Nucleophilic Reactivities and Lewis Basicities of 2-Imidazolines and Related N-Heterocyclic Compounds

European Journal of Organic Chemistry, 2013, 16, 3369-3377 published on 09.04.2013
Eur. J. Org. Chem.
The nucleophilicity parameters N and sɴ, as defined by the linear free-energy equation log k(20 °C) = sɴ (N + E), of the 2-imidazolines and the related N-heterocyclic compounds have been determined by studying the rates of their reactions with differently substituted benzhydrylium ions in dichloromethane at 20 °C by stopped-flow or laser flash photolysis techniques. It is demonstrated that the N and sɴ parameters thus obtained can be used to reliably predict the rate constants for their reactions with Michael acceptors of known electrophilicity E. A comparison of the nucleophilicity parameters of the imidazoline derivatives with other commonly used nucleophilic organocatalysts shows that they are 10 to 10³ times less nucleophilic than PPh₃, 1,8-diazabicyclo[5.4.0]undec-7-ene, or 4-(dimethylamino)pyridine. The structure–reactivity relationships of these heterocycles are discussed.
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