Dissociative elimination-addition mechanism

Theoretical calculations support a low-energy oxidative addition mechanism [26c], Reaction of the unsolvated cationic complex Cp Ir(PMe3)(CH3) with pentane, cyclohexane or benzene in the gas phase also gives Cp Ir(PMe3)(R) as the product. However, a mechanistic investigation of this process by electrospray tandem spectrometry has demonstrated that neither the oxidative addition-elimination mechanism nor the concerted a-bond metathesis mechanism is operative. Instead, the authors proposed a dissociative elimination-addition mechanism which proceeds through a series of 16-electron Ir(III) intermediates [26d]. 

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

By contrast, addition-elimination mechanisms in their simplest form begin with formation of an addition complex resulting from a well on the PES, followed by dissociation of the complex, yielding products. Both the entrance to and exit from the well may be hindered by barriers on the PES. Addition mechanisms are uncommon in radical -b saturated closed-shell reactions due to the difficulty of bond formation with the saturated species (ion-molecule reactions are exceptions). By contrast, additions are more common in radical -b unsaturated closed-shell species, where the double or triple bond allows a low barrier or barrierless pathway for addition of the radical into the 7i-bond of the stable species, such as the reaction... 

The elimination-addition route [Eq. (36)] constitutes a rare but important dissociative mechanism for displacement at silicon (150). Along the reaction coordinate one must assume the formation of unsaturated silicon species, which are now of interest to great number of organosilicon chemists (151). 

Sulfuryl transfer has available the same mechanistic pathways previously discussed for phosphoryl transfer (Figure 2). If a monoester follows a fully dissociative (D A ) mechanism, a sulfur trioxide intermediate would form, analogous to metaphosphate in the phosphoryl system. An addition—elimination (A D ) mechanism would form a pentacoordinate sulfurane intermediate. 

The previously accepted pathway consisted of P-H oxidative addition to Pt(0) to form 19 followed by coordination and insertion of the alkene in the Pt-P bond to form 20 and a final reductive elimination to furnish the product and regenerate the catalyst. Another possibility is the nucleophilic attack of phosphido complex 19 to the alkene ( Michael addition mechanism, as in anionic polymerisation) to generate the zwitterionic intermediate 21. This complex can yield the hydrophosphination product 11 via one of two complementary pathways. Carbanion attack at the cationic platinum hydride i.e. intramolecular hydrogen transfer) would yield the final phosphine complexed to Pt(0) that would be displaced by an equivalent of PHR R to furnish, after oxidative addition, starting complex 19. Alternatively, the anionic carbon atom in 21 could attack the platinum centre directly, forming the cyclic intermediate 22. From here Pt-P bond dissociation would generate 20, which would furnish the product after reductive elimination. 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Pseudo-first-order rate constants for carbonylation of [MeIr(CO)2l3]" were obtained from the exponential decay of its high frequency y(CO) band. In PhCl, the reaction rate was found to be independent of CO pressure above a threshold of ca. 3.5 bar. Variable temperature kinetic data (80-122 °C) gave activation parameters AH 152 (+6) kj mol and AS 82 (+17) J mol K The acceleration on addition of methanol is dramatic (e. g. by an estimated factor of 10 at 33 °C for 1% MeOH) and the activation parameters (AH 33 ( 2) kJ mol" and AS -197 (+8) J mol" K at 25% MeOH) are very different. Added iodide salts cause substantial inhibition and the results are interpreted in terms of the mechanism shown in Scheme 3.6 where the alcohol aids dissociation of iodide from [MeIr(CO)2l3] . This enables coordination of CO to give the tricarbonyl, [MeIr(CO)3l2] which undergoes more facile methyl migration (see below). The behavior of the model reaction closely resembles the kinetics of the catalytic carbonylation system. Similar promotion by methanol has also been observed by HP IR for carbonylation of [MeIr(CO)2Cl3] [99]. In the same study it was reported that [MeIr(CO)2Cl3]" reductively eliminates MeCl ca. 30 times slower than elimination of Mel from [MeIr(CO)2l3] (at 93-132 °C in PhCl). 

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