Computational studies on complex reaction mechanics

Molecular modeling with density functional and higher level methods was used to study mechanisms for the reactions of carbonyl compounds with ozone, the Doering-Moore-Skattebol Rearrangement, bis-lactam cyclizations, and thermal rearrangements of ortho-ethynyltoluene. Ozonation of both formaldehyde and acetone can proceed by one of two slow pathways: stepwise addition across the carbonyl group or hydrogen atom abstraction. The Doering-Moore-Skattebol Rearrangement proceeds as a triangular lithium-halogen carbenoid, opening stereospecifically to an allene, with lithium-halogen dissociation occurring after the transition state. Bis-lactam cyclization is rapid, reversible, and thermodynamically controlled. The experimentally observed major product is confirmed by computations as thermodynamically most stable. Thermal rearrangements of o-ethynyltoluene proceed through competitive [1,2] and [1,5] H-shifts. Chrysene is formed as a minor product by dimerization of a novel intermediate, orthoxylallene

Computational Studies on a Carbenoid Mechanism for the Doering–Moore–Skattebøl Reaction

The reaction of geminal dihalocyclopropanes with metals or alkyllithiums affords carbenoids which undergo low-temperature ring opening to allenes; this is known as the Doering–Moore–Skattebøl reaction. DFT and CCSD­(T)//DFT computations have been used to model the structure, coordination state, and ring opening of 1-bromo-1-lithiocyclopropane as a model for cyclopropylcarbenoid chemistry. Both implicit (PCM) and explicit solvation models have been applied. Carbenoid ring opening is similar to the process predicted in earlier studies on cyclopropylidene. The initial disrotatory stereochemistry becomes conrotatory en route to the allene–LiBr complex. Predissociation of the carbenoid to cyclopropylidene + LiBr is not supported by computations. DFT computations predict modestly exergonic dimerization of the carbenoid, with or without solvation, and the dimer appears to be the most likely reactive species in solution. Predicted barriers to ring opening are only modestly affected by solvation or by dimer formation, remaining in the range of 9–12 kcal/mol throughout