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

Abstract

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.

Diverting Radical-Anionic C-H Amidation to a C-N/C-O Cascade for the Construction of Hydroxyisoindolines from Unprotected Amides

“…The third electron avoids its apparent destiny of ending up at the high energy σ*C-N orbital by "hopping" to a lower energy π* orbital of the aromatic ring (Scheme 10). This state crossing, not unusual for radical-anionic reactions, 34 stabilizes the product by creating a delocalized π-type radical-anion.…”

Upconversion of Reductants

“…In reality, such reactions often happen in the presence of an orthogonal π system (reaction of a vinyl or an aryl radical), where the extra electron is intercepted by a π* orbital through an avoided state crossing between the σ*/π* states (Scheme ). In the latter scenario, a normal two‐electron σ bond is formed because the extra electron is caught in a “π* trap” that is populated at the state crossing . For the reactions of aryl radicals, this process leads to the formation of quite stable and highly delocalized radical anions but decreases the amount of energy stored by the upconversion.…”

Phenalenannulations: Three‐Point Double Annulation Reactions that Convert Benzenes into Pyrenes

“…The contrasting conformations of the two methoxy groups in 19 highlight the "stereoelectronic chameleon" [15] behavior of -OMe groups. [16] Pyrenes are among the most popular chromophores with a number of interesting properties useful for a broad range of applications. [17] The photophysical characterization of selected compounds reveal several interesting structural effects on absorption and emission (see the Supporting Information for additional details).…”