Computational Insight Concerning Catalytic Decision Points of the Transition Metal Catalyzed [2 + 2 + 1] Reaction of Allenes.

Rhodium and molybdenum catalyzed allenic [2 + 2 + 1] cycloaddition reactions give 4-alkylidene and α-alkylidene cylopentenones, respectively. The selective reaction of one double bond of the allene over another is controlled by the transition metal and not the substrate structure. Calculations were performed to explain this unique control element using the B3LYP functional as implemented in Gaussian 03. The 6-31G(d) basis set was applied to all elements except rhodium, which is described with the LANL2 effective core potential and the LANL2DZ basis set. The product-determining step for both reaction pathways is oxidative addition of the metal to the alkynyl allene to form the corresponding metallocycles B and B'. The transition state calculations strongly suggest that geometry constraints imposed by the metal in the transition state are the key controlling factor of the double bond selectivity. The transition state structure of rhodium-catalyzed oxidative addition has a distorted square planar geometry that affords a lower transition state energy when coordinated to the distal double bond of the allene. In turn, the distorted trigonal bipyramidal geometry of molybdenum in the transition state structure imposes conformational constraints upon binding to the distal double on the allene and thus leads to the energetically preferred complexation and reaction with the proximal double bond.