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.

Beyond frontier molecular orbital theory: a systematic electron transfer model (ETM) for polar bimolecular organic reactions.

Polar bimolecular reactions often begin as charge-transfer complexes and may proceed with a high degree of electron transfer character. Frontier molecular orbital (FMO) theory is predicated in part on this concept. We have developed an electron transfer model (ETM) in which we systematically transfer one electron between reactants and then use density functional methods to model the resultant radical or radical ion intermediates. Sites of higher reactivity are revealed by a composite spin density map (SDM) of odd electron character on the electron density surface, assuming that a new two-electron bond would occur preferentially at these sites. ETM correctly predicts regio- and stereoselectivity for a broad array of reactions, including Diels-Alder, dipolar and ketene cycloadditions, Birch reduction, many types of nucleophilic additions, and electrophilic addition to aromatic rings and polyenes. Conformational analysis of radical ions is often necessary to predict reaction stereochemistry. The electronic and geometric changes due to one-electron oxidation or reduction parallel the reaction coordinate for electrophilic or nucleophilic addition, respectively. The effect is more dramatic for one-electron reduction.

Microwave-based reaction screening: tandem retro-Diels-Alder/Diels-Alder cycloadditions of o-quinol dimers.

We have accomplished a parallel screen of cycloaddition partners for o-quinols utilizing a plate-based microwave system. Microwave irradiation improves the efficiency of retro-Diels-Alder/Diels-Alder cascades of o-quinol dimers which generally proceed in a diastereoselective fashion. Computational studies indicate that asynchronous transition states are favored in Diels-Alder cycloadditions of o-quinols. Subsequent biological evaluation of a collection of cycloadducts has identified an inhibitor of activator protein-1 (AP-1), an oncogenic transcription factor.