Oxidative addition Three-center mechanism

Experimental evidence points to two different modes of oxidative addition. For R -X, when R is sp2 hybridized, a three-centered mechanism seems to apply (see also Section 7-2-2). This leads to an intermediate where R and X are cis. Because of the high trans effect of R a rapid cis-trans rearrangement likely occurs to give the more stable stereoisomer (equation 12.58).121... 

A kinetic study by Harrod and Smith of oxidative addition to a square planar cationic iridium complex also supports the three-center mechanism (258). The rate law is first order in the iridium complex and first order in hydrosilane. Determination of the activation parameters indicated a moderate activation enthalpy (AH — 5-6 kcal/mol) and a large negative activation entropy (AS — -47 e.u.) No variations were observed on changing the solvent. Harrod and Smith concluded that oxidative addition proceeds via a concerted three-center transition state in which little bond-making or bond-breaking had occurred. The activation enthalpy was attributed to a deformation of the square planar complex on its approach to the transition state. 

The reaction sequence includes (1) an oxidative addition of C-X bond of the aromatic substrate Ar-X to a Pd° center, (2) a substitution of OR for X in the LnPd (Ar)X intermediate, and (3) a reductive elimination of the C-O bond from the Pd center. The ability of palladium(ll) alkoxides bearing p-hydrogen atoms to undergo p-hydride elimination imposes some limitations on the type of alkoxide groups that can be involved in these C-O coupling reactions [2]. The C(sp )-0 reductive elimination reactions from Pd and Pt centers have also been studied computationally [4]. The reactions were suggested to proceed via a concerted three-center mechanism. 

Two recent computational studies have examined the mechanism of crosscouplings involving aryl acetates, earbamates, and sulfamates. In a study of synthetic advances with aryl pivalates, oxidative addition of Ni(0) to the aryl acetate was found to proceed by a three-centered mechanism via an T arene complex. Alternatively, in a separate study, oxidative addition involving aryl carbamates and sulfamates was found to proceed by a five-centered transition state, with coordination of the carbonyl oxygen to nickel being a key interaction in the oxidative addition. ... 

The oxidative addition of methyl iodide to Vaska s complex, shown in Equation 7.2, is a classic example of the oxidative addition of alkyl halides by an mechanism. Strong electrophiles that are sterically accessible, such as methyl iodide, benzyl bromide, allyl halides, and chloromethyl ethers, react with Lj(CO)IrX species by this pathway. A series of data supports addition of these electrophiles by an mechanism. For example, the trans stereochemistry of the kinetic product from addition of methyl iodide is inconsistent with a concerted three-centered mechanism radical traps do not affect the rate or products of the reaction the reaction rates are faster in more polar solvents - and the reactions are first order in both metal and electrophile. Higher aUcyl halides add by more complex mechanisms presented below. 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

The concerted three-center mechanism is the most common. It is exactly symmetrical to that already described for oxidative addition, according to the microreversibility principle (see section 3.1). It could indeed be shown that the reaction is intramolecular. For instance, in the following example, no cross-product was obtained ... 

Three-center mechanism via a transient o complex oxidative addition... 

A kinetic analysis of the styrene hydrogenation catalyzed by [Pt2(P205H2)4]4 [66] was indicative of the fact that the dinuclear core of the catalyst was maintained during hydrogenation. However, three speculative mechanisms were in agreement with the kinetic data, which mainly differ in the H2 activation step. This in fact can occur through the formation of two Pt-monohydrides, still connected by a Pt-Pt bond, or through the formation of two independent Pt-monohydrides. The third mechanism involves the dissociation of a phosphine from one Pt center, with subsequent oxidative addition of H2 to produce a Pt-dihy-dride intermediate. 

Mechanisms for oxidative additions vary according to the nature of X—Y. If X—Y is nonpolar, as in the case of H3. a concerted reaction leading to a three-centered transition state is most likely. 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

The reaction mechanism commonly accepted to account for the double silylation of unsaturated substrates involves three key steps. First, the disli-lane undergoes oxidative addition to the metal center, forming a transition metal-bis(silyl) complex. The unsaturated moiety inserts into the metal-silyl bond, followed by Si-C reductive elimination to give the double sily-... 

