Photochemistry of some three-membered heterocycles

Abstract

-The solid-state photolysis of oxiranes and aziridines produces highly colared ylide intermediates. The stability of these intermediates is dependent an a combination of electronic and steric factors as well as the solid-state constraints of the environment.Flash photolysis studies indicate that two different ylides can be produced in solution at room temperature. These intermediates appear to be identical to those formed consecutively in the solid state or lowtemperature glasses. By the use of certain gas-solid reactions, it is possible to control the lifetime of the intermediate so that it is destroyed immediately or continues to exist almest indefinitely. INTRODUCTIONThe room temperature photolyses of aryloxiranes have been shown by Griffin and co-workers to involve a two-bond cleavage producing aryl carbenes and carbonyl compounds (3,4). On the other hand, the thermal transformations of aryl aziridines were interpreted by Reine (5) and Huisgen (6) as one-bond cleavages producing azomethine ylide intermediates which could be trapped with suitable dipolarophiles.Our previous interest in low temperature and solid-state photochemical techniques (7-9) suggested the possibility of obtaining direct physical evidence for the above intermediates an photoreactions carried out at 77°K. The results of several of these studies already have appeared (10-13) and this report will describe additional recent werk with special emphasis an the relation of the low-temperature studies to the unusual room•temperature solid-state photochemistry which is exhibited by many of the compounds. LOW TEMPERATURE PHOTOCHEMISTRY OF ARYL OXIRANESThe room temperature photochemistry of aryloxiranes has been studied extensively by Griffin and co-workers (3,4). The reactions involve a cycloelimination an photolysis in solution to give aryl carbenes and carbonyl compounds. H /H~ )c=otC , - Further reactions Ar ~Ar 261Unauthenticated Download Date | 5/9/18 9:17 PM

Crystal engineering of photochromic diarylethene single crystals

“…Molecules that exhibit photochromism have attracted much attention because of their potential applicability to optoelectronic devices, such as optical memory and optical switches 2. Some photochromic molecules undergo the reactions even in their crystalline phases 3–9. In most crystalline photochromic systems, however, photogenerated colored isomers are thermally unstable and the colored crystals return to the initial colorless ones even in the dark.…”

Comments on photochromism of 3-(3-pyridyl)sydnone and 4-alkenylsydnones

“…The latter are akin to 1-amino-3-alkylthio-diazirinium-3-carboxylate zwitterions proposed by Gotthardt and Reiter. 12 A different kind of diazonium carboxylate zwitterions RN=N + =CH-COO -proposed by Trozzolo et al 13 has been discounted. 6 Solution photolysis of sydnones is known to afford nitrile imines 8, which are probably also formed from the diazirines 5, and finally nitrile imines 8 rearrange both thermally and photochemically to the lower-energy carbodiimides 7.…”

Photoinduced electron transfer oxygenations

“…In a related work, Arnold [164] has utilized 1,4-dicyanonapthalene, an electron-transfer sensitizer, with epoxides in the presence of dipolarophiles to yield substituted tetrahydrofurans in which the major products have a cis-diphenyl relationship. Trozzolo has used spectroscopic methods to characterize carbonyl ylides produced by photochemical cleavage of epoxides [174] The re-examination of this reaction led Schaap's group to provide the first direct evidence for the formation of an ozonide from a singlet oxygenation of a photochemicaUy generated carbonyl ylide. Additional insight into the mechanism of DCA/BP cosensitized photooxygenation of epoxides can also be obtained considering an analogous process, i.e., the homogeneous redox catalysis of electro-reduction of aliphatic halides (RX).…”