Mechanistic cycle

The dissolution of palladium atoms from supported catalysts probably generates Pd species that are extremely active in the classic Heck cycle. The intrinsic nature 

Palladium colloids as active species can be excluded according to a series of experiments [20]. They act as a reservoir for molecular Pd species in solution. In addition, we do not see any reason why the surface of small Pd (nano)particles in solution should be active, the surface of the same Pd particles supported on a solid carrier, however, is not. 

Mechanistically, all catalytic systems, including heterogeneous catalysts, pal-ladacydes and pincers operate through the formation of soluble Pd(0) nanoparticles in solution. In the catalytic cycle the aryl halide oxidatively adds to the palladium atoms at the rim, leading to the formation of an anionic aryl-palladium dihalide complex, or possibly its dimer. This anionic species then completes the Heck cycle. 

Monopoli, A. Fomaro, L Sabbatini, N. Cioffi, N. Ditaranto, Organometallics 2004, 23, 5154. 

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Scheme 8.1 General mechanistic cycle proposed for Ni and Pd catalysed hydrodehalogenation...

Scheme 8.4 Proposed mechanistic cycle to account for the conversion of C F H to 1,2-C F H...

Having all the essential building blocks of the DeNO, mechanism well established and verified spectroscopically, quantum chemical modeling may be then used for providing a molecular rational for the observed structure-reactivity relationships. The first mechanistic cycle of the DeNO reaction, where NO reacting with Cu Z center is transformed into N20, involves the following steps ... 

A particular feature of the whole process is the trade-off between the key intermediates of both mechanistic cycles. While the N—N bond formation (controlled by thermal stability of the mononitrosyl intermediate) is favored by lower temperatures, the 0-0 bond formation step (constrained by endothermic decomposition of the nitrate intermediate) is favored by higher temperatures. Indeed, as revealed by operando IR studies (Figure 2.24), at low temperatures nitrates accumulate on the surface, whereas at high temperatures the surfaces is essentially depleted of the mononitrosyl complexes. The optimal reaction temperature corresponds, therefore, to a subtle balance between the rate of formation of the Cu NO Z surface complex in the early stages, and the rate of decomposition of the CuN03 Z complex in the late stages of the reaction. 

Several heteroaromatic compounds can be hydrogenated by [Rh(COD) (PPh3)2]+ species. Thus, this cationic complex has been reported to be a catalyst precursor for the homogeneous hydrogenation of heteroaromatic compounds such as quinoline [32] or benzothiophene [33]. Detailed mechanistic cycles have been proposed by Sanchez-Delgado and coworkers. The mechanism of hydrogenation of benzothiophene by the cationic rhodium(III) complex, [Rh(C5Me5) (MeCN)3]2+, has been elucidated by Fish and coworkers [34]. 

Y-chelate [Pd(C(0)CH7CH C(0)Me)(phen)] were determined through a combination of competitive and relative equilibria studies. Based on this multiform investigation, a complete mechanistic cycle of chain propagation was proposed (Scheme 7.11) [26]. 

Acetyl iodide is very reactive and it reacts efficiently with water or methanol leading to acetate compounds. Hydrolysis of acetyl iodide along with the subsequent conversion of methanol to methyl iodide are very rapid under the reaction conditions leading to a complete mechanistic cycle. 

Herzfeld and Langmuir-Hinshelwood-Hougen-Watson cycles, could be formulated and solved in terms of analytical rate expressions (19,53). These rate expressions, which were derived from mechanistic cycles, are phrased, however, in terms of the formation and destruction of molecular species without the need for computing the composition of reactive intermediates. Thus, these expressions are the relevant kinetics required for molecular models and are rooted to the mechanistic cycles only implicitly by the mechanistic rate constants. The molecular model, in turn, transforms a vector of reactant molecules into a vector of product molecules, either of which is susceptible to thermodynamic analysis. This thermodynamic analysis helps to organize these components into relevant boiling point or solubility product classes. Thus the sequence of mechanistic to molecular to global models is intact. 

