DIMERIZATION OF POLYPHENYLALLENE .2. FORMATION AND INTERCONVERSION OF CYCLOBUTANE DIMERS OF TRIPHENYLALLENE - STEREOSELECTIVITY OF ALLENE DIMERIZATION

Progressive heating of triphenyl-allene 1 leads first to the kinetic dimers 3b and 3c, then the less stable dimer 3c undergoes thermal opening, and so isomerizes to 3a and 3b, which can equilibrate further. Finally, the diradicals 2 produced by thermal reopening of these dimers 3 can give the dihydronaphthacene 4 through recombination involving the ortho position of a phenyl group and dehydrogenation. The kinetically formed diradical, which gives 3b and 3c by conrotatory closures, is shown to possess structure 2a, and this fact suggests that the meeting of two allene molecules 1 must lead to a transition state of T1,2 type, corresponding to optimum electronic and steric stabilization. The analysis of thermochemical data for the dimerization of aliene to 1,2-dimethylene cyclobutane demonstrates their compatibility with such a structure for the transition state. Stereochemistry of various polysubstituted allenes dimerizations appears to conform with the same factors of stability for the corresponding transition states. © 1979.