Catechol borane hydroboration

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... 

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). 

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. ... 

Late-metal complexes of Pd, Pt, and Rh can also catalyze hydrosilylation, hy-drostannylation, hydroboration, and diborylation reactions of 7r bonds. Both C=C and C=0 bonds may be hydrosilylated or hydroborated, whereas hydrostannyla-tion is usually carried out only on C=C bonds. (Some boranes add to C=0 and C=C bonds in the absence of catalyst, but less reactive ones, such as catechol-borane ((C6H402)BH), require a catalyst. Moreover, the metal-catalyzed reactions sometimes display different selectivities from the uncatalyzed variants.) The mechanisms of all these reactions are the same as hydrogenation, except that oxidative addition of H-H is replaced by oxidative addition of a R3Si-H (R3Sn-H, R2B-H, R2B-BR2) bond. 

Some effected the coupling of phenyl-, 2-fuiyl-. and 1-hexenylboronic acids with 4-thallated indole-3-carboxaldehyde (Pd(OAc),/DMF) to give 4-substituted 3-formylin-doles [148]. Regioselective thallation of indole-3-carboxaldehyde is achieved using thallium tris-trifluoroacetate in 77% yield. Indole 129, which is available by the Buchwald zirconium indoline synthesis, was used by Buchwald to synthesize 130 via a Suzuki protocol [149]. Boronate ester 130 is prepared by the hydroboration of3-methyl-1-butyne with catechol borane. Indole 131 had been used in earlier studies to synthesize the clavicipitic acids. 

There are a wide variety of hydroborating reagents, including BH3 complexes, pinacolborane, thexyl borane, and catechol borane. The following employs... 

A dramatic example of this technique is the last step of Heathcock s synthesis of myxalamide A 96. Hydroboration of the alkyne 94 with catechol borane gives the A-vinyl borane and this is combined with the Z-vinyl iodide 95 in a palladium-catalysed Suzuki coupling to give the natural product 96 with every alkene correct.17... 

Hydroboration of the alkyne 252 with catechol borane 235 is regioselective for steric reasons and the resulting -vinyl boronate 253 couples with the iodo-benzene 254 under palladium catalysis. 

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. 

Hydroboration of a,(3-unsaturated carbonyl compounds have been studied [14,15], mainly due to the synthetic utility of boron enolates [ 1 ]. Phenyl-1 -alke-nylketones undergo selective 1,4-hydroboration either with 9-BBN or catechol-borane to produce the corresponding (Z)-boron enolates (Chart 20.2) [16] in high yields in solvents like chloroform, dichloromethane, THF, or benzene and in high isomeric purity. However, this selectivity has not been observed in the reactions of 9-BBN or catecholborane with alkylalkenylketones (3). 

Alkenyl boranes are most commonly prepared by the cw-hydroboration of alkynes using catechol borane (Scheme 2.79). The resulting catechol boranes may be directly coupled, or hydrolysed to the boronic acid, prior to coupling. 

The boron-containing substrate (a boronic acid) is efficiently prepared by hydroboration of a terminal alkyne with a special reagent, catechol borane ... 

Witulski has reported the first hydroboration of 1-alkynylamides. Thus, the hydroboration of ynamide 112 with catechol borane in THF proceeded chemo- and regioselectively yielding only the monohydroboration product, alkenyl boronic ester 113 [90]. However, the isolation of the boronic ester 113 was complicated due to its instability and difficulties of storage and purification. Therefore, it was directly subjected to Suzuki-Miyaura cross-coupling yielding (E)-y3-arylenamide and 3-(2 -amidovinyl)indoles such as 114 (Scheme 3.58). 

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. 

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... 

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