Photochemical C−C Bond Cleavage of 1,2-Diarylcyclopropanes Bearing an Acetylphenyl Group. Generation and Observation of Triplet 1,3-Biradicals

Abstract

The photochemical properties of 1,2-diarylcyclopropanes bearing an acetylphenyl group were studied by product analysis and laser flash photolysis. All cyclopropanes showed efficient cis−trans photoisomerization followed by inefficient isomerization to 1,3-diarylpropenes. All the products were unquenched by the addition of triplet quencher, 2-methyl-1,3-butadiene (E T ≃ 60 kcal mol-1), whereas molecular dioxygen gave oxygenated products. Triplet 1,3-biradicals generated from a short-lived acetophenone-like triplet (≲1 ns) were observed as intermediates in these reactions through nanosecond laser flash photolysis. Polar substituent effects on the lifetime of the 1,3-biradicals were small, except for a heavy atom effect in the case of Br. Spin−orbit coupling calculations on model 1,3-biradicals show a negligible effect on polar substituent and thus predict a negligible effect on the intersystem crossing rate. Conjugated biradicals in general now seem unlikely to show the “ionic character effect” on intersystem crossing suggested by the work of Salem and Rowland.

A photochemical C=C cleavage process: toward access to backbone N-formyl peptides

“…Typical photochemical pathways result in cleavage of a benzylic C–X bond following initial benzylic H-atom abstraction [ 11 , 13 ]. In contrast, photorelease systems based on C–C or C=C bond photocleavage are quite rare [ 14 – 15 ]. We recently reported a vinylogous analogue of this photo-deprotection process, which allowed photocleavage of alkenyl sp 2 C–X bonds, rather than benzylic sp 3 C–X cleavage [ 16 – 17 ].…”

Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

“…As the first manifestation of this photoexcitation, significant changes of its electronic states occur, enabling an electron from the ground state to be transferred in the excited state, inducing consecutive reactions. Depending on the energy of the absorbed photons, a cleavage of a chemical bond or the production of radicals can occur [1][2][3][4]. In an effort to drastically reduce the use or the generation of hazardous substances during chemical reactions, photo-assisted chemistry, and thus photochemistry, has been identified as an environmentally friendly synthetic method which is now commonly used in organic chemistry [5].…”

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

“…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 •• *.…”