No abstract

No abstract

No abstract

The approximate rate and stoichiometry of the reaction of excess pure potassium triisopropoxyborohydride, KIPBH, with 56 selected compounds containing representative functional groups under standardized conditions (tetrahydrofuran, 0 °C) was examined in order to define the characteristics of the reagent for selective reductions. Primary, secondary, and tertiary alcohols evolve hydrogen partially, even after a long period of time. Phenol also generates partial hydrogen, and the reactions of those amines and thiols studied with the reagent are very slow. Aldehydes and ketones are reduced rapidly and quantitatively to give the corresponding alcohols. Unlike sodium and potassium borohydrides, KIPBH is very stereoselective. 2-Methylcyclohexanone can be reduced to the correspondingly less stable isomer, ds-2-methylcyclohexanol, in a high ratio (91% cis isomer). Cinnamaldehyde is rapidly reduced to cinnamyl alcohol, and further reduction is very slow under these conditions. Anthraquinone is cleanly reduced to 9,10-dihydro-9,10-anthracenediol. Carboxylic acids liberate hydrogen only

No abstract

No abstract

The reaction in tetrahydrofuran of potassium hydride with representative B-alkoxy-9-borabicyclo[3.3.1]nonanes (B-OR-9-BBN) containing alkoxy groups with increasing steric requirements was examined in detail to establish the generality of this synthesis of the corresponding potassium 9-alkoxy-9-boratabicyclo[3.3.1]nonanes (K9-OR-9-BBNH) and the stereoselectivities of these new reagents for the reduction of cyclic ketones. For R = Me and n-Bu, the reactions with potassium hydride are very fast, almost instantaneous, even at 0 °C. However, the products are unstable and rapidly undergo redistribution, even in the presence of excess potassium hydride. Moderately hindered alkoxy derivatives, R = 2-Pr and 2-Bu, react somewhat slower (1 h at 0 °C and 25 °C, respectively) and the products are stable to redistribution. More hindered alkoxy derivatives, R = t-Bu, t-Am, Thx, require 24 h at 25 °C. Even more hindered alkoxy groups, R = 3-ethyl-3-pentyl and 2,4-dimethyl-2-pentyl, require even longer reaction times and higher temperatures. All reagents show high stereoselectivities in the reduction of cyclic ketones, with the stereoselectivities generally increasing with increasing steric requirements of the alkoxy substituent. The thexyl derivative appears especially favorable, with the byproducts of the reaction readily removed from the reaction mixture.

No abstract