Allenoates in Enantioselective [2+2] Cycloadditions: From a Mechanistic Curiosity to a Stereospecific Transformation

Identification of a novel catalyst–allenoate pair allows
enantioselective [2+2] cycloaddition of α-methylstyrene. To
understand the origin of selectivity, a detailed mechanistic investigation
was conducted. Herein, two competing reaction pathways are proposed,
which operate simultaneously and funnel the alkenes to the same axially
chiral cyclobutanes. In agreement with the Woodward–Hoffmann
rules, this mechanistic curiosity can be rationalized through a unique
symmetry operation that was elucidated by deuteration experiments.
In the case of 1,1-diarylalkenes, distal communication between the
catalyst and alkene is achieved through subtle alteration of electronic
properties and conformation. In this context, a Hammett study lends
further credibility to a concerted mechanism. Thus, extended scope
exploration, including β-substitution on the alkene to generate
two adjacent stereocenters within the cyclobutane ring, is achieved
in a highly stereospecific and enantioselective fashion (33 examples,
up to >99:1 er).