Nucleophilic 1,2-Schifts of Alkoxycarbonyl and Carboxylate Groups in the Benzilic-Acid Type Rearrangement of α,β-Dioxobutiric Esters

Article abstract of DOI:10.1002/hlca.19850680121

tert-Butyl α,β-dioxobutyrate (hydrate; 1d) undergoes, at medium or high pH, the benzylic acid rearrangement with exclusive 1,2-shift of the COO(t-Bu) group; the same is the true for the corresponding isopropyl ester 1c and ethyl ester 1b at high pH, whereas the overall picture of these reactions is complicated by concurrent hydrolysis of the ester, followed by a 1,2-shift of the COO^1- group.Consequently, the shift of these electron-attracting groups cannot be considered to be systematically disfavoured (compared, e.g., with alkyl-group shifts).Kinetic measurements of the rearrangement show for both esters (as well as for the analogous ethyl ester 1b, and also for ethyl 3-cyclopropyl-α,β-dioxopropionate (4)) a characteristic rate profile: at relatively low pH, k is proportional to 1->, approaching saturation with increasing 1-> (interpreted as complete transformation of the substrate into the hydrate monoanion), which is followed at higher pH by another rate increase with k proportional to 1-> (probably due to the reaction of the hydrate dianion).The similarity of k values for 1b-d shows that in the shift of COOR steric hindrance caused by R is negligible.