Alkynes elimination with boranes

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. 

This reaction occurs rapidly at room temperature using a small excess of alkyne and either pinacol- (HBpin) or catecholborane (HBcat). When an excess of borane was used, the Z/E ratio of the products was slowly eroded, eventually attaining a thermodynamic distribution of isomers. Equilibration presumably occurs via addition/elimination of excess Rh-H. Miyaura s method provides a useful synthetic complement to knovm cis-hydroboration methods. Under optimized conditions, good yields and high stereoselectivity (>90 10) were achieved for a variety of alkenylboronates (Table 9.8). The best selectivities were generally obtained with the use of catecholborane and Et3N as an additive. As in related reactions, the presence of base seems to suppress undesired reaction pathways. 

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. 

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]. 

In another copper-catalyzed reaction, cross-coupling of alkynes with phosphi-ne-boranes was followed by surprising oxidation to yield ketones (Scheme 69) [122]. The active species was proposed to be a copper phosphido-borane complex, formed by proton transfer to a Cu-OH group. Formation of a Cu-acetylide followed by P-C reductive elimination would then yield a phosphino-alkyne, whose subsequent Cu-mediated air oxidation yields the ketone. 

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-... 

By far the most widely used method for making boron-carbon bonds is by the addition of boranes to alkenes or alkynes. Much of the work in this area is due to H.C. Brown. This reaction is termed hydroboration . Although it is reversible the equilibrium normally lies in favour of product rather than reactants in ether solvents at room temperature, so that the addition goes to completion. On heating above ca. 100°C organoboranes with a j5-C—H function eliminate alkene, that is, the reverse reaction takes place. Isomerization of alkylboranes occurs by a series of these addition-elimination steps, leading eventually to terminal boranes. The alkene can then be displaced by a less volatile alkene. The direction of this... 

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. 

A synthetic method for the introduction of an ethynyl group in cyclic structures is based on an intramolecular silicon-tethered radical cyclization with iodine transfer (Scheme 25.8). For example, starting from alkyne 10, a triethyl borane-initiated process allows a radical formation and cyclization to give the alkenyl radical 11. An iodine atom is abstracted from the starting material to give the alkenyl iodide 12, thus, propagating the chain. Treatment of 12 with tetra-/j-butylammonium fluoride (TBAF) results in elimination to furnish 13 in good yields. 

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