Three-centered concerted additions, mechanisms

From the kinetic data, the stereochemistry and structure of the <7-complex, the mechanism of formation of the -complex involves a rate-determining formation of a n-alkene Pt complex, followed by a rapid rearrangement rather than a single electron transfer, a three-centered concerted addition or a free radical pathway. However, attempts to prepare a 7c-complex from 2-propenyl triflate and Pt(PPh3)2(C2H4) failed but, two n-alkene-Pt halide complexes were fully characterized, (equation 129). 

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. 

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

The mechanism underlying the oxidative addition of C(sp )—X bonds to palla-dium(O) complexes is considered to involve a three-centered concerted pathway, whereas the C(sp )—X bonds are activated in an Sn2 fashion by the reactive palladium(0) complex (Scheme 1.5) [33, 34]. 

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. 

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. 

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]. 

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. 

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. 

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. ... 

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. 

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 ... 

During CCT, there are occasions where organometallic compounds form a Co—C bond that is stable at room temperatures. Investigation of the mechanism of this reaction with deuterated substrates proved that hydrogen transfer from cobalt porphyrin to a double bond (Scheme 8.3) occurred stereospecifically through cis-addition. Cis-addition supposes a concerted reaction mechanism. Stereospecific cis-addition hardly could be observed if reaction of LCo-H addition to a double bond proceeds by a three-center mechanism that requires formation of intermediate radical from the olefin ... 

There are three general mechanisms for insertions concerted, free radical, and heterolytic addition. In the 1,2-insertion, the concerted mechanism proceeds via interaction of the 7t system of the unsaturated compound directly with the intact E-H bond, with each end of the n system directed at either the E or the H atom (Scheme 1). This interaction may or may not be preceded by precoordination of the unsaturated molecule to the element. The transition state for this reaction is considered to be four-centered, and yields products that are cis-substituted on the reduced unsaturated substrate. 

Several useful reviews have appeared. Mondal and Blake have collected thermochemical data on oxidative addition, Halpern has investigated the formation of C-H bonds by reductive elimination, while in a thought-provoking article on activation of C[5/ ]-X bonds, Chanon stresses the importance of electron transfer in oxidative addition (among other topics). In a discussion of oxidation addition and reductive elimination involving two metal centers, Halpem classifies and gives examples of three mechanisms whereby binuclear reductive elimination can occur concerted two center (77), concerted one center (78), and free-radical [(79)-(81)] reactions given in Scheme 6. 

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