Sulfoxides substituted allylic systems

Allyl anion is too strongly basic to be studied as the free anion in solution. Bordwell developed an acidity scale based on equation 1 in dimethyl sulfoxide (DMSO) at 25 °C3 and applied the method to a number of more acidic substituted allylic systems. A summary of some results is shown in Table 10. DMSO is sufficiently polar that there is little ion... 

The first example of Pd-catalyzed asymmetric nucleophilic substitutions of chiral j8-sulfinyl allylic systems was reported in 1995. The reaction of a chiral olefinic sulfoxide (5s)-l with dimethyl sodiomalonate was carried out in THF at room temperature (r.t.) in the presence of Pd(OAc)2 (0.1 equiv) and a phosphine ligand (0.2 equiv), giving (5,5s)-(Z)-2 with 29-79% de (Scheme 1). The degree of the asymmetric induction was largely dependent on the phosphine ligand used. With dpph as a ligand, the highest enantioselectivity was obtained. 

Another type of substitution reaction is increasing in popularity—the use of an allylic substrate, such as an allyl acetate where the nucleophile is introduced with stereochemical control in the presence of a palladium catalyst and a chiral ligand. Reactions where a chiral anion, be it derived from a chiral heteroatom group, such as a sulfoxide, or an auxiliary, such as Evans s oxazolidinones, are not included in this chapter because the alkyl halide is usually relatively simple and the stereochemical selectivity is derived from the system itself. 

Induction in the chirality transfer from C-l to C-3 overrides that of the chirality transfer from the sulfur atom to C-3. Therefore, the chirality at the heteroatom only has an impact on systems lacking a stereogenic center at C-l. The two diastereomeric transition states for the rearrangement of such chiral sulfoxides are designated as exo and endo"1. These lead to the different enantiomers of the allylic alcohol. Their energy difference, which depends mainly on the substitution of the double bond, determines the enantioselectivity of the process. 

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