Generation of 1,2-Bisketenes from Cyclobutene-1,2-diones by Flash Photolysis and Ring Closure Kinetics1a

• Published in: Journal of the American Chemical Society Vol. 119; no. 50; pp. 12125 - 12130
• Main Authors: Allen, Annette D, Colomvakos, Jim D, Diederich, François, Egle, Ian, Hao, Xiaokuai, Liu, Ronghua, Lusztyk, Janusz, Ma, Jihai, McAllister, Michael A, Rubin, Yves, Sung, Kuangsen, Tidwell, Thomas T, Wagner, Brian D
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17.12.1997
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja9722685

The interconversion of cyclobutene-1,2-diones (1) and 1,2-bisketenes (RCCO)2 (2) has been surveyed for different combinations of substituents R = H, Me, t-Bu, Ph, Me3Si, CN, Cl, Br, R1O, alkynyl, and PhS. The bisketenes 2 have been generated by flash photolysis, and the kinetics of their conversion to 1 have been studied by time-resolved infrared and ultraviolet spectroscopy. The rate constants of the ring closure of 2 are correlated by the ketene stabilization parameters (SE) and with calculated barriers. The rate constant of ring closure of the di-tert-butyl bisketene 2g to cyclobutenedione 1g is only 40 times smaller than for the dimethyl analogue, showing a rather modest steric barrier. The quinoketene 2s has a fast rate of ring closure, but not as fast as anticipated on the basis of calculated geometric and thermodynamic factors. A lag in the attainment of aromatic stabilization in the transition structure for ring closure is a possible cause of this diminished reactivity.