Photoinduced electron transfer cyclopropanes

Preparation of mono-adducts of fullerene - for studies on electrostatic interactions - was undertaken by cyclopropanation of fullerene with appropriately functionalised malonic esters 1 (Bingel reaction) to form 2. Coupling with the tert-butyl protected oligoamide-amino-dendron 3 and subsequent hydrolysis lead to the water-soluble fullerene dendron 5, which can carry up to nine negative charges after depro to nation. After association with the zinc complex of cytochrome C, photoinduced electron transfer (PET) from the redox protein to the fullerene can be accomplished, which was studied by fluorescence spectroscopy. 

Arylcyclopropanes and their heterocyclic analogues are liable to electron transfer induced fragmentation of a carbon-carbon bond that in some cases leads to synthetically useful products. Thus, 1,2-diarylcyclopropanes [240-243] as well as 2,3-diaryloxirans [244-246] and -aziridines (in the last case, also 2-monophenyl derivatives) [247,248] are cleaved upon photoinduced electron transfer sensitization. The final result, after back electron transfer, is trans-cis isomerization of the ring. In the presence of a suitable trap, however, a cycloaddition reaction takes place, involving either the radical cation or the ylide. Thus, dioxoles, ozonides or azodioxoles, respectively, are formed in the presence of oxygen and oxazolidines have been obtained from cyclopropanes in the presence of nitrogen oxide (Sch. 23). 

Nucleophiles also add to radical cations of arylcyclopropanes to give anti-Markownikoff-type adducts [40-43]. There are two possible structures for the radical cations of arylcyclopropanes i.e., ring opened 1,3-radical cation and ring closed cyclopropane radical cation. Dinnocenzo reported that the photoinduced electron transfer reaction of chiral trans-l-methyl-2-phenylcyclopropane in methanol gives chiral 3-methoxy-l-phenylbutane via the radical cation [44], This result indicates that the radical cation in the ring closed form is attacked by methanol. The fact that this cyclopropane does not isomerize under the photochemical reaction conditions supports this conclusion (Scheme 11). 

Tazuke reported the carboxylation of the radical anions of aromatic hydrocarbons that are generated by photoinduced electron- transfer from the tertiary amines to the excited singlet aromatic hydrocarbons (Scheme 36) [119]. Toki and his coworkers reported the photofixation of COj with styrene using tertiary amines as electron donors [120]. Tomioka reported the photoaddition of tertiary amines to electrophilic cyclopropanes [115]. 

Nitrogen oxide NO also inserts into the cyclopropane ring of 1,2-diarylcyclopropanes under photoinduced electron transfer reaction conditions, giving 3,5-diarylisoxazolines (Scheme 60). 

Photoinduced electron transfer from the enone (89) to DCA in acetonitrile results in opening of the cyclopropane ring. The resultant radical (90) undergoes cyclization to the enone moiety to give the isomeric compounds (91)... 

PHOTOINDUCED ELECTRON TRANSFER REACTIONS OF CYCLOPROPANE DERIVATIVES Tsutomu Miyashi, Hiroshi Ikeda,... 

Mizuno, K. and Otsuji, Y, Addition and cycloaddition reactions via photoinduced electron transfer, in Topics in Current Chemistry, Vol. 169, Mattay, J., Ed., Springer-Verlag, Berlin, 1994, p. 301. Miyashi, X, Ikeda, H., Takahashi, Y, and Akiyama, K., Photoinduced electron transfer reactions of cyclopropane derivatives, in Advances in Electron Transfer Chemistry, Vol. 6, Mariano, P.S., Ed., Jai Press Inc., Stamford, CT, 1999, p. 1. 

Irradiation of dimethyl-substituted homobenzoquinone 6a in the presence of EtjN or diethylamine gave dihydrogenated 41a without cyclopropane ring cleavage (Scheme 22). In the case of DMA, only the aniline adduct 40a was produced as well as in the reaction of 6c. The stereoselective formation of 40 and 41 is rationalized by the stability of their half-chair conformations. In conclusion, irradiation of homo-quinones in the presence of donors resulted in a photoinduced electron transfer that is responsible for the subsequent dimerization, donor amine addition, and hydrogenation, depending on the nature of the homoquinone and the donor. 

A survey of the literature reveals some characteristic photochemical features of homoquinones (1) [2+2]-photocycloaddition with alkenes and alkynes, (2) hydrogen abstraction and/or ring opening of incorporated cyclopropane, and (3) photoinduced electron transfer reactions, as described below. The photocycloaddition with alkenes and alkynes took place exclusively at the ethylene linkage of homobenzoquinones to afford cyclobutanes (or cyclobutene derivatives) regio- and exo/endo stereoselectively, depending on the identity of both the homobenzoquinones as well as the alkenes. 

Cation radicals of arylcyclopropanes, produced by photoinduced one-electron transfer to an electron acceptor, undergo ring-opening reactions accompanied by regioselective polar additions. 1,4-Dicyanobenzene (5) sensitized photolysis of l,l-bis(4-tolyl)cyclopropane (4) in methanol/acetonitrile resulted exclusively in anti-Markovnikov addition of methanol to the cyclopropane. It is assumed that excitation of 5 followed by electron transfer from 4 gives a dicyanobenzene radical anion 5(- ) - cyclopropane radical cation 4(4-) pair. Attack by methanol on the radical cation 4(4-) followed by proton loss affords benzylic radical 7 that is reduced by 5( —) to give benzylic anion 8 and quenched by methanol to afford product... 

For example, the structure of the radical cation of 1,2-diphenylcyclopropane has been assigned on the basis of the analysis of CIDNP data formed in the act of photoinduced reversible electron transfer from cyclopropane to chloranil. The choice has been made between closed and open structures of the radical cation of 1,2-diphenylcyclopropane (Figure 4). The observed CIDNP effects of 1,2-diphenylcyclopropane (absorption of aromatic ortho-, para-, and Hd, and the emission of Hp) comply only with the open structure. 

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