Selenides, aryl vinyl alkylation

M-1 s-1 alkyl phenyl selenides, aryl bromides, vinyl bromides, a-chloro esters, a-thiophenyl esters 104-102M 1 s-1 alkyl chlorides, alkyl phenyl sulfides, a-chloro and a-thiophenyl ethers. 

Lithiated allylic sulfoxides may be a-alkylated and the resulting products subjected to [2,3]-sigmatropic rearrangement induced by a thiophile to give allylic alcohols (eq 43). In contrast, alkenyl aryl sulfoxides produce a-lithiated species which are alkylated with Mel or PhCHO in good yields (eq 44). LDA has also been used to metalate allylic and propargylic selenides as weU as aryl vinyl selenides. ... 

The a-deprotonation of vinylic aryl selenides is strongly dependent on the substituents [4]. For phenyl vinyl selenide, the reaction was successfully carried out with LDA or KDA at -78°C in THF (Scheme 50, reaction 1). When a alkyl group is present, the use of LDA in THF leads to the abstraction of both vinyl and allyl protons. The a-deprotonation was improved using KDA in THF at -78°C (Scheme 50, reaction 2). With two )9,)9 -alkyl groups, the metalation occurred only at an allylic position (Scheme 50, reaction 3). 

The effects of aryl groups were kinetically analyzed by comparing the rate constants of both steps (ki for oxidation step and /C2 for elimination step) which were determined by NMR analysis of the concentration of vinyl selenides, the intermediate selenoxide, and allenic sulfones [16b]. This kinetic study indicates that the rates of both oxidation and elimination steps were accelerated by the introduction of an electron-withdrawing group. Such acceleration has been known in the overall selenoxide elimination as well as in the selenoxide elimination step of alkyl aryl selenides. As a result, it was disclosed that the ratio of these rate constants (/C1//C2) was closely related to the enantiomeric excess of the products the smaller the ratio, the larger the enantiomeric excess becomes. Thus, the introduction of o-nitrophenyl group as an aryl moiety, which suppresses sterically the racemization of the intermediate chiral selenoxide and accelerates the selenoxide elimination step, is necessary to achieve a higher asymmetric induction. 

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