Selective Reductions. 38. Reaction of Thexylchloroborane-Methyl Sulfide Complex in Methylene Chloride with Selected Organic Compounds Containing Representative Functional Groups. Comparison of the Reducing Characteristics of Thexylchloroborane, Thexylborane, and Diborane

Article abstract of DOI:10.1021/jo00376a040

The approximate rate and stoichiometry of the reaction of excess thexylchloroborane-methyl sulfide complex, ThxBHCL*SMe2, with 56 selected organic compounds containing representative functional groups under standard conditions (methylene chloride, 0 deg C) were determined order to define the characteristics of the reagent for selective reductions.The selectivity of the reagent was also compared to the selectivities of thexylborane and diborane.Alcohols and phenol react with the reagent at a fast rate to evolve an equivalent of hydrogen without any further reduction .Amines and aliphatic thiols do not form any hydrogen, while benzenethiol shows partial hydrogen formation.Aldehydes and ketones are reduced rapidly and quantitatively to give the corresponding alcohols.Unlike thexylborane and diborane, the reagent shows good stereoselectivity toward cyclic ketones.For example, 2-methylcyclohexanone is reduced to the less stable isomer, cis-2-methylcyclohexanol, in a high ratio (99.9percent cis isomer at -78 deg C).Cinnamaldehyde is reduced rapidly to cinnamyl alcohol, and any further reduction of the double bond is very slow under these conditions. p-Benzoquinone reacts only partially with the reagent while anthraquinone is totally unreactive.Carboxylic acids liberate 1 equiv of hydrogen rapidly and are further reduced to the corresponding aldehydes in good yields and purity.Acid chlorides react sluggishly with the reagent to use 2 equiv of hydride, while acetic anhydride utilizes 3 equiv of hydride to yield acetaldehyde and ethanol.On the other hand, cyclic anhydrides, such as succinic anhydride and phthalic anhydride, react very slowly with the reagent.Esters are almost inert toward thexylchloroborane. γ-Butyrolactone and phthalide are only partially reduced under the reaction conditions.Isopropenyl acetate utilizes 3 equiv og hydride to yield the corresponding acetaldehyde and presumably the hydroboration product of propylene.Only a partial reduction of epoxides can be observed.Primary amides like caproamide and benzamide evolve 1 equiv of hydrogen, but further reaction is very slow.Tertiary amides are almost inert under these conditions.Capronitrile reacts with the reagent to use 2 equiv of hydride in less than 24 h, while the reaction between benzonitrile and thexylchloroborane is sluggish.Nitrobenzene and 1-nitropropane do not react with the reagent, while azobenzene reacts only partially.Azoxybenzene consumes 2 equiv of hydride in 48 h.Only a sluggish reaction between thexylchloroborane and cyclohexanone oxime or phenyl isocyanate can be observed.Pyridine does not react, while pyridine N-oxide utilizes 3 equiv of hydride.Of the sulfur compounds tested, only dimethyl sulfoxide is reduced by the reagent to form the corresponding sulfide, while other sulfur compounds, such as disulfide, sulfide, and sulfone, are inert under these conditions.Altough sulfonic acids evolve hydrogen, no further reduction is observed.