Catalytic Sml2 reagent systems

The majority of reactions using Sml2 require a stoichiometric excess of reductant. Due to the high molecular mass of Sml2 (MW = 404), a large amount of metal with respect to the substrate is required. Economic and environmental concerns have led to attempts to develop reagent systems that use a catalytic amount of Sml2 and stoichiometric quantities of an inexpensive 

The use of this protocol provided good to excellent diastereoselectivities in a range of intermolecular and intramolecular pinacol coupling reactions. 

Coordination of glyme to both Sm(II) and Sm(III) is believed to be important in enhancing the diastereoselectivity of the pinacol couplings in addition to facilitating the reduction of Sm(III) to Sm(II).28 

Representative procedure.2S Magnesium turnings (154 mg, 6.4 mmol) were added to a Schlenk tube and stirred under argon for 1 h. A solution of Sml2 (0.1 M solution, 4ml, 0.4 mmol, 10 mol%) in THF, dimethyldichlorosilane (0.05 ml, 

4 mmol) and tetraglymc (0.09 ml, 0.4 mmol) were added successively to the Schlenk tube. A combination of aldehyde (1 mmol) and dimethyldichlorosilane (0.1 ml, 0.8 mmol) in THF was added dropwise using a syringe pump at a rate such that the blue colour of the solution was maintained. After addition, the mixture was filtered and quenched with tetra-w-butylammonium fluoride (1.0 M in 10ml of THF) and washed with 10ml of brine. The solution was concentrated in vacuo and the crude product was extracted with diethyl ether and purified by silica gel chromatography. 

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