Unsaturated/dihydride mechanism

For these systems there are basically two general mechanisms, commonly known as unsaturated and dihydride mechanisms (Scheme 7 6) 69,79,82,83... 

These studies, although they did not deliver the much desired direct characterisation of a dihydride intermediate with the coordinated alkene, opened the door to the alternative mechanism, known as dihydride mechanism. An example of this mechanism, the hydrogenation of MAC with Rh/t-Bu-BisP complex, is depicted in Scheme 7.8. As with the unsaturated mechanism, this particular case is presented in the scheme, the mechanism is discussed in a broader context. 

This is illustrated in Scheme 1.8. Hydrogenation of BenzP -Rh catalyst 8 is characterized by relatively high activation barrier (Table 1.1, entry 2), and in the case of the substrate with unsubstituted p-carbon atom, the achvahon of dihydrogen proceeds via unsaturated mechanism with lower achvahon barrier. Nevertheless, with the substrate having p-subshtuent, the achvahon barrier for oxidahve addihon to a chelating catalyst-substrate complex becomes very high, and H2 achvahon proceeds via dihydride mechanism. ... 

The experimental data are nicely reproduced computationally for substrate 19. Although the unsaturated mechanism in this case is just moderately l -stereoselective, it probably only marginally interferes with the dihydride mechanism either at 298 or 173 K and the order of enanti-oselection is mostly determined by the AG values, which are quite close to the corresponding AG p. 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

Time-proven concepts for the reaction mechanisms of homogeneous hydrogenations follow two approaches which, according to Halperrfs step-wise analysis of hydrogenations using Wilkinsorfs catalysr [25] and the cationic catalyst DI-PHOS [26], respectively, can be grouped into the so-called dihydride or unsaturate routes [27] (Fig. 12.9). 

Figure 1.7. Catalytic hydrogenation of Al-acylated dehydroamino esters via dihydride/ unsaturate mechanism the p substituents in the substrates are omitted for clarity [P-P = (i ,i )-r-Bu-BisP S = solvent or a weak ligand].

Hydrogenation of unsaturated substrates. Hydrogenation of alkenes and alkynes is one of the most widely useful reactions involving dihydrogen activation. The mechanism of Wilkinson s complex offers a good example of the process (Fig. 2.3) [100]. The cycle shown is just one of several that operate in the real system, depending on the exact conditions. It serves to show a key point, that the activation step is the hrst step in the cycle. As in the Vaska case, the ligands fold back so that the cis-dihydride is formed. An early NMR experiment (Fig. 

The extraordinary results in Rh-catalysed enantioselective hydrogenation have run in parallel to tremendous progress into mechanistic understanding since the early days of the reaction. At present, the border between the classical unsaturated and dihydride reaction mechanisms is more blurred than ever, leading to an overall mechanism that is dependent on (and can be accommodated to) the exact substrate, ligand and reaction conditions. 

Heck has formulated a mechanism which accounts for hydroformylation of olefins catalyzed by cobalt carbonyl (68). A modification of this mechanism is presented in Fig. 5. Cobalt octacarbonyl reacts with hydrogen to form the tetracarbonyl hydride. It is proposed that this coordinatively saturated complex loses a CO group to form the four-coordinate hydride (LX). Coordination of an olefin yields the olefin complex (LXI). Migration of hydride yields an unsaturated alkyl complex (LXII). Further insertion of a CO group (undoubtedly by a migration mechanism) affords the four-coordinate acyl cobalt(I) complex (LXIII). Oxidative addition of hydrogen affords the hypothetical dihydride (LXIV), which eliminates the product aldehyde and regenerates the cobalt(I) hydride catalyst (LX). This latter... 

Reactions of solvate dihydrides with prochiral substrates The accessibility of the solvate dihydrides for some catalysts (Schemes 1.10 and 1.12) makes possible the study of their direct reactions with prochiral substrates. These experiments can provide information on the late intermediates in the catalytic cycle. Besides, since the substrate is introduced in the system at very low temperature after the hydrogen activation already took place, the input from unsaturated mechanism is effectively excluded, and checking the ee of the recovered product gives direct information about the effectiveness of the enantioselection by coordination of a substrate to the octahedral solvate dihydride. 

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