Spectroscopy and photochemistry of aryloxiranes

Abstract

The absorption spectra of several phenyl-substituted oxiranes in 3-methylpentane at -196°are highly structured and characterized by a 0-0 band at 271 ± 3 nm with a more intense, structureless band at ~230 nm.The fluorescence emission of these compounds occurs at 285-340 nm, with a maximum at 305-310 nm. The isomeric 2,3-bis-a-naphthyloxiranes and tetraphenylepisulflde have also been studied. Irradiation into the 0-0 or higher energy absorption bands causes formation of a colored intermediate. Subsequently, the parent alkene and arylcarbene(s), and carbonyl compound(s) are formed in a manner dependent on oxirane symmetry. Phenyloxirane behaves atypically and no divalent carbon fragments are formed. A consistent mechanistic pathway is proposed to explain the photochemistry and spectroscopy of the oxiranes and their photoproducts. Several aspects of the spectroscopy and photochemistry of phenyloxiranes have been studied.2 Griffin and coworkers3 have found that arylcarbenes are genii) (a) Taken from the thesis submitted by R. O. B. in partial fulfillment of the Ph.D. requirements

Ultrafast Photo‐ion Probing of the Ring‐Opening Process in Trans‐Stilbene Oxide

“…The appearance of the 167 amu fragment is interesting in its own right, as it may be an indication of a reaction which starts with the breaking of one of the C−O bonds in the oxirane ring (as depicted in Scheme 2), instead of the C−C bond. If this C−O break is initiated by the promotion to the first excited state, this fragment may confirm the idea that trans‐ stilbene oxide may have multiple reaction pathways in the first‐excited state [21–23] . In order for the 167 amu fragment to form, a 1,3‐diradical will likely be produced as an intermediate due to the photo‐dissociation of the C−O [5,11] This intermediate can then undergo a 1,2‐migration of one of the phenyl groups and eject an aldehyde group.…”

“…This argument is further supported by past studies which show that appearances of trans ‐stilbene from photo‐excited stilbene oxide is not due to the recombination of radical methylphenyl groups. This point is strengthened by that fact that these studies were performed in solution [23] offering even the possibility of geminate recombination [42] . Second, it is already well known that the breaking of one of the C−O bonds is the preferred process in the parent oxirane molecule, and that this is generally followed by the 1,2‐migration of a hydrogen atom within the molecule (known as the Gomer‐Noyes mechanism) [2,3,5,8,43] .…”

“…In an attempt to better understand what dynamic processes may be apparent in stilbene oxide after photo‐excitation, we have (to our knowledge) performed the first experiments using sub‐picosecond pump and probe lasers to study trans ‐stilbene oxide in the gas phase. We are looking to better document the main reaction pathway of trans ‐stilbene oxide after promotion to the first‐excited state (S 1 (ππ*), excited with ∼266 nm light) [21,23] on the ultrafast timescale, as well as any other competing photo‐induced reactions taking place. We performed single‐color UV/UV pump‐probe ion mass spectroscopy, as this type of spectroscopy has been proved successful in following similar molecular de‐excitation paths [24–26] .…”

The interaction of rhodium carbenoids with carbonyl compounds as a method for the synthesis of tetrahydrofurans

“…Many studies support the intermediacy of carbonyl ylides in reactions involving the interaction of a metallo carbenoid with a carbonyl oxygen [12][13][14][15][16]. Reactions of carboalkoxycarbenes with carbonyl compounds were described as early as 1885 [17] and the structures of the dioxolane products were proposed in 1910 [18].…”

Generation and characterization of carbonyl ylides from pyrazolinone spirooxiranes

“…Aryl oxiranes can undergo photochemical fragmentation reactions in solution to give aryl carbenes and carbonyl compounds [ 51. When photolyzed in matrices at 77 K, the oxiranes also give rise to colored species [6] characterized as car-bony1 ylides arising from the cleavage of the C-C bond in the oxirane ring [ 71. Oxiranes are isoelectronic with cyclopropyl anions which are predicted by the Woodward-Hoffman rules [8] to open photochemically to ally1 anions in a disrotatory manner.…”