Hydrogenation, catalytic cycles for

Figure C2.7.4. Catalytic cycle for hydrogenation of methyl-(Z)-a-acetamidocinnamate tire rate constants were measured at 298 K S is solvent [8],...

Figure 9.1 Catalytic cycle for hydrogenation of carbonyl compounds.

Simplified catalytic cycle for hydrogenation of C = C bonds by species derived from RhCl(PPh3)3 or from [(alkene)2RhCl]2 + PR3- Possible solvent coordination is disregarded. Cycle shows only major... 

Propose a catalytic cycle for hydrogenation step in the production of (S)-metolachlor using the cationic Ir complex described in Section 9-7-2. 

Figure 8.6 Catalytic cycle for hydrogenation of olefins using a Wilkinson catalyst (redrawn from Halpern et al., 1976). L = PPh3, = vacant coordination site, rds = ratedetermining step...

Fig. 5.20 Simplified catalytic cycle for hydrogenation of an alkene using Wilkinson s catalyst. (16), (18) mean 16-, 18-electron etc. Unsaturated species, e.g. L RhCl may have additional solvent ligands, r.d. = rate determining step. L = PPhj.

Figure 15.4 Catalytic cycle for hydrogenation of olefins using a Wilkinson s catalyst. L= PPhj,...

Enantioselection in asymmetric hydrogenation ofenamides The detailed computations of the catalytic cycles for hydrogenation of five different substrates catalyzed by Rh-BenzP complex convincingly showed... 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

Scheme 12 Proposed catalytic cycle for Casey s hydrogenation...

Scheme 38 Catalytic cycle for the hydrogen-mediated enyne cyclization...

Scheme 4.11 Proposed catalytic cycle for the hydrogenation of alkynes promoted by Pd(Ar-bian)(dmf) complexes.

An informative set of calculations was carried out by Brandt et al, coupled to experimental studies that demonstrated first-order dependence of the turnover rate on both catalyst and H2, and zero-order dependence on alkene (a-methyl-(E)-stilbene) concentration [71]. The incentive for this investigation was the absence of any characterized advanced intermediates on the catalytic pathway. As a result of the computation, a catalytic cycle (for ethene) was proposed in which H2 addition to iridium was followed by alkene coordination and migratory insertion. The critical difference in this study was the proposal that a second molecule of H2 is involved that facilitates formation of the Ir alkylhydride intermediate. In addition, the reductive elimination of R-H and re-addition of H2 are concerted. This postulate was subsequently challenged. For hydrogenation of styrene by the standard Pfaltz catalyst, ES-MS analysis of the intermediates formed at different stages in the catalytic cycle revealed only Ir(I) and Ir(III) species, supporting a cycle (at least under low-pressure conditions in the gas... 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

Scheme 8.2 Catalytic cycle for the hydrogenation of (v, ligand dissociation step 2, oxidative addition of jj /i-hydridc transfer step 5, reductive elimination...

Figure 1.2 Catalytic cycle for the Rh-catalyzed hydrogenation of methyl- Z)-cc-acetamidocinr mate, (50% C in the a-C, denoted by ) in MeOH.

Scheme 6.104 Key intermediates of the proposed catalytic cycle for the 100-catalyzed Michael addition of a,a-disubstituted aldehydes to aliphatic and aromatic nitroalkenes Formation of imine (A) and F-enamine (B), double hydrogen-bonding activation of the nitroalkene and nucleophilic enamine attack (C), zwitterionic structure (D), product-forming proton transfer, and hydrolysis.

Scheme 7 Catalytic cycle for the synthesis of hydrogen peroxide from dioxygen...

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

The generation of methane in the reaction was evidenced by the 111 NMR spectrum of the reaction mixture. It was also shown that the newly obtained complex 29 reacts catalytically with silanol 28 to give the trimer 30 (presumably from trimerization of 31) with the evolution of hydrogen gas. In the presence of MesSiOMe the same reaction resulted in the formation of an insertion product of the intermediate silanone 31 as shown in the lower part of Scheme 12. The proposed catalytic cycle for the dehydrogenation of 28 with 29 is shown in Scheme 13. It should be noted, however, that spectroscopic evidence for the proposed silanones was not presented. 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

---