Tetrahedron Letters vol. 38, no. 27, pp. 4787 - 4790
The Hammett plots obtained for the title-reactions exhibit a break, i.e. ρ acyl values decrease from 2.21 ~ 2.44 to 1.45 ~ 1.52 as the acyl substituent becomes a strong electron withdrawing group (σ > 0.6). Such a break in the Hammett plots is suggestive of a change in the reaction mechanism and strong evidence of a stepwise mechanism for the acyl-transfer reaction.