Regioreversed Thermal and Photochemical Reduction of 10-Methylacridinium and 1-Methylquinolinium Ions by Organosilanes and Oraganostannanes

Abstract

Irradiation of the absorption band of the 10-methylacridinium ion (AcrH+) in acetonitrile containing allylic silanes and stannanes results in the efficient and selective reduction of the 10-methylacridinium ion to yield the allylated dihydroacridines. In the photochemical reactions of AcrH+ with unsymmetric allylsilanes, the allylic groups are introduced selectively at the α position. Likewise, the reactions with unsymmetric allylstannanes afforded the α adducts predominantly, but the γ adducts were also obtained as minor products. In contrast to this, the thermal reduction of AcrH+ and the 1-methylquinolinium ion (QuH+) by unsymmetric allylstannanes gave only the γ adducts. The thermal reduction of QuH+ by tributyltin hydride or hydrosilanes in the presence of a fluoride anion also occurs to yield 1-methyl-1,2-dihydroquinoline selectively. On the other hand, the photoreduction of QuH+ derivatives by tributyltin hydride and tris(trimethylsilyl)silane yields the corresponding 1,4-dihydroquinolines exclusively. The difference in the mechanisms for the regioreversed thermal and photochemical reduction of AcrH+ and QuH+ is discussed in terms of nucleophilic vs electron-transfer pathways. The photochemical reactions proceed via photoinduced electron transfer from organosilanes and organostannanes to the singlet excited states of AcrH+ and QuH+, followed by the radical coupling of the resulting radical pair in competition with the back electron transfer to the ground state. The rate constants of photoinduced electron transfer obtained from the fluorescence quenching of AcrH+ and QuH+ by organosilane and organostannane donors agree with those obtained from the dependence of the quantum yields on the donor concentrations for the photochemical reactions. The electron-transfer rate constants are well analyzed in light of the Marcus theory of adiabatic outer-sphere electron transfer, leading to the evaluation of the reorganization energy (λ = 0.90 eV) of the electron-transfer reactions. The transient spectra of the radical pair produced by the photoinduced electron transfer from organosilanes to the singlet excited state of AcrH+ have been successfully detected in laser-flash photolysis of the AcrH+−organosilane systems. The rate constants of back electron transfer to the ground state have been determined, leading to the evaluation of the reorganization energy for the back electron transfer, which agrees with the value for the forward electron transfer.