Cope rearrangement electron-transfer

Characteristics and energetics of the photo-induced electron-transfer degenerate Cope rearrangement of 2,5-diarylhexa-l,5-dienes have been reported in detail. Oxygentrapping experiments have established the two-step nature of the rearrangement of... 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

A sub case in this category is the benzylation of enolates through the Srn 1 process. In this way, both a-nitro- and a-chloro-4-nitrocumene are alkylated by the enolates of 2-nitropropane, diethylmalonate, or diethyl 2-butylmalonate. A particular case of benzylic C-C fragmentation is the electron transfer photosensitized Cope rearrangement of 2,5-diphenyl-1,5-hexadienes [218,219]. 

TCNE takes place (Scheme 57). Tomioka reported the (3 + 2) photocycloaddition between 1,1,2-triarylcyclopropanes and vinyl ethers in the presence of p-DCB [162]. Mizuno and Otsuji reported the (4 -l- 2) photocycloaddition between 1,2-diarylcyclopropanes and DCA [23]. The 1,4-radical cation produced as an intermediate of the Cope rearrangement of 1,5-dienes via photoinduced electron transfer can be trapped by molecular dioxygen, giving bicyclic dioxanes (Scheme 58) [163]. This photooxygenation takes place in a stereospecific manner. 

A study of the photochemical Cope reaction of the hexadienes 40 has been carried out under photoinduced electron-transfer conditions. Evidence was gathered for the formation of a chair cyclohexane-1,4-radical cation 41 °. In snch systems, where the radical cation is formed using DCA as the sensitizer, a degenerate Cope process is operative. Thus when the tetradeuterio derivative 42 is used, rearrangement affords a (52 48) mixture of the two dienes 42 and 43. Related to this general problem, DCA-sensitized reactions of the isomeric dienes 44 and , -45 and the cyclization prodnct, the bicyclohexane 46, have been studied in considerable detail. At low conversions, the irradiation of 46 affords a mixture of the dienes 44 and , -45 in ratios that are independent of temperature. The influence of the position of the aiyl groups on the diene skeleton has also been studied. This does not appear to affect the conversion to a cyclic radical cation. Thus the SET-induced reaction of the diene 47 has shown that the open chain radical cation of the diene 48 cyclizes preferentially to the radical cation 49. ... 

GonzAlez-Navarrete P, Andres J, Berski S (2012) How a quantum chemical topology analysis enables prediction of electron density transfers in ehemieal reactions. The degenerated cope rearrangement of semibullvalene. J Phys Chem Lett 3 2500-2505... 

Allylic carbon-carbon bond cleavage occurs more effectively examples are the stereospecific Cope rearrangement observed with 2,5-diary-1,5-hexadienes upon electron transfer sensitization (see Equation 4.20) " and the cleavage of the four-membered ring in pinenes at the radical cation stage. 

Ikeda, H., Takasaki, T., Takahashi, Y, Konno, A., Matsumoto, M., Hoshi, Y, Aoki, T., Suzuki, T, Goodman, J., and Miyashi, T, Photoinduced electron-transfer Cope rearrangements of 3,6-diaryl-2,6-octadienes and 2,5-diaryl-3,4-dimethyl-l,5-hexadienes stereospecificity and an unexpected formation of the bicyclo[2.2.0]hexane derivatives,/. Org. Chem., 64,1640, 1999. 

Ikeda, H., Minegishi, T., Abe, H., Konno, A., Goodman, J.L., and Miyashi, T., Photoinduced electron-transfer degenerate Cope rearrangement of 2,5-diaryl-l,5-hexadienes a cation radical cyclization-diradical cleavage process,/.Am Chem. Soc., 120, 87,1998... 

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