Catechol borane

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

The adducts derived from catechol borane are hydrolyzed by water to vinylboronic acids. These materials are useful intermediates for the preparation of terminal vinyl iodides. Since the hydroboration is a syn addition and the iodinolysis occurs with retention of the alkene geometry, the iodides have the -configuration.214... 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

One common approach incorporates an oxazaborolidine-mediated catechol-borane reduction starting from a-ketophosphonates (146).155 The reaction proceeds with good yield and gives excellent ee (up to 99%). 

Catechol allenylboranes have also been used to synthesize homopropargylic alcohols [25], These reagents are prepared by hydroboration of an enyne with catechol-borane in the presence of a Pd(0) catalyst possessing monodentate phosphine ligands. Dienylboranes were formed as minor products. Optimum results were obtained by treatment of the catecholborane with molar equivalents of triphenylpho-sphine and the palladium catalyst. Although several allenylboranes were prepared, only the dimethyl reagent was further examined. Treatment of that borane with benzaldehyde afforded the homopropargylic alcohol in 62% yield (Eq. 9.21). 

Deoxygenation of carbonyl compounds (6, 98 7, 54 8, 79-80). This easily prepared borane is as effective as catechol borane for reduction of tosylhydrazones of carbonyl compounds to the corresponding methylene compounds. 

Obtained by catechol-borane reduction of 3,3-dimethyl-2-butanone-SAMP-hydrazone, followed by N-N bond cleavage. 

A new selective and very mild method for the reduction of ketone tosylhydrazones to hydrocarbons offers promise for steroid chemistry reduction with catechol-borane and buffered hydrolysis gives the hydrocarbon in good yield.193 2,4-Dinitrobenzenesulphonylhydrazine is superior to toluene-p-sulphonylhydrazine for... 

Catechol borane (50 mmol) was added dropwise to 1-bromo-l-propene at 80 C and the mixture refluxed 24 hours. The crude product was treated with (+) pinandiol (50 mmol) dissolved in 20 ml THF and the reaction stirred 2 hours at ambient temperature. The solvent was removed, the product purified by chromatography on silica gel using hexane, and isolated in 76% yield as a colorless oil. H- and C-NMR and MS data supplied. 

Reduction of tosylhydrazones of conjugated acetylenic ketones with catechol-borane (1) proceeds with migration of the triple bond to give allenes. ... 

As expected, many of the same hydride reagents which convert other imine derivatives to the saturated analogs also reduce hydrazones, although extensive surveys have not been conducted. Successful reagents with varying types of hydrazones include NaBH4, catechol borane, ... 

Reductions with Catechol Borane and Related Reagents... 

Table 10 Reduction of Carbonyl Tosylhydrazones with Catechol Borane and Bis(benzoyloxy)borane...

As with cyanoborohydride, very few functional groups are affected by catechol borane under the tosylhydrazone reduction conditions (25 °C), allowing highly selective conversions in the presence of most moieties, including alkenes and alkynes which are hydroborated at more elevated temperatures (70-100 The only exceptions to this appear to be aldehydes, carboxylates, sulfoxides, amine oxides and anhydrides, which are reduced at rates comparable to tosylhydrazones. ... 

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