Ligand, solvent, and deuterium isotope effects in Mn(III)-based oxidative free-radical cyclizations
作者:Barry B. Snider、Bridget A. McCarthy
DOI:10.1021/jo00075a015
日期:1993.11
Oxidation of beta-keto ester 1 with Mn(Pic)3 and Cu(OAc)2 affords bicycloalkane 8, not the expected alkene 7, which is formed in high yield with Mn(OAc)3 and Cu(OAc)2. A series of control experiments established that the most likely explanation for this observation is that Mn(PiC)2, but not Mn(OAC)2, reacts with the bicyclic radical 5 more rapidly than Cu(OAc)2 does. These studies also indicate the potential for improved yields from oxidative free-radical cyclization of deuterated substrates. For instance, reaction of beta-keto ester 1-d with 2 equiv of Mn(OAc)3 and no Cu(OAC)2 affords 65% of 8, whereas beta-keto ester 1 provides only 22% of 8 under the same conditions. Large kinetic isotope effects change the nature of the termination step and prevent the formation of acyclic radical 3 by intermolecular hydrogen transfer. Solvent and ligand effects on the oxidation of beta-keto ester 1, malonate 14, and acetylenic beta-keto ester 25 are described.