Boranes, with alkynes, reduction

Diborane or alkylboranes are used for reduction of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

Internal alkynes induce olefin loss from alkylboranes to form alkenyl boranes. Thus, (C HsjaB reacts rapidly and reversibly with alkynes at 160-200°C to form trial-kenylborane and C2H4. With (r-C4H9)3B, the reaction proceeds under milder conditions. It is not known whether such reactions proceed via B-H intermediates produced by -hydride elimination, or by intramolecular hydride transfer from the alkyl group to a coordinated acetylene. This reaction is useful for reduction of acetylenic hydrocarbons. 

Examples of alkene and alkyne insertions followed by transmetallation with organotin compounds as well as boranates, and eventually reductive elimination are shown in Scheme 16.[ 0l.ns].[37]-[39] reactions do not only require that the transmetalla-... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

CH,),QH,]2BH (1). Mol. wt. 250.20, m.p. 68°, air stable, in 70% yield by reduction of fluorodimesitylborane with LiAlHj. This borane is recommended for hydroboration of alkynes, par-e hydroboration of unsymmetrical alkynes (equation I). 1-Alkynes khydes in high yield. Since alkcnes react only slowly with this iwration of alkynes in the presence of alkenes is possible. 

Methyl alkynic ketones are reduced with slightly lower efficiency and f-butyl alkynic ketones are reduced very slowly. In the latter case, dehydroboration of Alpine-Borane to give 9-BBN competes with the rate of reduction and the liberated 9-BBN reduces the ketone to give products of lower enantiomeric purity. This problem may be overcome by using high pressure or by using 6-10-cij -myrtanyl-9-BBN (eqs 5 and 6). ... 

As with cyanoborohydride, very few functional groups are affected by catechol borane under the tosylhydrazone reduction conditions (25 °C), allowing highly selective conversions in the presence of most moieties, including alkenes and alkynes which are hydroborated at more elevated temperatures (70-100 The only exceptions to this appear to be aldehydes, carboxylates, sulfoxides, amine oxides and anhydrides, which are reduced at rates comparable to tosylhydrazones. ... 

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. ° ... 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

With hydride reagents such as dibal, Z-alkenes can be selectively obtained from alkynes in the phenolic lipid series (ref. 162), and related series of boron reagents greatly supplement the chemical methods of selective reduction and alkyiation. This selectivity has been achieved by the use of less reactive dialkylboranes such as bis(3-methyl-2-butyl)borane (di-isoamylborane), bis(2,3-dimethyl-2-butyl)borane (thexylborane), 9-boraUcyclo[3.kl]nonane (9-BBN) and dicyclohexylborane. Some applications in the polyethenoid field have been summarised (refs. 135,163) and the synthesis of alkenyl compounds generally reviewed (ref. 164). By the use of dibromoborane dimethylsulphide, an internal alkyne can be reduced selectively (ref. 165) as for example in the following way (R = n-alkyl). 

With bis(di-isoamyl) borane reduction of a terminal alkyne is selectively effected as shown (ref. 166). 

Over the past few years, the research groups of Marks and Piers have developed a number of new and effective perfluoroarylborane activators as well as bifunctional boranes. Bis(pentafluorophenyl)bo-rane [HB(CeF5)2]2 was synthesized by Piers et al. by reduction of monomeric chloroborane with Me2-Si(Cl)H (which also serves as the solvent for the reaction) in a quantitative yield. This white, crystalline material exists in a dimeric form in the solid state and in suspension in toluene or benzene promotes rapid hydroboration of a range of simple alkenes and alkynes. The details of the hydroboration chemistry as well as the reaction of the products with zirconocene dialkyls have been the subject of a recent review article. ... 

Although no mechanism was proposed, the Pd(Me-DuPhos)-catalyzed asymmetric hydrophosphination of an alkyne with a phosphine-borane under kinetic resolution conditions (Scheme 6) presumably involves similar insertion and reductive elimination steps [14]. 

---