Catechol borane reduction

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%). 

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

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 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... 

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. 

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... 

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

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. ... 

Table 11 Reduction of Conjugated Carbonyls with Catechol Borane...

The catalysts may be used in 5-10 mol% concentrations, with either borane or catechol borane [43] as the stoichiometric reductant. Use of the more reactive catechol borane allows one to conduct the reduction at lower temperature, a feature that may be advantageous in cases where selectivity at room temperature is not high enough. The reductions are sensitive to moisture Jones, et al. [45] found that the presence of 1 mg of water per gram of ketone lowered the enantioselectivity from 97% to 75% es. 

Six-membered Rings.—Cyclohexanones. Conversion of an alicyclic ketone into its corresponding methylene derivative is often necessary, but existing methods tend to require vigorous conditions. A mild method has been reported in which the ketone tosylhydrazone is treated with catechol borane in chloroform at 263 K and the reduction product decomposed with sodium acetate. Yields of 40—90% are claimed. 

It has been pointed out °° that caution should be exercised in planning alkene syntheses through the reduction of a/3-unsaturated tosylhydrazones with sodium cyanoborohydride. Alkene (imminium ion reduction) and alkane (Michael addition to the unsaturated system) production can be competing processes. However, the new, mild procedure using catechol borane as the reducing agent seems to circumvent these difficulties. ... 

Sodium borohydride in acetic acid is a convenient new system for the reductive deoxygenation of carbonyl tosylhydrazones [equation (43)], while reduction of a,/8-alkynyl tosylhydrazones using catechol-borane provides a useful route to allenes [equation (44)]. ... 

Another breakthrough came from Antilla et al., who employed a chiral phosphoric acid as an efficient catalyst for the reduction of prochiral ketones (83). As an extension of their success in the enantioselective allylation of aldehydes with allyl borate [84], they hypothesized that a chiral phosphoric could activate and direct the borane reduction of ketones. Initial studies showed that the asymmetric reduction truly works and, after screening various chiral phosphoric acids, found that 6 stands out as the most suitable catalyst. The enantioselectivity is further improved by using 4-(dimethylamino)pyridine (DMAP) as an additive (ee up to 95%) (Scheme 32.30). To gain mechanistic insight into this reduction, the authors performed several parallel B NMR experiments the results suggest that the complex formed between 6, catechol borate, and DMAP might be the real catalyst for the asymmetric reduction. 

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