ENANTIOSELECTIVE CYCLOPROPANE SYNTHESES USING THE CHIRAL CARBENE COMPLEXES (S(FE))-C5H5(CO)(PR3)FE=CHCH3(+) AND (R(FE))-C5H5(CO)(PR3)FE=CHCH3(+) - A MECHANISTIC ANALYSIS OF THE CARBENE TRANSFER-REACTION

Enantiomerically pure or enriched iron-carbene complexes of the type C5H5(CO)(PR3)Fe=CHCH3+ have been prepared by three routes: (a) Diastereomeric acyl complexes C5H5(CO)(PPh2R*)FeC(O)CH3 (R* = (S)-2-methylbutyl) have been prepared, separated by column chromatography, and converted by using standard techniques to (S(Fe),S(p)- and (R(Fe),S(p)-C5H5(CO)(PPh2R*)Fe=CHCH3+; (b) Enantiomerically enriched (76% ee) (R(c-alpha)-C5H5(CO)2FeCH(OCH3)CH3 has been prepared from (S)-(-)-ethyl lactate and converted to enantiomerically enriched diastereomers Cp(CO)(PR3)-FeCH(OCH3)CH3 (R = Me, Et). The individual diastereomers were then converted to enantiomerically enriched ethylidene complexes C5H5(CO)(PR3)Fe=CHCH3+ (R = Me, Et); (c) Racemic acyl complexes Cp(CO)(PR3)FeC(O)CH3 (R = Me, Et) have been conveniently resolved via fractional crystallization of diastereomeric hydroxy carbene salt generated by using (S)-(+)- or (R)-(-)-10-camphorsulfonic acid. The enantiomerically pure acyl complexes were converted to the corresponding enantiomerically pure carbene complexes (S(Fe))- and (R(Fe))-C5H5(CO)(PR3)Fe=CHCH3+ by using standard techniques. Enantioselective ethylidene transfer from these complexes to styrene, vinyl acetate, and isopropenyl acetate gave methylcyclopropanes in high optical yields. Ethylidene complexes C5H5(CO)(PR3)Fe=CHCH3+ (R = Me, Et), C5H5(CO)-(PPh3)Fe=CHCH3+, and C5H5(CO)(PPH2R*)Fe=CHCH3+ (R* = (S)-2-methylbutyl) were generated in the CD2CL2 solution and studied by H-1 and C-13 NMR spectroscopy. At very low temperatures (ca. -100-degrees-C) both anticlinal (major) and synclinal (minor) isomers could be detected. Equilibrium ratios and rates of interconversion of these isomers were determined by using variable temperature H-1 NMR spectroscopy. A mechanistic analysis of the transfer reaction is presented by using the stereochemical results obtained coupled with deuterium labeling and relative reactivity studies. It is concluded that the most likely mechanism for carbene transfer involves reaction of the olefin with the minor but more reactive synclinal isomer of C5H5(CO)(PR3)Fe-CHCH3+ followed by backside attack of the developing electrophilic center at C-gamma on the Fe-C-alpha bond. A rationale is offered for the differing diastereoselectivities of ethylidene transfer from C5H5(CO)(PR3)Fe=CHCH3+ versus C5H5(CO)2Fe=CHCH3+ to various olefins.