Solvent effects platinum complexes

The molecular modelling approach, taking into account the pyruvate—cinchona alkaloid interaction and the steric constraints imposed by the adsorption on the platinum surface, leads to a reasonable explanation for the enantio-differentiation of this system. Although the prediction of the complex formed between the methyl pyruvate and the cinchona modifiers have been made for an ideal case (solvent effects and a quantum description of the interaction with the platinum surface atoms were not considered), this approach proved to be very helpful in the search of new modifiers. The search strategy, which included a systematic reduction of the cinchona alkaloid structure to the essential functional parts and validation of the steric constraints imposed to the interaction complex between modifier and methyl pyruvate by means of molecular modelling, indicated that simple chiral aminoalcohols should be promising substitutes for cinchona alkaloid modifiers. Using the Sharpless symmetric dihydroxylation as a key step, a series of enantiomerically pure 2-hydroxy-2-aryl-ethylamines... 

The stereoselective catalyzed addition of water or methanol to dimethyl acetylenedicarboxylate (DMAD) was reported to yield oxalacetic acid dimethylester or dimethyl methoxyfumarate. The catalyst precursor cis-[Pd(PMe2Ph)2(solvent)2] [BFJj was prepared from ds-[PdCl2(PMe2Ph)2] and AgBp4 (Eq. 6.54). The analogous platinum complex was not effective, however [99]. 

Two features need to be noted. Firstly, platinum-195 chemical shifts are quite sensitive to temperature, and in order to obtain acceptably narrow lines the solution temperature should be kept constant over the data accumulation time. Secondly, solvent effects need to be considered. A study of cis- and fraraj-PtCl2(PBu )2 in 14 solvents shows a change in A<5(P) of only 0.83p.p.m., but a change in Av(PtP) of 84.2Hz between n-hexane and acetonitrile.1355 Further work on temperature, solvent, substituent, oxidation state and stereochemical effects on 31P and 195Pt NMR chemical shifts and complexes is needed, and further efforts to collect and correlate data will be very useful.1356-1358... 

Since these substitution reactions follow a two-term rate law, it is clear that solvent effects are very significant. Poorly coordinating solvents are benzene, carbon tetrachloride and sterically hindered alcohols and strongly coordinating solvents are water, lower alcohols, DMF, DMSO, acetonitrile and nitromethane. The first-order rate constants are greater in DMSO than in water. Since the majority of precursor platinum complexes used in synthetic and mechanistic studies are halo complexes, the replacement of halide ligands by solvent and the reversibility of this reaction are important features of platinum halide chemistry. 

Since it is known that the tetranuclear mixed-valent platinum blue and tan complexes such as 1 and 2 undergo disproportionation and reduction by water as Eqns. 1-3 and 7-9 show [54] [66], all the species appearing in Eqns. 1-3 and 7-9 are present in the solution. However, only one or several of the four species in the solution may in fact be active during catalytic olefin oxidation. To clarify this point, the effects of the Pt oxidation state in the platinum complexes were compared. The results are summarized in Table 2. It clearly shows that the dinuclear Pt111 complex is most effective, and is likely to be a true catalyst. Compound 1 also exhibits high activity, whereas the dinuclear Pt11 complex is ineffective. All other factors expected to affect the catalytic efficiency, including the presence of 02, the surfactant and the choice of solvent, have been examined and the results are summarized... 

Galbraith, J. A., Menzel, K. A., Ratilla, E. M. A. and Kostic, N. M. (1987) Study of stereodynamics by variable-temperature platinum-195 NMR spectroscopy. Diastereoisomerism in platinum(II) thioether complexes and solvent effects. Inorg. Chem., 26, 2073-2078. 

The mechanism of the final step of C-0 bond formation by a formal reductive elimination was included as part of Chapter 11. In particular, two model systems have provided information on the mechanism of this reaction. First, the reaction orders, solvent effects, and electronic effects on the reductive elimination of methyl acetate and methyl aryl ethers from methylplatinum(lV) acetate and phenoxide complexes (Equation 18.16 and Scheme 18.4) indicated that these reductive eliminations occur by backside attack on a platinum methyl. Second, a study of the stereochemistry of the attack of water on a Pt(lV) alkyl showed that the formation of alcohol occurred with the inversion of configuration that reflects a backside attack. ... 

First, Albano et al. presented a detailed study [90] of the mechanism of chloride abstraction from the complex [(NN)Pt(Me)(Cl)] (NN = 2,3-bis(2,6-diisopropyl-phenylimino)butane) by silver(I) salts, during which they isolated a trimetallic intermediate (Scheme 24, top) featuring a platinum-silver contact of 2.895 A. This intermediate was stable in THE but released AgCl in chloroform, illustrating how subtle solvent effects may influence the course of such reactions. 

Entering Groups. Rates of reaction of [PtCl4], [PtCl3(OH2)], and [PtCl2(OH2)2] with ethene are all rather similar. Rates and activation parameters for the reaction of a variety of uncharged platinum(ii) complexes with cyanide have been reported. The complexes in question are [PtCl(N02)(NH3)2], [Pt(N02)2(NH3>2], [Pt(CN)a-(en)], and rra s -[Pt(CN)(N02)(NH3)2] cyanide substitution follows the usual rate law [equation (1) above]. The variation of the second-order term with solvent composition for the reaction of [PtCl2(bipy)] with thiourea in aqueous dioxan and in aqueous THE has been discussed in terms of solvent effects on the initial and transition states (see Chapter 5 of this Part). ... 

An earlier investigation of phosphine-catalysed isomerization of platinum(n) complexes resulted in the proposal of a mechanism in which pseudorotation in a five-co-ordinate intermediate played a central role. The authors of a recent study of phosphine-catalysed isomerization of -[PtLaXa] prefer a consecutive displacement reaction, with an ionic intermediate [PtLaL X]+X . Their conclusion is based on n.m.r. determinations both of isomerization and of ligand-exchange rates, on solvent effects on rates, and on the stabilities of species [PtLL Xa] relative to [PtLaXJ and [PtL XJ. Although these results are convincingly consistent with a consecutive-displacement mechanism, they do not seem to rule out the alternative pseudorotation scheme. 

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