Rates of hydride transfer from several hydride donors to benzhydrylium ions have been measured at 20 °C and used for the determination of empirical nucleophilicity parameters N and s(N) according to the linear free energy relationship log k(20 °C) = s(N)(N+E). Comparison of the rate constants of hydride abstraction by tritylium ions with those calculated from the reactivity parameters s(N), N, and E showed fair agreement. Therefore, it was possible to convert the large number of literature data on hydride abstraction by tritylium ions into N and s(N) parameters for the corresponding hydride donors, and construct a reactivity scale for hydride donors covering more than 20 orders of magnitude.

Rate constants of activation (k act) for the reactions of tertiary alkyl halides with the ATRP catalyst CuIBr/1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) have been determined in 14 different solvents. The measurements have been performed at 25 °C by spectrophotometrically following the time-dependent absorbances of the CuII species. A large excess of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), which quantitatively trapped the alkyl radicals, ensured the irreversible generation of CuII. The rate constant for the least active solvent butanone is 30 times smaller than that of the most active solvent DMSO. In addition, the effect of increasing amounts of monomer in a solvent on the activation rate has been analyzed. A linear correlation of activation rate constants with previously determined equilibrium constants (K ATRP) provides a Leffler–Hammond coefficient of 0.45. However, the activation rate constants do not correlate with dielectric constants and Dimroth’s and Reichardt’s E T(30) values. Application of the linear solvation energy relationship of Kamlet and Taft revealed that the dipolarity/polarizability π* of the solvent, i.e., nonspecific solvent–solute interactions, mainly accounts for solvent effects on k act, while the ability to donate a free electron pair is important for some solvents. Quantum chemical calculations showed that more polar solvents stabilize the CuII product complex to a higher degree than the CuI starting complex.

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The nucleophilicity parameters (N) of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene borane and 1,3-dimethylimidazol-2-ylidene borane are 9.55 and 11.88. This places N-heterocyclic carbene boranes (NHC-boranes) among the most nucleophilic classes of neutral hydride donors. Reductions of highly electron-poor C═N and C═C bonds provide hydrogenation products along with new, stable borylated products. The results suggest that NHC-boranes have considerable untapped potential as neutral organic reductants.