Hydrogenation of diphenylacetylene

Scheme 8.5 Main species involved in the hydrogenation of diphenylacetylene catalyzed by ruthenium clusters, as determined by PHIP methods (CO ligands omitted for clarity).

Fig. 38.1 Product distribution in the hydrogenation of diphenylacetylene as a function of the pH. [ RuCI2(mtppms)2 2] = 6.6 mg (6.79X10-3 mmol ruthenium), mtppms = 8.1 mg (2.03xlCT2 mmol), diphenyl acetyle ne = 89.1 mg (0.5 mmol), 1 bar H2,...

Phenylacetylene is completely converted to ethylbenzene under the reaction conditions used. No hydrogenation of the phenyl group was detected. This shows a considerable degree of selectivity of the catalyst. This selectivity was further illustrated in the hydrogenation of diphenylacetylene which gave both stilbene (predominently trans-) and bibenzyl.8 Careful kinetic studies at 20 bar hydrogen and 373 °C show an induction time of 60 minutes and an... 

A related cationic cluster [Ru3(/i-H)(/i3-ampy)(/r,77 77 -PhG GHPh)(GO)8][BF4] is a catalyst precursor for homogeneous hydrogenation of diphenylacetylene to cis- and /ra r-stilbene under very mild conditions, 333 K and less than 1 atm hydrogen. Reactivity, spectroscopic, and kinetic studies suggest that the active catalyst is a cationic trimetallic cluster." ... 

A dimeric T12-H2 metal complex has also been proposed as intermediate species in the selective hydrogenation of diphenylacetylene to cw-stilbene catalysed by the diiridium compound [Ir2( a.-H)( x-Pz)2H3(NCCH3)(P/-Pr3)2] [38-40]. As shown... 

Clusters 19 22 and 24 have also been tested as catalyst precursors for the hydrogenation of diphenylacetylene (Table 1, entries 16, 18, 20). Z-Stilbene (kinetic product) and f-stilbene (thermodynamic product) are formed with higher conversions (70-100% after 90 min) than in the case of terminal alkynes. As with tert-butylacetylene, similar activities and selectivities are observed for the five cluster complexes, suggesting the intermediacy of common catalytic species in solution. Compound 27 (which arises from the reactions of 21 or 22 with diphenylacetylene) and compound 28 (which arises from the reaction of 24 with the same alkyne) (Fig. 4) have been proposed as catalytic intermediates in these hydrogenation reactions. ... 

Because compounds 4 and 33 are efficient catalyst precursors for the hydrogenation of diphenylacetylene and both contain an alkyne bound parallel to a metal-metal bond, E. Sappa and coworkers have made the hypothesis that clusters with an alkyne bound parallel to one edge of a triangular metal array could act as catalyst precursors or as intermediates in the hydrogenation of alkynes. - i In fact, the unsaturated binuclear complex [Ru2(Cp)2(//-Ph2C2)(CO)] (35), which has the alkyne bonded in a perpendicular mode to the metal-metal bond (Fig. 5), is a poor hydrogenation catalyst. ... 

Figure 9. Mechanism of the hydrogenation of diphenylacetylene promoted by complex 40, at low [Ph2C2]/[40] ratios. All cluster intermediates contain eight CO ligands.

The hexanuclear derivative [Ru6(/<-H)6(//3-ampy)2(CO)i4] (41) (Fig. 7) also promotes the hydrogenation of diphenylacetylene under mild conditions (Table 1, entry A kinetic analysis of the catalytic reaction, which is first-order in the concentration of 41, suggests that the catalytic species are hexanuclear, but no catalytic intermediates have been characterized. 

The catalytic hydrogenation of diphenylacetylene promoted by cluster 53 is very slow under mild conditions (Table 1, entry 31). " The low rate, the occurrence of activation periods, and the deactivation of the catalyst after long reaction times (ca 500 min), have prevented a kinetic analysis of the reaction. Nevertheless, the observation of the dihydride [Ru3( -H)2( 3-ampy)( U-PhC=CHPh)(PPh3)2(CO)5] (54) (Fig. 17) in the catalytic solutions suggests that the catalytic hydrogenation of diphenylacetylene promoted by complex 53 follows a similar mechanism to that described above for complex 44 (Fig. 15). The slower reaction rate and the activation period are probably because the activation energy for the release of CO from 53 (to create the necessary vacant site for the subsequent reaction with hydrogen to... 

---