Carboxylic acids, reaction with boranes

Alternatively, borane in tetrahydrofuran (BH3/THI ) is a useful reagent for reducing carboxylic acids to primary iilcohols. Reaction of an acid with BH / l HF occurs rapidly at room temperature, and the proceaurc is often preferred to reduction with I iAlH4 because of its relative ease and safety. Borane reacts with carboxylic acids faster than with any other functional group, thereby allowing selective transformations such as tliat shown below on / -nitrophenylacetic acid. If the reduction of p-nitrophcnylacetic acid were done with LiAll-14, both nitro and carboxyl groups would be reduced. 

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

One hydrogen of the borane is wasted through reaction with the acidic hydrogen of the carboxyl group to give hydrogen. An example is... 

Acyloxyboranes. Yamamoto et a/.1 have used the known reactivity of borane with carboxylic acids to activate acrylic acids for Diels-Alder reactions. Thus addition of BH3-THF to acrylic acids at 0° furnishes an acyloxyborane formulated as 1, which undergoes cycloaddition (equations I and II). The reaction proceeds satisfactorily even when borane is used in catalytic amounts. A chiral acyloxyborane, BL, prepared from a tartaric acid derivative, can serve as a catalyst for an asymmetric Diels-Alder reaction (equation III). 

The rapid reaction between carboxylic acids and borane is related to the electrophilicity of the latter. The carbonyl group of the initially formed acyloxyborane intermediate, which is essentially a mixed anhydride, is activated by the Lewis acidity of the trivalent boron atom. Addition of 1/3 equiv of the Borane-Tetrahydrofuran complex to acrylic acid in dichloromethane followed by addition of a diene at low temperature results in the formation of Diels-Alder adducts in good yield (eq 1). Further, the reaction is successful even with a catalytic amount of borane. 

The reaction with acrylic acid deserves special attention, because acrylic acid is not usually a good Diels-Alder reagent. That the reaction proceeds catalytically and with high ee indicates the facile exchange of the (acyloxy)borane of the cycloadduct with the carboxylic group of unreacted acrylic acid, whereas the monoacrylated tartaric acid remains bound to boron (Fig. 1). 

Hydrazides (s. a. Carboxylic acid hydrazides) reduction with borane 26,56 Hydrazine (s. a. Wolff-Kishner reduction) 26,15 reaction with dicarboxylic acid anhydrides 8, 482... 

Acyloxy)dialkylboranes are best prepared from carboxylic acids with tri-alkylboranes. " Other methods involve reactions of halodiorganobo-ranes with carboxylic acids or with acetic anhydride. Alternative syntheses from acetic anhydride using dialkyl(alkylthio)boranes or dialkylamino compounds have also been described. ... 

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