Mechanistic Insight into Additions of Allylic Grignard Reagents to Carbonyl Compounds

Allylic Grignard reagents exhibit
high reactivity and low selectivity in additions to carbonyl compounds.
Additions of allylic Grignard reagents to carbonyl compounds were
investigated using prenylmagnesium chloride as a mechanistic probe.
When the carbonyl group is relatively unhindered, the addition proceeds
through a six-membered transition state with allylic transposition.
This process generally occurs with no diastereoselectivity because
the reaction rates approach the diffusion limit. With hindered ketones,
however, this pathway is disfavored, and the addition proceeds through
a transition state resembling that
of other Grignard reagents.