Rearrangements of Free Radicals, XIII^[1]. Thermal and Photochemical Rearrangements of Cyclic C₈H₉ Radicals in Adamantane Matrix

Abstract

The ESR spectra observed after X-irradiation of bicyclo-[5.1.0]octa-2,5-diene (homotropylidene) (12) in a [D,,]adamantane matrix at 210 K have been identified to be mainly due to the cyclooctatrienyl radical in adamantane, is thermally stable up to 370 K, but also undergoes a facile rearrangement to the radical 17 on UV irradiation. This process is reasonably explained to occur stepwise via radicals 7 and 8. The postulated reaction paths and the spectral assignments are supported by semiempirical (AM1, PM3), abinitio (UHF/3-21G*), and molecular mechanics (MM2ERW) calculations, which are in accord with the finding that 8 is energetically more stable than 7. The quantum mechanical calculations predict that a degenerate sigmatropic circumambulation of the cyclopropane ring in radical 7 should favorably compete with its ring opening.During the past two decades the adamantane matrix isolation technique12] has been established as a useful tool for the ESR investigation of rearrangement reactions of free radicals which, due to competing fast bimolecular decay reactions, cannot be observed in low-viscous fluid sol~tion [',~,~]. An intriguing example of that kind was the identification of a "walk" rearrangement (circumambulation) of the cyclopropyl group in the (deuterated) bicyclo-[3.l.O]hexenyl radical ( l ) [ 3 a 3 b 1 from the observation of a statistical distribution of deuterium in the cyclohexadienyl radical (2) which is formed by electrocyclic ring opening of 1 at temperatures above 210 K. This ring opening is photochemically reversible r3a,b1. According to these observations the electrocyclic ring opening of the bicyclo[5.l.0]octa-2,5-diene-4-yl (homotropylidenyl) radical (7) and the ring closure of the cyclooctatrienyl radical (8) are expected to proceed exceedingly faster than the corresponding reaction 1+2. Therefore, it would be of interest to learn if radical 7 undergoes a mutual interconversion with radical 8 and if this isomerization is com-

Combined Quantum Chemical/RRKM-ME Computational Study of the Phenyl + Ethylene, Vinyl + Benzene, and H + Styrene Reactions

“…However, they are separated from 5 by high barriers, inaccessible from the C 6 H 5 + C 2 H 4 reactants. The rearrangement of 13 to 6 and many other isomerizations involving these radicals have been studied by semiempirical (AM1, PM3) and molecular mechanics methods . Although the energetic parameters predicted by those methods are only semiquantitatively accurate, none of the secondary isomerization channels of radicals 6 and 13 appear to be important for the mechanism of reactions R1−R3.…”

“…The rearrangement of 13 to 6 and many other isomerizations involving these radicals have been studied by semiempirical (AM1, PM3) and molecular mechanics methods. 53 Although the energetic parameters predicted by those methods are only semiquantitatively accurate, none of the secondary isomerization channels of radicals 6 and 13 appear to be important for the mechanism of reactions R1-R3. Hence, the only kinetically important transformation of 5 is the ring opening via TS5 that brings it back to 1.…”

Programming Organic Molecules: Design and Management of Organic Syntheses through Free-Radical Cascade Processes

“…Delicate balances of rearrangement possibilities can easily be redirected by structural changes. This was illustrated by the fact that the vinylogous bicyclo[5.1.0]octa‐2,4‐dienyl radical did not undergo an analogous walk 163, 164…”

Programmierung organischer Moleküle: Design und Management organischer Synthesen über radikalische Kaskaden

“…In einem empfindlichen Gleichgewicht stehende Umlagerungsmöglichkeiten kann man leicht durch strukturelle Veränderungen in eine andere Richtung leiten. Dies wird dadurch belegt, dass das vinyloge Bicyclo[5.1.0]octa‐2,4‐dienylradikal keinen analogen Rundgang beschreitet 163, 164…”