Selectivity in the photoisomerization of 3H-azepines

Abstract

Szlwnzary Some 2-substituted-3H-azepines are reported to photoisomerize to 2-azabicyclo [3,2,0] hepta-2,g-dienes.PHOTOCHEMICAL intramolecular cycloadditions of 3H-azepines (I) should be disrotatory processes because of the constraints imposed by the seven membered ring. Simple considerations of the initial interactions developing in the highest occupied orbital of the first excited state on proceeding along the reaction co-ordinates? leads to the conclusion that 2-azabicyclo [3,2,0]hepta-2,6-dienes (11) and 6-azabicyclo [3,2,0]hepta-2,6-dienes (111) are allowed products. Irradiation of dilute pentane solutions of (Ia) (medium pressure Hanovia lamp, Corex filter) yielded (IIa) (70%). Similar reactions of (Ib) and (Ic) using low-pressure mercury lamps gave (IIb) (45%) and (IIc) (60%). No products of the type (111) were identified in any irradiations.: A brown polymer accounted for the remainder of the reactant.The structures assigned to (IIa), (IIb), and (IIc) are consistent with elemental analyses and u.v., i.r., and n.m.r.$ spectral data. The ring-proton portions of the n.m.r.(I I )x (111) (la) X = -NMe2 (I b) X.= -NH2(Ic) X =: -0Et2 spectra, which are strikingly similar, is particularly decisive in distinguishing between the isomeric 2-aza-bicyclo-[3,2,0 J hepta-2,6-dienes (11) and 6-aza-bicyclo [3,2,0]hepta-2,g-dienes (111). Each compound has a 1H doublet in the range of T 3-60-3.64 and a 1H triplet in the range of T 3-8-3.89 with no additional absorptions below T 5-00. Since the t Arguments for the validity of such an approach when dealing with molecules in which there is little symmetry were presented in a $ No volatile products other than those reported were detected except on photolysis of 2-ethoxy-SH-azepine, when an Its structure cannot alter the basic observation of selectivity in the Spectra of (IIa) and (IIc) in CCI, and of (IIb) in review article (ref. l), "Conservation of Orbital Symmetry").uncharacterised impurity in the product was present (<3%). photoisomerization.[ %H,] pyridine.N.m.r. spectra were measured at 60 MHz with Me,Si as the internal standard.

Continuous flow photolysis of aryl azides: Preparation of 3H-azepinones

“…1 ), although this was accompanied by increased concomitant formation of the byproduct 10 . Lactam 10 was obtained upon re-exposure of purified 9a to the reaction conditions and is believed to result from photochemical disrotatory electrocyclization of the 3 H -azepinone diene moiety [ 51 – 54 ] ( Scheme 3 ). Instead of making a direct comparison between reaction progress in batch and flow, the effects of longer residence times reported for batch photolysis were evaluated in a stopped-flow experiment by irradiating a solution of 8a for 3.5 h in the same FEP photoreactor ( Fig.…”

Photoisomerization of 3H -Azepines

“…Concise reviews on the photochemistry of azepines have appeared. Although a preliminary account of our work has been given, 55 full details of the syntheses have not been reported. Direct irradiation of dilute pentane solutions of 2-dimethylamino-3H-azepine (1a), 2-amino-3H-azepine (1b) and 2-ethoxy-3H-azepine (1c) with medium-pressure mercury lamps gave the corresponding 3-substituted 2-azabicyclo[3.2.0]hepta-2,6-dienes 2.…”

“…Direct irradiation of dilute pentane solutions of 2-dimethylamino-3H-azepine (1a), 2-amino-3H-azepine (1b) and 2-ethoxy-3H-azepine (1c) with medium-pressure mercury lamps gave the corresponding 3-substituted 2-azabicyclo[3.2.0]hepta-2,6-dienes 2. 55 Product yields were determined by GC analysis and were based on reactant consumed. Highest yields were obtained when pentane was employed as solvent and the azepines were irradiated at wavelengths corresponding to their maximum absorptions in the ultraviolet.…”

“…Photoelectrocyclization reactions of 3H-azepines 1 should be disrotatory processes, but the reasons for their observed selectivity are unknown. We have made the simple suggestion 55 that in order to avoid the loss of resonance energy the preferred pathway for the amidine or imidate ester systems is that leading to the corresponding 3-substituted 2-azabicyclo-[3.2.0]hepta-2,6-dienes 2. The direction of the photo-ring closure of unsymmetrical cycloheptatrienes to bicyclo-[3.2.0]heptadienes may be controlled by the electronic nature of the substituent and the charge distribution in the excited state.…”

Photochemical behavior of 2,5-di-tert-butyl-3H-azepine: unexpected formation of a dicyclopenta[b,e]pyridine derivative and a cis,cis-cyclodeca-1,6-diene derivative

“…The reaction was rationalized by the formation of a cyclobuta[b]pyrrole derivative via ring closure between the 4 and 7 positions, exclusively. 4 Although 3H-azepine without any substituents has been found to be a labile substance 5 an electron donating group, imino ether or amidine conjugation, has been found to stabilize it.…”