Enolboration. 7. Dicyclohexyliodoborane, a Highly Stereoselective Reagent for the Enolboration of Tertiary Amides. Effects of Solvent and Aldolization Temperature on Stereochemistry in Achieving the Stereoelective Synthesis of either Syn or Anti Aldols

Abstract

Abstract : A highly stereoselective enolboration of tertiary amides has been accomplished for the first time with dicyclohexyliodoborane, Chx2BI. A systematic study of the enolboration of representative N,N-dialkylpropionamides (CH3CH2CONR2) with Chx2BI in the presence of various tertiary amines of variable steric requirements revealed an unusual aldol stereoselectivity in different solvents and at different aldolization temperatures. Both the nature of solvent and the aldolization temperature influence the stereochemistry of enolboration with the solvent effect being greater than that of the temperature. Aliphatic and alicyclic hydrocarbon solvents favor formation of the syn aldols from the enol borinates by aldolization at lower temperature (-78 deg C), whereas most of the other solvents examined, such as aromatic and chlorinated aliphatic solvents, favor formation of the anti aldols by aldolization at relatively higher temperatures (0 or 25 deg C). The remarkable effects of both temperature and solvent in the case of tertiary amides raise a question about the validity of the previously assumed constancy of the Z to syn and E to anti relationship, suggesting either a possible isomerization of enol borinates with temperature or a different aldolization transition state with different solvent. jg