Dissociative ligand 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... 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

This activity dependence on CO pressure could be attributed to a dissociative-type mechanism, i.e., one that necessitates loss of a carbonyl ligand as a kinetically limiting step ... 

Scheme 17.3 Mechanisms of C02 insertion into a metal-hydrogen bond. L represents a potentially dissociable ligand. Ancillary ligands are not shown.

A multiple-path mechanism has been elaborated for dissociation of the mono- and binuclear tris(hydroxamato)-iron(III) complexes with dihydroxamate ligands in aqueous solution. " Iron removal by edta from mono-, bi-, and trinuclear complexes with model desferrioxamine-related siderophores containing one, two, or three tris-hydroxamate units generally follows first-order kinetics though biphasic kinetics were reported for iron removal from one of the binuclear complexes. The kinetic results were interpreted in terms of discrete intrastrand ferrioxamine-type structures for the di-iron and tri-iron complexes of (288). " Reactivities for dissociation, by dissociative activation mechanisms, of a selection of bidentate and hexadentate hydroxamates have been compared with those of oxinates and salicylates. ... 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

Although this was at first thought to indicate that an associative mechanism was indeed operative in these reactions, over the years a body of further data accumulated to suggest that this was not the case. It is now clear that the deprotonation of a co-ordinated amine is the key step in this mechanism, which is based upon dissociative ligand loss from an amido intermediate. This process is known as the SN1 cZ mechanism, and was mentioned briefly in Chapter 2, and illustrated in Fig. 2-13. The first step involves the deprotonation of the co-ordinated amine (Fig. 5-40). 

Steps 1-3 of the catalytic cycle correspond to various steps of other catalytic cycles already discussed in Chapter 13. Step 1 tt-complex formation by combination of the aryl triflate and a sufficiently valence-unsaturated and thus sufficiently reactive Pd(0) species. Step 2 oxi-dative addition of the aryl triflate to Pd with formation of a Cspi—Pd(II) bond. Steps 3a and 3b exchange of a PPh3 ligand by an acrylic acid methyl ester via a dissociation/addition mechanism. The newly entered acrylic acid ester is bound as a 77 complex. 

The dissociation/recombination mechanism was further confirmed by studying the ligand facial exchange in [La(III)(N03)3 18C6] with the value for k of 35 5 s 1 which is close to k i of 50 s 1. The exchange equilibrium may be written as ligand facial exchange ... 

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