Catalysts alkynes

Alkenes and alkynes can also add to each other to give cyclic products in other ways (see 15-61 and 15-63). The first exclusive exo-dig carbocyclization was reported using HfCU as a catalyst. Alkynes also add to alkenes for form rings in the presence of a palladium catalyst or a zirconium catalyst. " Carbocyclization of an alkene unit to another alkene unit was reported using an yttrium catalyst and alkenes add to alkynes to give cyclic compounds with titanium catalysts. ... 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

In the presence of Co(I)-catalysts alkynes and nitriles can be co-trimerized in organic solvents to yield substituted pyridines under rather harsh conditions. The reaction is biased by formation of large quantities of benzene derivatives and with acetylene gas as much as 30 % of all products may arise from homotrimerization. It has been found recently, that with cobalt(I) catalysts heterotrimerization of various nitriles and C2H2 could be achieved under ambient conditions using aqueous/organic biphasic systems and irradiating the reaction mixture with visible light (Scheme 7.12) [39,40]. 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

When applied to triple bonds, hydrocarboxylation gives a,p-unsaturated acids under very mild conditions. Triple bonds give unsaturated acids and saturated dicar-boxylic acids when treated with carbon dioxide and an electrically reduced nickel complex catalyst. Alkynes also react with NaHFe(CO)4, followed by CuCl2 2 H2O, to give alkenyl acid derivatives. A related reaction with CO and palladium catalysts in the presence of SnCE also leads to conjugated acid derivatives. Terminal alkynes react with CO2 and Ni(cod)2, and subsequent treatment with DBU (p. 1132) gives the a,p-unsaturated carboxylic acid. ... 

The olefin metathesis reaction is also suitable for alkynes. In the presence of a metathesis catalyst alkynes can undergo one of the three following reactions (1) cyclo-trimerization (e. g., trimerization of propyne into 1,3,5- and 1,2,4-trimethylbenzene) (2) polymerization, e. g., of phenylalkynes (eq. (9)) and (3) metathesis (rupture and reformation of carbon-carbon triple bonds), e. g., eq. (10). 

The preparation of a,p-unsaturated ketones by direct acylation of vinylcopper reagents has proven more problematic, since lithium cuprates do add to the product enones. Better results are obtained with the less reactive monovinyl copper compounds in the presence of a palladium catalyst. Alkynic ketones have been prepared by a variation of the Stephens-Castro coupling. ... 

In the presence of a nickel catalyst, alkynes can be inserted into bonds between tin and an alkynyl, allyl, or acyl group, to give new vinylstannanes.54-60 Examples from this rapidly developing field are shown in equations 8-23 to 8-25 57... 

Catalyst Alkyne Activity (mmol min 1 g ) Hydrogenation products (%)... 

With an ordinary nickel or platinum catalyst, alkynes are hydrogenated all the way to alkanes (eq. 3.1). However, a special palladium catalyst (called Lindlar s catalyst) can control hydrogen addition so that only 1 mole of hydrogen adds. In this case, the... 

In the presence of concentrated sulfuric acid and Hg(II) salts as catalysts, alkynes undergo the addition of water in a reaction analogous to the oxymercuration of al-kenes (Section 6.3F). The Hg(II) salts most often used for this purpose are HgO, HgSO, or Hg(OAc)2- For terminal allies, addition of water follows Markovnikov s rule hydrogen adds to the carbon atom of the triple bond bearing the hydrogen. The resulting enol is in equilibrium with the more stable keto form, so the product isolated is a ketone (an aldehyde in the case of acetylene itself). 

In Summary Alkynes are very similar in reactivity to alkenes, except that they have two TT bonds, both of which may be saturated by addition reactions. Hydrogenation of the first TT bond, which gives cis alkenes, is best achieved by using the Lindlar catalyst. Alkynes are converted into trans alkenes by treatment with sodium in liquid ammonia, a process that inclndes two snccessive one-electron reductions. 

Hydrosilylation in the presence of a carbon electrophile is often accompanied by C-C bond formation. For example, three-component coupling of hydrosilane, alkyne, and y unsaturated aldehyde is suggested to proceed via oxanickelacycle intermediate to give (Z)-enol silyl ether (Scheme 3-28). Hydrosilylation of alkenes under a carbon monoxide atmosphere allows carbonyl incorporation, giving silyl enol ethers by using a cobalt or iridium catalyst (Schemes 3-29 and 3-30). Under similar reaction conditions in the presence of a rhodium catalyst, alkynes are converted to y silyl-substituted acroleins (Scheme 3-31). ... 

Depending on the catalyst, alkynes can undergo hydroamination via prior isomerization to allene followed by generation of 77 -allyl intermediate (Scheme 6)... 

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