Oxidation of Aryl- and Diarylcyclopropanes in a Pentasil Zeolite: Ring Opening with Deprotonation or Net Hydrogen Migration

Radical cations of disubstituted cyclopropanes: stereoelectronic effects on hyperfine coupling

“…To our knowledge, neither cis ‐ nor trans ‐ 6 •+ has been characterized by ESR spectroscopy. An attempt to observe the ESR spectrum of trans ‐ 6 •+ upon incorporation of trans ‐ 6 into a redox‐active zeolites resulted in the conversion to E,E ‐1,3‐diphenylallyl radical, E,E ‐ 7 • …”

Incorporation of 2-Arylhexa-1,5-diene into Pentasil Zeolite: A Distorted 1-Arylcyclohexane-1,4-diyl Radical Cation at Room Temperature

“…In an attempt to probe the identity of the secondary species observed in H−mor, we incorporated three potential products, 1- p -anisylcyclohexene, 11c , 1- p -anisylcyclohexa-1,3-diene, 12c , and p -methoxybiphenyl, 13c , into H−ZSM-5 and compared the resulting spectra with that spectrum (Figure , bottom). These substrates were chosen because results of previous studies suggested that a radical cation of structure 10c • + might undergo a combination of deprotonation-oxidation and dehydrogenation-oxidation in the zeolite interior. 8a,d The spectra obtained from 11 and 12 were essentially identical, suggesting that 11 • + is readily converted to 12 • + in the zeolite channels. Because the spectrum of 12 • + closely resembled the secondary spectrum in H−mor (Figure , bottom), that spectrum was assigned as due to 12 • + .…”

“…The persistence of 10a − c • + is all the more remarkable, as deprotonations, dehydrogenations, or net hydrogen migrations readily occur in zeolites. For example, oximes readily form iminoxyls, p -propylanisole gives rise to p -propenylanisole radical cation,8a and p -anisylcyclopropane is converted to 1- p -anisylpropene radical cation 8d. In light of these results, one might have expected radical cations 10a − c • + to rearrange to 1- p -arylcyclohexene radical cations, 11a − c • + , or be dehydrogenated to 1- p -arylcyclohexa-1,3-dienes, 12a − c • + , or p -substituted biphenyl radical cations, 13a − c • + .…”

“…Various organic radical cations can be generated spontaneously by inclusion of their precursors into zeolites. , The rigid microporous solids stabilize the otherwise highly reactive radical cations due to the combined effects of (a) the intense electrostatic fields inside zeolites, (b) topological restrictions that prevent the access of external reagents, and (c) the limiting dimensions of the zeolite channels, which may restrict the shape of the enclosed intermediates. , Typically, radical cations generated in zeolites have extended lifetimes and can, therefore, be studied by conventional spectroscopic techniques. On the other hand, some radical cations sequestered in zeolites undergo rapid conversions, which are without precedent in cryogenic matrices or in solution . For example, p -propylanisole gave rise to p -propenylanisole radical cation;8a 2-phenyl-1,3-dithiane formed 1,2-dithiolane radical cation;8b trans -1,2-diphenylcyclopropane was converted to exo , exo -1,3-diphenylallyl radical;8c and p -cyclopropylanisole gave rise to p -propenylanisole radical cation 8d…”

“…On the other hand, some radical cations sequestered in zeolites undergo rapid conversions, which are without precedent in cryogenic matrices or in solution . For example, p -propylanisole gave rise to p -propenylanisole radical cation;8a 2-phenyl-1,3-dithiane formed 1,2-dithiolane radical cation;8b trans -1,2-diphenylcyclopropane was converted to exo , exo -1,3-diphenylallyl radical;8c and p -cyclopropylanisole gave rise to p -propenylanisole radical cation 8d…”