PHOTOINDUCED ELECTRON-TRANSFER REACTIONS OF ARYL OLEFINS .2. CIS-TRANS ISOMERIZATION AND CYCLOADDUCT FORMATION IN ANETHOLE-FUMARONITRILE SYSTEMS IN POLAR-SOLVENTS

In acetonitrile, the photoreactions of cis-anethole, cA, or trans-anet hole, tA, with fumarodinitrile, FN, lead to isomerization of both subs trate 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 yield s regardless of whether the substrate is cA or tA. The configuration o f the anethole-derived moiety in these adducts is always trans, wherea s all possible configurations of the cyano groups occur, From Stern-Vo lmer experiments it was concluded that the quenching mechanism is elec tron transfer, not exciplex formation. Electron-transfer quenching is also the pathway leading to the cycloadducts, as was established by ph otoinduced 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 cycl oadducts. Radical pairs A(.+)FN(.-) (RP I) consisting of the radical c ation of the anethole and the radical anion of fumarodinitrile were id entified 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 (3)RP I to give eith er triplet anethole, (3)A, or triplet fumarodinitrile, (FN)-F-3, occur s and constitutes the main pathway to isomerization of substrate and q uencher. The cycloadducts are also formed via (3)RP I; a significant p articipation of free radicals in their generation was ruled out. The s tereochemistry of the products and the different ratios of polarizatio n intensities of the isomerized olefin and the cycloadducts can only b e explained by the intermediacy of a triplet biradical (3)BR. After in tersystem crossing to the singlet state, ring closure of (1)BR compete s with biradical scission. The latter process provides an additional i somerization pathway, which differs from the pathway via triplet olefi ns by leading only to one-way cis-trans isomerization of the substrate . (3)BR is formed by geminate combination of triplet radical ion pairs (3)RP I; an indirect pathway via back electron transfer of (3)RP I to give 3A or 3FN followed by attack to the other olefin, as has been pr oposed for similar photocycloadditions, could be excluded unambiguousl y. 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-Buchi r eaction between donor olefins and electron-deficient carbonyl compound s which we recently reported. The CIDNP experiments at variable quench er concentration revealed the presence of an additional radical pair R P II, also containing A(.+) but an anion other than FN.-. It was shown that RP II results from a biphotonic process. These findings were exp lained by two-photon ionization of the anethole and formation of an ol igomeric anion of acetonitrile.