Coordination of the Substrate

Homogeneous catalysis dissociation association oxidative addition reductive elimination 

In square-planar, 16-electron complexes (i.e. coordinatively unsaturated) as found for many group 9 (Co, Rh, Ir) and 10 (Ni, Pd, Pt) metals, the associative process is most common. The trans effect and trans influence ligand series [19-23] are also useful measures in the study of homogeneous catalysis. Apart from very small ligands, such as CO, H2, and NO, steric repulsion between ligands, as well as complexes and incoming substrates, plays a dominant role in determining the kind of intermediates and complexes formed and their equilibria in solution. 

Carbon monoxide and ethylene are common substrates involved in homogeneous catalysis. The bonding of carbon monoxide to a transition metal has been depicted in Fig. 4.4. The bonding of alkenes to transition metals is described by the Chatt-Dewar-Duncanson scheme involving c donation by the filled it orbital of the alkene, and n back donation from the metal into the n orbital of the alkene (see Fig. 4.6). 

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In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

It was shown that dibenzothiophene oxide 17 is inert to 1-benzyl-l,4-dihydro nicotinamide (BNAH) but that, in the presence of catalytic amounts of metalloporphyrin, 17 is reduced quantitatively by BNAH. From experimental results with different catalysts [meso-tetraphenylporphinato iron(III) chloride (TPPFeCl) being the best] and a series of substituted sulfoxides, Oae and coworkers80 suggest an initial SET from BNAH to Fe1 followed by a second SET from the catalyst to the sulfoxide. The results are also consistent with an initial coordination of the substrate to Fem, thus weakening the sulfur-oxygen bond in a way reminiscent of the reduction of sulfoxides with sodium borohydride in the presence of catalytic amounts of cobalt chloride81. 

As a mechanistic hypothesis, the authors assumed a reduction of the Fe(+2) by magnesium and subsequent coordination of the substrates, followed by oxidative coupling to form alkyl allyl complex 112a. A ti—c rearrangement, followed by a syn p-hydride elimination and reductive elimination, yields the linear product 114 with the 1,2-disubstituted ( )-double bond (Scheme 29). This hypothesis has been supported by deuterium labeling experiments, whereas the influence of the ligand on the regioselectivity still remains unclear. 

Although the major manifold is neglected dne to high enantioselectivity, the intermediate in the major manifold formed from the coordination of the substrate with the active catalyst Xq should be included in the kinetic model. This is because this intermediate is the resting station for bulk of the catalyst but in a rapid exchange with Xi in the minor manifold The intermediates in the kinetic model and the relationship among the intermediates are expressed in Figure 3.2B. 

Allylic halides (example 19, Table VII) carbonylate under very mild conditions. An inverse effect of the CO pressure was observed in reaction with Ni(CO)4, CO dissociation being required to allow coordination of the substrate (168). 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis ... 

An essential step in processes utilizing soluble transition metal catalysts is the coordination of the substrate to the transition metal (5.). A corequisite is the availability of a vacant site in the coordination sphere of the metal for substrate binding, a provision often met by dissociation of a bonded... 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

The initial coordination of the substrate is rate-limiting however, the reductive elimination of the alkane is considered to be rapid ... 

Motorina and Grierson (224) examined the use of bis(oxazoline)-Cu(II) complexes as chiral Lewis acids in the intramolecular heterocycloaddition of azadienes, Eq. 184. Very low selectivities are observed in the cycloaddition of 326. The authors speculate that monodentate coordination of the substrate to copper is responsible for the low selectivity. 

When the diphosphine is chiral, binding of a prochiral alkene creates diastereomeric catalyst-alkene adducts. (Diastereomers result because binding of a prochiral alkene to a metal center generates a stereogenic center at the site of unsaturation.) Through a powerful combination of3lP and l3C NMR methods, Brown and Chaloner first demonstrated the presence of two diastereomeric catalyst-enamide adducts with bidentate coordination of the substrate to the metal (Figure 1) [19]. 

Another way of looking at the question of creation of a vacant site and coordination of the substrate is the classical way by which substitution reactions are described (Figure 2.1). Two extreme mechanisms are distinguished, an associative and a dissociative one. In the dissociative mechanism the ratecontrolling step is the breaking of the bond between the metal and the leaving ligand. A solvent molecule occupies the open site, which is a phenomenon that does not appear in the rate equation. Subsequently the solvent is replaced by the... 

