Synthesis of tricarbonyl(N-methylisatin)chromium(0) and an unanticipated transformation of a N-MEM to a N-MOM group

The synthesis of tricarbonyl(N-methylisatin)chromium(0) (6) was achieved by protection of the keto functionality of the ligand as an acetal followed by complexation with tricarbonyl(naphthalene)chromium(0) and subsequent deprotection with formic acid. In order to obtain a removable substituent at nitrogen, N-methoxyethoxymethyl (N-MEM) substituted isatin 12 was prepared. Upon acetalization with trimethylformiate in methanol under acidic reaction conditions the corresponding methoxymethyl (N-MOM) derivative was unexpectedly obtained. This substitution was highly accelerated by microwave irradiation. Complexation of the N-MEM and N-MOM substituted isatin afforded only poor yields. The addition of vinylmagnesium bromide at 6 takes place not only at the keto group but also at the lactam carbonyl group. Divinylation at either one of the carbonyl functions was also achieved with the product distribution being highly dependent on the reaction time.