The cycloaddition reaction of N-phenylmaleimide (NPMI) with an excess of enantioenriched 1,3-dimethylallene (13DMA) has been carried out with varying initial enantiomeric excesses (ee's) of the 13DMA and differing concentrations of the reactants. The recovered unreacted 13DMA showed no loss in ee. The four cycloadducts have been separated by HPLC, and their ee's have been determined by the use of a chiral NMR chemical shift reagent. Two of the cycloadducts have retained approximately 80% of the ee of the starting 13DMA, one approximately 50%, and the fourth approximately 10%. Molecular modeling calculations have been carried out on the conformational energy surface for the approach of reactant models to the activated complexes for diradical intermediate formation, and on models for the diradical intermediates. A mechanism is proposed for the formation of the diradical intermediates and the cycloadducts involving three separate minimum-energy reaction channels for the formation of specific conformations of the diradical intermediates which undergo ring closure to specific cycloadducts. These minimum-energy reaction channels differ in the extent of the facial selectivity of attack on the NPMI during the irreversible formation of the diradical intermediates. The cycloaddition of 13DMA with dimethyl fumarate (DMFM) produces two major cycloadducts which have been formed involving > 96 and > 70% transfer of the ee of the starting 13DMA. Two very minor cycloadducts are also formed, one possessing approximately 74% of the ee of the starting 13DMA with the other minor cycloadduct being formed in such small quantities that is isolation and measurement of its ee could not be accomplished. Molecular modeling calculations have been carried out on the conformational energy surface of the two reactants in approaching the activated complexes for diradical intermediate formation and on the conformational preferences of the diradical intermediates. The detailed stereochemical analysis of the formation of the cycloadducts suggests that there are two major minimum-energy reaction channels involved in diradical intermediate and cycloadduct formation arising from the same low-energy conformation for approach of the reactants to two different activated complexes and diradical intermediates.
