Preference of 4-exo ring closure in copper-catalyzed intramolecular coupling of vinyl bromides with alcohols

The copper-catalyzed intramolecular O-vinylation of gamma-bromohomoallylic alcohols was investigated. With 10 mol % of CuI as the catalyst and 20 mol % of 1,10-phenanthroline as the ligand, the reactions of 3-bromo-3-buten-1-ols in refluxing CH3CN led to the convenient formation of the corresponding 2-methyleneoxetanes in good to excellent yields via a 4-exo ring closure. The configuration of the CC bond was nicely retained. This methodology was then successfully extended to the cyclization in 5-exo, 6-exo, and even 6-endo modes. Moreover, the competition experiments revealed that 4-exo cyclization is fundamentally preferred over other modes of cyclization. On the other hand, the corresponding Pd(0)-catalyzed O-vinylation showed the predominance of 5-exo over 4-exo cyclization.