Intramolecular addition reactions stilbenes

Lewis, F. D., Bassani, D.M., Reddy, G.D., Styrene Amine and Stilbene Amine Intramolecular Addition Reactions, Pure Appl. Chem. 1992, 64, 1271 1277. 

Aminoalkyl)stilbenes 7 and 8, in which a primary amine is appended to the stilbene ortho position with a di- or trimethylene linker, undergo DCNB-sensitized intramolecular addition reactions, yielding isoquinoline and benzazepine products 7a and 8a, respectively (Scheme 8). In both cases, ring formation occurs via intramolecular nucleophilic addition to the end of the stilbene double bond proximal to the point of attachment of the aminoalkyl group. Addition to the distal end of the double bond presumably would form a less stable distonic cation radical intermediate. 

Treatment of key intermediate 119 with methoxide led to a toluene-type compound 120. An intramolecular Wittig reaction could also be achieved to yield a 2-substituted indole 121. The addition of an aldehyde resulted in a stilbene, e.g., 122 [142]. 

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

The photochemical behavior of the orr/to-(aminoalkyl)slil-benes 92—94 is also dependent upon the polymethylene linker (Scheme II). Irradiation of 93 and 94 results in formation of the benzazepines 96 and 97. These adducts are presumably formed by regioselective N—H transfer to the proximal end of the stilbene double bond in 93 and the distal end in 94, in both cases resulting in the formation of a 1,7-biradical intermediate. Since intramolecular stilbene-amine addition reactions are non-regioselective, the regioselectivily of N-H transfer must be subject to exciplex conformational control. The biradical inter-... 

The addition reactions of ring-substituted stilbenes with EtjN are nonregioselective. (Aminoalkyl) stilbenes in which a trialkylamine is appended to the stilbene ortho position with a mono-, di-, or trimethylene linker form fluorescent intramolecular exciplexes but fail to undergo intramolecular addition, even in polar solvents. It is possible that the exciplex or radical ion pair geometry is not suitable for a least-motion proton transfer process. 

Other reactions involving the cleavage of a carbon heteroatom bond include a promising method for the deprotection of benzyl ethers by irradiation in the presence of acceptors (Scheme 46) [240-241] and the liberation of alkyl radicals (capable of initiating a polymerization) from alkyltriarylborate salts [242-243], The PET induced decomposition of phenyldiazomethane leads to cis-stilbene the reaction however appears to involve addition of the radical cation to a neutral molecule prior to nitrogen loss [244]. The detachment of a halogen after intramolecular electron transfer from the a C-X bond to an electron-rich... 

Stilbene and its derivatives have often been used in photochemical [2-1-2] cycloadditions. The intermolecular dimerization presented by H. Meier of 2,3-fcij(2-phenylethenyl)-naphthalene directly leads to cyclophane via two [2-1-2] cycloadditions in one step. The yield is surprisingly very high and comparable with those of intramolecular reactions presented by W. H. Laarhoven. Various examples of vinylstilbenes are photolyzed to form cyclobutanes via a "crossed" addition. Even [2a-H2jt] cycloadditions and rearrangements involving H-transfer are utilized to construct unusual bicyclic or tricyclic compounds. 

Data on photophysics and photochemistry of intramolecular stilbene-amine exciplexes and reactions have been reported [34]. The obtained data indicated that the photophysical and photochemical behavior of a series of traws-(aminoalkyl) stilbenes in which a primary, secondary, or tertiary amine is appended to the stUbene ortho position with a Me, Et, or Pr linker. The tertiary (aminoalkyl) stilbenes formed fluorescent exciplexes and underwent trans cis isomerization but failed to undergo intramolecular N—H addition. The secondary (aminoalkyl)stilbenes did not form fluorescent exciplexes but underwent the addition to the stilbene double bond. Intramolecular reactions were highly selective, providing an efficient method for the synthesis oftetrahydrobenzazepines. Direct irradiation of the primary (aminoalkyl) stilbenes resulted only in trans-cis isomerization, while irradiation in the presence of the electron acceptor p-dicyanobenzene resulted in regioselective intramolecular N—H addition to the stilbene double bond. 

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