Thermal C²−C⁶ Cyclization of Enyne−Allenes. Experimental Evidence for a Stepwise Mechanism and for an Unusual Thermal Silyl Shift

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.