Kinetic isotope effects enyne-allene cyclization

A study of kinetic isotope effects, theoretical calculations, and dynamic trajectories of the C(2)-C(6) (Schmittel)/ene cyclization of enyne-allenes has been reported.178 For (74), the isotope effect l=h/ l=d = 1-43 is consistent with a highly asynchronous... 

Kinetic isotope effects have been reported that suggest that the thermal cyclization of enyne-allenes proceeds through a stepwise diradical mechanism (Scheme 121). This is even true if steric bulk at the alkyne terminus is large, contrary to theoretical predictions.182... 

The evidence from kinetic isotope effect allows stepwise and synchronous mechanisms to be distinguished. A large kinetic isotope effect indicates that the thermal enyne-allene C -C cyclization proceeds along the diradical stepwise mechanism. This mechanism is even more pronounced when the alkyne is terminated by bulky substituents, which is... 

Scheme 3.36 Kinetic isotope effects in the thermal C -C cyclization of enyne-allenes.

The enyne-allene/cumulene transformation is not restricted to the Myers-Saito cyclization. The general analysis showed that the course of the cyclization of allene 3.968a (Scheme 3.146, where A = B = carbon) is determined by substitution in the acetylene branch and depends on electronic and steric factors, and the effect of stress in the cycle [424]. According to the study of the kinetic isotope effect, the formation of structure 3.970a can occur if there is at least one phenyl substituent in position 1 or 7. Depending on the substitution in the allene branch, there are several possible routes for the intramolecular reaction (formal [4+2] or [2+2] cycloaddition, or formation of ene-compound [424], which makes it possible to design a variety of ring systems. 

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