Crystallographic Determination of the Preferred Geometry for Intramolecular Hydrogen Atom Abstraction in 1,8-Cyclohexadecanedione
作者:
ThillairajJ. Lewis,
StevenJ. Rettig,
JohnR. Scheffer,
James Trotter,
期刊:
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals
(Taylor Available online 1992)
卷期:
Volume 219,
issue 1
页码: 17-28
ISSN:1058-725X
年代: 1992
DOI:10.1080/10587259208032113
出版商: Taylor & Francis Group
关键词: Solid state organic photochemistry;Crystal structure-reactivity relationships;Hydrogen abstraction geometry;Norrish type II reaction;Macrocyclic diketone
数据来源: Taylor
摘要:
The two carbonyl groups in 1,8- cyclohexadecanedione (1a) are separated by eight methylene groups on one side and six on the other, thus giving rise to two nonequivalent γ-hydrogen atoms for abstractionviathe Norrish type II reaction. X-ray crystallography reveals that, in the solid state, the γ-hydrogen atom on the longer methylene chain is more favorably situated for abstraction than that on the shorter chain, both in terms of abstraction distance (2.72 Å vs 2.99 Å) and angular relationship. In accord with this analysis, photolysis of diketone 1a in the solid state leads exclusively to a single cyclobutanol diastereomer resulting from abstraction of the stereo electronically favored hydrogen atom. In contrast, irradiation of compound 1a in solution affords a complex mixture of all six possible type II cyclization and cleavage products resulting from non-selective abstraction of both γ-hydrogens. The solid state results are interpreted in terms of the crystallographically determined conformation of diketone 1a, which favors stereoselective cyclization over cleavage.
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