Symmetry-adapted reaction electronic flux in cycloaddition reactions

The electronic activity along a reaction coordinate xi can be rationalized in terms of the reaction electronic flux [REF, J(xi)]. In this paper, its symmetry-adapted extension, J(s)(xi), is used to characterize the mechanism of two Diels-Alder reactions: butadiene plus ethylene to produce cyclohexene following a reaction path of C-s symmetry, and acetylene plus diacetylene to form o-benzyne in a C-2v reaction path. In the case of the former reaction, the electronic activity is captured in terms of the symmetry-adapted REF (SA-REF) according to irreducible representations A' and A ''. J(A)'(xi) characterizes the electronic activity due to pi electronic reordering, while J(A)''(xi) encompasses the formation of the new sigma bonds. The more complex o-benzyne formation displays four SA-REFs associated with each of the irreducible representation of the C-2v group. Results show that SA-REFs appear to be very useful for identifying electronic activity that displays an specific symmetry character. In the first reaction, the pi electronic activity drives the formation of cyclohexene, whereas in the second reaction, the in-plane electronic activity drives the formation of o-benzyne.