Origins of regio- and stereoselectivity in additions of phenylselenenyl chloride to allylic alcohols and the applicability of these additions to a simple 1,3-enone transposition sequence
Asymmetric Catalysis by Vitamin B<sub>12</sub>. The Mechanism of the Cob(I)alamin-Catalyzed Isomerization of 1,2-Epoxycyclopentane to (<i>R</i>)-Cyclopent-2-enol
作者:Pierre Bonhôte、Rolf Scheffold
DOI:10.1002/hlca.19910740706
日期:1991.10.30
The isomerization of 1,2-epcxycyclopentane (1) to enantiomerically enriched (R)-cyclopent-2-enol (2) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)-2 is usually ca. 60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B12
quantitative two-term description of memoryeffects arising in Pd-catalysed allylicalkylation reactions formally proceeding through 'meso'-type pi-allylpalladium intermediates is presented. The utility of this description (stereochemical convergence (sc) and global enantiomeric excess (ee(g))) is demonstrated by application to a series of Pd-catalysed allylicalkylation reactions involving racemic cyclopentenyl
H-2-Labelled and O-18-labelled cyclopentenyl esters (+/-)-4 and (+/-)-5 are used as probes for memory effects in Pd-catalysed allylic alkylation. H-2-Labelled alkylation product 6 arising from stereospecific Pd-catalysed reaction of (+/-)-4 was analysed by a novel C-13 NMR method involving H-2-isotope shifts and paramagnetic diastereotopic shifts. When catalysts bearing the Trost modular ligand (R,R)-3 were employed, variable memory effects were observed with the slower reacting chirality mismatched (R)-4 substrate-catalyst pairing. The memory effect is dependent on nucleofuge steric bulk and not pK(a). Attack by [LiCH(CO2CH3)(2)] occurs with reversed site selectivity but (R)-4 remains the mismatched substrate. Mismatched ionisation leading to a Pd-pi-allyl in which (R,R)-3 acts as a monophosphine ligand may explain the memory effect.
Mechanisms of the Base-Induced Isomerizations of Cyclopentene and Cyclohexene Oxides: Influence of Structure and Solvent on α and β Proton Removal