Alkenes 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). 

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). ... 

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. 

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). 

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. 

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

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... 

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). 

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. 

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... 

Zf-Alkenes can be prepared by several related reactions.29 Hydroboration of a bromoalkyne generates an a-bromoalkenylborane. On treatment with methoxide ion these intermediates undergo B C migration to give an alkyl alkenylborinate. Protonolysis generates an Zi-alkene. 

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. ... 

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. 

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. 

However, it is possible to compare the relative rate of protonolysis of (9-BBN)j by an alcohol with that of hydroboration of an alkene of suitable reactivity, such as 1-decene, employing the competition method. Consequently, equimolar quantities of an alcohol and an alkene are allowed to react with an insufficient amount of 9-BBN, and the amount of Hj evolved is measured. The amount of alcohol reacted is calculated from the hydrogen evolved. From this quantity the amount of alkene reacted is deduced. The relative rates are then obtained by employing the following Ingold-Shaw expression [3]. 

Other catalytic hydroelementations, E-H (E = P, B, Si, H) addition to C C multiple bonds, are efficiently mediated by organolanthanide complexes, and many are highly diastere-oselective. Hydrophosphination/cyclization appears to follow the catalytic patiiway similar to hydroamination/cyclization, whereas hydroboration, hydrosilylation, and hydrogenation proceed via different catalytic cycles, in which a lanthanide-hydride is the intermediate that is generated from protonolysis of the precatalyst. The proposed catalytic cycle of hydrosilylation of alkene using Cp LnCH(TMS)2 is presented in Scheme 5. 

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