Photochemical Behavior of Cyclopropyl-Substituted Benzophenones and Valerophenones

Abstract

p-Cyclopropylbenzophenone, 20, gives no photoreduction when irradiated in i-PrOH solvent. This is a general phenomenon and a number of cyclopropyl-substituted benzophenones, including 4-(endo-6-bicyclo[3.1.0]hexyl)benzophenone, 19, 4-(cis-2,3-dimethylcyclopropyl)benzophenone, 21, 4-(cis-2-vinylcyclopropyl)benzophenone, 22, and 4-(endo-7-bicyclo[4.1.0]hept-2-enyl)benzophenone, 23, also fail to undergo photoreduction. Instead these latter compounds undergo cis-trans isomerization when irradiated. A mechanism involving formation of an (n, π*) triplet, which subsequently fragments the strained cyclopropane bond to give a lower energy and unreactive open triplet, has been suggested. p-Cyclopropylvalerophenone, 25, and p-(endo-6-bicyclo[3.1.0]hexyl)valerophenone, 24, also undergo photoisomerization and fail to undergo the Norrish Type II photoreactions. Triplet energy dissipation by fragmentation of the cyclopropane bond is also proposed. In addition to the Norrish Type II reaction, p-cyclobutylvalerophenone, 27, undergoes a photofragmentation to give ethylene and p-vinylvalerophenone, 60, by an energy dissipation mechanism involving a 1,4-biradical derived from cyclobutane bond fragmentation.

Adiabatic process of higher electronically excited states: luminescence from an excited state biradical generated by irradiation of benzophenone‐substituted cyclopropanes

“…We refer to this phenomenon as “excited state C–C bond cleavage‐luminescence.” Analyses of transient absorption and emission results, obtained using double laser flash photolysis (LFP), showed that 3 4 •• * is formed adiabatically from 3 3 *, which itself is generated by rapid intersystem crossing (ISC) in 1 3 * owing to the presence of the BP moiety. This observation suggests that introduction of 1 BP moiety into the methylenecyclopropane system is sufficient to enable ISC to be competitive with other undesirable reactions of 1 3 * …”

“…This observation suggests that introduction of 1 BP moiety into the methylenecyclopropane system is sufficient to enable ISC to be competitive with other undesirable reactions of 1 3*. [26][27][28] In the current investigation, we used the enhanced ISC effect of a BP moiety on a diarylcyclopropane system [29][30][31] to control the nature of electronic excited states and luminescence properties of biradicals generated by homolytic C-C bond cleavage. For example, n,π*-excitation (λ EX = 355 nm) of the BP moiety in trans-and cis-1-(4-benzoylphenyl)-2phenylcyclopropane (5, Scheme 1C) should lead to production of biradical-derived luminescence, because it is expected that 3 5*, formed by rapid ISC, will undergo adiabatic bond cleavage reaction to give excited biradical 3 6 •• *.…”

Thermal Elimination of Ethylene from Cyclobutyl Groups Characterized by X‐ray Crystallography in a Metal–Organic Framework Matrix

“…The release of ethylene from the cyclobutyl ring will generate the desired vinyl‐substituted product (see Scheme )). While this has generated considerable interest in alkyl systems, we are aware of only one report where it has been used as a tool in aromatic cases . Developing methods to address this challenge will be useful from a fundamental standpoint and as a synthetic tool.…”

Elimination of Ethylene from Cyclobutyl Groups Characterised by X-ray Crystallography in a Metal-Organic Framework Matrix

“…While this has generated considerable interest in alkyl systems, [12] we are aware of only one report where it has been used as a tool in aromatic cases. [13] Developing methods to address this challenge will be useful from a fundamental standpoint and as a synthetic tool. The reverse reaction, the photochemical formation of cyclobutyl compounds from alkenes via [2 + 2] cycloaddition reactions, has been reported frequently.…”