A mechanistic cycle has been proposed21 for the reaction on Au/Fe2C>3 (Scheme 10.3) that bears some resemblance to that suggested for Au/Ce02 (Scheme 10.2) in that an anion vacancy near to a gold particle is required. 

Scheme 10. Proposed mechanistic cycle for Suzuki-Miyaura cross-coupling based on tbe observation of binuclear palladium complexes. L=l.l -(cyclohexylpbospbanediyl)dipiperidine.

Figure 28 Representation of the mechanistic cycle involved in the Wacker reaction the conversion of ethylene into acetaldehyde.

Scheme 6 Possible mechanistic cycles for Suzuki-Miyaura reactions...

Cyclodimerization of disubstituted alkynes R C=CR" mediated by this type of precatalyst (Ln = La, Ce Y is unreactive) seems to be limited to alkynes bearing at least one a-methyl group (R ) and a small second alkyl group (eq. (6)) [97]. Already use of R" = Et, n-Pr results in formation of two isomers. For example, formation of l,2-dimethyl-3-ethylidenecyclobutene from excess 1- butyne is complete in ca. 10 h at 80 °C, giving a TOF of 2 h. Unprecedented propargylic metalation/alkyne insertion are the key steps of the proposed mechanistic cycle. 

Schrock and Grubbs together with Chauvin who unraveled the mechanistic cycle of... 

Figure 13. Postulated mechanistic cycle for the electrocatalytic reduction of CO2 into CO by Nicyclam in water. Reprinted with permission from Ref. 184, Copyright (1986) American Chemical Society.

Again using PEGs as PT catalysts, the Alper group reinvestigated the carbonylation of benzyl halides in the presence of Co2(CO)8. They were able to characterize and investigate -benzyl, 3-benzyl, and ( -phenylacetyl) cobalt carbonyls as intermediates, and arrived at an elaborate mechanistic cycle [84],... 

The overall mechanism in Fig. 19 is satisfying on several counts. First, as mentioned above, a proper stoichiometry of one proton and three electrons needed for 0—0 bond cleavage is satisfied. Second, it clearly identifies the importance of the superoxo as the resting state for catalysis, a fact long known for the O2 reduction chemistry of Pacman porphyrins (54). Finally, the mechanistic cycle... 

C. acidiurici enzyme, and transfer of a second electron in the H. halobium reaction. The mechanistic cycle is then completed with the reoxidation of both enzymes by suitable electron acceptors. 

Figure 20 A chemical mechanism for the action of 5,6-LAM on (S)-lysine. In this mechanism, the 5 -deoxyadenosyl radical from coenzyme B12 abstracts hydrogen from C5 of the lysyl side chain in the internal PLP aldimine. The resultant free radical 1 undergoes isomerization by internal cyclization to the azacyclopropyl carbinyl radical 2, which opens to the primary C6 radical 3. Hydrogen abstraction from the methyl group of 5 -deoxyadenosine and release of 2,5-diaminohexanoate completes the mechanistic cycle.

Mechanistic studies of the diacetoxylation of alkenes using (diacetoxyiodo)benzene have demonstrated a protio-catalytic nature of this reaction [255]. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing additives indicate that strong acids act as catalysts in the reaction. When trifluoromethanesulfonic acid is used as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts, such as Pd(II) and Cu(II) metal cations. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction was proposed in this work [255]. 

The resulting mechanistic cycle is shown in Figure 6.9. Analogous to the Schrock cycle, a strictly alternating sequence of protonation and reduction steps is derived. In the first half of the cycle the N-N bond is weakened step by step, leading to cleavage of the N-N bond and release of the first molecule of ammonia after formation of the hydrazidium complex. In the second half of the cycle, the Ru-N bond is progressively weakened and the... 

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