Photochemistry of an azido-functionalized cryptand: controlling the reactivity of an extremely long-lived singlet aryl nitrene by complexation to alkali cations.
Photolysis of the cryptand 1, bearing an intraannular azido substituent, results in a complex photochemistry. Low-temperature photolysis yields the triplet nitrene (3)2, which has been characterized by EPR spectroscopy. Small differences in ZFS parameters are detected between the uncomplexed nitrene-functionalized ligand (in EtOH: D' = 0.93 cm(-1)) and its sodium (NaBr@(3)2 in EtOH: D' = 0.88 cm(-1)) and potassium (KBr@(3)2 in MTHF: D' = 0.89 cm(-1)) complexes. If the photolysis of the free ligand is conducted at ambient temperature, a derivative of o-aminobenzaldehyde 4 is found to be the main product, which is formed by reaction of the o-iminoquinone methide 9 with water. The latter can be detected by UV/vis spectroscopy. Its lifetime is tau = 254 s in acetonitrile solution at ambient temperature. In the presence of diethylamine, the methyleneazepine derivative 5 is formed, which is indicative of didehydroazepine formation (7). Room-temperature photolysis of acetonitrile solutions of the sodium or potassium complexes also results in formation of the o-aminobenzaldehyde derivative. In the presence of diethylamine, however, no methyleneazepine 5 is found. Formation of the aniline derivative 8 instead points to free radical processes. Laser flash photolysis (LFP) of acetonitrile solutions of 1 leads to the detection of a short-lived (tau = 1.4 mus, lambda(max) = 445 nm plus weak absorption at lambda > 500 nm) intermediate A, which decays to transient B (tau = 8 ms, lambda(max) = 295 and ca. 350-400 nm). LFP of acetonitrile solutions of complexes NaBr@1 and KBr@1 gives similar transient spectra. In the presence of sodium and potassium cations, the lifetime of the short-lived transient A is reduced (Na(+): A', tau = 200 ns; K(+): A", tau = 160 ns). Transients A' and A" decay to long-lived transients B' + C' (B" + C"). Based on the results of our product studies, a comparison with the low-temperature results, and quantum mechanical calculations, the transients A, A', and A" are identified as singlet nitrenes (1)2, NaBr@(1)2, and KBr@(1)2, while the long-lived transients B, B', and B" are assigned to didehydroazepines 7, NaBr@7, and KBr@7. Transients C' and C" can be assigned to aminyl radicals NaBr@16 and KBr@16.