Rhodium monohydrides

Figure 9.15 Rhodium monohydrides in enantioselective enamide hydrogenation that were characterized by conventional NMR spectroscopy.

With respect to the formation of mono- and dihydride sites, Schrock and Osborn (16) observed an interesting equilibration between a rhodium monohydride complex and a rhodium dihydride complex as expressed by Eqs. (7) and (8). 

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... 

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... 

Monohydride (MH) catalysts, such as [RhH(CO)(PPh3)3], react with substrates such as alkenes, according to Scheme 1.1, yielding rhodium-alkyl intermediates which, by subsequent reaction with hydrogen, regenerate the initial monohydride catalyst. This mechanism is usually adopted by hydrogenation catalysts which contain an M-H bond. 

The characterization of the rhodium complexes formed under hydroformylation conditions by NMR techniques and in situ IR spectroscopy showed that there is a relationship between the structure of the [HRh(CO)2 (BINAPHOS)] species and their enantiodiscriminating performance. Thus, (R,S)- and (S,R)-BINAPHOS ligands show high equatorial-axial (ea) coordination preference with the phosphite moiety in the axial position. Meanwhile, the characterization of the (R,R)- and (S,S)-BINAPHOS ligands suggests that there is either a structural deviation of the monohydride complexes from an ideal TBP structure or an equilibrium between isomers [20,34],... 

Th effect of pH on the rate of hydrogenation of water-soluble unsaturated carboxylic acids and alcohols catalyzed by rhodium complexes with PNS [24], PTA [29], or MePTA r [32] phosphine ligands can be similarly explained by the formation of monohydride complexes, [RhHPJ, facilitated with increasing basicity ofthe solvent. 

From the sum of these NMR investigations, the chemical shift and the corresponding coupling constants for a hypothetical rhodium-substrate-dihydride complex can be rationalized. One of the two hydrogen signals should fit the scheme of the well-known monohydride complexes (5 -20 ppm being trans to an oxygen... 

Monohydrides play an important role in the following rhodium-complex-catalyzed hydrogenations in aqueous media. The catalyst precursor is [RhCl(PTA)3], which gives the catalytically active species (HRh(PTA)3] formed by dehydrochlorination of the primary product of H2 oxidative addition (88). The complex is an active catalyst for several reactants, e.g., olefinic and oxo adds, allyl alcohol, and sulfostyrene. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

This hypothesis is supported by Lin s studies of the reactions of the Cg to C13 1,2-cycloalkadienes with HRh(PPh3)4 and ClRh(PPh3)3, complexes classified by Collman et al. as monohydride and dihydride hydrogenation catalysts, respectively. The monohydride complex unites with allenes to form TT-allyl compounds which have characteristic NMR spectra in solution. 1,2-Cyclononadiene yields almost the same proportion of cis- and trani-cyclononene with either Pd/Al203 or HRh(PPh3)4 as catalyst little of the trans isomer is formed when ClRh(PPh3)3 is used. The X-ray crystal structure of the complex formed from the reaction of ClRh(PPh3)3 with 1,2-cyclononadiene shows that only one double bond of the diene is coordinated with rhodium. ... 

Similarly, there are examples of rhodium(I) and iridium(I) tertiary phosphine complexes that form isolable dihydrides, which with separate treatment with external base yield monohydrides, equation (k) . Hydrogenations catalyzed by rran5-RhCl(CO)(PPh3)2 " may involve rrani-RhH(CO)(PPh3)2 formed according to equation (1) via an undetected dihydride intermediate. In some aminophosphine analogues, a coordinated N atom may act as proton acceptor s. 

Pathways corresponding to those of Scheme 3 are applicable for hydrogenation of alkynes by monohydride catalysts, the key intermediate now is a vinyl rather than an alkyl (10). Some rhodium complexes effective for hydrogenation of alkynes to alkenes were mentioned earlier and are listed in Table 3 with other rhodium catalysts. 

Coordination of the -SOT group of the ligand to the rhodium may, indeed, be important in the observed effect. It was found by Buriak and Osborn [146,147] that in microemulsions, prepared with the surfactant AOT (Scheme 3.11) the sulfonate group of AOT did coordinate to rhodium in the [Rh (-)-DBPP (NBD)]+ complex. It was suggested that this led to an easier deprotonation of an intermediate dihydride species in case of RS03 than for example in case of T (Scheme 3.28), i.e. to a switch from a dihydride route of hydrogenation to a monohydride pathway. How this would lead to high enantioselection still remains elusive. 

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