ENOLBORATION .3. AN EXAMINATION OF THE EFFECT OF VARIABLE STERIC REQUIREMENTS OF R ON THE STEREOSELECTIVE ENOLBORATION OF KETONES WITH R2BCL/ET3N - BIS(BICYCLO[2.2.2]OCTYL)CHLOROBORANE TRIETHYLAMINE, A NEW REAGENT WHICH ACHIEVES THE SELECTIVE GENERATION OF E-ENOLBORINATES FROM REPRESENTATIVE KETONES

A smooth, rapid, quantitative and stereoselective enolboration of a variety of ketones to E enolborinates is achieved with bis(bicyclo[2.2.2]octyl)chloroborane, Bco2BCl, a new reagent, in the presence of triethylamine in simple solvents such as diethyl ether, hexane, carbon tetrachloride, and methylene chloride. Representative R2BCl reagents with variable steric requirements have been examined, with 3-pentanone and propiophenone as model ketones, in order to understand the effect of the steric requirements of R in controlling the enolate geometry: (1) B-chloro-9-borabicyclo[3.3.1]nonane (B-Cl-9-BBN); (2) di-n-butylchloroborane (n-Bu2BCl); (3) bis(exo-norbornyl)chloroborane (exo-Nrb2BCl); (4) dicyclooctylchloroborane (Coc2BCl); (5) disiamylchloroborane (Sia2BCl); (6) dicyclohexylchloroborane (ChX2BCl); and (7) bis(bicyclo[2.2.2]octyl)chloroborane (Bco2BCl). R2BCl reagents with smaller R groups favor the formation of Z enolborinates, whereas those with bulkier R groups favor the formation of E enolborinates. The phenyl group also plays a significant role in favoring the E enolborinate in the case of propiophenone. The reagent 7, Bco2BCl, with appropriate steric requirements, provides the best results in generating E enolbonnates for both the model ketones. Consequently, this new reagent Bco2BCl, 7, was compared for a variety of ketones with the reagent 6, Chx2BCl, the best previously available reagent, to give E enolborinates. Surprisingly, Bco2BCl, achieves the E enolborinates either exclusively or with a higher selectivity than Chx2BCl. The enolborinates generated with Bco2BCl in the presence of triethylamine are highly reactive with aldehydes at temperatures as low as -78-degrees-C. The impressive efficiency of Bco2BCl in achieving the preferred synthesis of E enolborinates, together with its ease of preparation, handling, and greater stability, makes it a valuable reagent for the stereoselective enolboration of ketones. The important effect of variable steric requirements of R in R2BCl for the stereoselective enolboration of the model ketones and the application of Bco2BCl to achieve optimum formation of the E enolborinates from a variety of ketones are emphasized in this exploratory study.