These results indicate a concerted reaction between all three reaction components rather than initial deprotonation or formation of K through a (3+2) cycloaddition in pathway VIII. Deuterium-labelled VBX reagents 1a-D and 1a-D2 were utilized in a series of experiments to study whether any rearrangement or protonation took place (Scheme 5). The reaction of 4a with 1a-D delivered monodeuterated products 5a-D in a 7:1 Z:E mixture as the only vinylation products observed (Scheme 5a).