An alternative syn-addition pathway in which 1b approaches the more hindered face of 2a was calculated to proceed by a slightly higher energy transition state TS-2 (G ‡ (298K) = 9.3 kcal mol -1 ). The concerted anti-addition pathway, while leading to the most thermodynamically stable product, G°(298K) = -34.2 kcal mol -1 , is prohibitively high in energy requiring distortion of the reactants into a twisted geometry in TS-3 (G ‡ (298K) = 30.7 kcal mol -1 ).