A regioselective and stereospecific synthesis of allylsilanes from secondary allylic alcohol derivatives
Primary and secondary allylic acetates and benzoates react with the dimethyl (phenyl)silyl-cuprate reagent to give allylsilanes, provided that the THF in which the cuprate is prepared is diluted with ether before addition of the allylic ester. The reaction is reasonably regioselective in some cases: (i) when the allylic system is more-substituted at one end than the other, as in the reactions 4→5 and 9→10; (ii) when the steric hindrance at one end is neopentyl-like, as in the reactions 15→16; and (iii) when the disubstituted double bond has the Z configuration, as in the reactions Z-19→E-21 or, better, because the silyl group is becoming attached to the less-sterically hindered end of the allylic system, Z-20→E-22. The regioselectivity is better if a phenyl carbamate is used in place of the ester, and a three-step protocol assembling the mixed cuprate on the leaving group is used, as in the reactions 23→24 and E- or Z-29→E-21, or, best of all, because the silyl group is again becoming attached to the less-sterically hindered end of the allylic system, E- or Z-30→E-22. This sequence works well to move the silyl group onto the more substituted end of an allyl system, but only when the move is from a secondary allylic carbamate to a tertiary allylsilane, as in the reaction 38→39. Allyl(trimethyl)silanes can be made using alkyl- or aryl-cuprates on trimethylsilyl-containing allylic esters and carbamates, as in the reactions 40→41, and 43→44. The reaction of the silyl-cuprate with allylic esters and the three-step sequence with the allylic carbamates are stereo-chemically complementary, the former being stereospecifically anti and the latter stereospecifically syn. Homochiral allylsilanes can be made by these methods with high levels of stereospecificity, as shown by the synthesis of the allylsilanes 54, 58 and 59.