The reactions can be carried out in aqueous solutions or biphasic mixtures of the substrates with no additional solvent, in the presence of NaOAc (pH s 11.5) at 100 °C. At this pH the resting state of the catalyst is probably the dinuclear species depicted on Scheme 8.1, which falls apart upon coordination of the substrate alcohol. In this respect the catalyst system as very similar to that for the oxidation of terminal olefins [10,11]. Good results were obtained with 30 bar of air, however, an 8 % O2/N2 mixture can also be used, which further improves the safety of the process. Recycling of the aqueous catalyst solution is possible and is especially easy in case of biphasic reaction mixtures. Taking all these features, this Pd-catalyzed oxidation of alcohols is a green process, indeed. 

The proposed catalytic cycle is outlined in Scheme 9.9 [14]. Dimeric complex 23 is cleaved to give the monomeric complex 24 by solvation, substrate binding, or reaction with the nucleophile. Reversible exo-coordination of the substrate is followed by oxidative insertion with retention into a bridgehead C-O bond to give the 7r-aUyl or u-rho-dium aUcoxide complexes 26 or 27. It is likely that the formation of these rhodium] 111) aUcoxide complexes is irreversible due to the release of the ring strain present in the oxabicyclic aUcene substrate. The oxidative cleavage of the C-O bond is proposed to be the enantiodiscriminating step in the catalytic cycle. 

As far as the epoxidation of allylic alcohols with chiral titanium derivatives is concerned, there is agreement in the literature that the oxygen transfer step can be described as in equation 16. An /j -bound alkylperoxo moiety is present in the coordination sphere of the metal, and a preliminary coordination of the substrate through its hydroxylic function to the titanium center is required. 

The hydrogenation reaction mechanisms may be classified according to the role played by the substrate in the coordination sphere of the metal catalyst. Thus, those mechanisms proceeding with coordination of the substrate to the metal center can be labeled as inner-sphere mechanisms, whereas those with no direct coordination of the substrate to the metal center can be labeled as outer-sphere reaction mechanism (see Scheme 4). Hydrogenation reactions belonging to the so-called hydrogen transfer reactions (where the hydrogen source is usually an alcohol) can be also classified within these two families of reaction... 

Thus, for alkene hydrogenation, after the coordination of the substrate a metal-alkyl intermediate is formed by insertion of... 

The previous section described a two-step mechanism. The first step is the coordination of the substrate into the metal coordination sphere. The second is the most characteristic step within the inner-sphere mechanism the insertion of the substrate into the M-H bond. Nevertheless there are other mechanistic options that include neither substrate coordination nor M—H insertion. They are outer-sphere mechanisms and in turn can be classified as bifunctional and ionic mechanisms. 

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

Step 1. Coordination of the substrate immediately after mixing the Cu(II) catalyst with the substrate, a rapid change in the absorption is observed within several decaseconds [Fig. 27(b)] this is believed to be caused by the coordination of the substrate to the Cu(II) complex. We measured this rapid change spectroscopically by the stopped-flow method, and calculated the apparent rate constant k. When an insoluble polymer complex is used as a catalyst, a decrease in the XOH concentration in the liquid phase corresponds to the coordination of XOH to the Cu catalyst in the solid phase155. ... 

As Table 15 shows, the rate constant k of the polymer catalyst system is smaller than that of the monomeric pyridine-Cu catalyst, because of the bulky polymer ligand obstructing the coordination of the substrate to the catalyst153. ... 

On the other hand, an attempt to accelerate the step of coordination of the substrate to the Cu catalyst was successful because it used the hydrophobic domain of the polymer ligand156 That was the oxidation catalyzed by polymer-Cu complexes in a dilute aqueous solution of phenol, which occurred slowly. The substrate was concentrated in the domain of the polymer catalyst and was effectively catalyzed by Cu in the domain. A relationship was found to exist between the equilibrium constant (Ka) for the adsorption of phenol on the polymer ligand and the catalytic activity (V) of the polymer-ligand-Cu complex for various polymer ligands K a = 0.21 1/mol and V = 1(T6 mol/1 min for QPVP, K a = 26 and V = 1(T4 for PVP, K a = 52 and V = 10-4 for the copolymer of styrene and 4-vinylpyridine (PSP) (styrene content 20%), and K a = 109 and V = 10-3 for PSP (styrene content 40%). The V value was proportional to the Ka value, and both Ka and V increased with the hydrophobicity of the polymer ligand. The oxidation rate catalyzed by the polymer-Cu complex in aqueous solutions depended on the adsorption capacity of the polymer domain. 

It is apparent that coordination of the substrate with the metal ion would increase its reactivity toward hydroxyl ions and other nucleophilic reagents. 

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