Borane catechol, reaction with

For many purposes, borane does a great job in terms of both reactivity (high) and selectivity (not always so high). However, there are processes where a less reactive and more selective borane will be preferred, and numerous compounds are available. If borane is reacted with 2,3-dimethyl-2-butene, only a single addition takes place, because of the steric hindrance that would result from a further addition. The product is termed thexylborane (11.27). Similarly, disiamylborane (SiajBH, 11.28) is prepared from 2-methyl-2-butene. The number of times the borane reacts is clearly a function of steric hindrance. 9-Borabicyclo[3.3.1]nonane (9-BBNH, 11.29) is prepared from 1,5-cyclooctadiene, and catecholborane (HBcat, 11.30) from catechol (1,2-benzenediol) and borane. All of these have been used to improve selectivity for specific reactions—in general, the more hindered the borane, the more selective the reaction. 

Several oxygen- and sulfur-substituted boranes have been reported (125—130). 1,3,2-Benzodioxaborole [274-07-7] (catecholborane, CB) (15) is the one best studied. It is commercially available or can be prepared by the reaction of catechol with borane-THF (57,131), or by other procedures (132). The product is a Hquid existing as a monomer, remarkably stable to disproportionation. No... 

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%). 

Palladium-catalyzed reactions of arylboronic acids have been utilized to craft precursors for constructing indole rings. Suzuki found that tris(2-ethoxyethenyl)borane (149) and catechol-derived boranes 150 readily couple with o-iodoanilines to yield 151, which easily cyclize to indoles 152 with acid [158]. Kumar and co-workers used this method to prepare 5-(4-pyridinyl)-7-azaindoles from 6-amino-5-iodo-2-methyl-3,4 -bipyridyl [159], A similar scheme with catechol-vinyl sulfide boranes also leads to indoles [160]. A Suzuki protocol has been employed by Sun and co-workers to synthesize a series of 6-aryloxindoles [161]. 

A further synthetically useful borane, catecholborane (1,3,2-benzo-dioxaborole) is obtained by the reaction of borane with 1,2-dihydroxybenzene (catechol). 

It is prepared as follows.9 A 2 m solution of borane in tetrahydrofuran (100 ml, 200 mmol), maintained under nitrogen, is placed in a dry 500-ml flask which is vented to a fume cupboard through a mercury bubbler. The flask is immersed in an ice bath and a solution of 1,2-dihydroxybenzene (catechol) (22 g, 200 mmol) in tetrahydrofuran (50 ml) is added over 30 minutes with efficient stirring to the borane solution at 0°C. After the completion of the addition, the reaction mixture is stirred at 25 °C for an additional 30 minutes. Distillation provides 19.2 g (80%) of catecholborane, b.p. 76-77 °C/100mmHg. An example of the use of catecholborane in synthesis is described in Expt 6.4. 

The variety of C2-bridged PBs was further extended by Muhoro via hydroboration of diphenyl(vinyl)phosphine with catechol- and pinacol-boranes (Scheme 29).56 To compensate for the low Lewis acidity of these boronates, the hydroboration reactions were carried out in the presence of 5 mol% of titanocene bis(catecholborane) as catalyst. The desired products 40g and 40h were obtained with complete anti-Markovnikov selectivity. The spectroscopic data and the crystallographic study performed on 40h showed the expected monomeric open structure. 

Important examples of this type of compounds include pinacolborane, catecholborane, etc. These compounds are readily available by the reaction of pinacol 331 or catechol with either borane or boron halides. Transesterification also provides an attractive alternative for the synthesis of these compounds (Scheme 55). 

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. 

The reagent (1) is prepared in 80% yield by the reaction of catechol with borane in T11F. The borole reacts with olefins at 100° to give the corresponding 2-alkyl-... 

Catecholborane is somewhat more reactive than (1), though still far less reactive than dialkylbo-ranes. 27 28 It is readily prepared by the reaction of catechol with borane-THF, is stable at 0 "C, and hydroborates alkenes slowly and alkynes (equation 49) more rapidly in refluxing The rate of... 

Yang et al. used catechol to immobihze borane and boronic acids [58], since support-bound boronic catachol ester hnkage is stable enough to perform amidation reactions. Custom-derivatized boronic acids could be obtained, which could be hberated from the support with THF/H2O/ACOH 90 5 5 (v/v/v). Hebei et al. reported Suzuki-mediated release of biaryls from polyglycerol esters of various boronic acids and aryl bromides [59]. 

There are a wide variety of hydroborating reagents, including BH3 complexes, pinacolborane, thexyl borane, and catechol borane. The following employs 9-borabicyclo[3.3.1]nonane (9-BBN), which places the boron on the less sterically hindered carbon with high regioselectivity however, completely removing the cyclooctane by-products can be problematic. The alkylborane can be isolated, but is typically used directly in the next reaction, in this case oxidation to the primary alcohol. 

Evans and co-workers developed rhodium catalyzed hydroboration reactions, which enable the use of catechol borane and other boronate esters as hydroborating agents to afford organoboronic esters as products.9 These products have increased stability and can be used directly in palladium cross-coupling applications. However, the rhodium-catalyzed method is most effective for the hydroboration of monosubstitued olefins (i.e., 16), as lower reactivity is observed with more substituted alkenes. 

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