Vinyl sulphones nucleophilic addition

The formation of cyclopropanes from 7C-deficient alkenes via an initial Michael-type reaction followed by nucleophilic ring closure of the intermediate anion (Scheme 6.26, see also Section 7.3), is catalysed by the addition of quaternary ammonium phase-transfer catalysts [46,47] which affect the stereochemistry of the ring closure (see Chapter 12). For example, equal amounts of (4) and (5) (X1, X2 = CN) are produced in the presence of benzyltriethylammonium chloride, whereas compound (4) predominates in the absence of the catalyst. In contrast, a,p-unsatu-rated ketones or esters and a-chloroacetic esters [e.g. 48] produce the cyclopropanes (6) (Scheme 6.27) stereoselectively under phase-transfer catalysed conditions and in the absence of the catalyst. Phenyl vinyl sulphone reacts with a-chloroacetonitriles to give the non-cyclized Michael adducts (80%) to the almost complete exclusion of the cyclopropanes. 

Vinyl sulphones (384) are subjected to nucleophilic addition by a-lithionitriles and give cyclopropanes (385) together with 3-oxothian-1,1-dioxides (386). The mechanism is given in equation 127 . It is remarkable that reaction of the sulphone with sodium... 

KOMe [93% (all- )]. Notably, /-BuOK does not promote the elimination in this case. The mechanism of this process is different from that with the alkoxysulphones. The initial step is formation of a vinyl sulphone 50. Isomerization of this intermediate to the allylic sulphone 48 via 51 is effected by Michael addition and elimination of methanol. The ineffectiveness of t-BuOK is explained by this mechanism. This bulky reagent cannot abstract the proton from the a-carbon of the sulphonyl group because this is embedded in a sterically highly crowded environment. Moreover, r-BuOK does not participate in the Michael addition because of its weaker nucleophilic character. 

Hammett treatment showed that substitution in the phenyl ring of the sulphone influenced the reaction more than substitution in the thiol, indicating that the transition state resembles a carbanion intermediate. The second-order rate constant for the nucleophilic addition of p-MeCgH4S-to phenyl vinyl sulphone has been detected at 0-45°C, the energy, free... 

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