Alkynes hydroboration/protonolysis

The Lindlar catalyst is a deactivated hydrogenation catalyst that stops alkyne hydrogenation at the alkene stage, allowing conversion of allies to ds-alkenes. Hydroboration followed by an acid workup (instead of basic peroxide) also gives cfs-alkenes from alkynes. Hydroboration-protonolysis is an alternative way to prepare a ds-alkene from an alkyne. 

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. 

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

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). ... 

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

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. 

Three types of reactions are used to convert alkynes to alkenes and alkanes catalytic reduction, hydroboration-protonolysis, and dissolving-metal reduction. 

The net effect of hydroboration of an internal alkyne followed by treatment with acetic acid is reduction of the allgme to a ds-alkene. Thus, hydroboration-protonolysis and catalytic reduction over a Lindlar catalyst provide alternative schemes for conversion of an alkyne to a ds-alkene. 

Thus, by the proper choice of reagents and reaction conditions, it is possible to reduce an alkyne to either a ds-alkene (by catalytic reduction or hydroboration-protonolysis) or to a frans-alkene (by dissolving-metal reduction). 

Hydroboration-Protonolysis (Section 7.8B) Hydroboration of an alkyne followed by protonolysis also converts an alkyne to a cis-alkene. 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Protonolysis of alkenylboranes by carboxylic acids occurs readily. The stereochemistry of the alkenyl group is retained during the reaction and so hydroboration/protolytic cleavage of alkynes leads to cis alkenes. Deuterated... 

Alkenyldialkylboranes undergo simple protonolysis of the alkenyl group on treatment with hydrochloric acid, but other alkenyldialkylboranes rearrange under these conditions. This problem does not arise on protonolysis with a carboxylic acid, and hydroboration with a dialkylborane followed by protonolysis with a carboxylic acid is therefore of general utility for conversion of alkynes into alkenes. Internal alkynes cleanly give (Z)-alkenes (Scheme 12, Section 3.10.7.1). The process can also be used for stereospecific production of ( )-1-deuterio-1-alkenes from terminal alkynes (equation 57). Tritiated alkenes have been prepared similarly. ... 

Hydroborations of alkynes directed toward the synthesis of alkenylboranes furnishes versatile intermediates for a wide array of chemical transformations. For example, protonolysis of alkenylboranes provides a noncatalytic method for the semireduction of alkynes. 

Alkynes can be reduced by hydroboration and protonolysis. The internal acetylene 3-hexyne reacts with diborane to form an unsaturated boron derivative which is... 

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. 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

As mostly discussed earlier. Type III alkenyl derivatives, that is, ( )-R CH= CHM(or X), are widely and satisfactorily generated by (i) alkyne hydrometallation (M = B, Zr or, in some cases, Al, etc.) (Table 3.2, Scheme 3.6), (ii) polar halogenation reactions ofalkynes (Eqs. (1), (2), and (7), Scheme 3.15), and additionally, (iii) anti bromoboration of ethyne [53] followed by Negishi coupling (Eq. (1), Scheme 3.12). On the other hand. Type IV alkenyl derivatives may be prepared by (i) Normant alkylcupration of ethyne [67, 68] (Eqs. (5) and (6), Scheme 3.11), (ii) Zr-catalyzed alkylalumination of ethyne, (iii) syn hydroboration of 1-halo-l-alkynes followed by hydride-induced inversion of configuration [59] (Scheme 3.9), (iv) hydroboration of 1-alkynes followed by brominolysis (but not iodinolysis) with inversion [95], and (v) syn hydrozirconation or syn hydroalumination of 1-boryl- or 1-silyl-l-alkynes followed by protonolysis of the C-Al or C-Zr bond [96-98]. 

As already mentioned, the internal triple bond is hydroborated (90-96%) with equimolar quantity of 9-BBN, while twofold excess of terminal alkyne is used for monohydroboration (90-100%) with 9-BBN [1]. Protonolysis ofB-vinyl-9-BBN derivatives provides the corresponding alkenes. Thus, protonolysis of the product from 5-decyne and 9-BBN yields only ds-5-decene (Eq. 24.1X... 

The alkylborohydrides thus produced are converted to alcohols on oxidation, and alkenylboranes to alkanes by protonolysis. The hydroboration occurs preferentially in an anti-Markownikoff fashion. In contrast to what occurs with diborane, the Ti catalyzed hydroboration affords almost exclusively monohydroborated products (mixture of isomers) with alkynes or alkadienes, even in the presence of an excess of NaBH4. 

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