Vapor-Phase Photochemistry of Dimethylpyridines

Abstract

Irradiation of dimethylpyridine vapors (2-5 Torr) at 254 nm results in the formation of two sets of isomerization products. One set, formed in the larger yield, is substantially quenched when the irradiations are carried out in the presence of 15-21 Torr of nitrogen and is not formed when the irradiations are carried out with light of λ > 290 nm. In addition, a second set of reactions, which involve the interconversion of 2,3-and 2,5-dimethylpyridines, is enhanced by the addition of nitrogen, and these reactions are the only photoisomerization reactions observed when the irradiations are carried out with light of λ > 290 nm. In addition to the photoisomerizations, four of the dimethylpyridines also undergo demethylation to yield monomethylpyridines, and 2,6-dimethylpyridine undergoes methylation to yield a trimethylpyridine product. A variety of crossover experiments confirmed that the photoisomerizations are intramolecular. Based on the major phototransposition products, the six dimethylpyridines can be divided into two triads. Interconversion of the three members of each triad results in the major phototransposition products. These intra-triad interconversions are suggested to occur via 2,6-bonding, originating in a vibrationally excited S 2 (π,π*) state of the dimethylpyridine, followed by nitrogen migration and rearomatization. This allows nitrogen to insert within each carbon-carbon bond. Phototransposition of 2,6-dideuterio-3,5-dimethylpyridine to a mixture of 5,6-dideuterio-2,4-dimethylpyridine and 3,4-dideuterio-2,5-dimethylpyridine is consistent with this mechanism. In addition to these intra-triad reactions, 2,5-dimethylpyridine, a member of triad 1, was observed to interconvert with 2,3-dimethylpyridine, a member of triad 2. These inter-triad reactions are suggested to occur via interconverting Dewar pyridine intermediates, formed from the triplet state of the dimethylpyridines. These Dewar pyridine intermediates were also observed by 1 H NMR spectroscopy after irradiation of the dimethylpyridines in CD 3 CN at -30 °C.

Theoretical Characterization of Photoisomerization Channels of Dimethylpyridines on the Singlet and Triplet Potential Energy Surfaces

“…The photophysics and photochemistry of these nitrogen heterocycles have both received considerable attention experimentally. [1±6] In recent experiments, Pavlik et al [6] studied the photochemistry of dimethylpyridines 1 ± 6 in the vapor phase and in solution by irradiation at 254 nm and discovered new features in the photoisomerization reactions. For example, when 2,3-dimethylpyridine (2) is irradiated at 254 nm in CD 3 CN at À 30 8C, only the Dewar pyridine isomer 7 resulting from 3,6-bonding (Scheme 1) was observed by 1 H NMR spectroscopy.…”

“…In consideration of the mechnanistic aspects, Pavlik et al [6] proposed a scheme for the photochemical reaction of 6 to 4, involving electrocyclic ring closure, nitrogen migration around the sides of the cyclopentenyl ring, and rearomatization. In this scheme, an azaprefulvene species with biradical character, originating from the excited state S 2 (pp*), is suggested to be a precursor in the interconversion of the dimethylpyridines (Scheme 3).…”

“…The reaction path on the S 2 (pp*) potential energy surface (PES) was found to be the lowest path for the formation of the azaprefulvene isomer at the CASSCF(8,7)/ STO-3G level. [5] Formation of Dewar pyridine intermediates was assumed to occur [6] by intersystem crossing with a triplet excited state T 1 .…”

Vapor phase phototransposition chemistry of dimethylpyrazines and dimethylpyrimidines

“…6 In the latter case, the six isomers consisted of two photochemical triads. Although leakage between the two triads was observed, in the present study, no interconversion between the triad and the tetrad was detected.…”

“…18 This species reverts to pyridine in greater than 2 ns. 18 Considering the observed phototransposition reactions of deuterium, 19 methyl, 6 and cyano 20 substituted pyridines, migration of the N atom around the cyclopentenyl ring during the lifetime of the azaprefulvene diradical must be possible.…”