The mechanism and design of asymmetric co-arctate Br+ atom transfers between alkenes. A computational study

Ab initio calculations at the correlated MP2 and B3LYP/pVTZ level reveal th at a bromine cation transfer between two alkenes involves a symmetrically 4 -coordinated species which is an energy minimum in the gas phase energy pro file. Electron withdrawing and donating substituents on one alkene distort the position of the bromine respectively away from and towards the substitu ted alkene. Alkenes strained by incorporating a trans configuration into 6- 8 membered rings preferentially bind the Br cation, an equilibrium which ca n be tuned by using a combination of substituents and strain. The classific ation of such an atom transfer as a concerted coarctate reaction is discuss ed.