Electrophilic Reactivities of 1,2-Diaza-1,3-dienes

• Published in: Chemistry : a European journal 2010-10, Vol.16 (39), p.12008-12016
• Main Authors: Kanzian, Tanja, Nicolini, Simona, De Crescentini, Lucia, Attanasi, Orazio A., Ofial, Armin R., Mayr, Herbert
• Format: Article
• Language: English
• Subjects: Amines; azo compounds; Chemistry; cycloaddition; CC bond formation; Ethers; Free energy; kinetics; linear free energy relationships; Mathematical analysis; Rate constants; Reaction kinetics
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201000828

The kinetics of the reactions of 1,2‐diaza‐1,3‐dienes 1 with acceptor‐substituted carbanions 2 have been studied at 20 °C. The reactions follow a second‐order rate law, and can be described by the linear free energy relationship log k(20 °C)=s(N+E) [Eq. (1)]. With Equation (1) and the known nucleophile‐specific parameters N and s for the carbanions, the electrophilicity parameters E of the 1,2‐diaza‐1,3‐dienes 1 were determined. With E parameters in the range of −13.3 to −15.4, the electrophilic reactivities of 1 a–d are comparable to those of benzylidenemalononitriles, 2‐benzylideneindan‐1,3‐diones, and benzylidenebarbituric acids. The experimental second‐order rate constants for the reactions of 1 a–d with amines 3 and triarylphosphines 4 agreed with those calculated from E, N, and s, indicating the applicability of the linear free energy relationship [Eq. (1)] for predicting potential nucleophilic reaction partners of 1,2‐diaza‐1,3‐dienes 1. Enamines 5 react up to 102 to 103 times faster with compounds 1 than predicted by Equation (1), indicating a change of mechanism, which becomes obvious in the reactions of 1 with enol ethers. Electrophilicities quantified: The kinetics of the reactions of 1,2‐diaza‐1,3‐dienes 1 with carbanions have been studied photometrically at 20 °C (see figure). The second‐order rate constants were used to calculate the electrophilicity parameters E of compounds 1 according to the linear free energy relationship log k2(20 °C)= s(N+E). These parameters were employed to define the scope and limitations of the reactions of 1,2‐diaza‐1,3‐dienes 1 with various nucleophiles.