Study of the Substituted Vinylallene-Methylenecyclobutene Electrocyclic Equilibria. Comparison with the Butadiene-Cyclobutene and Bisallene-Bismethylenecyclobutene Electorcyclic Equilibria

Article abstract of DOI:10.1021/jo00278a010

A number of substituted vinylallenes and 3-alkylidenecyclobutenes have been prepared and their electrocyclic reactions studied.The electrocyclic ring-closure equilibria of the parent vinylallene, 1,2,4-pentatriene (5), has been determined at a number of temperatures in the gas phase, and the thermodynamic parameters have been calculated.Theoretical calculations have been carried out on 5a, 5s, and 6 at the fully geometry optimized 6-31G* level with further single-point calculations being carried out at the MP2/6-31G^* level and with zero-point energy corrections.Thermodynamic calculations on the 5-6 system have also been carried out using Benson's method.The correlation between the experimental and calculted thermodynamic parameters is excellent, indicating that 6 is enthalpically favored, while 5 is entropically favored.These results are compared with similar calculations on the butadiene-cyclobutadiene and bisallene-3,4-bisalkylidenecyclobutene systems in which in the former the butadiene is heavily enthalpically favored and in the latter the bisalkylidenecyclobutene is heavily enthalpically favored.The observed trends in the electrocyclic equilibria are discussed in terms of the relative heats of formation of the open-chain compounds which differ substantially.With the 2-substituted 1,3,4-hexatrienes 9, 11, and 13, equilibrium constants were obtained in the gas phase at 360 and 435 deg C, allowing for the calculation of the equilibrium thermodynamic parameters.However, with the 1-substituted systems 15 and 19 the corresponding ring-closed products 16 and 20 could not be detected.Heating 15c or 15t produces 18.The transformation of 15t to 18 involves isomerization of 15t to 15c,presumably via 16, which undergoes electrocyclic ring closure to 17 followed by a <1,5>-hydrogen sigmatropic rearrangement.Heating a mixture of 19c and 19t at 360 deg C produces a mixture of 21 and 22, and a third compound tentatively assigned structure 23.The transformation of 19t to 19c is believed to proceed via 20, which then undergoes a <1,5>-sigmatropic rearrangement to 21 followed by ring closure to 22.Substituents in the 4-position of the 3-isopropylidenecyclobutene apparently destabilize the cyclic structures, which shifts the equilibria in favor of the acyclic trienes, which when having a group at the 1-position are able to undergo other types of reactions.