Generation of 1,2-bisketenes from cyclobutene-1,2-diones by flash photolysis and ring closure kinetics

The interconversion of cyclobutene-1,2-diones (1) and 1,2-bisketenes (RC-C-O)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.