CHELE-CHELESELECTIVITY AND ANTI-CHELESELE CTIVITY, .5. ON THE STRUCTURE AND CHEMOSELECTIVITY OF SOME TRANSITION-METAL DERIVATIVES OF CARBOXYLIC-ACID ESTERS

On account of the IR spectra of EtO2CCH2FeCl (2a), EtO2CCH2CoCl (3a), and EtO2CCH(Et)CoCl (3b), prepared in situ by transmetalation of the c orresponding Li enolates, and EtO2CCH(Et)CrCl2 (5b), obtained in situ by one-electron oxidative addition of EtO2CCH(Et)Br to CrCl2, it is as sumed that these and similar complexes (2b, 2c, 3c) form 8-membered cy clic dimers of type 11 in THF in analogy to the Reformatzky reagents. Representatives of these complexes and of corresponding Ti complexes w ere treated (THF, mol ratio 1:1:1) with substrate pairs, consisting of a normal ketone and a beta-functionalized ketone (Table 2) or alterna tively of a normal ketone and an alpha-functionalized ketone (Table 3) . In the first case high anti-cheleselectivity was observed, but in th e second case high cheleselectivity. The reasons for this different be haviour are discussed.