Associative mechanism amines

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). 

Other cases in which second-order kinetics seemed to require an associative mechanism have subsequently been found to have a conjugate base mechanism (called S ICB, for substitution, nucleophilic, unimolecular, conjugate base in Ingold s notation ). These reactions depend on amine, ammine, or aqua ligands that can lose protons to form amido or hydroxo species that are then more likely to lose one of the other ligands. If the structure allows it, the ligand Irons to the amido or hydroxo group is frequently the one lost. 

Much of the reactivity of amido ligands involves proton exchange processes that eliminate amine. The exchange is believed to occur by an associative mechanism consequently, the rate of reaction decreases for sterically congested metal complexes. For example, treatment of V[N(CH3)2]j with terf-butanol at room temperature forms V(O-f-Bu) in good yield, while at this temperature the more encumbered complex W[N(CH3)2] reacts only slowly with methanol and ethanol, and not at all with tert-butanoV° The use of bulky N-alkyl substituents allows for the isolation of low-coordinate complexes, such as Cr[N( -Pr)2]3- Amido complexes derived from primary amines can also serve as precursors to imido complexes. In many cases, amido halide complexes form imido complexes by loss of hydrogen chloride in the presence of a base (Equation 4.17). ... 

The oxidation reaction (Eq. 9) occurs by an associative mechanism at the surface sihcon atom (Lehmann 2002) attack by a nucleophile generates a 5-coordinate intermediate which then induces Si—Si bond cleavage. In basic solutions the nucleophile is usually hydroxide (OH, Eq. 9), but other nucleophiles such as amines are also effective (Sweryda-Krawiek et al. 1996). The susceptibility to nucleophilic attack increases if the surface silicon atom has electron-withdrawing substituents like oxygen, and so oxidation of silicon by hydroxide is self-accelerating. The rate of aqueous dissolution can also be substantially enhanced if cationic surfactants are present in the solution (Canaria et al. 2002). 

Notice that the more methyl groups there are (and therefore the bulkier the bidentate amine), the faster is the rate of aquation. To see if this is consistent with either a dissociative or associate mechanism, consider the rate-determining step in each case as shown in Figure 5.7. 

In Figure 5.7a, the associative rate-determining step is shown. What would happen to the rate of such a reaction as the bulk of the bidentate amine increased Simply put, the incoming water ligand would find it more difficult to make its way into the metal to form the seven-coordinate intermediate, and therefore the rate would decrease. This is not what the data in Table 5.3 indicate, and therefore such data are not consistent with an associative mechanism. 

Contrary to expectations based on an associative mechanism, the reaction of t-BuP(0)Cl2 with amines such as t-BuNH2 gave only t-BuP(O)-(NHt-Bu)2 and unchanged starting material without detectable amounts of /-BuP(0)Cl(NH/-Bu)(36). It was shown that when independently prepared, (36) reacted with amines in a process that was unusually insensitive to steric eifects, which the author interpreted in terms of an elimination-addition process (34). Less bulky alkylphosphonic dichlorides (RP(0)Cl2 R = Me, Et, i-Pr) reacted as expected by an associative process. 

Bidentate coordination of the imine was also demonstrated in crystal structures of Ir(i) bis(ethylene) and 1,5-cyclooctadiene complexes. An analogous chelate iridium-amine adduct was also synthesized and structurally characterized. The kinetics of substitution of amine by imine (Scheme 14) were investigated to probe the mechanism of the product/substrate exchange reaction in the proposed catalytic cycle. The data were interpreted in terms of an associative mechanism in which the imine binds reversibly to Ir, prior to the release of amine. However, since the catalytic mechanism may well involve octahedral Ir(m) rather than square-planar Ir(i) complexes, a different mechanism could well operate during catalysis. 

Pd(OH2)Cl3] follows the two-term rate law [equation (4)] more usually encountered for square-planar complexes. An associative mechanism is also proposed for the reaction of [Pd(OHa)4] + with aromatic amines. 

The kinetics of ring closure of the complex cw-[Co(en)2(OH2)-(NHaCHaCHaOH)] have been studied, and are consistent with the non-observance of this species in the aquation of cw-[Co(en)2(NH2CHaCH20H)X] + cations. An associative mechanism for the ring closure of the aquo-ethanol-amine complex is proposed. The kinetics and mechanisms of analogous ring-closure reactions of similar platinum(n) complexes are discussed in the appropriate section of Chapter 2 of this Part. Kinetic parameters for aquation of the isomeric cations cw-[Co(en)2LBr]2+ with L = 3- or 4-methylpyridine show the expected similarity. ... 

The original monoamine hypothesis of depression states that depressions are associated with a deficiency of catecholamines, particularly norepinephrine, at functionally important adrenergic receptor sites in the brain. Elation conversely may be associated with an excess of such amines. The hypothesis was articulated in 1966 only after the mechanism of action of the tricyclic antidepressant desipramine and of the psychostimulants... 

Some of the details of the mechanism may differ for various catalytic systems. There have been kinetic studies on two of the amination systems discussed here. The results of a study of the kinetics of amination of bromobenzene using Pd2(dba)3, BINAP, and sodium r-amyloxide in toluene were consistent with the oxidative addition occurring after addition of the amine at Pd. The reductive elimination is associated with deprotonation of the animated palladium complex.166... 

One of the more important protective mechanisms is probably the ability of these substances to interact not only with various oxygen-centered radicals but also with hydroperoxides. This ability is supplemented by the formation of associates between the amine light stabilizer and species responsible for polymer degradation. 

Perhaps because of these effects on metabolism of brain amines CYP2D6 status has been linked to personality traits [34]. These personality traits are themselves associated with tobacco dependence and hence provide another possible mechanism finking allelic variation at the CYP2D6 locus to smoking behavior. 

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