Allylic substitution deuterium labelling

The reaction of vinylic phenyliodium salts (57) with cyanide anions could be mistaken for a simple substitution reaction.59 However, the presence of both allylic (58) and vinylic (59) nitrile products suggests a more complex picture. Deuterium labelling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of iodobenzene and a 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate (60). H-shift occurs from the methylene carbon in preference to the methine carbon. The effects of substitution and different nucleophiles were examined. 

The addition processes are characterized as Type II allylation reactions and featnre open transition states. Denmark and Keck have described mechanistic studies, which have provided basic information shaping current views regarding the Type II allylations of organostannanes. Denmark s models are briefly summarized by the cyclization of deuterium-labeled 4 (Scheme 5.2.2). Intra-molecular allylation proceeds with Lewis and Brpnsted acids to yield 6 with high selectivity. The alcohol 6 is rationalized throngh the synclinal transition state 5 via anti-Su substitution. To a lesser degree, various Lewis acids also produce the... 

Desulfurization through a-cleavage was shown to be the major product for a series of compounds 34 illustrated below [30]. These compounds, both benzylic and allylic, consistently showed cleavage on the benzylic side but suffered E/Z isomerization in the enal products 36 and 37 when not constrained by substitution. Deuterium labeling was consistent with the internal hydrogen transfer proposed to reach the sulEne 35. 

FIGURE 9.25 Deuterium labeling is used to study allylic substitution. 

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