Trimethylenemethane derivatives stabilized by conjugation II - Concerted or nonconcerted generation

The concerted formation of the orthogonal trimethylene-type diradicals 3, 6 , and 12 has been demonstrated for the three homofulvenes 2, 4, and 7 and s hown to proceed preferentially in a disrotatory manner. The same type of or thogonal diradicals are formed, but in a nonconcerted fashion, from the bic yclic methylenecyclopropanes 26, 30, 33, and 40, through the planar diradic als 28, 31, 35, and 50, respectively. The different reaction modes are view ed as being caused by the different coupling probabilities of the underlyin g vibrations. The orthogonal or planar geometries of the diradicals were de rived from stereochemical and kinetic arguments. Additional thermodynamic c haracterization was achieved for three diradicals [3c, (E)-19, and 41] by o xygen-trapping experiments.