Alkylboranes protonolysis

Protonolysis of alkenylboranes generates an alkene and this reaction is faster than the analogous protonolysis of alkylboranes.57 Once again, protonolysis proceeds stereospecifically with retention of configuration. In Negishi s synthesis of the sex pheromone of Lobesia botrana (65), hydroboration of 64 was followed by protonolysis with acetic acid at 0°C to give 65, for a net conversion of an alkyne to an alkene.58 As with alkylboranes, protonolysis of vinylboranes with deuteroacetic acid allows the synthesis of deuterated alkenes. 

Protonolysis of Alkylboranes with Carboxylic Acids Synthesis cf Alkanes 3.10.73 Protonolysis cf Alkenylboranes Synthesis ofAlkenes and Dienes... 

Protonolysis of Alkylboranes with Carboxylic Acids Synthesis of Alkanes... 

The protonolysis reaction tolerates functionalities such as halide or ether groups in the alkylborane. However, p-dialkylaminoalkylboron compounds undergo elimination to give alkenes under these conditions (see Section 3.10.1.2). ° " Also, systems which are intrinsically labile to either acid or heat, such as some terpenoids, may give problems. For example, enantiomerically pure limonene produces 1 -men-thene which is substantially racemized on hydroboration-protonolysis (Scheme 15). 

Protonolysis of alkylboranes is much more difficult than protonolysis of, say, Grig-nard reagents. The course of reaction (4) is evidently not equilibrium-controlled, but rate-controlled it is not the stronger base, R , that gets the proton, but instead the resonance-stabilized carbanion [RCHCOCH3I". 

The usefulness of the hydroboration reaction in synthesis arises from the fact that the alkylboranes formed can be converted by further reaction into a variety of other products. On hydrolysis (protonolysis), for example, the boron atom is replaced by a hydrogen atom. Particularly important is the oxidahon to alcohols. Some of the transformations are described below. 

Numerous other reactions include the synthesis of trialkylcarbinols from trialkylboranes (Brown, H. C. Acc. Chem. Res. 1969, 2, 65) and hydroboration-protonolysis leading to alkanes (Brown, H. C. Murray, K. /. Am. Chem. Soc. 1959,81,4108). Isomerization and disproportionation of the alkylboranes was also seen (reference 230b). 

Protonolysis of an alkylborane takes place with retention of configuration hydrogen replaces boron where it stands in the alkylborane. 

The overall stereochemistry of hydroboration—protonolysis, therefore, is syn (hke that of the oxidation of alkylboranes). 

Hydroboration followed by protonolysis of the resulting alkylborane can be used as an alternative method for hydrogenation of alkenes, although catalytic hydrogenation (Section 7.12) is the more common procedure. Reaction of alkylboranes with deuterated or tritiated acetic acid also provides a very useful way to introduce these isotopes into a compound in a specific way. 

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