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The deepest minima, however, are localized above and below C 3 in (TTfZ)' and C5 (TfZT)', respectively.With this particular behaviour of the electrostatic potentials one may associate a more or less restricted torsional motion of carbanion and proton donor in the transition state. The deepest minima correspond to a restricted torsional motion whereas the cylindrical potentials allow for almost free internal rotation, the vinylic minima representing intermediate situations. The additional degree of rotational freedom associated with proton approach to Cs in (TITl)' and to C3 in (TI2-I)' implies a relative increase of activation entropy (approximately A S = + 12 J K -' mol-' '3 and consequently a decrease of Gibbs energy ( -TAS of the order of 4 kJ/mol at room temperature). So the "acetylenic" approach channels relative to the others should lead to higher rate constants (approximately 3 4 times ) if barrier height differences are negligible. ConclusionsWith respect to the kinetics of the isomerization of (2113) and (1222) conclude that within the framework of our simple model we do not observe a positive correlation of experimental rates of product formation with a qualitative estimate of relative barrier heights. However, a qualitative estimate of activation entropies does agree with the experimental observations. Other authorsz4 have also stressed the importance of the shape of the potential although they did not explicitly _ _ _ _ rdate this to the entropy term in transition state theory. Whether entropy terms alone are responsible for the observed behaviour still remains to be seen, since reliable conclusions ought to be based upon a quantitative treatment of the activation parameters, particularly in this case where energy and entropy terms tend to oppose each other. From that point of view it should be realized that the chosen model is unable to lead to quantitative results due to the oversimplification of the proton transfer process. A more correct treatment includes the interaction of carbanion and proton donor which may affect both assumptions mentioned above. Structural changes in the carbanion associated with transfer of the proton might become important as well as differences in carbanion structure as a function of the position of the proton donor before transfer actually takes place. The possibilities are now being explored of handling this problem in a proper way within the restrictions imposed by the available computer time. AcknowledgementThe authors are indebted to Prof. Dr. J . F. Arem and Dr. P. Vernieer for their cooperation and to Dr. F. B. van Duijneoeldt for his interest and comments. Abstract. The synthesis of several plumbole derivatives is reported. Monocyclic compounds may be prepared by the reaction of organdailithio compounds with diorganolead dibromides. Spirocyclic compounds may be formed from the reaction of dilithio compounds with lead(l1) chloride. The compounds are characterized by 'H NMR and IR spectroscopy.

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