STEREOCHEMICAL FEATURES OF THE (2+2) CYCLOADDITION REACTIONS OF CHIRAL ALLENES .1. THE CYCLOADDITION OF ENANTIOENRICHED 1,3-DIMETHYLALLENE WITH THE MONOSUBSTITUTED ALKENES ACRYLONITRILE AND METHYL ACRYLATE

The various stereochemical features of the (2 + 2) cycloaddition reactions of symmetrically and unsymmetrically 1,3-disubstituted allenes with 1,1- and 1,2-disubstituted and monosubstituted alkenes are discussed. Experimental studies have been carried out for the case of the cycloaddition reaction of a symmetrically substituted chiral allene (1,3-dimethylallene, 13DMA) with the monosubstituted alkenes acrylonitrile (ACN) and methyl acrylate (MAC). The cycloaddition reaction of an excess of enantioenriched 13DMA with ACN produces four optically active cycloadducts, two retaining only a small amount of the enantiomeric excess (ee) of the starting 13DMA, and two cycloadducts retaining a significantly higher level of the ee of the starting 13DMA. The recovered unreacted 13DMA possesses within experimental error the same ee as that of the starting 13DMA, implying irreversible formation of the diradical intermediates. The results are discussed in terms of the preferred conformation of approach of the ACN and 13DMA to the activated complexes for the formation of the diradical intermediates, the preferred conformations of the intermediate diradicals and their intrinsic total asymmetry, possible racemization modes of the intermediates, and the effect of the regioselectivity of ring closure to form the cycloadducts. The ee's of the cycloadducts derived with MAC are significantly higher than those of the cycloadducts derived with ACN, which is attributed to a greater diastereoselectivity in progressing to the activated complexes with MAC.