Diastereoselectivity in the McLafferty Rearrangement of Photoionized 3-Methyl Valeramide

The McLafferty rearrangement of photoionized 3-methyl valeramide proceeds quasi-barrierless and with high regioselectivity. The mass spectra of the stereospecifically labeled syn- and anti-[4-D(1)]-diastereomers reveal a strong preference for activation of the gamma-hydrogen/deuterium in anti-position relative to the methyl group at C(3), which serves as a steric marker. Quantitative analysis of the fragmentation patterns of other photoionized isotopomers permits the determination of primary and secondary kinetic isotope effects (KIEs), the branching ratios of competing McLafferty reactions, and the steric effect (SE) associated with transfer of the diastereotopic H(D) atoms at C(4). While the associated KIEs of the title reaction are negligible, the steric effect (SE = 2.9) is remarkably large for the otherwise flexible, monofunctional compound. The findings can be explained by a preferentially chairlike transition structure for the initial gamma-H atom transfer.