Sulfones, vinyl stereoselective reduction

Transition-metal-catalyzed stereoselective reductions of vinylic sulfones with Grignard reagents are achieved with excess n-BuMgCl in tetrahydrofuran at room temperature.196,48 Better yields and selectivities are obtained with palladium catalysts [Pd(acac)2] than with nickel complexes such as [Ni(acac)2], especially if external ligands such as DABCO, triethylamine (TEA), or (n-Bu)2P are used.196,48 Nickel catalysts are used more often for reducing sulfonyl-1,3-dienes than palladium catalysts. When using this method, it is very important to remove the catalyst before isolation of the products in order to avoid isomerization of the... 

An interesting observation was made in the case of the 5-exo dig radical cyclization of the glucosyl pyridyl sulfone derivative 6 (Fig. 9b). An ( ) (Z)-stereoselectivity of greater than 50 1 was observed in the product, which was a fourfold increase in selectivity, compared with the previously reported tin hydride method. Closer examination of this reaction revealed that the stabilized vinylic radical was reduced to an alkenyl organosamarium species, as shown by a remarkably high incorporation of deuterium at this position (91%) when the reaction was performed in the presence of MeOD (Fig. 12). Compared with other studies on similar reductions of stabilized alkenyl radicals, this... 

In contrast, reductions using Al/Hg seem to be stereoselective at high reaction temperatures to afford exclusively the more stable E-alkene product. The equilibration of the anionic intermediates under the reaction conditions is responsible for this isomerization. The temperature is a crucial factor in controlling the stereoselectivity of the process, since the geometry of the vinylic sulfone can be maintained when the reduction is carried out at low temperature. Partial scrambling of configuration is observed upon increasing the reaction temperature (Eq. 23).41... 

A highly stereoselective synthesis of 1,2-disubstituted alkenes can be also accomplished via reductive desulfonylation of 1,2-disubstituted vinylic sulfones since both E- and Z-vinylic sulfones can be stereoselectively prepared by several methods.8 Although different reducing agents have been used for this purpose (Eqs. 24-28), the reduction of 1,2-disubstituted alkenyl sulfones by Na2S2C>4 is particularly efficient and highly stereoselective (Eq. 168). 

Both ( )- and (Z)-vinyl sulfones can be readily prepared by several methods, and it is therefore little wonder that their transformation to stereodefined alkenes has been investigated. Both isomers of vinyl sulfones can be obtained by elimination from erythro- or threo-JJ-tosyl sulfones [46], generated either by controlled reduction of j8-keto sufones [46, 118] or by varying the base in the coupling step [131]. The reduction of disubstituted vinyl sulfones by Na2S204, discovered by Julia et al. in 1982, is particularly efficient and highly stereoselective [93] (Table 3.9). 

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