Enolboration. 4. An examination of the effect of the leaving group (X) on the stereoselective enolboration of ketones with various R2BX/triethylamine. New reagents for the selective generation of either Z or E enol borinates from representative ketones

Abstract

Abstract : A smooth, rapid, quantitative and stereoselective enolboration of representative ketones to either Z or E enol borinates is achieved with many new R2BX/Et3N reagents. Representative B - X-9-BBN and Chx2BX reagents with various leaving groups, such as triflate, mesylate, iodide, bromide, and chloride, have been examined with representative ethyl ketones, such as diethyl ketone, ethyl isopropyl ketone, ethyl tert-butyl ketone, and propiophenone, as model ketones, in order to attain an understanding of the effect of the leaving group in controlling the enolate geometry. R2BX reagents with better leaving groups, such as triflate, mesylate, and iodide, favor the formation of Z enol borinates, whereas those with relatively poorer leaving groups, such as bromide, and chloride, favor the formation of E enol borinates. The steric requirements of R in R2BX and R'in C2H5COR' also contribute substantially to the control of enolate geometry. An unusual behavior of the iodide reagents, favoring the exclusive formation of the Z enol borinates, has been observed in the enolboration of EtCOt-Bu and EtCOPh. The achievement of an understanding of this important effect of the leaving group in R2BX, as well as the effects of steric requirements of the substituents on boron and ketone in controlling the enolate geometry, and also the discovery of new R2BX reagents for the stereoselective generation of either Z or E enol borinates from representative ethyl ketones, are emphasized in this exploratory study.