In a previous post, I described the work of Singleton on a simple hydroboration reaction. He found less regioselectivity than predicted by transition state theory. Further, trajectory computations suggested that dynamic effects were at play, and that some non-selective fast reactions were leading to the lower regioselection.

Pilling offers an alternative explanation based solely on RRKM (statistical) theory.¹ (Actually what is utilized is the stochastic energy grained master equation.) What he suggests is that there are hot intermediates (formed of a loose associate of BH₃ with propene) that react non-selectively before cooling. The cooled intermediates react very selectively (around 99%) to give the anti-Markovnikov product.

The upshot is that hydroboration – and by implication a whole lot of other seemingly ordinary chemistry – may in fact be much more complicated than we had previously thought. Standard transition state theory may not always apply, and trajectory analysis may not be enough!

References

(1) Glowacki, D. R.; Liang, C. H.; Marsden, S. P.; Harvey, J. N.; Pilling, M. J., "Alkene Hydroboration: Hot Intermediates That React While They Are Cooling," J. Am. Chem. Soc., 2010, 132, 13621-13623, DOI: 10.1021/ja105100f