A Practical Solution to Stereodefined Tetrasubstituted Olefins

Olefins, compounds with a carbon–carbon
double bond, are
of fundamental importance, and stereodefined construction of tetrasubstituted
carbon–carbon double bond is a significant challenge. Here
we show a unique and practical method for the preparation of stereodefined,
fully substituted olefins via conjugate addition of organozinc reagents
to readily available 2,3-allenals. Through mechanistic studies, it
is confirmed that the geometry of the newly formed double bond is
controlled by unique regiospecific oxygen-protonation of the enolate
intermediates, generating 1,3-alkadienols. Such alkadienols would
undergo a concerted 1,5-H-transfer reaction via a six-membered transition
state to ensure the configuration of the carbon–carbon double
bond in the final products. Using the readily available organozinc
reagents and 2,3-allenals provides a very rapid access to a wide range
of tetrasubstituted olefins with defined stereochemistry, bearing
an extremely versatile aldehyde functionality.