Oxidative addition mechanisms

In one possible mechanism, oxidative addition of iodobenzene to Pd(0) gives Pd(II) intermediate 74, which subsequently inserts into thiazole regioselectively at the C(5) position to form the a-adduct of arylpalladium(II) 75. The order of reactivity is similar to the electrophilic substitution, which is known to be C(5) > C(4) > C(2) [74]. Treatment of the insertion adduct 75 with a base regains the aromaticity after deprotonation, giving rise to 73 along with Pd(0) for the next catalytic cycle. 

Mechanism Oxidative addition of the thioester to Pd(0) complex and then transmetallation followed by reductive elimination gives the final product (Scheme 5.22). 

Alkyl halides that do not readily undergo nucleophilic attack may oxidatively add to a metal by radical mechanisms. Oxidative addition reactions that occur by radical mechanisms show loss of stereochemistry, nomeproducible rates, inhibition by radical inhibitors, and acceleration by O2 or light. Reactions of lr(Cl)(CO)(PMe3)2 with methyl and benzyl halides showed no indication of radical behavior, but other saturated alkyl halides, vinyl, and aryl halides showed characteristics consistent with a radical-chain pathway. 

Palladium(O) complexes also add alkyl halides by 5, 2 mechanisms. Oxidative additions of Mel to Pd(0) complexes were some of the early examples of this reaction, and a subsequent study demonstrated that oxidative addition of an alkyl tosylate and a higher alkyl bromide occurs by an S 2 path. - Equation 7.3 shows the stereochemical evidence for an Sj 2 pathway. This equation shows the individual steps that occur during the catalytic addition of an arylborane to a stereochemically defined alkyl tosylate. The product forms with overall inversion of configuration. As is noted in Chapter 8, the final step, reductive elimination to form a C-C bond, occurs with retention of configuration. Thus, the first oxidative addition step must occur with inversion of configuration, and this inversion of configuration signals an S 2 reaction. 

The mechanism of hydrocyanation of ethylene catalyzed by the combination of Ni(0) and P(0-o-To1)3, as deduced by Tolman, is shown in Scheme 16.27 The L2Ni(ethylene) complex has been isolated and shown to add HCN. The Ni(0) complexes of higher olefins are less stable. In this mechanism, oxidative addition of HCN to a Ni(0) olefin complex forms a cyanometal-hydride complex. In the presence of ethylene, this complex contains olefin, but in the presence of higher olefins this complex has the composition L3Ni(H)(CN). Insertion of an olefin into the metal hydride occurs by a migratory insertion mechanism initiated by coordination of olefin to the cyanometal hydride. Reductive elimination of the alkyl cyanide completes the cycle, and this step is accelerated by Lewis acids, as presented later in this chapter. 

The classic Chalk-Harrod mechanism is shown in Scheme 16.6, and the modified Chalk-Harrod mechanism " is shown in Scheme 16.7. In the Chalk-Harrod mechanism, oxidative addition of silane occurs to form a silyl hydride complex. Migratory... 

In the Chalk-Harrod mechanism, oxidative addition of silane occurs to a Pt(0) complex. The ancillary ligands on the active catalyst when reactions are initiated with "Speier s catalyst" are unknown, but are likely to be the olefin substrate. The ancillary ligands on the active form of Karstedt s catalyst or the related PtfCOD) complex could be the original diene ligands or the olefin substrate. Details on the mechanism of oxidative additions of silanes are provided in Chapter 6. In brief, this reaction occurs by coordination of silane to an open site to form a silane a-complex, followed by cleavage of the Si-H bond to form a silyl hydride species. 

In his review [7] and feature article [8] (entitled I wonder where the ligand went ) Garrou emphasizes the first mechanism, oxidative addition of the phosphorus-carbon bond to low-valent metal complexes. More recently experimental support for the other two mechanisms has been reported. In Figure 3 the three mechanism are briefly outlined. 

The main focus of this chapter is homogeneous catalysis which involves the cleavage of C-C=0 and C-CN bonds. Catalytic reactions are categorized in the following order (i) type of bond cleaved (ii) mechanism (oxidative addition, others, and unclear) and (iii) strategy (directed, non-directed, and others). Related stoichiometric reactions are also given when necessary. For catalytic reactions via decarboxylation of a-keto carboxylic acids, refer to Chapter 4. 

