Mechanism monohydride

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... 

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)... 

Further mechanistic insights into hydrogenations catalyzed by HRuCl(PPh3)3 (7, p. 83) have been obtained indirectly, from studies on hydrogenation of some ruthenium(III) phosphine complexes (83). A frequently considered mechanism for hydrogen reduction of metal salts involves slow formation of an intermediate monohydride, followed by a faster reaction between the hydride and starting complex (/, p. 72), Eqs. (2) and (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... 

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... 

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 presence of [RhH(TPPMS)3] causes substantial changes in the mechanism of hydrogenation, that most probably follows a conventional monohydride mechanism as shown in Scheme 1.1. This is also reflected in the rates and the hydrogenation selectivities [27]. 

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. 

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... 

Tetraneopentyl zirconium reacts in the same way as tetraneopentyl titanium to give, on a silica (soo), a tris(neopentyl) monografted species [32]. Treatment under H2 of this surface species yields silica-supported zirconium hydrides [33], which have been characterized as a mixture of mono- (65-70%) and bis- (35-30%) hydrides based on double quanta NMR experiments (Scheme 2.11) [34]. Interestingly, the double quantum experiment allows us to prove not only the presence of the two hydrides and the monohydride of zirconium by the presence or the absence of the double quanta correlation but also to detect the through space magnetic interaction between the zirconium monohydride and the silicon di-hydride, proving thus the spatial arrangement on the surface. This confirms the mechanism by which these hydrides have been formed on the surface. 

Figure 1.10. Catalytic cycle of BfNAP-Ru-catalyzed hydrogenation of methyl (Z)-a-aceta-midocinnamate involving a monohydride/unsaturated mechanism. The p substituents in the substrates are omitted for clarity.

Figure 1.14. Catalytic cycle of BINAP-Ru-catalyzed hydrogenation of P-keto esters involving a monohydride mechanism [P—P=(i )-BINAP S = solvent or a weak ligand].

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

The widely known Wilkinson catalyst is proposed to operate through this reaction mechanism. Computational evaluation of the full catalytic cycle showed that the rate-determining step implies the insertion and the subsequent isomerization process (27). Moreover, this catalyst has the particularity that the reaction mechanism depends on the hydrogen source since a monohydridic route has been proposed when 2-propanol is the hydrogen source (28). 

Within the monohydridic route, apart from the already explained inner-sphere mechanisms, there is another possibility involving the concerted outer-sphere transfer of one hydride and one proton to the corresponding substrate (Scheme 4b). This mechanism is very common to the so-called bifunctional catalysts. This term was proposed by Noyori for those catalysts having one hydrogen with hydridic character directly bonded to the metal center of the catalyst, a hydride ligand, and another hydrogen with protic character bonded to one of the ligands of the metal complex (20). In Scheme 9, examples of bifunctional catalysts that are synthesized... 

Scheme 18. DFT-derived mechanism for the alkyne hydrogenation by a palladium monohydride bisphospine cation.

The addition-elimination mechanism, however, is strongly preferred for monohydride systems such as [HCo(CO)4]187 and the Vaska complex193,194 promoting extensive isomerization. Hydroformylation of 2-pentenes in the presence of [HCo(CO)4], for instance, yields mainly the nonbranched aldehyde resulting from double-bond migration.195 Nickel hydride complexes are one of the most active... 

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