On the Catalytic Effect of Ethers during Hydroboration of Alkenes
East, Allan E. E.
Sandbeck, Daniel J. S.
Coupled-cluster and density-functional computations are used to test Brown’s hypothesis for the catalytic effect of ether solvents upon hydroboration of alkenes. Two diboranes are tested: B2H6 and Brown’s dimer (9-BBN)2 (9-BBN = 9-borabicyclo [3.3.1] nonane, C8H15B). Comparisons to Brown’s data are made for (i) infrared and 11B-NMR spectral features, and (ii) an equilibrium constant for adduct formation. Transition-state searches were performed for four SN2-type reaction steps, exploring a hypothesis published by Schleyer long ago. We concur with Brown regarding the value for the equilibrium constant for adduct formation, and explain why this should not be computed with traditional gas-phase ΔG estimates from computational chemistry codes. All 4 SN2-type transition states were located for B2H6 + tetrahydrofuran, but not all are expected to exist for the 9-BBN dimer due to steric hindrance for SN2 approach.