Catalysis cycles

LLB, a so-called heterobimetallic catalyst, is believed to activate both nucleophiles and electrophiles.162 For the hydrophosphonylation of comparatively unreactive aldehydes, the activated phosphite can react with only the molecules precoordinated to lanthanum (route A). The less favored route (B) is a competing reaction between Li-activated phosphite and unactivated aldehyde, and this unfavored reaction can be minimized if aldehydes are introduced slowly to the reaction mixture, thus maximizing the ratio of activated to inactivated aldehyde present in solution. Route A regenerates the catalyst and completes the catalysis cycle (Fig. 2-9). 

Scheme 4-38. Proposed mechanism for asymmetric aminohydroxylation. Sequence of steps in the first catalysis cycle (left) (1) addition (a1), (2) reoxidation (O), (3) hydrolysis (h1) in the second catalysis cycle (right) (1) addition (a2), (2) hydrolysis (h2), (3) reoxidation (O). The first cycle proceeds with high ee, the second with low ee. L = chiral ligand X = CH3SO2. ... 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

Besides proving the formation of p-chelates [Pd(CH7CH7C(0)Me)(P-P)] at room temperature, the spectra showed the occurrence of chain-transfer by protonolysis with adventitious water to give the p-hydroxo compounds cis/trans [Pd(p-OH)(P-P)]2 as well as the conversion of the latter compounds into cis/trans bis-chelates [Pd(P-P)2] (Chart 7.2) [5f]. Independent experiments with isolated compounds showed that the p-OH and bis-chelate complexes are not dead ends, and can reenter the catalysis cycle to give alternating polyketones. 

Mechanistically, the authors favored a thiourea 9-assisted heterolysis of the orthoester through hydrogen bonding as the entry into the catalysis cycle of the organocatalytic acetalization. The orthoester was suggested to serve as the source of the alcoholate, which rapidly attacks the carbonyl compound to form a... 

Scheme 48 Proposed tandem catalysis cycle for 13/Cp2ZrCl2. Reagents (i) MAO activation.

Fig. 12.3 (a) Tandem catalysis cycle coordinating the actions of three well... 

Fig. 8.1 Nucleophilic catalysis cycle for a Type I acyl transfer process [35],...

Fig. 8.7 Lewis base and Bmnsted acid catalysis cycles for Type II acyl transfer processes [30, 62],...

A single muon stopped in a target of deuterium-tritium mixture can catalyze more than 100 fusions, but this number is limited by two major bottle-necks. One is the rate at which a muon can go through the catalysis cycle before its decay (cycling rate), and another is a poisoning process called p-a sticking in which, with a probability u)s < 0.01, the muon gets captured after the fusion reaction to atomic bound states of the fusion product 4He, and hence lost from the cycle (see Section 5). 

The catalysis cycle comprises the following steps oxidative addition of H2 to the metal coordination of the benzothiophene in the 77 -mode hydride migration and, finally, elimination of dihydrobenzo[3]thiophene by reductive coupling of the hydride and dihydrobenzothienyl ligands (Scheme 73). Based on this, various ruthenium and rhodium complexes have been developed, which exibit good catalytic activity. 

Scheme 3. Catalysis cycles tor coball-catalyzed hydroformylation.

Scheme 4, Catalysis cycle for the cobalt-catalyzed hydrogenation of aldehydes ...

Recently, open-tipped M0S2 nanotubes were prepared by the decomposition of ball-milled ammonium thiomolybdate powder under a H2-thiophene atmosphere, and used as catalyst for the methanation of CO with H2 [260]. The conversion of CO to CH4 was achieved at a much lower temperature compared to polycrystalline M0S2 particles, and there was no deterioration even after 50 h of consecutive catalysis cycles. This observation is of importance in the context of energy conversion of global CO2. 

Figure 2.23. Catalysis cycle for the steam reforming of methanol involving different kinds of surface sites A and B. Adapted from Frank et al.188...

The catalysis cycle proposed for the opening and hydrogenation of BT to ETPNa does not diverge much from those previously reported for the hydrogenolysis of BT to 2-ethylthiophenol [4, 13 d] and of thiophene to 1-butanethiol [7] catalyzed by [(TRIPHOS)RhH]. The mechanism illustrated in Scheme 4 involves the usual steps of C—S insertion, hydrogenation of the C—S-inserted BT to 2-ethylthiopheno-late, and base-assisted reductive elimination of the thiol to complete the cycle. 

While the very rigid and, with respect to the type of donor groups and their geometric disposition (two trans-disposed pyridine donors and two cis-oriented tertiary amines), enforced and inflexible geometry precludes an accurate structural and spectroscopic modeling of copper proteins, it was especially feature (3.) that lead to the isolation and characterization of novel model complexes with hemocyanine- and catechol oxidase activities properties (81, 192, 196, 213). In the latter case, it was possible to isolate and structurally characterize complexes with coordinated catechol model substrates with structural features, which have been proposed to be of relevance in the enzyme catalysis cycle, but have not been observed before in low molecular weight complexes (192, 213). 

Pd(ll) precursors are often used in these catalytic reactions, which lead to the ongoing question about how reduction to the Pd(0) species occurs. Alcohol solvent, NEts base, and phosphine ligands have all been proposed to be reducing agents for producing the active Pd(0) catalyst species. There also exists the possibility of having a Pd(u)-Pd(iv) catalysis cycle with a similar sequence of steps. ... 

The other significant difference between the two bimetallic catalysts is that the monocationic monohydride dirhodium catalyst needs to oxidatively add Hj in order to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic dihydride system has the second hydride already present and ready to go for the acyl reductive elimination step. H2 then oxidatively adds to the dicationic catalyst to regenerate the dihydride llr/llr. But since the monocationic catalyst system has a low activation barrier for H2 oxidative addition (8.7 kcal), this is not a bottleneck in the catalysis cycle. 

Employing the high potential of iminium ion activation, MacMillan and coworkers were able to directly access the key intermediate 320 in a single operation cascade reaction between the tryptamine-derived indole derivate 321 and propynal (226) in the presence of their trademark catalyst 227. As illustrated in Scheme 74, the reaction is assumed to proceed via two iminium catalysis cycles (Diels-Alder cyclization first, followed by conjugate addition). It is worth noting that the authors considered either the possibility of intermediate 322 to directly enter the second... 

Fig. 3 Generic Pd(0)/Pd(II) catalysis cycle for oxidative C-H cross-coupling...

Pd(0) to Pd(II) by an external oxidant would close the catalysis cycle. While biaryl Pd(ll) intermediate 5 is typically proposed to undergo direct C-C reductive elimination, in the presence of an external oxidant, 5 could potentially be oxidized to a higher-valent species prior to the C-C bond-forming event [54, 55]. 

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