Stability rhodium catalysis

The asymmetric hydrogenation of prochiral ketones is often an important step in the industrial synthesis of fine and pharmaceutical products. Several noble metal nanoparticles have been investigated for asymmetric catalysis of prochiral substrates but platinum colloids have been the most widely studied and relevant enantiomeric excesses have been reported (>95%). Nevertheless, the enantioselec-tive hydrogenation of ethyl pyruvate catalyzed by PVP-stabilized rhodium nanocluster modified by cinchonidine and quinine was reported by Li and coworkers (Scheme 11.7) [68]. 

Leger, B. Nowicki, A. D. Roucoux, A. Bourbigou, H. O. (2008). Synthesis of bipyridine-stabilized rhodium nanoparticles in non-aqueous ionic hquids A new efficient approach for arene hydrogenation with nanocatalysts. Advanced Synthesis Catalysis, Vol.350, No.l, Oanuary 2008), pp. 153 -159, ISSN 1521-3765... 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

Larpent, C., Brisse-le-Menn, F., and Patin, H., New highly water-soluble surfactants stabilize colloidal rhodium(O) suspensions useful in biphasic catalysis, J. Mol. Catal., 65, L35-L40, 1991. 

A few years ago, a new class of ligands namely the sulfonated phosphites (for examples see Table 7, 132, 133) was described.283 287 They show remarkable stabilities in water compared to conventional phosphites such as P(OPh)3 and rhodium catalysts modified with 132 exhibited much higher catalytic activities in the hydroformylation of 1-tetradecene than conventional Rh/P(OPh)3 or Ph/PPh3 catalysts even at lower reaction temperatures.285,286 Sulfonated phosphite ligands may play a role in the emerging field of biphasic catalysis in ionic liquids15 22 or in combination with membrane separation of the metal complexes of these bulky ligands. 

The same reaction scheme can be written for (Z) -2-phenyl-2-butene, except that paths B and E would lead to erythro and threo aldehydes. In cobalt catalysis this isomerization could explain both the lack of stereospecificity and the lack of influence of the sterochemistry of the starting olefin on the distribution of aldehydes 26 and 27. This hypothesis agrees well with results with a-ethylstyrene. On the other hand, when rhodium is used, extensive isomerization occurs less readily probably because of a better stability of alkyl- and acylrhodium carbonyls, and one can thus achieve a high degree of stereospecificity. 

Catalyst decomposition is, overall, receiving little attention in academic work on homogeneous catalysis, and only in recent years has research on decomposition and stabilization of organometallic catalysts started to expand (116), with emphasis on reactions of significant commercial interest such as hydroformylation (117), metathesis 118), crosscoupling, and polymerization 119). Ligand decomposition seems to be a key issue for industrial application, because it affects the total number of turnovers, TON. Phosphine decomposition is an unavoidable side reaction in metal-phosphine complex-catalyzed reactions and the main barrier for commercial application of homogeneous catalysts. There are a few exceptions to this statement for example, the rhodium tppts-catalyzed hydroformylation of propene, a process developed by Ruhrchemie-Rhone Poulenc (now Celanese). 

---