Photoinduced Electron Transfer Reactions of Aryl Olefins. 2. Cis−Trans Isomerization and Cycloadduct Formation in Anethole−Fumaronitrile Systems in Polar Solvents

Abstract

In acetonitrile, the photoreactions of cis-anethole, c A, or trans-anethole, t A, with fumarodinitrile, FN, lead to isomerization of both substrate and quencher and to mixed [2 + 2] cycloaddition. By NMR analysis and by NOE measurements it was shown that the same four stereoisomers of 1-anisyl-2-methyl-3,4-dicyanocyclobutane are formed in equal yields regardless of whether the substrate is c A or t A. The configuration of the anethole-derived moiety in these adducts is always trans, whereas all possible configurations of the cyano groups occur. From Stern−Volmer experiments it was concluded that the quenching mechanism is electron transfer, not exciplex formation. Electron-transfer quenching is also the pathway leading to the cycloadducts, as was established by photoinduced electron-transfer sensitization. These photoreactions give rise to strong nuclear spin polarizations (CIDNP) in the starting and isomerized forms of both substrate and quencher as well as in the cycloadducts. Radical pairs (RP I) consisting of the radical cation of the anethole and the radical anion of fumarodinitrile were identified as the predominant source of the polarizations. The starting materials are regenerated by back electron transfer of singlet pairs; likewise, back electron transfer of triplet pairs 3RP I to give either triplet anethole, 3 A, or triplet fumarodinitrile, 3 FN, occurs and constitutes the main pathway to isomerization of substrate and quencher. The cycloadducts are also formed via 3RP I; a significant participation of free radicals in their generation was ruled out. The stereochemistry of the products and the different ratios of polarization intensities of the isomerized olefin and the cycloadducts can only be explained by the intermediacy of a triplet biradical 3 BR. After intersystem crossing to the singlet state, ring closure of 1 BR competes with biradical scission. The latter process provides an additional isomerization pathway, which differs from the pathway via triplet olefins by leading only to one-way cis−trans isomerization of the substrate. 3 BR is formed by geminate combination of triplet radical ion pairs 3RP I; an indirect pathway via back electron transfer of 3RP I to give 3 A or 3 FN followed by attack to the other olefin, as has been proposed for similar photocycloadditions, could be excluded unambiguously. Despite the different precursor multiplicity, the mechanism of the [2 + 2] photocycloaddition of donor and acceptor olefins investigated in this work is thus identical to the mechanism of the Paterno−Büchi reaction between donor olefins and electron-deficient carbonyl compounds which we recently reported. The CIDNP experiments at variable quencher concentration revealed the presence of an additional radical pair RP II, also containing A •+ but an anion other than FN •-. It was shown that RP II results from a biphotonic process. These findings were explained by two-photon ionization of the anethole and formation of an oligomeric anion of acetonitrile.

Photochemistry of substituted cyclic enones. Part 12.1 Photocycloaddition of 3-phenylcyclopentenone and 3-phenylcyclohexenone to (E )- and (Z )-1-phenylpropene

“…Adducts are formed with quantum yields that are approximately equal whether starting with the (E)-or (Z)-1-phenylpropene. It has been argued that the equal ratios of adducts formed from the photocycloaddition reaction of cis-or trans-anethole with fumarodinitrile, 32 similar to that which we observe for (E)-and (Z)-1-phenylpropene with 1, may be caused by the approach of the alkene to form biradicals.…”

“…Evidence for the same regiochemistry has been observed for the cycloaddition reactions of cis-or trans-anethole to fumarodinitrile. 32 Biradicals formed which lack the benzylic stabilisation of one radical centre have been ruled out by thermodynamic arguments.…”

Use of electrospray ionization mass spectrometry for the investigation of radical cation chain reactions in solution: detection of transient radical cations

“…However, radical cation 2 ț + could be detected and characterized through pulse radiolysis of anethole (2) by UV spectroscopy [13,14,15], photoinduced electron transfer of 2 on triplet quinone by CIDNP [16,17] and in zeolite Na ZSM-5, by ESR and CIDNP [18], and by cyclic voltammetry [19]. Despite of some efforts, radical cation 3 ț + could not be detected directly by any method in solution so far [13,14].…”

Chemical Transformations within the Paramagnetic World Investigated by Photo‐CIDNP

“…12 The described problem was encountered in investigations of cis-trans isomerizations and cycloadditions of donor olefins D and acceptor olefins A in acetonitrile. 24,25 All polarizations, both of the cycloadducts and of the starting and isomerized olefins, could be traced to radical ion pairs D . þ A .…”