Alkene-Stabilising Groups

Overall, the transformations are equivalent to carbene additions to the styrenes. However, a carbene mechanism can be ruled out since the only alkenes which are successful are those carrying anion-stabilising groups. 

Substituted alkenes, e.g. (13>, can be made by the Wittig reaction the more reactive phosphonate ester (14) is often used when there is a stabilising group present. 

In a case where betaine formation is reversible, the thermodynamically more stable isomer will be formed before elimination occurs. This is normally (3.112b), which leads to trans alkene. Generally trialkylphosphonium yUds, or those containing stabilising groups, give mainly trans alkenes. 

Depending on the nature of the sulfur or phosphorus compound used, the product R2S = O or R3P = O may undergo a number of further reactions with ROOH groups, all of which convert the hydroperoxide group into an alcohol. These compounds tend to be only weakly effective so are generally used in conjunction with synergistic promoters. Suitable mixtures are used to stabilise a variety of polymers including poly(alkenes), ABS, and poly(stryrene). 

Finally, the major structural features in the substrate promoting E2 elimination are those that serve to stabilise the resultant alkene or, more particularly, the T.S. that precedes it. Such features include increasing alkyl substitution at both a- and //-carbon atoms (leading to alkenes of increasing thermodynamic stability), or introduction of a phenyl group that can become conjugated with the developing double bond. 

An excellent application of the distinction between stabilised and unstabilised ylids is in the synthesis of leukotriene antagonists.10 The intermediate 39 (R is a saturated alkyl group of 6, 11 or 16 carbon atoms) was needed and disconnection of the Z-alkene with a normal Wittig reaction in mind followed by removal of the epoxide exposed a second alkene with the E configuration that could be made from the aldehyde 43 and the stabilised ylid 42. 

As discussed later, others have noted formation of three- and four-membered ring products from phenyl-, silyl or alkynyl-substituted alkenes, which generate more stabilised product organolithiums, or from compounds bearing allylic leaving groups.111... 

Stepwise reactions by way of diradical intermediates are also possible, as in the coupling of the halogenated alkene 6.102 with butadiene 6.103. As we saw in Chapter 2 (see pages 67-68), any group, C, Z or X, stabilises a radical. Both radical centres in the intermediate 6.104 are stabilised, the one at the top by the o-chlorines and the /3-fluorines, and the one below because it is allylic. These combine rapidly 6.104 (arrow on the upper drawing) to give the cyclobutane 6.105. 

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