Alkynes semi-hydrogenation

On balance, palladium offers the best combination of activity and selectivity at reasonable cost, and for these reasons has become the basis of the most successful commercial alkyne hydrogenation catalysts to date. Because of their inherently high activity, these catalysts contain typically less than 0.5 % (by weight) of active metal-to preserve selectivity at high alkyne conversion. Despite the prominence of these catalysts, other active metals are used in fine chemicals applications. Of particular utility is the nickel boride formulation formed by the action of sodium borohydride on nickel(II) acetate (or chloride). Reaction in 95 % aqueous ethanol solution yields the P2-Ni(B) catalyst and selectivity in alkyne semi-hydrogenation has been demonstrated in the reaction of 3-hexyne to form cw-3-hexene in 98 % yield [15,16] ... 

Perhaps the best known alkyne semi-hydrogenation catalyst is that developed by Lindlar which comprises calcium carbonate-supported palladium, modified by addition of lead acetate and, often, quinoline to improve selectivity [51]. Selective hydrogenation of 1-bromo-ll-hexadecyne (Eq. 8) has been shown to occur in high yield and without hydrogenolysis of the carbon-bromine bond, over Lin-dlar s catalyst treated with aromatic amine oxides such as pyridine A-oxide... 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

The selective semi-hydrogenation of alkynes, is a particularly important reaction in the context of fine chemieals manufaeture. The acetylenic group, RC CR, readily participates in substitution reactions enabling the formation of new carbon-carbon bonds, for example, and selective hydrogenation, leading to alkene or alkane species, further enhances the synthetic utility of the alkynes and has been exploited in the synthesis of biologically active compounds, e.g. insect sex pheromones (pest control) and vitamins [1-3]. 

The use of secondary modifiers, e. g. quinoline, and the choice of solvent also play important roles in directing semi-hydrogenation selectivity. For example, in the hydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts metal loading had no effect on selectivity when the reaction was performed in n-heptane as solvent. When the same experiment was conducted in n-propanol, however, an inverse relationship between selectivity and catalyst metal loading was observed [56], This effect has been interpreted as a polar solvent-induced modification of the Pd active sites, which alters the relative adsorption behavior of the alkyne and alkene species [57], Modification by addition of quinoline is reported to benefit the selective production of a cij-vitamin D precursor from the related disubstituted alkyne [58] ... 

Similarly, the effect of quinoline addition has been found to benefit the selectivity of other palladium formulations, as demonstrated in the semi-hydrogenation of an alkyne diester [59] ... 

As mentioned earlier, the particular geometric arrangement around the alkyne triple bond can also play a great part in controlling semi-hydrogenation selectivity. In molecules with both alkenic and alkynic functionality it is possible to preserve the original alkene group only if its approach to the catalyst surface can be restricted in some way. This effect was demonstrated in the reaction shown by Eqs (17) and (18) [67] ... 

Stereoselective functional-group-tolerant semi-hydrogenation of alkynes by Hj in CH2CI2 in the presence of AgOTf has been catalysed by [Cp Ru(cod)Cl] to afford the corresponding alkenes in 89% yield with excellent E/Z selectivity (98 2). It is further suggested that the use of mononuclear metal catalysts is superior to that of multinuclear metal catalysts. ... 

Sea sponge Negombata magnifica, [Mo]-VII/CH2Cl2 system catalyzed the chemoselective alkyne RCM and the subsequent semi-hydrogenation of the cyclic conjugated enyne brought about the formation of cyclic Z,E-1,3-dienes. 

They react with terminal alkynes by electrophilic addition of the empty p-orbital to the unsubstituted end of the triple bond 83. The intermediate would then be the more substituted vinyl cation 84. It is easier to draw this mechanism with R2BH than with the full structure for 9-BBN. The intermediate 84 is not fully formed before hydride transfer begins so that the reaction is semi-concerted and the transition state is something like 86. The result is a regioselective and stereospecific cis hydroboration of the triple bond to give the A-vinyl borane 85. The intermediate 84 is quite like the radical intermediate in hydrostannylation but the difference is that hydrogen transfer is intramolecular and stereospecific in hydroboration. 

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