Oxidative addition adduct stability

A step forward in the design of catalysts enabling the N-arylation of amines is to eliminate the activation step (reduction of palladium(II) to palladium(O) prior to oxidative addition). One approach is to use well-defined palladium(O) complexes of (NHC)2Pd or mixed phosphine/NHC,(R3P)Pd(NHC) type [80]. These complexes are efficient catalysts for this transformation at mild temperatures. A second approach is to sidestep two required activation stages in the catalytic cycle and eliminate the need for the preactivation and the oxidative addition processes by using well-defined catalysts that are oxidative addition adducts such as NHC-stabilized palladacycles (Scheme 25) [81]. 

Extensive computational calculations have been performed by using molecular mechanics (MM) [79], quantum mechanics (QM) [80], or combined MM/QM methods [81]. As major contributions, these theoretical studies predict the greater stability of the major isomer, explain the higher reactivity of the minor diastereomer, introduce the formation of a dihydrogen adduct as intermediate in the oxidative addition of H2 to the catalyst-substrate complexes, and propose the migratory insertion, instead of the oxidative addition, as a turnover-limiting step. 

There is abundant evidence that, in any particular system, the stability of adducts formed by oxidative addition using a series of silanes HSiXs increases as X becomes more electronegative a typical order is... 

The energy input is accomplished by a proton gradient/flux for the oxidation of a phosphate base [HO transfer from HOP(O)O )2]. The oxidized product is stabilized by the nucleophilic addition of ADP to give ATP. Scheme 8-9 outlines the several steps of the energy-input and energy-output cycles in oxidative phosphorylation. The net transduction is the neutralization (via a singleelectron transfer) of a hydroxide adduct [HOP(O)(O )2] (the one-electron reductant) by a hydronium ion (H3O+)... 

In this section, Pd(0)-catalyzed reactions of allenes with nucleophiles are treated, which are clearly different mechanistically from the reactions explained in the above. Attack of nucleophiles may occur at C-1, C-2, and C-3 carbons of the allenes 63. Among them, attack at C-3 to give 64 is predominant. Most importantly, reactions of allenes with pronucleophiles start by the oxidative addition of pronucleophiles to Pd(0) to generate H-Pd-Nu 65. The formation of 64 by hydro-carbonation can be explained in two ways in the case where Nu-H is the carbon pronucleophile. As one possibility, hydropalladation of one of the two double bonds occurs to afford the terminal palladium intermediate 66, which is stabilized by the formation of 7r-allyl complex 67, and reductive elimination provides the C-3 adduct 68. Another possibility is carbopalladation to generate 69, and subsequent reductive elimination provides 68. Of these two possibilities, the hydropalladation mechanism is preferable. 

The order of stabilities of the adducts was the same as that observed previously for additions of the hydrosilanes to complexes [14, 32c, 18], i.e., negative substituents such as alkoxy groups or chlorine atoms on silicon stabilize the adducts. Furthermore, rate measurements have indicated that the structure of hydrosilanes does not affect markedly the rate ( i) of the forward reaction (oxidative addition), but affects strongly the rate ( i) of the reverse reaction (reductive elimination). This latter fact, in addition to a possible dependence of the stability of metal-silicon bonds on metal species (c.g., rhodium vs, platinum) will be reflected in the catalysis by particular metal complexes, which is clearly shown in the following sections. Another approach has been to study the stereochemistry of an optically active... 

As we have seen, oxidative addition is the inverse of reductive elimination and vice versa. In principle, each reaction is reversible, but in practice the reactions tend to go in the oxidative or reductive direction only. The position of equilibrium in any particular case is governed by the overall thermodynamics this in turn depends on the relative stabilities of the two oxidation states and the balance of the A—B versus the M—A and M—B bond strengths. Alkyl hydride complexes commonly eliminate alkane, but only rarely do alkanes oxidatively add to a metal. Conversely, alkyl halides commonly add to metal complexes, but the adducts rarely reductively eliminate the alkyl halide. Third-row elements, which tend to have stronger metal-ligand bonds, tend to give more stable adducts. Occasionally, an equilibrium is established in which both the forward and back reactions are observed. 

Aldehydes are stoichiometrically decarbonylated by reaction with (XL) under mild conditions (77, 98,110,113). Aromatic aldehydes yield aromatic hydrocarbons whereas aliphatic aldehydes form saturated hydrocarbons and olefins. The latter minor products can be considered to arise from a reverse hydroformylation reaction. The initial step of this reaction is probably the oxidative addition of an aldehyde C—H bond to the rhodium(I) complex. However, a stable adduct of this type has not yet been reported. The driving force in these reactions is derived from the stability of the carbonyl (LXIX). 

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