Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 72, Issue 6, Pages 2166-2173Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo062448+
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Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C-2-C-6 cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C-2-C-6 cyclization making it possible to suggest a lifetime for the intermediate diradical of > 1 x 10(-10) s (at 170 degrees C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT.
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