Dissociative mechanism chelates

Dr. Margerum I want to develop briefly another point directly related to the dissociative mechanism—namely the opening of a chelate ring. 

When K[PtX(acac)2] (X = Cl, Br) is treated with a strong proton add the uncoordinated 0,0 site is protonated and complex (170) is formed.1606,1607 If the alkyl groups on the 0,0 -bonded acac ligand are non-equivalent, exchange can be observed. Although the mechanism has not been definitively proven, a dissociative mechanism is favored.160 This proposal correlates with the observations that the 0,0 -bonded chelate complex Pt(acac)2 will react with tertiary phosphines and nitrogen bases with substitution of one of the oxygen-bonded chelate arms.1609,1610 A variety of products are formed as outlined in Scheme 17. 

From temperature-dependent NMR studies it has been shown that uncomplexed diamine 6 exhibits ligand exchange with complexed 6 in (Li-6)2 6 by a dissociative mechanism with AG 228 = 7.8 kcalmol-1. The (Li-6)2 6 chelate undergoes a fast intra-aggregate diamine-amide interconversion via degenerate proton transfer between diamine and amide with AG 268 = 10.9 kcalmol-1 (Scheme 8)39. 

Evidence Tor a dissociative mechanism has been reported Lanza, S. Minnili, D. Moore. P. Sachinidis. J. Romeo, R. Tube, M. L. Inary. Chem. 1984, 23, 4428-4433. The reaction, which involves substitution of dmso in PtRitdmsofo. proceeds by loss of one dmso ligand. However, because it is possible that the dmso ligand that remains coordinated can function in a chelating capacity, it can be debated, as these workers acknowledge, whether this reaction can be colled a true dissociation. 

Quadridentate Ligands. The effect of extent of chelation on aquation rates has been studied for the compounds raK5-[Co(N02)2(LLLL)]+, where LLLL = (NH3)4, (en>2, (pnja, or cyclam (5), and ci5-[Co(N02)2(LLLL)]+, where LLLL = (NHg)4, (en)a, or trien (6). Rates of aquation increase as the number and size of chelate rings decrease, which is consistent with a dissociative mechanism. Rates of aquation are also sensitive to steric crowding around the leaving group. This is well illustrated by the 10 -fold difference in aquation rates of [Co(C03)(LLLL)]+ when LLLL = (en)a or tet-b (7), and by rate variations in the series... 

For the aqua- and chelate-complexes described as Fe NO (S=3/2, derived from high-spin metal centers), water-exchange measurements and activation parameters support a dissociative-interchange mechanism, with high values of and variable values of koff as a function of the type of chelate (i31). Dissociative mechanisms have been proposed for the prototypical low-spin [Fe(CN)5(NO)] complex (S=l/2, described as Fe NO"), showing much lower values of feoff at The... 

The mechanism of this process has been studied in detail. The identity of the palladium(O) species that lies on the catalytic cycle/ the effect of anions on the oxidative addition step/ " the effect of amines in the dissociation of chelating ligands from the palladium(O) complex during the oxidative addition/ the mechanism of formation of the amido complex/ and the mechanism of reductive elimination of amine - - have all been studied. The oxidative addition of aryl chlorides and bromides is generally the turnover-limiting step of the catalytic cycle. 

As a fast, albeit incomplete, probe of potential catalyst activity, the initiation rates of the catalysts presented in Figure 3.1 were measured via reaction with butyl vinyl ether (BVE) [27]. This assay does not describe the behavior of the catalysts in full detail and certainly says nothing about Z selectivity nevertheless, it was a relatively quick experiment that we hoped would shed light on the structural factors governing catalyst activity. It should also be noted that unlike the initiation behavior of phosphine-based Ru catalysts, which is easily explained by the steady-state approximation, the initiation behavior of catalysts with Hoveyda-type oxygen chelates, such as 1 and 7, is much less well defined. In these cases, initiation rates depend heavily on the nature of the catalyst, the reacting olefin, and several other factors [28]. Thus, it can be challenging to explain the initiation of these catalysts in terms of a simple associative or dissociative mechanism. 

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