Activation rate constants (k act ) for a variety of initiators for Cu-mediated ATRP were measured with Cu(I)Br(PMDETA) at various temperatures (i.e., -40 to þ60°C). Reactions of less active alkyl halides were more accelerated by increased temperatures than reactions of more active initiators. Straight Eyring and Arrhenius plots were obtained, from which the activation parameters (i.e., ΔH q , ΔS q , E a , and ln A) were determined. The activation enthalpies ΔH q are in between 26.0 and 38.7 kJ mol -1 with highly negative activation entropies (ΔS q =-156 to -131 J mol -1 K -1 ), which indicate greatly ordered structures of the transition states for these reactions.

Detailed mechanistic studies reveal that halogen exchange (HE) in ATRP can occur not only by a radical pathway (atom transfer) but also by an ionic pathway (SN2 reaction) because Cu(I)(L)X and Cu(II)(L)X2 complexes contain weakly associated halide anion that can participate in the SN2 reaction with alkyl halide (ATRP initiator). Both pathways were kinetically studied, and their contributions to the HE process were quantitatively evaluated for seven alkyl halides and three Cu(I)(L)Cl complexes. Radical pathway dominates the HE process for 3° and 2° alkyl bromides with more active complexes such as Cu(I)(TPMA)Cl. Interestingly, ionic pathway dominates for 1° alkyl bromides and less active ATRP catalysts. These studies also revealed that degree of association of alkyl halide anion depends on the structure of copper complexes. In addition, radical pathway is accompanied by the reverse reactions such as deactivation of radicals to alkyl bromides and also activation of alkyl chlorides, reducing the efficiency of halogen exchange.

The kinetics of reactions of acceptor-stabilized carbanions 2a-m with benzylidenebarbituric and -thiobarbituric acids 1a-e has been determined in a dimethyl sulfoxide solution at 20 degrees C. Second-order rate constants were employed to determine the electrophilicity parameters E of the benzylidenebarbituric and -thiobarbituric acids 1a-e according to the correlation equation log k(20 degrees C) = s(N + E). With E parameters in the range of -10.4 to -13.9, the electrophilicities of 1a-e are comparable to those of analogously substituted benzylidenemalononitriles.

The rates of the reactions of the colored para-substituted phenylacetonitrile anions 1a-c and the phenylpropionitrile anions 2a-c with Michael acceptors (3a-u) were determined by UV-vis spectroscopy in DMSO at 20 degrees C. The reactions follow second-order kinetics, and the corresponding rate constants k(2) obey the linear-free-energy relationship log k(2)(20 degrees C) = s(N + E), from which the nucleophile-specific parameters N and s of the carbanions 1a-c and 2a-c have been derived. With nucleophilicity parameters from 19 < N < 29, they are among the most reactive nucleophiles which we have so far parametrized. In DMSO, the nucleophilicity of the tert-butoxide anion is comparable to that of the p-cyanophenylacetonitrile anion 1b.

Kinetics of the reactions of four 2-benzylidene-indan-1,3-diones (1a-d) with carbanions (2a-I) have been studied photometrically in dimethyl sulfoxide solution at 20 degrees C, and the electrophilicity parameters E were determined by the linear free energy relationship log k(2)(20 degrees C) = s(N + E) (eqn (1)). The rate-determining step of these reactions is the nucleophilic attack of the carbon nucleophile at the double bond of the Michael acceptor. Comparisons with literature data show that the linear free energy relationship (eqn (1)) allows the semiquantitative prediction of the reactivities of 2-benzylidene-indan-1,3-diones towards various nucleophiles.

The relative rate constants for the vicarious nucleophilic substitution (VNS) of the anion of chloromethyl phenyl sulfone (1-) with a variety of nitroheteroarenes, for example, nitropyridines, nitropyrroles, nitroimidazoles, 2-nitrothiophene, and 4-nitropyrazole, have been determined by competition experiments. It was shown that nitropyridines are approximately four orders of magnitude more reactive than nitrobenzene. Among the five-membered heterocycles 2-nitrothiophene is the most active followed by nitroimidazoles and 4-nitropyrazole. Nitropyrroles are the least electrophilic nitroheteroarenes with reactivities comparable to nitrobenzene. Quantum chemically calculated methyl anion affinities (B3LYP/6-311G(d,p)//B3LYP/6-31G(d)) of the nitroarenes correlated only moderately with the partial relative rate constants. The correlation of these activities with the LUMO energies of nitroarenes is even worse. By measuring the second-order rate constants of the addition of 1- to nitroarenes and to diethyl arylidenemalonates 10, it was possible to link the electrophilic reactivities of nitroheteroarenes with the comprehensive electrophilicity scale based on the linear-free-energy-relationship log k(20 degrees C)=s(N+E).

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Kinetics of the reactions of four benzenesulfonyl-stabilized carbanions (1a-d)- with reference electrophiles (quinone methides 2 and diarylcarbenium ions 3) have been determined in dimethyl sulfoxide solution at 20 degrees C in order to derive the reactivity parameters N and s according to the linear free-energy relationship logk(20 degrees C) = s(N + E) (eqn (1)). The additions of (1a-d)- to ordinary Michael acceptors (e.g., benzylidene Meldrum's acid 4a, benzylidenebarbituric acids 5a-c, and benzylidene-indan-1,3-diones 6a-d) were also studied kinetically and found to be 5-24 times slower than predicted by eqn (1).