Alkylaryl selenoxides

The thermal decomposition (pyrolysis) of alkylaryl selenoxides (selenoxide pyrolysis) to an alkene and an aryl selenic acid Ar—Se—OH often takes place even at room temperature (Figure 4.10). This reaction is one of the mildest methods for introducing a C=C double bond by means of a /3-elimination. The mechanism is described by the simultaneous shift of three electron pairs in a five-membered cyclic transition state. One of these electron pairs becomes a nonbonding electron pair on the selenium atom in the selenic acid product. The Se atom is consequently reduced in the course of the pyrolysis. 

Conveniently, the required alkylaryl selenoxide does not have to be isolated. Instead, it is produced in solution by a low-temperature oxidation of the corresponding alkylaryl selenide and eliminated in the same synthetic operation by thawing or gently heating the reaction mixture. 

Other phosphorous compounds, as shown in Table 8, were resolved by the same procedure. The three isomeric phosphinates 27b-d containing a methyl group attached to the aryl substituent could also be resolved, irrespective of the methyl position. From the related phosphine oxides 28a d, however, only those with R=H (28a) and R=m-CH (28c) could be well resolved no satisfactory resolution could be obtained for the other isomers of 28. The efficiency of the optical resolution of alkylaryl-substituted sulfoxides and selenoxides was found to depend similarly on the type of substitution on the aryl ring. 

Certain alkylaryl selenides can be prepared by the electrophilic selenylation of eno-lates (Figure 4.12 also see Table 10.4). With a subsequent H2Oz oxidation to produce the selenoxide followed by the elimination of Ph—Se—OH, one proceeds in a total of two synthetic steps from a carbonyl or carboxyl compound to its a,/3-unsaturated analogue. 

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