Factors affecting facial selectivity in the hydroboration of steroidal ?5-Alkenes

A comparison between the a- and ß-facial selectivity observed in the hydroboration of some androst-5-enes and B-norandrost-5-enes does not parallel the difference between the calculated force field energies for a- and ß-cyclobutane models suggesting that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial p-complex between the alkene and the borane.