Stereochemical Aspects in the Insertion by Alkylidenemethylene Carbenoids into the α-C-H Bond of Alkoxides

Article abstract of DOI:10.1021/jo00248a034

Primary alkoxides (R¹CH2OM; M=K or Li) when treated with haloalkenes (R³R⁴C=CXY; X=Cl or Br, Y=H or Cl) in the presence of n-BuLi in THF at 0 deg C gave allylic alcohols (R¹CH(OH)CH=CR³R⁴) through the insertion reaction of the corresponding alkylidenemethylene carbenoid () into the α-C-H bond of alkoxides.Secondary alkoxides (R¹R²CHOM), under similar reaction conditions, gave butyl adducts (R¹R²C(OH)C4H9) in addition to the insertion products.In particular, the C-H insertion of menthyl oxide proceeded without stereospecificity to give a mixture of the axial and equatorial insertion products.These results provided evidence for the hydride abstraction-recombination mechanism in the carbenoid insertion reaction.The regioselective, nonstereospecific insertion reaction was also observed when alkoxides were treated with separately prepared ((2,3-benzo-2-cyclohexylidene)chloromethyl)lithium at temperatures from -95 to -40 deg C.The absence of H-D scrambling in crossover experiments under these reaction conditions clearly showed that the hydride abstraction-recombination mechanism proceeded within a solvent cage.An inversion of configuration on the carbenoid carbon in the hydride abstraction step was proposed on the basis of the E/Z stereoselectivity in the insertion products.