Explaining Regio-Divergent Vinylations with Vinylbenziodoxolones

Vinylbenziodoxolones have recently been identified as efficient hypervalent iodine(III) reagents for electrophilic vinylations under transition metal-free conditions. The regiochemistry of the products have been found to vary with the nucleophile class, with thiols giving internal alkenes whereas phosphine oxides and similar P-nucleophiles give terminal alkenes. This paper constitutes the first mechanistic investigation of VBX vinylations, and makes use of NMR studies, deuterium label-ling and computations to rationalize the observed regio- and stereochemical outcome. While the S-vinylation was found to proceed through the ligand coupling mechanism typical of diaryliodonium salts, the P-vinylations took place via a phos-phinous acid-coordinated VBX complex, which underwent concerted deprotonation and Michael-type addition. Subsequent base-assisted protonation and E2 elimination delivered the terminal alkene. The findings can be used to predict the regiose-lectivity in vinylations of other nucleophile classes.