Hydroboration, alkenes acidity

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

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). 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alcohols can be prepared by hydration of alkenes. Because the direct hydration of alkenes with aqueous acid is generally a poor reaction in the laboratory, two indirect methods are commonly used. Hydroboration/oxiclation yields the product of syn, non-Markovnikov hydration (Section 7.5), whereas... 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

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. 

The adducts derived from catechol borane are hydrolyzed by water to vinylboronic acids. These materials are useful intermediates for the preparation of terminal vinyl iodides. Since the hydroboration is a syn addition and the iodinolysis occurs with retention of the alkene geometry, the iodides have the -configuration.214... 

Silicon substituents can be introduced into alkenes and alkynes by hydrosilation.70 This reaction, in contrast to hydroboration, does not occur spontaneously, but it can be carried out in the presence of catalysts such as H2PtCl6, hexachloroplatinic acid. Other catalysts are also available.71 Halosilanes are more reactive than trialkylsilanes.72... 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

Hydroboration of alkenes generates organoboranes, which react with sodium azide in the presence of an aqueous acid to give primary amines (equation 58). Thus 1-nonene... 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

While the introduction of aryl groups has been well documented, the use of alkylboronic acids to decorate the pyridazine core is hitherto not well explored. Wermuth and co-workers nicely showed that hydroboration of alkenes with 9-BBN followed by Suzuki coupling with 3-iodopyridazines 180 and 182 yielded the corresponding 3-alkylpyr-idazines 181 and 183 in good yield (Schemes 43 and 44) <2002SL1123>. 

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