A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes.

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.