Hydrido mechanism

Most researchers currently agree that the hydrido mechanism is more common than the alkoxycarbonyl path in the alkoxycarbonylation of alkenes with palladium systems. However, carbalkoxy complexes are putative intermediates in carbonylation reactions giving succinates and polyketone diesters, with metals like Co, Rh, or Pd.137... 

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... 

Recently, Y. Yamamoto reported a palladium-catalyzed hydroalkoxylation of methylene cyclopropanes (Scheme 6-25) [105]. Curiously, the catalysis proceeds under very specific conditions, i.e. only a 1 2 mixture of [Pd(PPh3)4] and P(o-tolyl)3 leads to an active system. Other combinations using Pd(0 or II) precursors with P(o-tolyl)3 or l,3-bis(diphenylphosphino)propane, the use of [Pd(PPh3)4] without P(o-tolyl)3 or with other phosphine ligands were all inefficient for the hydroalkoxylation. The authors assumed a mechanism in which oxidative addition of the alcohol to a Pd(0) center yields a hydrido(alkoxo) complex which is subsequently involved in hydropal-ladation of methylenecyclopropane. 

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

The most plausible mechanism proceeds through oxidative addition of the aldehyde to an active Ru(0) species to form (acyl)(hydrido)ruthenium(ll) complex 155. Insertion of the less-substituted double bond of the 1,3-diene into the Ru-H bond occurs to generate an (acyl)( 73-allyl)ruthenmm(ll) intermediate of type 156. Successive regioselective reductive eliminations between the acyl and the 73-allyl ligands provide the desired product with regeneration of the... 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

A tentative mechanism includes ruthenium-induced isomerization of the initial allylic alcohol via (hydrido) (7T-allyl)ruthenium complex 167 to the corresponding Ru-bound enol 168. This in. ( ////-generated nucleophile complex can then add to aldehydes or imines under formation of the desired products. 

The influence of hydrogen pressure, substrate and catalyst concentration has briefly been mentioned. The reaction rate is dependent upon the catalyst concentration and hydrogen pressure, but appears to be independent of substrate concentration. The mechanism is proposed to involve the activation of the parent [Pd(allyl)] species producing an unstable hydrido-Pd(II) species (71), ensued by a fast reaction with the diene to restore the [Pd(allyl)] moiety (72) (Scheme 14.21). The observation that most of the starting material is isolated after the reaction suggests that only a small portion of the catalyst is active under the reaction conditions. Although a complete selectivity for the monoene is observed (even after full conversion), the presence of catalytically active colloidal palladium has not been completely excluded. 

The combination of these findings with the aforementioned results of Beckhaus, Rosenthal, and Mach may also be interpreted in terms of an alternative mechanism for the polymerization of acetylene, which differs from that of Alt [12] (Scheme 10.3). In the absence of coupling, as in 11, or of twofold C—H activation as found in 12, the steps after substitution of MeiSiC=CSiMci by HC=CH and formation of Cp2Ti(i]2-HC2H) are (i) oxidative addition to give the hydrido-acetylide Cp2Ti(H)(C=CH),... 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

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