Cyclization of terpenoid dicarbonitrile polyalkenes upon photoinduced electron transfer to 1,4-dicyano-2,3,5,6-tetramethylbenzene and other cyanoarenes

Photoinduced electron transfer from terpenoid polyalkenes bearing electron-withdrawing groups, i.e., 2,6-dimethyl-1,5-heptadiene-1,1-dicarbonitrile (D1) and its higher homologues D2 and D3, as well as from 1,1'-biphenyl (BP) to 1,4-dicyano-2,3,5,6-tetramethylbenzene (Al), 1,4-dicyanonaphthalene (A3), 1-cyanonaphthalene (A4), and 9,10-dicyanoanthracene (A5), was studied in polar solvents by time-resolved UV-vis spectroscopy and conductivity. The transients observed for Al in deoxygenated acetonitrile in the presence of BP by laser flash photolysis with lambda(exc) = 308 nm are the radical cation of BP (BP.+, lambda(max) = 380 and 660 nm) and the radical anion of the acceptor (A(.-), lambda(max) = 360 nm). Fluorescence quenching of Al and the formation of BP.+ depend on the BP concentration; at low [BP], triplet quenching of Al prevails, leading to (BP)-B-3* via energy transfer. The transients for several other accepters are characterized; for example, the radical anions of A3 and A5 absorb at lambda(max) = 390 and 312 nm, respectively. The rate constants for secondary electron transfer from Dn (n = 1-3) or other donors to BP.+ were determined. Secondary electron transfer to the polyalkenes from the radical anion of Al, but not the other accepters, was observed. The transient conductivity of the acceptor/BP/polyalkene systems in acetonitrile in the presence or absence of water indicates formation and disappearance of charges, in particular of protons. In addition, the effects of water and methanol on the yields for consumption of D1 and formation of the products were studied by pulsed and continuous irradiation. A representative reaction scheme is suggested for the cyclization of the smallest polyalkene, D1, which, on the basis of the kinetic results, is proposed also for D2 and D3. Furthermore, optimized reaction conditions for synthetic applications are discussed.