Divergent oxidative rearrangements in solution and in a zeolite: distal vs proximal bond cleavage of methylenecyclopropanes.

Irradiation of 9,10-dicyanoanthracene (DCA) or p-chloranil in the presence of E-1-benzylidene-2-phenylcyclopropane (E-5) in CH(2)Cl(2) causes E-5 to undergo methylenecyclopropane rearrangement. An adduct, Z-7, between DCA and 5 firmly supports the involvement of a bifunctional trimethylenemethane radical cation. In contrast, incorporation of E-5 into HZSM-5 produces trans,trans-1,4-diphenyl-1,3-butadiene radical cation sequestered in the HZSM-5 interior, tt-8(.+)@HZSM-5, identified by ESR and diffuse reflectance spectroscopy. In addition, low yields of tt-8, its cis,trans-isomer (ct-8), and 1-phenyl-1,2-dihydronaphthalene (9) were isolated from the supernatant solution. The sharp contrast between the photoinduced electron-transfer reaction with photosensitizers in solution and the spontaneous reaction with redox-active acidic zeolite offers the prospect of further zeolite-induced regiodivergent reactions in a range of additional substrates.

An evolved explanation for the molecular geometry and electronic structure of diphenyl-substituted cyclic trimethylenemethane in the ground state: a nearly planar conformation with a considerably localized electronic state.

[structures: see text] We reinvestigated the molecular geometry and electronic structure of the diphenyl-substituted, five-membered cyclic trimethylenemethane (TMM) diradical (Berson's TMM, 3**) using UV/VIS absorption and emission spectroscopy combined with density functional theory (DFT) and time-dependent (TD)-DFT calculations. Two intense absorption bands, A and B, with lambda(ab) at 298 and 328 nm, respectively, a weak absorption band C, with lambda(ab) at 472 nm, and an intense emission band D, with lambda(em) at 491 nm, were observed for 3**. By comparing the spectrum of 3** with those of the 1,1-diphenylethyl (7*) and cyclopent-2-en-1-yl (9*) radicals, it was found that bands B, C, and D originated from the diphenylmethyl radical moiety (subunit I), while band A should most likely be assigned to an electronic transition related to an interaction between subunit I and residual subunit II, the cyclopentenyl radical moiety. An UB3LYP/cc-pVDZ calculation indicated that, in the ground state, the two unpaired electrons of 3** are mainly localized in subunits I and II, respectively, and the interaction between them is inefficient, despite the nearly planar conformation (theta = +23.5 degrees). Furthermore, a TD-UB3LYP/cc-pVDZ calculation suggested that absorption band A is assigned to an electronic transition involved with enhancement of the electron density of the C-2-C-3 bond. Substituent effects on the absorption and emission spectra of 3** using 11** and 13** support the conclusion based on the experiments and calculations. Therefore, we propose an evolved explanation for the molecular geometry and electronic structure of the ground state of 3** in a low-temperature matrix, a nearly planar conformation with a considerably localized electronic state, which alone accounts for the spectroscopic characteristics.

Incorporation of 2-arylhexa-1,5-diene into pentasil zeolite: A distorted 1-arylcyclohexane-1,4-diyl radical cation at room temperature.

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.

Photoinduced electron-transfer bicyclopropenyl-benzene rearrangements of 2,2',3,3'-tetraphenylbicyclopropenyls: a new mechanism via Dewar benzene.

[reaction: see text] The 9,10-dicyanoanthracene-sensitized photoreaction of 1-methyl- and 1,1'-dimethyl-2,2',3,3'-tetraphenylbicyclopropenyl gives the corresponding benzene and Dewar benzene derivatives, indicating that their photoinduced electron-transfer bicyclopropenyl-benzene rearrangements proceed via Dewar benzenes.

Rearrangement of a 2-methylenecyclobutanone derivative triggered by photoinduced electron transfer: an unprecedented oxa analogue of the tetramethyleneethane radical cation.

On irradiation of p-chloranil (CA), 2,2-bis(p-methoxyphenyl)-3,3-dimethyl-4-methylenecyclobutanone (1) gives 2,2-bis(p-methoxyphenyl)-4-isopropylidenecyclobutanone (2), whereas 2 affords a CA adduct (4), suggesting that a novel rearrangement of 1 to give 2 occurs irreversibly via intermediate 3*+, a radical cation variant of an unprecedented oxa analogue of tetramethyleneethane.

Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer.

2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.

Photoinduced Electron-Transfer Cope Rearrangements of 3,6-Diaryl-2,6-octadienes and 2,5-Diaryl-3,4-dimethyl-1,5-hexadienes: Stereospecificity and an Unexpected Formation of the Bicyclo[2.2.0]hexane Derivatives.

Under the 9,10-dicyanoanthracene-sensitized photoinduced electron-transfer conditions, (Z,Z)-, (E,E)-, (E,Z)-3,6-diaryl-2,6-octadiene and (d,l),(meso)-2,5-diaryl-3,4-dimethyl-1,5-hexadiene stereospecifically undergo the Cope rearrangement to give a Cope photostationary mixture. Remarkably, the photoinduced electron-transfer Cope rearrangements of the 4-methylphenyl derivatives are concurrent with the formation of trans- or endo,cis-1,4-bis(4-methylphenyl)-2,3-dimethylbicyclo[2.2.0]hexane in a Cope photostationary mixture. Observed stereospecificity of the Cope rearrangement and the formation of the bicyclo[2.2.0]hexane derivatives demonstrate the intermediacies of both the chair and boat 1,4-diaryl-1,2-dimethylcyclohexane-1,4-diyl and cation radical intermediates in a Cope rearrangement cycle. Photoreactions of trans- and exo,cis-1,4-diaryl-5,6-dimethyl-2,3-diazabicyclo[2.2.2]oct-2-enes further support the interventions of the diyl intermediates in the Cope rearrangement cycle. By photoacoustic analysis, a cation radical cyclization-diradical cleavage mechanism is proposed for the photoinduced electron-transfer Cope rearrangement of the title dienes.