Hydrogenation III Alkynes

The selective hydrogenation of a triple bond to give an alkene without concomitant positional or geometric isomerization is particularly important in synthetic procedures and many industrial processes. In the absence of any isomerization, selective partial hydrogenation of a disubstituted alkyne produces the cis alkene. Small amounts of the trans alkene are sometimes formed in these reactions, but catalytic processes do not lead to the production of the irans olefin as the primary product. The Irons alkenes can be produced as a primary product by metal-ammonia reduction of disubstituted alkynes.2 

The hydrogenation of an alkyne is an example of a Type III selectivity (Chapter 5) (Eqn. 16.4). As a result, when diffusion, particularly that of the 

It was shown that with a Pd/C catalyst in the liquid phase terminal triple bonds were saturated faster than internal ones, and both hydrogenated faster than terminal or internal double bonds in competitive processes (Eqn. 16.5). Further, alkene isomerization generally does not take place over palladium catalysts when alkynes are present. This selective hydrogenation depends on the stronger adsorption of an alkyne compared to an alkene. It is also possible that steric factors can influence the selectivity in the competitive semihydrogenation of an acetylene and an olefmic group in the same molecule. When the double bond and the triple bond are c/s to each other as in 7, selective adsorption of the acetylene 

All of the common hydrogenation catalysts can effect the complete saturation of the alkyne to alkane, but all catalysts are not equally effective in the selective hydrogenation to produce alkenes. Selectivities for cis 2-pentene formation from 2-pentyne decreased in the order Pd Rh Pt Ru Ir. Palladium is the most selective of the noble metal catalysts for alkyne semihydrogenation with respect 

While unmodified Pd/Al203 was an effective catalyst for the semihydrogenation of the amino alkyne, 9, (Eqn. 16.8), the hydrogenation of the amino diyne, 10, over palladium in absolute ethanol gave the products resulting from the cyclization of the partially saturated triple bonds on the catalyst surface (Eqn. 16.9).  

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The rhodium (O) complex, (VIII), reacts with an alkyne to form a 1 1 addition complex which is catalytically active for the hydrogenation of alkynes and (weakly) alkenes. Complexes of types (III), but with chloro-ligands, and (VII, X = Cl) also form complexes with acetylenes, which can be subsequently hydrogenated. A series of complexes of structure (VH) with or... 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Hydrogenation of Dienes and Alkynes with Croup III and Lanthanide Complexes... 

The alkynes are bonded in essentially the same way as, but less firmly than, the olefins (see Section III,R). In the hex-3-yne series, substitution of an a-hydrogen atom by a methyl group reduces the argentation constant (a measure of the silver-alkyne bond strength) by a factor of roughly 1 /3 this influence of methyl substitution on complex formation is opposite to that observed in the platinum(II) complexes (see Section IV,J). 

The first instance involves the Pt(II) fragment "TpPdMe," used not as a catalyst, but rather as a protecting function for alkynes during catalytic, and indeed stoichiometric, processes, a role that followed from the noted stability of TpPtMe(r 2-RC=CR) complexes, and their capacity to release the alkyne by carbonylation.56 63 Thus, ji-complexes with a series of bis (amide)acetylenes (144-149, Scheme 12, Section III.B.l), formed from the polymeric TpPtMe (126), could be subjected to conditions of catalytic hydrogenation, or basic hydrolysis of the pendant functions, without... 

The mechanism of the Au(III) catalysis proposed in Scheme 5 implies the stereoselective formation of the new C-C bond which, of course, cannot be observed in the final product when terminal alkynes are used (the aryl group and the former alkyne hydrogen are situated at the same side of the double bond in the vinyl-Au intermediate). For the reaction of 1-phenyl-l-propyne and mesitylene 1 (see below, Table 1) the proposed mechanism should lead to preferential formation of the Z isomer which is, in fact, observed [2]. The formation of a small amount of E isomers can be explained by isomerization of the initially formed Z compound. Such isomerization was, in fact, observed directly in the case of related electron-poor alkynes [4],... 

Table 19. Hydrogenation of alkenes and alkynes by Rh(III) and Pd(II) chelates of 120). Reaaion ...

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