1.2-Eliminations thioaldehyde formation

A novel synthesis of a-unsaturated sulfines has been introduced by Bra-verman et al. [99]. Et3N or DABCO treatment of allylic and benzylic tri-chloromethyl sulfoxides triggered the elimination of chloroform and formation of the sulfines. It must be stressed that these sulfines are thermally relatively stable, and this stands in high contrast to the corresponding thio-carbonyl compounds unsaturated thioaldehydes cannot be monitored under the same experimental conditions and have to be used at very low temperature or trapped in situ. The first synthesis of thioacrolein S-oxide was achieved by flash vacuum thermolysis of an anthracene allyl sulfoxide [100], and both isomers in a (Z E) ratio of 78 22 were characterised by NMR spectroscopy at -60 °C. 

In the a-elimination mechanism (39) as shown in Equation 9, hydrogen extraction by the base alpha to the sulfur results in formation of a carbanion which may rearrange via route a or b c to give cysteine and a thioaldehyde which decomposes in alkaline solution to an aldehyde and hydrogen sulfide. Thus, the stoichiometry of this reaction is (Equation 10) ... 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

A valuable route to metallocene complexes is through a cyclometalla-tion reaction with removal of a p-proton from the substrate and elimination of an alkane or arene. Typical of this approach is the formation of zirconocene 1-butene 4 (Scheme 5.3). The C-H activation method avoids the problem of dimerisation of the ir-component since there is none of this initially free of the metal. The method also facilitates the formation of complexes of components which are unstable in the free state, such as ben-zyne, cycloalkynes, and thioaldehydes. It is normally successful for crosscouplings when the component generated on the metal is strongly bound. If this is not the case, exchange with the second ir-component may take place, as described for zirconocene 1-butene (Scheme 5.3), giving the unwanted dimer or mixtures (see Section 5). 

---