Reactivity stereochemical change

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

The isomers cis- and trans-[Cr(CN)2(NH3)4]+ as well as m-[Cr(CN)(NH3)4DMS0](G04)2 have also been synthesized by the anation reactions shown in Scheme 29.325 While anation in DMSO is accompanied by stereochemical change, the reaction in H20 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. 

Arguments have been presented to suggest that the mechanism of catalyzed nucleophilic displacement at a phosphoryl or thiophosphoryl centre may not be as had been previously thought. Thus far, nucleophilic displacements have been thought to involve double Sj 2 displacements involving the catalyst and/or, pentacoordinate intermediates. Measurements of reaction rates and stereochemical changes for a variety of linear and cyclic phosphorochloridates (and also phosphonochloridates and phosphonochloridothioates) in alcoholysis reactions catalyzed by hexamethylphosphorous triamide, pyridine, or N-methylimidazole (the order of increasing reactivity) have been used to question the earlier ideas, and it has now been proposed that the... 

Reactions of binuclear complexes 29 Stereochemical changes at an unsaturated carbon centre 30 Reactions of coordinatively-saturated complexes 30 Reactions of coordinatively-unsaturated complexes 31 Stereochemical changes at the metal 32 The iodide catalysis effect 38 The reactions of binuclear complexes 39 The reactivity of the carbon centre 41 The reactions of a,o>-dihalogenoalkanes 44 Activation parameters 47... 

When fused to sugars, butenolide reactivity may change because of stereochemical hindrance issues and sugars also become conformationally restricted, possibly altering the biological properties, for example, by enhancing selectivity or activity. 

Although the rationalization of the reactivity and selectivity of this particular substrate is distinct from that for chiral ketals 92-95, it still agrees with the mechanistic conclusions gained throughout the study of Simmons-Smith cyclopropa-nations. StOl, the possibility of the existence of a bimetallic transition structure similar to v (see Fig. 3.5) has not been rigorously ruled out. No real changes in the stereochemical rationale of the reaction are required upon substitution of such a bimetallic transition structure. But as will be seen later, the effect of zinc iodide on catalytic cyclopropanations is a clue to the nature of highly selective reaction pathways. A similar but unexplained effect of zinc iodide on these cyclopro-panation may provide further information on the true reactive species. 

Bohle and co-workers (133) have demonstrated that varying the electronic and stereochemical properties of porphyrin substituents can strongly influence the rates of NO labilization (Eq. (11)). For example, the displacement of NO from Fe(TPP)(NO) by pyridine is many orders of magnitude slower than from Fe(OBTPP)(NO) (OBTPP = octabromo-tetraphenylporphyrin). An analysis of the kinetics of the latter reaction indicated saturation in [L], and the mechanism was suggested to involve reversible formation of Fe(OBTPP)(L)(NO) followed by NO dissociation (Eq. (50)). Clearly changes in porphyrin properties can lead to enhanced reactivity toward NO loss. 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

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