Skipped diynes—VII : Stereospecific thermal isomerization of a tetraethynyldimethylenecyclobutane

1,1-Bis(t-butylethynyl)-3-t-butylchloroallene (1) dimerizes stereospecifically at ca 25° to give Z - 1 - (t - butylchloromethylene) - 2- bis(t - butylethynylmethylene) - 3 - t - butyl - 3 - chloro - 4,4 -bis(t - butylethynyl)cyclob Z-I in the solvent bis(2 - ethoxyethyl) ether. Rate data for 100° are k = 111 × 10^?6 sec^?1, ΔH^≠ = 30·8 ± 1 kcal/mole and ΔS^≠ = 15 ± 3 eu. It is proposed that the dimerization of 1 produces a bisallyl diradical in an orthogonal conformation from which conrotatory paths lead to Z-I or trans-II. The observations on this system are used to construct a useful, if simplistic, approach to rationalize or predict product distributions in reactions such as allene dimerizations, 1, 2 - dimethylenecyclobutane rearrangements, etc, which proceed through the bisallyl diradical. On the graph which connects all of the species (or on the energy surface which contains all of the species) the first products should be those which are one allowed reaction step away from a given diradical. Applications and exceptions to this concept of favored kinetic control are discussed.