Diimines reaction mechanism

Imhof et al. [22] studied the reaction mechanism of the [2+2+1] cycloaddition reactions of diimines, CO, and ethylene catalyzed by iron carbonyl complexes on the basis of density functional theory (Scheme 4). The catalytic reaction does not start when CO dissociates from 10 followed by the addition of ethylene, but instead the associative pathway to 11 is proposed. In addition, it can be concluded that the insertion of CO in 11 takes place into a C-Fe bond but not... 

Shilov and coworkers discovered the oxidation of methane to methanol by mixtures of Pt and Pt, and aroused the Holy Graft-pursuing for electrophilic C H activation and subsequent alkane oxidation. The diimine complexes of Pt(II) methyl are indeed found to facilitate smooth benzene activation, resulting in formation of methane via Pt (Me)(Ph)(H) intermediates (Scheme 10). Such Pt(II)/Pt(IV) involved C-H activation reactions have been widely extended to a variety of nitrogen-donor ligands, whose electronic and steric effects shed light on the reaction mechanisms (see Section 7.1). 

Ab initio calculations have indicated that a new reaction mechanism, viz. a suprafacial [5,5]-sigmatropic rearrangement, is possibly involved in the formation of leuco-indoaniline from the reaction of p-benzoquinone diimine with phenolate. 

Methyl acrylate ethene copolymerization The copolymer stmcture and the reaction mechanism of a-diimine complex-catalyzed copolymerization reactions with functionalized a-olefins are now described in detail for the ethene-MA copolymerization system.This will serve as a general example for a copolymerization of ethene with polar comonomers. In other specific cases, the nature of the rdevant steps may vary, but the general concept is applicable. 

The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

Activity in the area of medium effects (27) has declined greatly in recent years, though there has been some interest in kinetics and mechanisms in supercritical fluids (28). Indeed activation volumes for ring closure reactions of diimine-carbonyls M(CO) (diimine) show some of the most dramatic medium effects. Thus AF values range from +66 to +4 cm3 mol-1 on going from 7% benzene in supercritical C02 (at 35 °C) to 100% benzene (at 25 °C) (29). 

Diimines such as bpy and phen replace 2-(arylazo)pyridine ligands (aap) in [Pd(aap)Cl2] by a simple second-order process, whose detailed mechanism may depend on the nature of the incoming ligand (254). Three phthalocyanine units, each containing Zn2+, can be bonded to tetrahedral phosphorus, to give [PPh(pc-Zn)3]+. Mechanistic proposals are advanced for this novel exchange reaction in which palladium-bound phthalocyanine replaces phenyl on phosphorus (255). 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

The mesityl diimine 88d was as effective a ligand in the aziridination as the 2,6-dichlorophenyl diimine 88a ( 65% ee vs 66% ee) (61). The bound face of the styrene undergoes aziridination (in contrast with Fu s selective crystallization of the wrong face of styrene in his copper-catalyzed cyclopropanation reaction, cf. Section II.A.8). Unfortunately, the potential racemization of 118 (by the mechanism... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

The synthesis of metal-coordinated 1-azirines and the reactions of azirines induced by metals have opened a new area in the chemistry of this small ring heterocycle. Many of the reactions encountered bear resemblance to previously discussed thermally and photo-chemically induced reactions of 1-azirines. The reaction of a series of diiron enneacarbonyls in benzene results in coupling and insertion to give diimine complexes and ureadiiron complexes as well as pyrroles and ketones (76CC191). A mechanism for the formation of these products which involves initial 1,3-bond cleavage and generation of a nitrene-iron carbonyl complex as an intermediate was proposed. 

Oxidative dehydrogenation reactions of alcohols and amines are widespread in enzymatic biochemistry, and are of potential importance with regard to the operation of fuel cells based on simple alcohols such as methanol. The nature of products, and their rates of formation, may vary depending on the reaction conditions, and a role of metal ions has been recognized. The oxidation of amines may lead to a variety of products (nitriles, nitro species, etc.) although dehydrogenated diimine products are obtained quantitatively when the oxidation of the amine occurs via coordination to metal centers. A review is available on the mechanisms of oxidative dehydrogenations of coordinated amines and alcohols (93). 

Mechanism 1 is a simple rate-determining outer-sphere electron transfer between hydroxide and the tris(diimine)metal(III) reactant. This mechanism is, however, not very plausible for the reactants in Table II, because, first, the enthalpy of activation is significantly less than the enthalpy of reaction (6). Second, the standard reduction potential for Eq. (5)... 

Mechanism 2 requires deprotonation of a ligand, and dissociation of the hydrogen bound to the C(3) atom was suggested to be the slow step for the reaction of tris(2,2 -bipyridine)ruthenium(III) in base (12). This would require a mechanism for the 1,10-phenanthroline complexes different from that of the 2,2 -bipyridine complexes, but, from the data in Table II as illustrated in Figs. 3-5, this seems unlikely. The requirement of a different mechanism is based upon the significant differences in rates of D/H exchange as measured by 1H NMR for the tris(diimine)... 

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