Mechanism rhodium-based catalysts

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

In fact, mechanism of hydroformylation is even more comphcated than shown in Fig. 5.30. The experimental observations collected for rhodium-based catalysts with different hgands (triphenylphosphine and cyclohexyl diphenylphosphine) demonstrated... 

The rhodium catalyst has several distinct advantages over the cobalt catalyst it is much faster and far more selective. The higher rate is in process terms translated into much lower pressures (the cobalt catalyst is operated at pressures of 700 bar). Nickel- and palladium-based catalysts have also been reported, but no applications have resulted from these. The mechanism for group 10 metals has not been studied (see Section 9.3.2.3). 

Detailed aspects of the catalytic mechanism remain unclear. However, influence of basic additives on the partitioning of the conventional hydrogenation and reductive cyclization manifolds coupled with the requirement of cationic rhodium pre-catalysts suggests deprotonation of a cationic rhodium(m) dihydride intermediate. Cationic rhodium hydrides are more acidic than their neutral counterparts and, in the context of hydrogenation, their deprotonation is believed to give rise to monohydride-based catalytic cycles.98,98a,98b Predicated on this... 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Monsanto also discovered significant catalytic activity for iridium/iodide catalysts however, they chose to commercialize the rhodium-based process due to its higher activity under conventional high water conditions. Despite this, detailed mechanistic studies by Forster and his colleagues were undertaken at Monsanto and revealed a catalytic mechanism for iridium which is similar to the rhodium system in many respects, but with additional complexity due to participation of both anionic and neutral complexes (see below). 

Highly stereospecific hydrogenation catalysts have thus been developed, particularly rhodium based complexes bearing a chiral ligand the synthesis of L-Dopa with RhCl (DIOP" ") catalyst is the most successful realization in this field. The mechanism of this reaction has been extensively investigated, particularly by Halpern. Bidentate coordination of the prochiral unsaturated precursor is a key factor to explain the induction of chirahty. 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The insertion of a carbene into a Z-H bond, where Z=C, Si, is generally referred to as an insertion reaction, whereas those occurring from Z=0,N are based on ylide chemistry [75]. These processes are unique to carbene chemistry and are facilitated by dirhodium(II) catalysts in preference to all others [1, 3,4]. The mechanism of this reaction involves simultaneous Z-H bond breaking, Z-car-bene C and carbene C-H bond formation, and the dissociation of the rhodium catalyst from the original carbene center [1]. 

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