Reduction of ketosulfoxides with DIBAH

Reduction of ketosulfoxides with DIBAH in the presence of ZnCl2 

To a solution of p-ketosulfoxide (2 mmol) in THF (20 ml) was added anhydrous zinc chloride (1.1 eq) in solution in THF (20ml). After lh at room temperature the reaction mixture was cooled at -78°C, treated with DIBAH and then worked up as above. The p-hydroxysulfoxidc (R,R)-(3a) was obtained in an 80% yield, m.p. 99-102°C, (R,R)/(R,S) = 95 5. 

This step was necessary for the following metallation and alkylation. 

To a solution of p-hydroxysulfoxide (3a) (lmmol) in dichloromethane (20 ml) at room temperature was added m-CPBA (1.1 eq) in dichloromethane (20 ml). After 6h at room temperature, the reaction mixture was washed with a 5% aqueous solution of sodium hydroxide (20 ml) and then dried over sodium sulfate. After evaporation of the solvent the crude sulfone obtained in a 56-58% yield was used without further purification (m.p. 69°C). 

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The presence of a chiral sulfoxide group in the P-position allows the highly selective reduction of imines by Li 5-BU3BH or LiBEt3H [482, 1120] (Figure 6.20). Reduction of P-sulfonylimines with DIBAH/ZnCl2 at -78°C is also very efficient, and produces nonracemic amines after Raney nickel desulfurization [1121, 1122] (Figure 6.20). A chelated complex that is similar to the one involved in the reduction of p-ketosulfoxides ( 6.1.2.2) is proposed as a reaction intermediate. 

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