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

Abstract

Incorporation into a redox-active pentasil zeolite [(Na,H)-ZSM-5] converted 2-arylhexa-1,5-dienes (9a-c; aryl = phenyl, tolyl, anisyl) into 1-arylcyclohexane-1,4-diyl radical cations, 10a-c*+. The ESR spectra of 10a-c*+ (six lines, g = 2.0026; a = 9.0 G) indicated the presence of five essentially equivalent nuclei, indicating limited delocalization of spin and charge into the phenyl group. Sequestered in the pores of ZSM-5, the three species 10a-c*+ are stable at room temperature, in striking contrast to the parent radical cation in cryogenic matrices: cyclohexane-1,4-diyl radical cation is converted to cyclohexene radical cation above 90 K. The structures of radical cation 10a*+ (X = H) and of the unsubstituted parent were probed by density functional theory (DFT) and ab initio calculations.

Brønsted Acid Catalyzed Cyclization of C7 and C8 Dienes in HZSM-5: A Hybrid QM/MM Study and Comparison with C6 Diene Cyclization

“…In the gas phase the diene molecules undergo thermal rearrangements such as Cope and Claisen rearrangement via a cyclic radical-cation transition state. A recent publication 18 regarding the Cope rearrangement inside the zeolite channels reports the stabilization of the radical-cation intermediate even at room temperatures. Unsaturated alkenes can undergo Diels-Alder [4 + 2] cycloaddition to form cyclohexene with reported energy barrier of 27.5 kcal/mol.…”

Photoinduced Electron Transfer between the Anionic Porphyrins and Viologens in Titania Nanosheets and Monodisperse Mesoporous Silica Hybrid Films

“…This new absorption band gradually decreased with further UV irradiation for 80 min, however, this intermediate is stable even under the aerated condition. Although the detailed mechanisms have yet to be clarified, stabilization of nonpersistent chemical species within the inorganic nanospace have been reported in various systems. ,,,,− The stability in the present system may be explained by the rapid diffusion of the photogenerated electrons or holes within the TNS layers, the difficult and negligible diffusion of atmospheric oxygen gas or water molecules in the nanospaces, ,, and/or the charge neutralization of these species within the nanospaces. , More detailed investigations are now underway on this issue such as the identification of the unknown intermediates and the determination of their stability.…”

“…The hybridization of nanostructured inorganic hosts with organic photofunctional molecules has been widely and extensively studied. − The photofunctional organic functional molecules hybridized with nanosized inorganic hosts have been observed to have unique redox properties in photochemical and photophysical processes. ,− In our previous studies, − consecutively stacked hybrid thin films of mesoporous silica films ( cubic -MPS) − or monodisperse mesoporous silica spheres (MMSS) − and layered titania nanosheets (TNS) ,,− were shown to exhibit unique photoinduced electron transfer between the cationic porphyrins (MTMPyP 4+ ; M = H 2 , Zn, and Co) and methyl viologen (MV 2+ ), which were separated and were present in different inorganic hosts. , Upon UV-light illumination of the photocatalytic TNS, , the (MV 2+ -TNS)/(H 2 TMPyP 4+ -MMSS), MV 2+ radical ions (MV +• ) were formed at the expense of the oxidative consumption of H 2 TMPyP 4+ , resulting in electron transfer through the interface between the TNS and mesoporous silica, thus leading to charge separation at the interface between the two nanostructured inorganic hosts. ,, The charge separated states obtained in these materials were highly stabilized and lasted for several hours even in the ambient atmosphere …”

“…26,47,49,60,71À79 The stability in the present system may be explained by the rapid diffusion of the photogenerated electrons or holes within the TNS layers, 75 the difficult and negligible diffusion of atmospheric oxygen gas or water molecules in the nanospaces, 47,78,79 and/or the charge neutralization of these species within the nanospaces. 26,72 More detailed investigations are now underway on this issue such as the identification of the unknown intermediates and the determination of their stability.…”