A nickel-catalyzed
reductive dimerization of bromocyclobutenes
to produce unusual and unprecedented cyclobutene dimers was developed.
In a stereoconvergent procedure, various bromocyclobutenes were readily
dimerized in good yields, with good diastereoselectivities and broad
functional group tolerance. Notably, the presence of a carbonyl group
in the starting material appears to dictate diastereoselectivity.