AN ABINITIO STUDY OF VINYLALLENE CONFORMATIONS

Ab initio optimizations and MP2/6-31G* single-point calculations are performed for the polyene compounds (Z)-l,3-pentadiene (5), 1,2,4-pentatriene (6), and (Z)-l,2,4-hexatriene (7). The s-trans conformer is found to predominate in all cases; however, the preference is less for the vinylallenes than for either butadiene or pentadiene. Compound 6 was found to be planar in both the s-trans and s-cis conformations; however, nonplanar twist structures for 5 and 7 were found at lower energy than the planar s-cis. As further evidence of steric crowding in the s-cis conformation, the methyl group of compounds 5 and 7 are found to stagger rather than eclipse the double bond in the s-cis conformation. The reduced methyl rotation barrier in the s-trans pentadiene and hexatriene is found to be a result of steric repulsions and reduced π interactions between the out of plane methyl hydrogens and the π bond. Geometries, orbital energies, and intramolecular interactions are examined with a view to understanding the enhanced reactivities of the vinylallenes. In these, few differences between the normal dienes and the vinylallenes are found. © 1990, American Chemical Society. All rights reserved.