Hydroboration-protonolysis

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

A hydroboration-protonolysis procedure for the conversion of conjugated enynes to dienes is far superior to partial hydrogenation over Lindlar s catalyst, in terms of stereoselectivity and yields280. Ratovelomanana and Linstrumelle reported the synthesis of methyl a-eleostearate (equation 163) and methyl punicate by employing this strategy280. 

Hydroboration-protonolysis, 23 648 Hydroboration reaction, 23 632-646 Hydroborofluoric acid, 4 150 Hydrobromic acid, 4 319 22 823 use in selenium analysis, 22 94... 

Steps a-c (hydroboration-protonolysis-oxidation) represent a method for the selective reduction of a terminal alkyne in the presence of an alkene. 

Combination of reactions seen in Sections B4.1 and B4.2 with hydroboration/protonolysis reactions provides syntheses of stereodefined dienes. 

Hydroboration/protonolysis of ( )-enynes completes a route to (E, Z)-dienes. For example, the ( )-enyne generated in Figure B4.6 was converted into (IE, 9Z)-7,9-dodecadien-l-yl acetate (a natural sex pheromone of the European grape vine moth Lobesia botrana) by hydroboration with Sia2BH followed by acetic acid protonolysis (Equation B4.7). 

Conjugated diynes can be reduced to cis enynes by the well-known hydroboration-protonolysis sequence . cw-5-Dodecen-7-yne (215) is obtained from the diyne 213 in 76% yield as shown in equation (23). The orientation of addition of the disiamyl-borane was established by using CH3CO2D in the second step. Addition of a second mole of disiamylborane to 214 is very slow, but reduction to the cis,cis diene can be accomplished by using dicyclohexylborane instead . 

All three alkenyl groups of a trialkenylborane can be removed at 0 °C with a carboxylic acid, and alkenyldialkylboranes undergo preferential protonolysis of the alkenyl group.Stereochemistry is retained during protonolysis, so the sequence hydroboration-protonolysis, applied to internal alkynes, provides a useful, general method for synthesis of (Z)-alkenes (Scheme 12). ... 

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). 

Hydroboration-protonolysis is thus a general, stereospecific and versatile route from alkynes to alkenes, enynes and dienes. 

The mildness and selectivity of the hydroboration-protonolysis procedure allows for the presence of functional groups and its ready adaptability to the synthesis of natural products. 

Diastereomerically enriched P-hydroxy silanes are also accessible from a,P-epoxy silanes and reaction of these with organocuprate reagents. The epoxy silanes are synthesized by epoxidation of (E)- or (Z)-vinylsilanes with m-chloroperbenzoic acid. The required (Z)- and ( )-vinylsilanes can be obtained by hydroboration-protonolysis of 1-trimethylsilyl-l-alkynes or by hydrosilylation of 1-alkynes, respectively. 

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). 

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

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