Catalytic hydrogenation of alkynes

Catalytic hydrogenation of alkynes on a metal surface provides cis alkenes (see Chapter 7, Problem 13), while treatment with sodium in liquid ammonia nearly always leads to trans alkenes, e.g., hydrogenation of 2-butyne. 

Although catalytic hydrogenation of alkynes can be accomplished in many other ways, this catalytic system was significant because it was the first time an f element complex had been observed to cataly-tically activate hydrogen in homogeneous solution. The result had... 

Catalytic hydrogenation of alkynes by a monomeric transition metal complex was thought to yield exclusively the Z-alkene. However, Bargon and coworkers [213, 214], who have been active in PHIP research, observed the formation of -alkenes using [RuCp (alkene)[ as catalyst. Figure 1.36 shows the NMR spectrum after... 

Woelk and Bargon have studied the catalytic hydrogenation of alkynes in scCOj [34], using catalysts based on transition metal colloids, carried out in the TCA setup. Under mild experimental conditions (15 bar hydrogen partial pressure, at 323 K) they have found extremely high TOFs (as high as 4 X 10 h" ... 

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. 

The catalytic hydrogenation of alkynes is similar to the hydrogenation of alkenes, and both proceed with syn stereochemistry. In catalytic hydrogenation, the face of a pi bond contacts the solid catalyst, and the catalyst weakens the pi bond, allowing two hydrogen atoms to add (Figure 9-2). This simultaneous (or nearly simultaneous) addition of two hydrogen atoms on the same face of the alkyne ensures syn stereochemistry. 

Catalytic hydrogenation of alkynes using Lindlar s catalyst. 

CHj, R = C2H5]. Indeed, [(C5H4R)2Y( t-H)( ]2 and (CsH4R)2Y-[C(R )=CHR ](thf) can be interconverted in this way and both initiate catalytic hydrogenation of alkynes to cis alkenes ( 5). Hence, one plausible rationale exists for the lanthanide-based catalytic hydrogenation reaction which originally stimulated the study of molecular lanthanide hydrides (50, 60). 

Since (CsMe5)2Sm(thf)2 was a crystallographically characterized low-valent lanthanide complex with catsJytic activity in hydrogenation reactions, it was an ideal complex to study with respect to the catalyses which were initiated by metal vapor products as discussed earlier. liiis system was of further interest since it initiated catalytic hydrogenation of alkynes at rates 1000 times faster than the previously studied organolanthanide... 

A The alkynyl hydrogen of propyne is acidic and hence the propyne will form an organometallic derivative. The Grignard derivative will react with carbon dioxide to give the carboxylic acid. Cis alkenes are formed by the catalytic hydrogenation of alkynes using a Lindlar catalyst. Hence a propynyl alcohol is a precursor to compound (b). This could be obtained by the addition of a propyne anion to propanone ... 

The property that the stereochemical result of an electrocyclic reaction is absolutely predictable is called stereospecificity. A stereospecific reaction will give you one stereochemical result when a cis starting material is used, and the opposite result when a trans starting material is used. Other examples of stereospecific reactions include Sn2 substitutions, catalytic hydrogenation of alkynes or alkenes, and dihydroxylation and bromination of alkenes. 

You will already be aware that m-alkenes can be formed by catalytic hydrogenation of alkynes. This example illustrates how acetylene, a double-ended nucleophile, can be used to build the skeleton of a Z-allyl silane 235 of the type used in chapter 12. The disconnection 236 is next to the alkene (now an alkyne) rather than across it. 

Catalytic hydrogenation of alkynes takes place in a stepwise manner, and both the alkene and the alkane can he isolated. Complete reduction of alkynes to the saturated compound is easily accomplished over platinum, palladium or Raney nickel. A complication which sometimes arises, particularly with platinum catalysts, is the hydrogenolysis of hydroxyl groups a- to the alkyne (propargylic hydroxyl groups) (7.15). 

Catalytic hydrogenation of alkynes to cis-olefins occurs with dinuclear molybdenum complexes " and the synthesis and reactions of binuclear molybdeno-cene and tungstenocene derivatives have been reported. ... 

---