Cyclization of Terpenoid Dicarbonitrile Polyalkenes upon Photoinduced Electron Transfer to 1,4-Dicyano-2,3,5,6-tetramethylbenzene and Other Cyanoarenes

Abstract

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 (A1), 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 A1 in deoxygenated acetonitrile in the presence of BP by laser flash photolysis with λexc = 308 nm are the radical cation of BP (BP•+, λmax = 380 and 660 nm) and the radical anion of the acceptor (A•-, λmax = 360 nm). Fluorescence quenching of A1 and the formation of BP•+ depend on the BP concentration; at low [BP], triplet quenching of A1 prevails, leading to 3BP* via energy transfer. The transients for several other acceptors are characterized; for example, the radical anions of A3 and A5 absorb at λmax = 390 and 312 nm, respectively. The rate constants for secondary electron transfer from D n (n = 1−3) or other donors to BP•+ were determined. Secondary electron transfer to the polyalkenes from the radical anion of A1, but not the other acceptors, 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.

Concentration-Dependent Isotope Effects in Electron Transfer-Mediated Reactions Photocyanation of Biphenyl and Dimethylbiphenyls

“…In oxygen-free samples, a value of k SV of 141 M -1 has been measured. 11 The presence of oxygen in our samples reduces the lifetime of the singlet excited state of DCN. This accounts for the lower Stern-Volmer constant, since k SV ) k q × τ S where k q is the rate of quenching by DCN and τ S is the DCN singlet lifetime.…”

“…14,17,28 Singlet excited DCN acquires an electron from BP to yield an IP. 11 Back ET within the IP yields the ground states of DCN and BP, and separation of the IP gives two free radical ions. 12,13 Cyanide attack can occur both at the aromatic radical cation within the IP and at the free radical cation, to yield the cyanohydrogen radical.…”

“…BP efficiently quenches the fluorescence of DCN by an ET mechanism. 11 To study whether there is an IE on this ET step, the Stern-Volmer constants for quenching of singlet excited DCN with BP-h 10 and BP-d 10 were measured in acetonitrile. In both cases, linear plots were obtained ( Figure 1) and the Stern-Volmer constants for BP-h 10 (k SV ) 93.8 ((0.6) M -1 ) and BP-d 10 (k SV ) 91.9 ((2.4) M -1 ) are identical within experimental error (IE ) 1.02 ( 0.03).…”

“…int. ): 179 (100), 152 (11). 4-Cyanobiphenyl-d 9 was prepared accordingly from 4-bromobiphenyl-d 9 .…”

“…Biphenyl (BP) was chosen as the model substrate for several reasons: (i) Its radical cation (BP +• ) can be easily photogenerated by selective excitation of 1,4-dicyanonaphthalene (DCN) in the presence of BP. 11 In this way, exciton exchange as depicted in eq 1 cannot occur. (ii) Although initially ion pairs (IPs) are formed, the quantum yield of separated BP +• is high.…”