934009-08-2Relevant academic research and scientific papers
Thermal C2-C6 cyclization of enyne-allenes. Experimental evidence for a stepwise mechanism and for an unusual thermal silyl shift
Schmittel, Michael,Mahajan, Atul A.,Bucher, Goetz,Bats, Jan W.
, p. 2166 - 2173 (2007/10/03)
Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C2-C6 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 C2-C6 cyclization making it possible to suggest a lifetime for the intermediate diradical of > 1 × 10-10 s (at 170 °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.
