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

A highly stereoselective enolboration of tertiary amides has been,acco mplished for the first time with dicyclohexyliodoborane, Chx(2)BI. A s ystematic study of the enolboration of representative N,N-dialkylpropi onamides (CH(3)CH(2)CONR'(2)) with Chx(2)BI in the presence of various tertiary amines of variable steric requirements revealed an unusual a ldol stereoselectivity in different solvents and at different aldoliza tion temperatures. Both the nature of solvent and the aldolization tem perature influence the stereochemistry of enolboration, with the solve nt effect being greater than that of the temperature. Aliphatic and al icyclic hydrocarbon solvents favor formation of the syn aldols from th e enol borinates by aldolization at lower temperature (-78 degrees C), whereas most of the other solvents examined, such as aromatic and chl orinated aliphatic solvents, favor formation of the anti aldols by ald olization at relatively higher temperatures (0 or 25 degrees C). The r emarkable effects of both temperature and solvent in the case of terti ary amides raise a question about the validity of the previously assum ed constancy of the Z to syn and E to anti relationship, suggesting ei ther a possible isomerization of enol borinates with temperature or a different aldolization transition state with different solvent. While the effect of steric requirements of the dialkylamino group of the ter tiary amide does not contribute significantly to the stereochemistry, that of the amine exerts a considerable influence. The present study e stablishes a simple procedure for the stereoselective synthesis of eit her syn or anti aldols from representative tertiary amides merely by c hanging the : solvent and the aldolization temperature.