Challenges in predicting ΔrxnG in solution: the mechanism of ether-catalyzed hydroboration of alkenes

Ab initio (coupled-cluster and density-functional) calculations of Gibbs reaction energies in solution, with new entropy-of-solvation damping terms, were performed for the ether-catalyzed hydroboration of alkenes. The goal was to test the accuracy of continuum-solvation models for reactions of neutral species in nonaqueous solvents, and the hope was to achieve an accuracy sufficient to address the mechanism in the "Pasto case": B2H6 + alkene in THF solvent. Brown's SN2/SN1 "dissociative" mechanism, of SN2 formation of borane-ether adducts followed by SN1 alkene attack, was at odds with Pasto's original SN2/SN2 hypothesis, and while Brown could prove his mechanism for a variety of cases, he could not perform the experimental test with THF adducts in THF solvent, where the higher THF concentrations might favor an SN2 second step. Two diboranes were tested: B2H6, used by Pasto, and (9BBN)2 (9BBN = 9-borabicyclo[3.3.1]nonane, C8H15B), used by Brown. The new entropy terms resulted in improved accuracy vs traditional techniques (∼2 kcal mol(-1)), but this accuracy was not sufficient to resolve the mechanism in the Pasto case.