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-D1]-diastereomers reveal a strong preference for activation of the γ-hydrogen/deuterium in anti-position rel...

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Bibliographic Details
Published in:Journal of organic chemistry 2005-02, Vol.70 (3), p.1073-1076
Main Authors: Loos, Jessica, Schröder, Detlef, Schwarz, Helmut
Format: Article
Language:English
Subjects:
Chemistry
Exact sciences and technology
Kinetics and mechanisms
Organic chemistry
Reactivity and mechanisms
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Summary: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-D1]-diastereomers reveal a strong preference for activation of the γ-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 γ-H atom transfer.
ISSN:0022-3263
1520-6904
DOI:10.1021/jo0482458