Hydrogenation, catalytic, alkene poisons

There are several ways to perform the cis reduction of alkynes. For example, catalytic hydrogenation using a poisoned catalyst and hydroboration followed by protonation yields ds-alkenes. For examples and mechanism, see Chapter 6, section 6.2. 

The reagent Is expensive and poisonous, consequently the hydroxylation procedure is employed only for the conversion of rare or expensive alkenes (e.g., in the steroid field) into the glycols. Another method for hydroxylation utilises catalytic amounts of osmium tetroxide rather than the stoichiometric quantity the reagent is hydrogen peroxide in tert.-butyl alcohol This reagent converts, for example, cyc/ohexene into cis 1 2- t/ohexanedlol. 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

Addition of 1 mol of hydrogen to the carbon-carbon triple bond can be accomplished stereospecifically. Catalytic reduction leads to the cis isomer. This is most often carried out using Lindlar catalyst, a lead-poisoned palladium-on-calcium carbonate preparation. Palladium on BaS04 is an alternative. Some examples are recorded in Scheme 3.10. Numerous other catalyst systems have been employed to effect the same reduction. Many specific cases are cited in reviews of catalytic hydrogenations. If the trans alkene is desired, the usual method is a dissolving-metal reduction in ammonia. This reaction is believed to involve two successive series of reduction by sodium and protonation ... 

Catalytic hydrogenation is slowed or even stopped altogether when certain contaminants are present. Such contaminants are usually specific compounds that are called poisons because they poison the hydrogenation reaction. Poisons include amines, phosphines, sulfur compounds, carbon monoxide, etc. These molecules coordinate to the metal catalyst and compete with the reaction of the metal with the hydrogen gas, the alkene, or the alkyne. In most cases, compounds known as poisons bind to the metal more or less irreversibly, and the metal is less reactive toward hydrogen and to the alkene or alkyne. In effect, the poison blocks the surface of the metal and prevents binding of hydrogen gas or the alkene or alkyne. 

Alkynes can be reduced to either a cis or trans alkene. Catalytic hydrogenation of an alkyne using a poisoned catalyst (H2, Lindlar catalyst) results in the syn addition of one equivalent of H2 to give a cis alkene product. Dissolving metal reduction (Li, NH3) of an alkyne produces the corresponding trans alkene. This strategy is well suited for synthesizing monosubstituted alkenes and disubstituted alkenes with a specific stereochemistry. 

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