Oriented External Electric Fields Affect Rate and Stereoselectivity of Electrocyclic Reactions

We carried out a computational investigation at the density functional theory (M06-2X) level on the effects of oriented external electric fields (OEEF) on activation barriers and stereo chemical output of the thermal ring opening of 3-substituted cyclobutenes (C4H5X) to butadienes. It is well known that with pi-electron-donor substituents (X = CH3, NH2), the conrotatory outward rotation is preferred, while with pi-electron-acceptor substituents (X = CHO, NO, BH2), the conrotatory inward process becomes favored. In the presence of the OEEF applied along the three axes x, y, and z in either positive or negative direction, for both pi-donor and pi-acceptor substituents, we observed either catalysis or inhibition. Both effects were consistent with the change of the induced dipole along the direction of the applied field. An interesting effect was observed for X = CHO and NO. The simultaneous catalysis and inhibition of the outward and inward transformation leads to a reversed ratio between outward and inward transformation, with the former being favored (stereochemical inversion). Such effect was not observed for X = BH2 (the strongest pi-acceptor examined here). In this case, in the absence of the applied field, the difference between the inward and outward barriers is too large and the simultaneous catalysis and inhibition of the outward and inward transformation is not capable of determining the stereochemical inversion.