Orbital Crossings Activated through Electron Injection: Opening Communication between Orthogonal Orbitals in Anionic C1–C5 Cyclizations of Enediynes

Generally,
the long-range electronic communication between spatially
orthogonal orbitals is inefficient and limited to field and inductive
effects. In this work, we provide experimental evidence that such
communication can be achieved via intra­molecular electron transfer
between two degenerate and mutually orthogonal frontier molecular
orbitals (MOs) at the transition state. Interaction between orthogonal
orbitals is amplified when the energy gap between these orbitals approaches
zero, or at an “orbital crossing”. The crossing between
two empty or two fully occupied MOs, which do not lead to stabilization,
can be “activated” when one of the empty MOs is populated
(i.e., electron injection) or one of the filled MOs is depopulated
(i.e., hole injection). In reductive cycloaromatization reactions,
such crossings define transition states with energies defined by both the in-plane and out-of-plane π-systems. Herein,
we provide experimental evidence for the utility of this concept using
orbital crossings in reductive C1–C5 cycloaromatization reactions
of ene­diynes. Communication with remote substituents via orbital
crossings greatly enhances regio­selectivity of the ring closure
step in comparison to the analogous radical cyclizations. We also
present photophysical data pertaining to the efficiency of electron
injection into the benz­annelated ene­diynes.