Side Reactions in the Catalytic Cycle

Successful work with peroxidases requires knowledge about the possible side reactions in the catalytic cycle. The most important are shown in Scheme 5 [39]. The pathway from 18 to 21 covers the normal catalytic cycle. If the local phenol concentration is too low and/or the local concentration of hydrogen peroxide is too high, compound I is converted into an intermediate (Scheme 5, 23) [39,69]. This intermediate can follow three different paths of decomposition. First it can react back to the na-... 

Equations 12, 14 and 17 require the presence of H20. Thus H20 plays an important role in promoting the catalytic activity, but can also cause deactivation. Catalysis will be more efficient when all the reactions directly involved in the catalytic cycle are faster than the side reactions subtracting active species. Deactivation is related to the requirement of the palladium centre to have a vacant coordination site to ensure high catalytic activity. However, palladium tends to achieve the usual coordination number four, for example through dimerisation. Dimerisation/deactivation can be prevented by coordination of labile ligands, like H20, which acts also as an efficient hydride source. Also deprotonation leads to dimerisation/deactivation an acid can prevent it. 

Since the hydride based isomerisation is such a facile reaction it often occurs even when it is not desired. Thus, if hydrides play a role in the catalytic cycle and alkenes are present or formed during the reaction, isomerisation may represent an undesired side reaction. For instance in the Heck reaction (see Chapter 13) an alkene and a palladium hydride are formed and thus, if the alkene can isomerise, this may happen at the end of the cycle as a secondary process. 

M(n+1)+. For oxygenation involving oxometal species, M(" 2)+o, the regeneration mode in the catalytic cycle is substrate oxidation via equation 6. Occasional side reactions with M-based or other redox active species can lead to the intermediate oxidation state of the catalyst, Equations 9-12 are routes to in the... 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

In addition to this, there is another lithium salt promoted pathway (Fig. 4.9) that contributes signihcantly to product formation. Here the product-forming reaction between lithium acetate and acetyl iodide is followed by the reaction between Lil and methyl acetate. These reactions are shown by the inner loop on the left-hand side. In fact, the inner loop is the dominant product-forming pathway, and lithium salts play a crucial role in the overall catalysis. Note that the right-hand-side loop of the catalytic cycle is exactly the same as in Fig. 4.1(a). 

Further investigations on reductive elimination processes showed that this reductive elimination could be the side reaction leading to degradation of active species in C-C cross-coupling reactions. As illustrated in Scheme 31, palladium-based catalyst (93) underwent oxidative addition in the presence of iodobenzene, providing the reaction intermediate (183), which could be involved in the catalytic cycle but also affords the imidazolium salt (184) by direct reductive elimination. Since then, a few other examples of... 

As all steps in the catalytic cycle are reversible, the alkene metathesis reaction is an equilibrium and all arrows are considered to be equilibria. Despite this nature of the metathesis, the use of two terminal olefins makes it possible to shift the equilibrium fully to the side of the products as one equivalent of ethene is produced and removed from the equilibrium. 

The stability of the triarylamine cation radicals in redox catalytic reactions strongly depends on their substitution pattern, the reaction medium, and also on the overall rate of the catalytic reaction. If the catalytic cycle is slow, higher stationary concentrations of the cation radicals are favoring side reactions. For example 4-bromo- and 4-iodo-substituted cation radicals can undergo 4,4 -coupling under loss of the halo substituents, leading to... 

Quite stable catalytic reaction solutions were obtained in THF with the starting pressure for ethylene of 6-6.5 MPa at a reaction temperature of 120 °C. Under these conditions and with the ratios piperidine/rhodium of 100 1 and 1000 1 in 36 and 72 h, yields of 70 and 50 % ethylpiperidine were reached, which correspond to TONs of 2 and 7 mol amine/(mol Rh) per h, respectively. Total conversion is also possible if the reaction time is prolonged further. As a side reaction, ethylene dimerization to butene was observed. This indicates the formation of a hydrido rhodium(III) complex in the hydroamination reaction, as formulated in Scheme 3, route (b). Hydrido rhodium(III) complexes are known as catalysts for ethylene dimerization [19], and if the reductive elimination of ethylpiperidine from the hydrido-y9-aminoethyl rhodium(III) complex is the rate-limiting step in the catalytic cycle of hydroamination, a competitive catalysis of the ethylene dimerization seems possible. In the context of these mechanistic considerations, an increase of the catalytic activity for hydroamination requires as much facilitation of the reductive elimination step as possible. 

All of the steps in the catalytic cycles just illustrated are reversible. In principle, a M-H species can undergo a series of insertions and jS-hydride eliminations to give a product whose 77 bond has been isomerized to a different position. In fact, this is an occasional side reaction in Pd-catalyzed hydrogenations. The partial hydrogenation of fatty acids containing cis double bonds gives a small amount of trans fatty acids by this very mechanism. 

As discussed above, the postulation of an equilibrium in step (1.65b) is essential to the reaction mechanism. This in turn has implications on the possible structure of the intermediate species Au2(C0)02 in the catalytic cycle (1.65) which might be a simple coadsorption of the two molecules, e.g., on different sides of Au2 or already a reacted carbonate COs-like species adsorbed onto the gold dimer. Apparently, the observed intermediate can... 

A nucleophilic attack on the acylated enzymes by water is the important rate-limiting step in the catalytic cycle or the restoration of inhibited enzymes. The nucleophilic oxygen attacks the bond between phosphorus and the enzyme, leading to the restoration of the free enzyme (EH). As a side reaction, an alkyl group of the phosphorylated enzyme may be removed, as shown below. When this occurs, the enzyme cannot be restored. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

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