Competence CASS scheme

The iron(II)-iron(III) form of purple acid phosphatase (from porcine uteri) was kinetically studied by Aquino et al. (28). From the hydrolysis of a-naphthyl phosphate (with the maximum rate at pH 4.9) and phosphate binding studies, a mechanism was proposed as shown in Scheme 6. At lower pH (ca. 3), iron(III)-bound water is displaced for bridging phosphate dianion, but little or no hydrolysis occurs. At higher pH, the iron(III)-bound OH substitutes into the phosphorus coordination sphere with displacement of naphthoxide anion (i.e., phosphate hydrolysis). The competing affinity of a phosphomonoester anion and hydroxide to iron(III) in purple acid phosphatase reminds us of a similar competing anion affinity to zinc(II) ion in carbonic anhydrase (12a, 12b). 

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

Products with mass equal to the sum of the reagent masses also form, to different extents, in the reactions of 02 with ketones, namely acetone, CF3COCH3 and (CF3)2C0264. These adducts were tentatively assigned the structure of the bound tetrahedral adduct of nucleophilic carbonyl addition. While this reaction is the only one observed with acetone, it competes with H+ abstraction in the case of CF3COCH3 to form the stabilized enolate [CH2=C (CT)CF3] and with ET in the case of (CF3)2CO (electron affinity is ca 33.7 kcal moF1). In this latter case it was concluded that reaction of (CF3)2CO with Of occurs exclusively via ET and that the adduct forms in a secondary process via reaction of the primary product, the radical anion of (CF3)2CO with 302 present in the flow from the preparation of 02 (see Scheme 39). 

The initial disclosure of a [6 + 4] cycloaddition between tropone (1) and a 1,3-dipole involved diphe-nylnitrilimine as depicted in Scheme In this instance the [6 + 4] pathway competed rather poorly with various alternative [4 + 2] pathways and only a small quantity of the adduct in which the dipole added across the 2- and 7-positions of (1) was recovered. The three [4 + 2] adducts (45)-(d7) that were isolated from the reaction mixture presumably arose from a base-catdyzed hydrogen shift that occurred subsequent to the initial cycloaddition. Efforts to account for the divergent behavior of dienes and 1,3-dipolar species in their reactions with tropone have included invoking a dipole repulsion between the large positive charge located on the central atom of the 1,3-dipole and the partial positive charge on tropone, which must come into close proximity in the transition state of a concerted [6 + 4] cycloaddition. A ca-... 

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