Cycloaddition Reactions of 1,3-Dimethylallene. Assignment of Stereochemistry of the Cycloadducts and Interpretation of Product Distributions

Article abstract of DOI:10.1021/jo00012a007

The structures of the cycloadducts formed in the (2 + 2) cycloaddition reactions of 1,3-dimethylallene (13DMA) with 1,1-dichloro-2,2-difluoroethene (1122), acrylonitrile (ACN), methyl acrylate (MAC), N-phenylmaleimide (NPMI), and diethyl fumarate (DEF) and maleate (DEM) and the product distributions have been interpreted on the basis of favored conformations for diradical formation and ring closure to the cycloadducts.The stereochemistry of the ring substitution in the cycloadducts derived from ACN and MAC has been assigned on the basis of the relative chemical shifts of H2, the relative magnitudes of the vicinal coupling constants between H1 and H2, the relative magnitudes of the cross-ring coupling constants between H1 and H3 and H4, which are highly dependent on the conformations of the substituted methylenecyclobutane ring systems, and, in the case of the ACN cycloadducts, the results of NOE studies.The stereochemistry about the exocyclic double bonds in the cycloadducts derived from 13DMA with 1122, ACN, and MAC has been assigned on the basis of the relative magnitudes of the allylic and homoallylic coupling constants between H1, H3, and H4 with H5 and CH3b, the interpretation of which is based on the results of conformational studies carried out by molecular mechanics and ab initio MO calculations on methylenecyclobutane (28), pseudoaxial and pseudoequatorial 2-methylmethylenecyclobutane (29a and 29e), ethylidenecyclobutane (30), and pseudoaxial and pseudoequatorial 2-methylethylidenecyclobutane (31a and 31e).The results of these calculations indicate that with 29 the pseudoaxial conformation 29a is significantly populated, while with 31 the pseudoaxial conformation 31a should be overwhelmingly populated.The trends in the relative chemical shifts and the magnitudes of the vicinal and long-range coupling constants observed in the NMR spectra of the cycloadduccts derived from the reaction of 13DMA with NPMI and DEF have allowed for the assignment of the stereochemistry about the exocyclic double bonds and the ring substitution of the cycloadducts formed in these reactions.The product distributions formed in the cycloaddition reactions of 13MA with ACN and NPMI have been analyzed by molecular mechanics calculations on the conformational preferences for formation of the diradical intermediates and on the conformational preferences of the diradical intermediates which lead directly to cycloadduct formation.The predictions of the molecular mechanics calculations are in agreement with the observed configurations in the cycloadducts formed from (R)-(-)-13DMA and ACN and consistent with the observed product distributions.Detailed analyses of the observed product distributions in the cycloaddition reactions of 13DMA with the various radicophiles lead to the conclusions that in the cycloaddition reaction with 1122 the anti,anti diradical intermediate is preferentially formed, while in the cycloadditions reactions with ACN, MAC, and NPMI the anti, syn diradical intermediates are preferentially formed.The differences in stereochemical ...