Monohydride catalysts

Monohydride Hydrogenation Catalysts. Monohydride (MH) catalysts react with alkenes, according to Figure 5, to yield metal-alkyl intermediates, which by subsequent reaction with hydrogen regenerate the initial monohydride catalyst. This mechanism is usually adopted by hydrogenation catalysts that contain an M—H bond (Fig. 5). 

Since the first reports on Wilkinson s catalyst,19,20 many transition-metal-based catalytic systems for hydrogenation of unsaturated organic molecules have been developed. Two major pathways seem to occur, one involving monohydride (M—11) species, and the other, dihydride (MH2)... 

A monohydride mechanism is not operating in reactions catalyzed by these complexes. Noyori observed that the presence of an NH or NH2 in the auxiliary ligands was crucial for catalytic activity, the corresponding dialkylamino analogs being totally ineffective. These findings indicate a novel metal-ligand bifunctional cycle (Scheme 28) KOH reacts with the pre-catalyst (87)... 

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... 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

Recently, the dissimilar complex [RuCl2(p-cymene)]2 has also demonstrated excellent selectivity for the (Z)-vinylsilane products for a variety of substrates (see Table 5). Whether or not this complex also acts as a monohydride-type hydrosilylation catalyst—as do the vast majority of well understood systems—is an open question. The selectivities for (Z)-/3-vinylsilane products are some of the best yet reported, especially for a-branched substrates (entry 4). From a synthetic point of view, the catalyst is exciting, but at present is limited to trialkyl- and triphenylsilanes in intermolecular applications (see Scheme 21), presenting problems for some applications. 

A kinetic analysis of the styrene hydrogenation catalyzed by [Pt2(P205H2)4]4 [66] was indicative of the fact that the dinuclear core of the catalyst was maintained during hydrogenation. However, three speculative mechanisms were in agreement with the kinetic data, which mainly differ in the H2 activation step. This in fact can occur through the formation of two Pt-monohydrides, still connected by a Pt-Pt bond, or through the formation of two independent Pt-monohydrides. The third mechanism involves the dissociation of a phosphine from one Pt center, with subsequent oxidative addition of H2 to produce a Pt-dihy-dride intermediate. 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

A similar mechanism was postulated for the Ti-catalyzed reactions by Buch-wald [21, 87]. The active catalyst was proposed to be the monohydride species ebthi-Ti-H, produced by reacting ebthi-TiR2 with n-BuLi followed by phenylsi-... 

In the case of hydrogenation using [Ru(BINAP)Cl2]n as the catalyst precursor, the reaction seems to occur by a monohydride mechanism as shown in Scheme 6-31. On exposure to hydrogen, RuC12 loses chloride to form RuHCl species A, which in turn reversibly forms the keto ester complex B. Hydride transfer occurs in B from the Ru center to the coordinated ketone to form C. The reaction of D with hydrogen completes the catalytic cycle.67... 

The Rh dimer after H2 adsorption exhibited similar EXAFS oscillation and Fourier transform to those for the fresh imprinted catalyst Detailed analysis of the EXAFS data confirmed retention of the local conformation of the Rh dimer with a Rh-Rh bond (CN = 1.3 0.4), two Rh-O bonds (CN = 1.7 0.5) and a Rh-P bond (CN = 1.2 0.2). No formation of Rh metallic particles was observed. However the Rh-Rh bond contracted from 0.268 0.001 to 0.265 0.001 nm with the hydride dimer, indicating stabilizahon of the dimer structure by electronic rearrangement due to monohydride coordination on both Rh atoms in the dimer. After reaction of the Rh-dimer hydride species with 3-methyl-2-pentene, the shrunken Rh-Rh bond of the monohydride species expanded again to recover the... 

The hydrogen atom is then transferred to the terminal carbon atom. It is of interest to note that, according to this scheme, the ligand acts as co-catalyst for its own formation. That the isomerisation does not occur with, for example platinum(II), is presumably due to the great difficulty of forming a monohydride of platinum(IV) as the reaction intermediate. 

All of the intermediates in the mechanistic picture of Figure 9.14, i.e the catalyst, both substrate complexes [29], and both monohydrides [30-32], have been characterized by NMR spectroscopy, except one vital key intermediate the catalyst-sub-strate-dihydrido complex. Without characterization of this molecule, the whole mechanistic model remains uncertain. In the following, the hunt for the dihydride is described. 

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