus and retention at the migrating center, wherein a significant level of mutual enantiomer recognition is observed in the radical recombination process.
The catalytic applications of Nafion-H, a superacidic perfluororesinsulfonic acid in organic synthesis are reviewed. Nafion-H, the acid form of a commercial Nafion-K ionomer, is a highly acidic (H0≥-12) solid catalyst superior to conventional resinsulfonic acids such as sulfonated polystyrenes in its catalytic activity, thermal stability and chemical resistance. Nafion-H catalyst is effective in a wide range of liquid and gas phase reactions, including electrophilic substitutions on aromatic nuclei, dehydration of alcohols, rearrangements, polymerizations and so on. Nafion-H and related perfluoroalkanesulfonic acids are not only much stronger acids but are also stable in corrosive environments and at temperature up to 210°C. Furthermore, specific advantages of using insoluble resins as catalysts include i) ease of by-product separation from the main reaction product usually by simple filtration; ii) prevention of intermolecular reaction of reactive species or functional groups by simulating high dilution conditions; iii) the possibility of reusing recovered reagents as well as eliminating the use of volatile or noxious substances.
Optically active bis(oxazolinyl)pyridine (Pybox) was developed as a chiral nitrogen ligand for asymmetric hydrosilylation of ketones. Pybox reacted with RhCl3 in ethanol solution to produce a stable RhCl3 (Pybox) complex, which exhibited powerful catalytic activity for the hydrosilylation of ketones with diphenylsilane in the presence of silver tetrafluoroborate. Acetophenone was reduced to 1-phenylethanol in 91% yield and in 94% ee (S). Other simple ketones were reduced to optically active secondary, alcohols in ca. 90% ee on the average. Substituents on the oxazoline rings and on the pyridine ring were also examined to verify the yield and the enantioselectivity due to their steric and electronic effects.
Details of a study on the intramolecular bis-silylation of C-C multiple bonds promoted by a new catalyst system, palladium-tert-alkyl isocyanide, are described. With a disilanyl ether derived from a homoallylic alcohol, intramolecular regioselective addition of the Si-Si linkage to the C=C bond took place to furnish an exo-ring closure product, i.e., 1, 2-oxasilolane. The bis-silylation of alkenes having substituents α to the C=C bond gave trans-3, 4-disubstituted oxasilolanes, while substitution β to the C=C bond favored cis-3, 5-disubstituted oxasilolanes. The 1, 2-oxasilolanes thus produced stereoselectively were oxidatively converted to the corresponding 1, 2, 4-triols. The present methodology for the synthesis of 1, 2, 4-triols was successfully extended to the stereoselective synthesis of 1, 2, 4, 5, 7- and 1, 2, 4, 6, 7-pentaols through a sequence of the intramolecular bis-silylation. The bis-silylation was also performed with alkenes linked to disilanyl groups through a 3-carbon chain and through an amide linkage. Alkenes tethered to disilanyl groups through chains of 2 atoms underwent similar intramolecular bis-silylation. The intramolecular bis-silylation of an alkyne tethered to a disilanyl group furnished a bis-silylated exocyclic olefin. Subsequent hydrogenation from the less-hindered side of the ring produced cis-disubstituted oxasilolane, providing an alternative method for the stereoselective synthesis of 1, 2, 4-triols. The intramolecular bis-silylation under high pressure led to the efficient synthesis of a new cyclic tetrakis(organosilyl)ethene, which possessed a non-twisted structure.
The importance of the stereodivergent synthesis of both enantiomers of β-lactams is ever increasing in connection with the structure-activity relationship study and the development of new derivatives in the β-lactam antibiotics. In this account stereocontrol at the C-3 and C-4 carbons of β-lactams using metal ester enolate-imine condensation reactions is discussed. The addition of an achiral ester enolate with a chiral imine possessing a dioxolane ring derived from (2S, 3S)-1, 4-dimethoxy-2, 3-butanediol as a chiral auxiliary is presented, in which 4-monosubstituted, 3, 4-disubstituted, and 3, 3, 4-trisubstituted β-lactams are prepared in a highly stereodivergent manner simply by switching metal ester enolate species. The application to the stereoselective synthesis of 1β-methylcarbapenem from 4-monosubstituted β-lactam is conducted by the use of selective hydrogenation reaction as a crucial step. The addition of an achiral ester enolate with a chiral imine having an auxiliary derived from phenylglycine at the nitrogen and the reaction of a chiral ester possessing arylsulfinyl group as an auxiliary at the α-position to the carbonyl with an achiral imine are also described for the stereodivergent synthesis of each of β-lactams from a single chiral source.
Transition-metal complexes containing low-valent silicon and germanium ligands have attracted much attention because of their unique bonding modes as well as their possible intermediacy in various transformation reactions of organosilicon and -germanium compounds mediated by transition metals. This article describes (1) some reactions involving silylene and germylene complexes, in particular the fast intramolecular scrambling of substituents on the silicon atoms in silyl(silylene)iron complexes, and (2) the synthesis, structures, and properties of some new types of such complexes ; namely, donor-stabilized bis(silylene), (silylene)(germylene), and bis(germylene) complexes.