Ruthenium-catalyzed arylation seems to favor the proton abstraction mechanism. We studied the coupling of an aryl group to a phenylpyridine moiety [64]. The first step consists of cleavage of a C- H bond to form a Ru-C bond. The two mechanisms, oxidative addition on the ruthenium or H-abstraction, were evaluated. As for palladium, the oxidation of Ru(II) to Ru(IV) is energetically disfavored with a barrier of 133 kJ/mol. The H-abstraction possesses a much lower barrier, 35 kJ/mol with HCOs" as proton abstractor (Fig. 11.13). The barrier of H-abstraction is sensitive to the nature of the base it is only of 22 kJ/mol with acetate. 

The hydride complex can be formed via three mechanisms oxidative addition (1), homogeneous (2) or heterolytic (3) addition. These three processes are presented below. 

Pd-catalyzed coupling reaction of vinyl haUdes with diazo compounds has been proven to be an efficient process to synthesize substituted olefins with good stereoselectivity [78-80], In 2010, Barluenga s group reported the Pd-catalyzed coupling reaction of vinyl halides with fV-tosylhydrazones for the preparation of dienes in moderate yields (Fig. 26) [98]. The reaction follows a similar mechanism oxidative addition and the... 

W C, A Tempcz)rrk, R C Hawley and T Hendrickson 1990. Semianalytical Treatment of Solvation for Molecular Mechanics and Dynamics. Journal of the American Chemical Society 112 6127-6129. ensson M, S Humbel, R D J Froese, T Matsubara, S Sieber and K Morokuma 1996. ONIOM A Multilayered Integrated MO + MM Method for Geometry Optimisations and Single Point Energy Predictions. A Test for Diels-Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition. Journal of Physical Chemistry 100 19357-19363. 

Wylation under neutral conditions. Reactions which proceed under neutral conditions are highly desirable, Allylation with allylic acetates and phosphates is carried out under basic conditions. Almost no reaction of these allylic Compounds takes place in the absence of bases. The useful allylation under neutral conditions is possible with some allylic compounds. Among them, allylic carbonates 218 are the most reactive and their reactions proceed under neutral conditions[13,14,134], In the mechanism shown, the oxidative addition of the allyl carbonates 218 is followed by decarboxylation as an irreversible process to afford the 7r-allylpalladium alkoxide 219. and the generated alkoxide is sufficiently basic to pick up a proton from active methylene compounds, yielding 220. This in situ formation of the alkoxide. which is a... 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Chemisorption of alkanethiols as well as of di- -alkyl disulfides on clean gold gives indistinguishable monolayers (251) probably forming the Au(l) thiolate species. A simple oxidative addition of the S—S bond to the gold surface is possibly the mechanism in the formation of SAMs from disulfides ... 

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

The mechanism of the carhonylation reaction is thought to involve a frrst-step oxidative addition of the methyl iodide promotor to the Rh(I) complex, followed hy a carhonyl cis insersion step ... 

A plausible mechanism accounting for the catalytic role of nickel(n) chloride has been advanced (see Scheme 4).10 The process may be initiated by reduction of nickel(n) chloride to nickel(o) by two equivalents of chromium(n) chloride, followed by oxidative addition of the vinyl iodide (or related substrate) to give a vinyl nickel(n) reagent. The latter species may then undergo transmetala-tion with a chromium(m) salt leading to a vinyl chromium(m) reagent which then reacts with the aldehyde. The nickel(n) produced in the oxidative addition step reenters the catalytic cycle. 

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

A great number of articles related to the mechanism of this reaction has been published. It can be considered as certain that the silanes react with the platinum center by an oxidative addition to the metal with formation of a silylplatinum hydride and subsequent transfer of the silyl group to the coordinated alkene. 

The three steps 32-34 have been suggested77 to be equilibria, and the overall equilibrium must lie far to the left because no adduct 23 is found in the reaction mixture when the reaction of sulfonyl chloride with olefin is carried out in the absence of a tertiary amine. A second possible mechanism involving oxidative addition of the arenesulfonyl halide to form a ruthenium(IV) complex and subsequent reductive elimination of the ruthenium complex hydrochloride, [HRulvCl], was considered to be much less likely. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

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 general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

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