Benzylidenebarbituric acids have been termed "electrically neutral organic Lewis acids" because of their highly polarized double bond [1]. By measuring kinetics of the addition reactions of stabilized carbanions, the electrophilicities of these Michael acceptors could be quantified according to the correlation equation log k (20 °C) = s(E + N), where s and N are nucleophile-specific parameters and E is an electrophilicity parameter [2].

Scheme 1. Reaction of an acceptor-stabilized carbanion with a benzylidenebarbituric acid.

It is found that the electrophilicities of benzylidenebarbituric acids are in between those of stabilized benzhydrylium ions and quinone methides [3].

Figure 1. Electrophilicity parameters of benzhydrylium ions and Michael acceptors.

[1] P. Schuster, O. E. Polansky, F. Wessely, Tetrahedron 1966, Suppl. 8 (II), 463-483.
[2] H. Mayr, M. Patz, Angew. Chem. Int. Ed. 1994, 33, 938-957.
[3] R. Lucius, R. Loos, H. Mayr, Angew. Chem. Int. Ed. 2002, 41, 91-95.

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1. Nucleophilicities of Triflinate-Stabilized Carbanions
2. Quantifying Electrophilicity and Nucleophilicity
3. Nucleophilicities of Amines and Amino Acids
4. Towards a General Scale of Nucleophilicity?