A -Epoxysilanes

The preparation and applications of a,0-epoxysilane a-anions have been described in detail105, as have the preparation and applications of a,0-epoxysilanes themselves106. 

In an unusual example of the epoxide moiety acting as a nucleophile, Hodgson and co-workers <02TL7895> have reported on a convenient method of deprotonating terminal oxiranes with lithium 2,2,6,6-tetramethylpiperidide (LTMP), followed by trapping of the anion with silyl-based electrophiles, to provide a,p-epoxysilanes in good yield. For example, chloro-epoxide 56 underwent clean conversion to epoxysilane 57 at 0 °C. This approach improves upon an earlier method, which employed sparteine derivatives at very low temperature (-90 °C) <01OL461>. 

Cyclohexanone (5.7 mmol) was added to a solution of the reagent (6.2 mmol) at -78 to -50°C. After 0.5 h at this temperature, it was allowed to warm to ambient temperature over 3 h. The mixture was poured into dilute HC1 (25ml, 0.5m), extracted with dichloromethane (3 x 30ml), and the organic extracts dried and concentrated, to give the epoxysilane (4.7mmol, 83%). 

To a stirred solution of the epoxysilane (1 mmol) in THF/water (3 ml, 4 1) was added perchloric acid (0.1 ml, 70%), and stirring was continued for 4 h at ambient temperature. The mixture was then poured into water (20ml), and extracted with dichloromethane (3 x 20 ml). Drying and concentration gave cyclohexane carboxaldehyde (0.71 mmol, 71%). 

This epoxysilane rearrangement can also be effected by use of a catalytic amount of MABR. Interestingly, treatment of epoxysilane 143 with 0.2 equiv. MABR facilitated smooth rearrangement at 25 °C to furnish silyl enol ether 144 in 74 % yield reaction of 143 with 2 equiv. MABR at -40 °C gave a-silylaldehyde 145 in 93 % yield (Sch. 116). 

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