Theoretical studies on alkyne addition to molybdenum alkylidenes: The preference of alpha- and beta-additions

The addition of alkynes HC=CR to Mo(NH)(CH2)(OR')(2) (R = H, Me, Ph; R' = C H3, CF3) has been studied with both ab initio molecular orbital and density functional calculations. Geometry optimizations were carried out with the HF/3-21G, HF/HW3, and B3LYP/HW3 methods. The transition structures for thes e addition reactions are in distorted trigonal bipyramidal geometries, simi lar to those of alkene additions. The calculated activation enthalpy for HC =CH addition to Mo(NH)(CH2)(OR')(2) is about 10.3 kcal/mol for R' = CH3 and about 2.3 kcal/mol for R' = CF3, indicating a significant preference for a cetylene addition to Mo-(NH)(CH2)(OCF3)(2) over Mo(NH)(CH2)(OCH3)(2). These barriers are higher than those of the corresponding ethylene addition by a bout 2-4 kcal/mol, even though the reaction of acetylene is much more exoth ermic. The a-addition of HC=CR (R = Me, Ph) is found to be considerably mor e favorable than the beta -addition to Mo(NH)(CH2)(OR')(2). Interestingly, the a-addition has a lower activation energy, while the beta -addition has a higher activation energy, compared to that of the parent acetylene additi on. Thus, a-addition is intrinsically favored over beta -addition by over 4 kcal/mol. This preference is reduced by solvent effect. All these can be e xplained by a destabilizing interaction between the nonreacting pi -orbital of alkyne and one of the lone pairs on the imido nitrogen. The steric effe ct of the bulky ligands in the real catalysts is also investigated qualitat ively by the PM3 method. These studies give results in good accord with the experimentally observed regioselectivity.