Vinylation of cyclohexanone enolates using vinyl ether-iron complexes. Diastereoselectivity of carbon-carbon bond formation

Article abstract of DOI:10.1021/om00154a021

Cationic vinyl ether-iron complexes 1, isolated as stable BF₄^- salts, are readily available from α-halo acetals and ketals. The olefin ligand in these complexes is unsymmetrically bound to the metal, which accounts for the relatively low barrier for rotation about the formal double bond, which is observed in these substances (15-23 kcal/mol), and for the high regioselectivity with which nucleophiles add to the activated olefinic center. The salts 1 serve as vinylating, isopropenylating, and cis-propenylating reagents with cyclohexanone enolates. Reaction of 1a with cyclohexanone enolate gave a single diastereomeric product 5. This has been converted to the lactone 10, which establishes the relative configuration of the adjacent chiral centers in 5. The relative configuration of analogous chiral centers in the major diastereomer 16a, formed by the alkylation of 3-methylcyclohexanone enolate with 1a, has been established by a single-crystal X-ray diffraction study and shown to be identical with that in 5. Complex 1a also gives a single diastereoisomeric adduct with 6-methylcyclohexanone enolate, and this has been shown by ¹³C NMR studies to have the same relative configuration at the newly formed chiral centers as in 5 and 16a. Alkylation of 5-methylcyclohexanone enolate with 1b leads to a mixture of isopulegone and isoisopulegone, while alkylation of the kinetic enolate derived from 3-methyl-2-cyclohexenone with this complex yields isopiperitenone.