Associative substitution mechanism

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

The ki term in the rate law for square planar substitution is very clearly connected with an associative mechanism. The k term may be also. Consider the pathway (S = solvent)... 

FIGURE 20.2 The energy profile for substitution ofY for X in an associative mechanism. 

In a CO matrix, therefore, the primary product involves an expanded coordination nunber ("ring-slippage") and it is argued that such a species is consistent with the associative mechanism proposed for room temperature substitution reactions of (13 -05115)Co-... 

For the phosphine-substituted cobalt carbonyl hydroformylations, it is probable that the mechanism follows the pathway of Heck and Breslow (77, 18), although the possibility of an associative mechanism has been raised (7). The increased stability of the HCo(CO)3PR3 complexes toward loss of CO was cited as being suggestive of a nondissociative pathway. 

Replacements of ferrocene-substituted p-diketone ligands, p-dik, in cyclooctadiene-rhodium(I) complexes [Rh(p-dik)(cod)] by 1,10-phenan-throline are characterized by large negative activation entropies, indicating the operation of the expected associative mechanism, although the... 

The use of 77-acceptors can also achieve a higher rate of substitution. In our group we were able to compare the substitution behavior of the Pt(II) aqua complexes of ethylenediamine and 1,10-phenanthro-line, and although the reactivity of the phenanthroline complex is —102 higher than the ethylenediamine complex, the activation parameters strongly indicate that an associative mechanism is operative (65). 

The reaction of l-fluoro-2,4,6-trinitrobenzene and 2,4-dimethoxyaniline, in cyclohexane, shows a negative activation enthalpy274 (—SOkJmoU1), in agreement with a desolvative association mechanism in which the nucleophile competes with the solvent in associating with the substrate in an equilibrium preceding the substitution process. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

The situation is quite different with tetrahedral complexes of Ni(0), Pd(0) and Pt(0). We might anticipate that an associative mechanism would be deterred, because of strong mutual repulsion of the entering nucleophile and the filled d orbitals of the d system. Thus a first-order rate law for substitution in Ni(0) carbonyls, and M (P(OC2H5)3)4M = Ni, (Sec. 1.4.1) Pd and Pt, as well as a positive volume of activation ( + 8 cm mol ) for the reaction of Ni(CO)4 with P(OEt)3 in heptane support an associative mechanism. 

Only limited data are available for substitution in this state. An associative mechanism is favored and Au(I) appears to be less reactive than Ag(I) or Hg(II). ... 

The aqua ion Au(H20)4+ has not been characterized either in solution or in the solid state. Most of the substitution studies have involved the halide complexes AuXj and Au(NH3) (Ref. 319). A number of earUer generalizations have been confirmed. Rates are very sensitive to the nature of both entering and leaving ligands and bond formation and breaking are nearly synchronous. The double-humped energy profiles witnessed with Pd(II) and Pt(II) are not invoked the five-coordinate species resulting from an associative mechanism is the transition state ... 

Solvolysis studies of meta- and para-substituted phenyl phosphates (240) in anhydrous Bu OH and in Am OH have revealed that generally reactions of dianions are much faster in alcohols than in water. For example, the dianion of p-nitrophenyl phosphate (240 X = 4-NO2) reacts 7500- and 8750-fold faster in Bu OH and Am OH, respectively, than in water." The results of a theoretical study of the reactivity of phosphate monoester anions in aqueous solution do not support the generally accepted view that Brpnsted coefficients fhg = —1.23 and jSnuc = 0.13 determined more than 30 years ago for the uncatalysed reaction of water and a monophosphate dianion (241) represent conclusive evidence for the dissociative mechanism. It is suggested that, instead, the observed LFERs could correspond to a late transition state in the associative mechanism." ... 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

When (concave) pyridines with identical a-substitution but different basicity (caused by 4-substitution) were compared, a linear basicity-reactivity dependence was found. For different types of pyridines the lines run more or less parallel [47]. The slopes of these lines are approx. 0.3. This argues for the association mechanism between the alcohol and the pyridine (via 56) because for... 

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