Guanidines: Highly Nucleophilic Organocatalysts

The nucleophile-specific parameters N and sN, as defined by log k20 degrees C=sN(N+E) have been derived for the guanidines 1?ah from the second-order rate constants of their reactions with diarylcarbenium tetrafluoroborates in CH2Cl2 at 20 degrees 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.