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Concerning the Stereochemical Course and Mechanism of the Photochemical 1,3‐Acetyl Shift in a β,γ‐Unsaturated Ketone. Preliminary Communication
Author(s) -
Sadler David E.,
Hildenbrand Knut,
Schaffner Kurt
Publication year - 1982
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19820650714
Subject(s) - chemistry , racemization , singlet state , stereospecificity , ketone , radical , photochemistry , atmospheric temperature range , methyl radical , enantiomer , excited state , medicinal chemistry , stereochemistry , organic chemistry , thermodynamics , catalysis , physics , nuclear physics
Enantiomerically enriched samples of 1,2‐dimethyl‐3‐( 2 H 3 )methyl‐2‐cyclopentenyl (1) and 1‐( 2 H 3 )methyl‐2,3‐dimethyl‐2‐cyclopentenyl methyl ketones ( 2 ) have been irradiated at 313 nm in methanol in the temperature range +50 to −45°. The 1,3‐acetyl shift, which interconverts the two isomeric ketones, occurs with a small change in the enantiomeric composition and independently of temperature in the range studied. This change corresponds to an upper limit of approximately 20% reaction with racemization. It is proposed that reaction occurs from both the S 1 ( n , π*) and T 2 ( n , π*) excited states with stereospecific reaction from S 1 (rapid primary geminate recombination of a singlet radical pair, with a possible contribution by a concerted 1,3 shift) dominating throughout the temperature range, but with the proportion of reaction from T 2 increasing as the temperature is lowered. The racemization results from secondary geminate recombination of the singlet and triplet radical pairs and the random recombination of free radicals. Viscosity effects are proposed to explain the independence of the racemization on temperature.