STRUCTURAL EFFECTS ON [1,5]-SIGMATROPIC HYDROGEN SHIFTS OF VINYLALLENES

This study was directed toward developing a more quantitative understanding of ring size and sulfoxide anti-directing effects on the reactivity profile and/or stereochemical course of [l,5]-sigmatropic hydrogen shifts of vinylallenes. Regarding ring size effects, vinylallenes 13a-c were synthesized and subjected to kinetic studies. The results were not in simple agreement with an earlier prediction from this laboratory that the ring size which influences the migrating hydrogen trajectory distance is the primary determinant in the rate of vinylallene isomerization to hexatrienes. It is apparent that additional factors such as ring strain have to be taken into account. Regarding substituent effects on the π-facial stereochemical course of [1,5]—shifts, vinylallenes with various terminal allenic substituents were studied and it is concluded that the anti-directing effect imparted by a sulfoxide group and enhancement of this effect by a geminal alkyl substituent are general. Of all the substituents studied previously and in this investigation (alkyl, sulfide, sulfoxide, sulfone, phosphine oxide, phosphonate, and carbonyl), the sulfoxide remains the only substituent which imparts significant 7r-facial selectivity. © 1990, American Chemical Society. All rights reserved.