Oxidative addition see Oxidative Addition) ofH-C, H-Si, H-Sn, Cl-C, and other a-bonded atom pairs have been used to add group 14 donor ligands to metal clusters. Additions of H-C, H-Si, and H-Sn bonds require an unsaturated metal center. A number of studies of the mechanism are consistent with a mechanism involving addition at a single metal atom through a three-center, synchronous process, the same as commonly occurs for monometallic complexes. Some examples of H Si addition to unsaturated (equation 10) and lightly stabihzed (equation 11) OS3 clusters are shown. Oxidative additions of unactivated C-H bonds are rarer. Preferential addition of terminal C H bonds to... 

It may be assumed, that the reaction of the chloroaquoplatinum(ll) complex with atkanes begins as oxidative addition, proceeds through a three-center transition state, and terminates by the synchronous formation of a platinum-carbon bond with elimination of a proton (which can be transferred to a molecule of water). A similar mechanism has been proposed [35a] for the cyclometalation of 8-alkylquinolines by palladium(II). It has also been suggested that the reverse process (the protolysis of the platinum-carbon bond in alkyl complexes) involves a three-center transition state [35b], and the concerted oxidative addition to Ir(I) complex has been proposed [35c]. 

If a concerted three-center oxidative addition mechanism is realized (path b), first a lone electron pair of an atom X (X = O, N) is used to form the M-X bond resulting in a complex 5 (step bi). Formation of the M-C bond and cleavage of the C-X bond constitute a concerted process resulting in a five coordinate intermediate, which can be stabilized by coordination of a solvent molecule (Sol) as in structure 6 (step b-2) similar to 4. Finally, by coordination of ligand L, the intermediate 6 can produce an octahedral reaction product (step b ) shown in Fig. 3, b. The configuration of the carbon atom present in R-Z is retained during oxidative addition to the metal. 

Typically, these reactions proceed by a two-step (so-called S v2 ) mechanism involving a charge-separated intermediate [LnMR] X . Three-center transition states have been considered, but these are likely to be highly polar. Lone-pair donation complexes of alkyl halides (e.g., CH3I) are known, but not as intermediates to oxidative addition. Recently reported reaction (9) is the first such... 

Mechanisms of Oxidative Addition 1.03.3.2.1 Three-center concerted additions... 

The mechanism of the reactions of aryl halides cannot occur by the common S 2 patii for the oxidative addition of methyl halides, and most aryl halides lack substituents that would make them sufficiently electrophilic to react by nucleophilic aromatic substitution pathways. As presented in the section on radical pathways for oxidative addition, aryl halides react with metal complexes that readUy imdergo one-electron oxidation by radical mechanisms. However, metal complexes that do not readily undergo one-electron processes tend to react by two-electron mechanisms. Thus, aryl halides typically react with tP" palladium(O) complexes by concerted pathways through three-centered transition states. No strong data for a radical pathway has been gained during the many studies on the oxidative addition of aryl halides to Pd(0). In contrast, evidence that oxidative addition of aryl halides to P, iridium, Vaska-t)q)e complexes occurs by a radical pathway has been published. ... 

Because oxidative addition and reductive elimination are the same process run in reverse directions, the intermediates and transition states are the same for the two classes of reactions. Thus, reductive eliminations occur through three-centered transition states, as weU as by stepwise mechanisms through cationic or radical intermediates. The mechanism depends on the metal center and the ligands in the complexes undergoing reductive elimination. 

In oxidative additions via a concerted mechanism (Fig. 1.4), the A-B molecule binds firstly to the metal center and then, the cleavage of the A-B bond and the formation of the new M-A and M-B bonds take place simultaneously through a three-centered transition state. This mechanism is normally found in oxidative additions of non-polar reagents [78-81] and aryl halides [82-85]. Experimental evidences for this mechanism are the retention of configuration at a stereogenic center in the case of chiral A-B reagents, and the relative cis disposition of the ligands A and B after the oxidative addition [86]. The latter, however, may be not observed in the cases in which the cis-to-trans isomerization reaction of the oxidative product is very fast [87],... 

As expected the calculated energy barrier for the oxidative addition reaction was rather low (17.0 kcal mol ) and involves the concerted formation of the Pd-I and Pd-C bonds, and the cleavage of the C-I bond through a three-centered transition state (OA-TS). This transition state results in the oxidative addition product OA-P, which evolves to the more stable trans isomer through a cis-to-trans isomerization. This isomerization is known that may take place following different pathways, [54] but in any case it is an easy process [64]. Thus, we focused our further analysis on the proposed mechanisms starting from the trans-[Pd(Ph)(I)(PH3)2] (1) complex. 

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