Challenges in Predicting Delta sub(rxn)G in Solution: The Mechanism of Ether-Catalyzed Hydroboration of Alkenes

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2014-02, Vol.118 (50), p.11768-11779
• Main Authors: Sandbeck, Daniel JS, Kuntz, Colin M, Luu, Christine, Mondor, Rachelle A, Ottaviano, John G, Rayer, Aravind V, Sumon, Kazi Z, East, Allan LL
• Format: Article
• Language: English
• Subjects: Accuracy; Adducts; Alkenes; Coupling (molecular); Damping; Hydroboration; Mathematical models; Solvents
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp507793v

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": B sub(2)H sub(6) + alkene in THF solvent. Brown's S sub(N)2/S sub(N)1 "dissociative" mechanism, of S sub(N)2 formation of borane-ether adducts followed by S sub(N)1 alkene attack, was at odds with Pasto's original S sub(N)2/S sub(N)2 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 S sub(N)2 second step. Two diboranes were tested: B sub(2)H sub(6), used by Pasto, and (9BBN) sub(2) (9BBN = 9-borabicyclo[3.3.1]nonane, C sub(8)H sub(15)B), used by Brown. The new entropy terms resulted in improved accuracy vs traditional techniques ( similar to 2 kcal mol super(-1)), but this accuracy was not sufficient to resolve the mechanism in the Pasto case.