Regioselectivity in 2-Methylbutane Hydroxylation Mediated by FeO⁺ and FeO²⁺

The regioselectivity in FeO + - and FeO 2+ -mediated hydroxylation reactions of 2-methylbutane that involves primary (1°), secondary (2°), and tertiary (3°) carbon atoms is discussed from theoretical calculations at the B3LYP DFT level. The energetics for four kinds of hydroxylation reactions of 2-methylbutane that lead to primary, secondary, and tertiary alcohols in a two-step concerted mechanism and in a radical mechanism are calculated and analyzed. With respect to H atom abstraction, a concerted mechanism via the four-centered transition state is energetically more favorable than a radical mechanism via the linear transition state C···H····O−Fe in most cases, except for a case generating a tertiary carbon radical intermediate. The regioselectivity in the concerted mechanism prefers a secondary C−H bond dissociation and declines in the order 2° > 1° > 3°, whereas the regioselectivity in the radical mechanism prefers a tertiary C−H bond dissociation and declines in the order 3° > 2° > 1°. The kinetic isotope effects (KIEs) for the H atom abstraction from 2-methylbutane are analyzed on the basis of the transition state theory. Computed k H / k D values in the concerted mechanism do not show site dependence, while those in the radical mechanism have significant site dependence, the order of k H / k D values being 1° > 2° > 3°.