A FLUORINE-MEDIATED BORON REDUCING AGENT - SODIUM TRIS(PENTAFLUOROPHENOXY)BOROHYDRIDE - PREPARATION AND REACTION WITH SELECTED ORGANIC-COMPOUNDS CONTAINING REPRESENTATIVE FUNCTIONAL-GROUPS - FACILE DIASTEREOSELECTIVE REDUCTION OF SUBSTITUTED CYCLOHEXANONES

Addition of 3 equiv. of pentafluorophenol to a tetrahydrofuran (THF) suspension of sodium borohydride at 0-degrees-C resulted in the rapid and quantitative formation of sodium tris(pentafluorophenoxy)borohydride (NaTPFPBH) with concurrent evolution of 3 equiv. of hydrogen gas. The reducing agent NaTPFPBH is stable at 0-degrees-C for an extended period of time without having to remain in equilibrium with an alkali metal hydride. The approximate rate and stoichiometry of the reaction of excess pure NaTPFPBH with 41 selected compounds containing representative functional groups was examined in order to characterize the reducing agent for selective reductions. Primary, secondary and tertiary alcohols evolved 1 equiv. of hydrogen slowly over a period of 24 h at 0-degrees-C. Phenol also liberated hydrogen slowly and the reaction of hexylamine was very slow. Aldehydes and reactive ketones are reduced readily and quantitatively to yield the corresponding alcohols. Carboxylic acids generated hydrogen quantitatively without undergoing any further reduction. Esters, lactones and phthalides are essentially inert towards the reagent. Epoxides are not reduced by NaTPFPBH. Primary aliphatic and aromatic amides evolved hydrogen but no significant reduction occurred. Unlike sodium and potassium borohydrides, NaTPFPBH is very stereo- and regio-selective. 2-Methylcyclohexanone is reduced predominantly to the corresponding less stable isomer, cis-2-methylcyclobexanol. Switching the order of addition of sodium borohydride and pentafluoropbenol to reduce substituted cyclohexanones gave the thermodynamically more stable alcohols with an equatorial hydroxy group. Cinnamaldehyde and 2-cyclohexen-1-one are reduced readily to cinnamyl alcohol and 2-cyclohexen-1-ol, respectively,