Photoinduced 1,2,3,4-tetrahydropyridine ring conversions.

Stable heterocyclic hydroperoxide can be easily prepared as a product of fast oxidation of a 1,2,3,4-tetrahydropyridine by (3)O2 if the solution is exposed to sunlight. The driving force for the photoinduced electron transfer is calculated from electrochemical and spectroscopic data. The outcome of the reaction depends on the light intensity and the concentration of O2. In the solid state the heterocyclic hydroperoxide is stable; in solution it is involved in further reactions.

Are reactions between metal cyanides and aryl diazonium ions really outer-sphere electron transfer processes?

Substitution-inert complexes such as Fe(CN)(6)(4-) are usually considered to react by outer-sphere electron transfer (ET) with most electron acceptors, including aryl diazonium ions (ZC(6)H(4)N(2)(+), where Z denotes a substituent on the benzene ring). However, in contrast to the conclusion drawn in a previous report ( J. Am. Chem. Soc. 1987 , 109 , 1536 - 1540 ), kinetic studies and identification of products from the reactions of 4-nitro- and of 4-methoxybenzenediazonium with an excess of Fe(CN)(6)(4-) show that this is not the case and that the reactions actually go via the formation of an adduct, a diazoisocyanide complex [ZC(6)H(4)N(2)(+) + Fe(CN)(6)(4-) --> ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-)]. The adduct decomposes heterolytically by expulsion of nitrogen either to form an isocyanide complex [ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) --> ZC(6)H(4)(NC)Fe(CN)(5)(3-) + N(2)] or the 4-substituted benzonitrile via a ligand exchange [ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) --> ZC(6)H(4)CN + Fe(CN)(5)(3-) + N(2)]. A competing homolytic decomposition resulting in an overall ET reaction occurs only to a minor extent, giving small amounts of Fe(CN)(6)(3-), ZC(6)H(5), and various organic compounds. In oxygenated solutions ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) decomposes to Fe(CN)(6)(3-) and ZC(6)H(4)OH. The measurements with Fe(CN)(6)(4-) were supplemented by the study of the analogous reactions of Os(CN)(6)(4-), Mo(CN)(8)(4-), and W(CN)(8)(4-). The observation that isocyanide and even short-lived diazoisocyanide complexes are formed is in accordance with an inner-sphere mechanism. Further support of this conclusion comes from the observation that the slope of the activation-free energy plots for the reactions of NO(2)C(6)H(4)N(2)(+) and MeOC(6)H(4)N(2)(+) with the four metal cyanides is higher than that expected for an outer-sphere ET mechanism. The implication of these results are discussed in the context of the previous report (vide supra) on the extraction of the self-exchange reorganization energies for substituted benzenediazonium salts from their reactions with Fe(CN)(6)(4-) and decamethylferrocene. Our conclusion is that Marcus theory is not applicable in the interpretation of the measured rate constants, thereby also precluding a determination of such energies.