Dihydrogen addition

Dihydrogen addition to the enamide complex is rate-limiting and irreversible. With para-enriched hydrogen, there is no ortho-para equilibration in a dehydroamino acid turnover system until hydrogenation is complete [18] (this last precept has come under recent close scrutiny). 

Fig. 31.13 An iridium analogue for the stereochemical course of dihydrogen addition in enantioselective hydrogenation.

Figure 11. The calculated rate-determining barriers and energies of the reaction Al + 3H2 + N2 - Al + N2 - 2NH3. Numbers given without parenthesis, and in (..) and [..] correspond to the first, second and third dihydrogen addition process, respectively.

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

The liquid-phase hydrogenation of cyclohexene to cyclohexane (in an inert solvent) is conducted over solid catalyst in a semibatch reactor (dihydrogen addition to keep the total pressure constant). 

The initial observation of remarkable size-dependent reactions of deuterium with clusters of cobalt and niobium was rapidly followed by other reports of dihydrogen addition reactions with clusters of many metals, in some cases containing more than 100 atoms. Further work was reported for nio-bium " and cobalt as well as for iron, vanadium, " nickel, platinum, rhodium, tantalum, and aluminum. ... 

Bosnich and coworkers analyzed asymmetric hydrogenation using molecular graphics with MODEL-MMX (30). Dihydrogen addition to both major and minor diastereomers was analyzed for the [(5,5-CHIRAPHOS)Rh(EAC)] complex. (EAC is ethyl-A-acetyl-a-aminocinnamate.) As with the Brown approach... 

Dihydrogen addition reaction (3) is, on a comparable basis, about 15-18 kcal/mol more exothermic when M = Ir than when M = Rh (Table 2). Thus, reaction (3) is exothermic by 26-27 kcal/mol at the DFT level and by more than 40 kcal/mol at the MPn levels the CCSD(T) value is 36.6 kcal/mol, essentially twice the value for reaction (3) when M = Rh. For reaction (4), the differential exothermicity increase (M = Ir vs. Rh) is larger by approximately 25 kcal/mol. We obtain exothermicities near 50 kcal/mol at the DFT level and well above 60 kcal/mol with the MO-based correlation methods. The calculation at the CCSD(T) level predicts an exothermicity of 64.0 kcal/mol, 29 kcal/mol larger than when M = Rh. The SQP-TBP enthalpy difference is again on the order of 10... 

Since cis-1 is considerably more stable than trans-1 (Table 1), a solution of 1 would contain almost exclusively c/ -l and hence presumably overwhelmingly form cis-2 upon oxidative addition of H2. However, trans-1 is more stable than cis-2 by = 10 kcal/mol (Table 3), and it is thus possible that the dihydrogen addition bypasses cis-2 altogether. Interconversion of TBP and SQP complexes often proceeds with low activation energies (33,48), and we have located the transition state for conversion of cis-2 to the thermodynamically favored product trans-2. When M = Rh (cis-2a — trans-2a), the transition state is about 10 kcal/ mol above cis-2a. The transition state for the SQP TBP interconversion when M = Ir (cis-2b —> trans-2h) is also about 10 kcal/mol above the SQP conformer (cis-2b), so with both metals the rearrangement should be facile at ambient temperatures. The activation energy for cis-2 - trans-2 interconversion should be even less with sterically bulky phosphines that selectively destabilize cis-2. 

Fig.12. Possible routes for the dihydrogen addition step starting with the reactive diastereomer of the Me-DUPHOS complex based on X-ray crystal structures of the ligand and (separately) enamide...

A qualitative approach will possibly be more fruitful. Fig. 12 illustrates how the dihydrogen addition step (late with respect to heavy atom locations, early with respect to dihydrogen) might appear for the two diastereomeric pathways of the 16-electron route, with CHIRAPHOS as the ligand. In the alternative 14-electron route, dissociation of the alkene is assumed to occur, followed by irreversible H2 addition. The process in then consummated by reformation of the alkene-rhodium bond, or by a sigma bond metathesis which bypasses the dihydride state. 

The reactivity of the bis-dppm-bridged complexes described above, although diverse, suggests a common mechanism of dihydrogen activation initiated by a single-metal addition. After this common activation step, the feasibility of subsequent reactions such as hydride migrations, further dihydrogen additions, or reductive eliminations is determined by the specific features of each compound, which also define the nature of the final reaction product. 

Electron-richer dM compounds can also be considered as H2-activating alternatives to compounds with the unfavorable dM configuration. In the case of the bis-dppm bridged Rh(I)Ir(-I) complex 14, the d d configuration has been found to result in a metal-metal bonded species in which the coordination around the rhodium center is similar to that in planar homovalent d compounds. [47] The kinetic product of dihydrogen addition to 14 is consistent with the occurrence of a single-metal oxidative addition to the Rh(I) (Scheme 12). This kinetic product is thermally unstable and reductively eliminates methane from the iridium center. The overall reaction constitutes a clear example of bimetallic cooperation, since the oxidative addition to one center provokes a reductive elimination in the other metal. 

Mechanistic Studies Involving Dihydrogen Addition to Late Metal Centers 431... 

Formation of 14 is thought to be the rate-limiting step. The rate of dihydrogen addition to each isomer of 13 can be very different, with the minor diastereomer, thermodynamically disfavoured, reacting much faster than the major diastereomer. For the exact system of Scheme 7.7 the minor isomer re-13 reacts about 600 times faster than -13. This different reactivity and not the isomeric ratio of 13 is the parameter that dictates, to a large extent, the... 

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