Proposed Catalytic Cycle

Once 26 or 27 has been formed, the rhodium-aUcoxide complex is protonated by a nucdeophile molecule, generating the cationic rhodium complex 29 and an alkoxide or phenoxide nucdeophile. This proton transfer step is supported by kinetics experiments and has two effects [14]. Firstly, the organorhodium species is made more electrophilic as a result of the positive charge, and secondly, the nucleophile is rendered more nu-cdeophihc by becoming deprotonated. 

The positioning of the rhodium metal on the n -allyl moiety will influence the regios-electivity of nucleophilic attack. Nucleophilic attack with inversion is proposed to occur adjacent to the alkoxy group in an 8 2 fashion relative to the rhodium metal. The product is subsequently liberated and the regenerated rhodium monomer will either reform the dimer (if another rhodium monomer is encountered) or continue the catalytic cycle. 

32 by an ammonium salt will give the cationic complex 33. Intermediate 33 will react with the nucleophile to give the ring-opened product and regenerate the catalyst. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

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The palladium-catalyzed hnear telomerization of 1,3-bntklienes provides a useful method for thepreparation of functionalized alkenes. A proposed catalytic cycle for the paliadinm-catalyzed... 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

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

The proposed catalytic cycle for the dehydrogenation of alcohols to ketones is shown in Scheme 15. The initial reaction of 17 with H2O affords the hydride complex a and C02- Dehydrogenation of a by acetone gives the active species b and 2-propanol. The subsequent reaction of b with the alcohol yields the corresponding ketone and regenerates a to complete the catalytic cycle. 

Scheme 15 Proposed catalytic cycle for dehydrogenation of alcohols to ketones...

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

Scheme 34 Proposed catalytic cycle for the C-C bond formation...

The proposed catalytic cycle is based on that reported by Bergman et al. with zir-conium-imido complexes (see above, Scheme 4-15). 

Scheme 5-14 Stoichiometric reactions of Pt(Me-Duphos) complexes relevant to the proposed catalytic cycle for asymmetric hydrophosphination...

Fig. 4. Proposed catalytic cycle for the hydroxylation of methane by MMO. The reductase and B components may interact with the hydroxylase in one or more steps of the cycle. Protons are shown in the step in which intermediate Q is generated, but other possibilities exist (see Fig. 3 and the text). The curved line represents a bridging glutamate carboxylate ligand.

Scheme 22. Proposed catalytic cycle for carbonic anhydrase.

Scheme 1.52. Proposed catalytic cycle for the cationic domino rearrangement/hetero-Tishchenko reduction process of secondary a-hydroxy epoxides in the presence of Sml2.

Scheme 2.25. Proposed catalytic cycle and stereochemical model.

Figure 9 The proposed catalytic cycle of asymmetric aminohydroxylation.

The laccases, classed as polyphenol oxidases, catalyze the oxidation of diphenols, polyamines, as well as some inorganic ions, coupled to the four-electron reduction of oxygen to water see Fig. 12.4 for the proposed catalytic cycle. Due to this broad specificity, and the recognition that this specificity can be extended by the use of redox mediators [27], laccases show promise in a range of applications [28], from biosensors [29-32], biobleaching [27, 33-35] or biodegradation [36], to biocatalytic fuel cells [1-3, 18, 26, 37-42]. 

FIGURE 12.4 Proposed catalytic cycle for laccase, where S represents substrate. (From [44], with permission from the American Chemical Society.)... 

Only recently (2002) have the very first examples of the transition metal-catalyzed incorporation of acrylate monomers into linear polyethylene been demonstrated. In our opinion the most notable report is that of Drent and coworkers [48] who describe the use of a neutral palladium catalyst with a chelating P-O ligand to generate linear copolymers that included the incorporation of acrylate monomers (Drent s catalyst and proposed catalytic cycle are shown in Scheme 2). In these early results, there was only minor acrylate incorporation (limited to some 3-17 mol%) and the resulting polymers were of very low molecular weight (Mn 4000-15,000). 

Figure 8.1 Representation of proposed catalytic cycle for reaction to produce CsHgO (Chong and Sharpless, 1977)...

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

The proposed catalytic cycle of the ruthenium-catalyzed intermolecular Alder-ene reaction is shown in Scheme 21 (cycle A) and proceeds via ruthenacyclopentane 100. Support for this mechanism is derived from the observation that the intermediate can be trapped intramolecularly by an alcohol or amine nucleophile to form the corresponding five-or six-membered heterocycle (Scheme 21, cycle B and Equation (66)).74,75 Four- and seven-membered rings cannot be formed via this methodology, presumably because the competing /3-hydride elimination is faster than interception of the transition state for these substrates, 101 and 102, only the formal Alder-ene product is observed (Equations (67) and (68)). 

A variety of six-membered carbocycles and heterocycles were synthesized by Shibata et al.81 using Wilkinson s catalyst (Equation (79)). The proposed catalytic cycle (Scheme 24) rationalizes the exclusive formation of the (Z)-isomer. Additionally, the mechanism is supported by the results of a isotope-labeling study reported by Brummond... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

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

Fig. 32.32 Proposed catalytic cycle of hydrogenation of simple ketones with the TolBINAP/DPEN-Ru catalyst.

More recently, Kobayashi and co-workers reported on Zr-catalyzed additions of ketene and thioketene acetals to a range of aromatic and aliphatic aldehydes (Scheme 6.25) [83], As in the Erker study, the presence of protic additives proved critical here as well. As the example in Scheme 6.25 illustrates, the addition of larger amounts of iPrOH improved the yield and ee it was reported that in the absence of the alcohol additive much lower yield and enantioselectivities" were attained. The proposed catalytic cycle, depicted in Scheme 6.25, provides a plausible rationale for the role of the additive Si transfer is facilitated by iPrOH to regenerate the chiral catalyst. Finally, it is worthy of mention... 

Figure 32 displays the proposed catalytic cycle for model complex E, which stresses the resemblance to that given for the enzyme GO in Fig. 8. 

Figure 13.2 Proposed catalytic cycle for the selective delignification of wood (lignocellulose) fibres with an equilibrated ensemble of POMs. (From Weinstock, I. A. et al., Nature, 414, 191,2001.)...

Figure 13.4 Proposed catalytic cycle for activation of molecular oxygen and substrate by the ruthenium-substituted sandwich-type [WZnRu23 (XW90 34)2]11 (X=Zn2+ or Co2 ). (From Neumann, R., and Dahan, M., Nature, 388, 353, 1997 and Neumann, R., and Dahan, M., J. Am. Chem. Soc., 120, 11969, 1998.)...

Figure 13.5 Proposed catalytic cycle for the epoxidation of alkenes with H202 by H3PW12O40. (From Ishii, Y. et al J. Org. Chem., 53, 3587, 1988.)...

Figure 13.8 Proposed catalytic cycle for the epoxidation of alkenes by [jc-C5H5NC16H33]3[P04(WO)4] catalyst coupled with the 2-ethylanthraquinone/2-ethylanthrahydroquinone redox process. (From Xi, Z. et al., Science, 292, 1139, 2001.)...

Scheme 10 Proposed catalytic cycle of the CuF-Tol-BINAP catalysis...

Fig. 10.1. Proposed catalytic cycle of copper-catalyzed conjugate addition.

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