The nucleophile-specific parameters N and sN, as defined by log k20 °C=sN(N+E) have been derived for the guanidines 1 a–h from the second-order rate constants of their reactions with diarylcarbenium tetrafluoroborates in CH2Cl2 at 20 °C. The applicability of these parameters for predicting rate constants of the reactions of guanidines with ordinary Michael acceptors has been demonstrated. Comparison with other organocatalysts shows that 1,5,7-triazabicyclo[4.4.0]dec-5-ene, the strongest nucleophile of this series, exceeds the nucleophilicity of diazabicyclononene and 4-(dimethylamino)pyridine by factors of 3 to 7.
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