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Stereoelectronic and electronic effects of methyl substitution on the regiochemistry of ring opening in the conversion of cyclopropylcarbinyl to but-1-en-3-yl radicals has been reinvestigated. cis,trans-and trans,trans-2,3-dimethylcyclopropyl-3-d-l-carbinyl alcohols and tri-n-butylstannyl ethers have been prepared and converted to their corresponding cyclopropylcarbinyl radicals. ESR detection of the homoallylic radicals produced from these materials through hydrogen atom abstraction by photogenerated tert-butoxy radicals allows determination of ring-opening selectivities on the basis of differing a and @ splitting constants for H vs. D. The cis,trans-dimethyl-substituted radicals undergo conversion to homoallylic radicals by nearly exclusive ring opening of the cyclopropane u bond bearing the cis-methyl substituent. This preference is also measured by the use of product-analysis techniques. The tri-n-butylstannyloxy-substituted 2,3-dimethyl-substituted cyclopropyl-3-dcarbinyl radicals are generated by reactions of the corresponding aldehydes with tri-n-butyltin hydride initiated by AIBN. The homoallylic radicals generated by rearrangement are rapidly trapped by the tin hydride reagent, giving stannyl enol ether. Mass spectrometric analysis of 3-methylvaleraldehyde arising after methanolysis of the reaction mixture is used to determine, in an indirect fashion, ring-opening selectivities which demonstrate a 3.5:l preference for opening of the cis-methyl-substituted bond in rearrangement of the &,trans-dimethylcyclopropylcarbinyl radical. The results above along with those obtained on related systems from other laboratories are discussed in terms of stereoelectronic effects and frontier molecular orbital controls.

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