Denitrogenation of bicyclic azoalkanes through photosensitized electron transfer: generation and intramolecular trapping of radical cations

Abstract

To elucidate the intermediates that intervene in the denitrogenation of bicyclic azoalkanes by photosensitized electron transfer (PET), the 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivative lb was synthesized, and its PET reactions were examined. With triphenylpyrylium tetrafluoroborate (TPT) or 9,10-dicyanoanthracene (DCA) as sensitizers and biphenyl as cosensitizer, the azoalkane lb gave through intramolecular cyclization the spiro ethers 5 and 6 as trapping products, in addition to the bicyclopentane 2b and the cyclopentenes 3b,b', the latter as rearrangement products.Comparison with 1,4-dimethyl-DBH (lc) revealed that trapping of a 1,3-diyl radical cation as the major pathway is unlikely. PET experiments with the regioisomeric cyclopentenes 3b,b', which both led to the spiro ether 5, imply the involvement of the allyl cation 3b(-H)+ as the decisive intermediate in the nucleophilic trapping reactions. Comparison of the PET chemistry of the azoalkane lb and the corresponding bicyclopentane 2b gave further insight into the mechanism of denitrogenation of azoalkanes through single electron transfer. The latter results lend additional support for the involvement of the diazenyl radical cation 1,+, which to date has escaped direct detection.

Dual Catalysis Strategies in Photochemical Synthesis

“…129,130 Addition of biphenyl as a redox mediator results in an increase in the rate of reaction and also changes the distribution of products. The authors proposed that BET from DCA •− to 71 •+ leads to 1,3-diradical 71 , which can rapidly form housane 67 .…”

Regio- and diastereoselective rearrangement of cyclopentane-1,3-diyl radical cations generated by electron transfer

“…Besides EPR spectroscopy under matrix-isolation conditions 23,24 and pulse-radiolysis 25,26 studies, also chemical trapping 7,27 proved helpful in detecting and characterizing the transient radical cations derived from azoalkanes and housanes. Thereby valuable mechanistic insight has been gained into the chemical behavior of these short-lived reactive intermediates.…”

Stabile Heterocyclopentan‐1,3‐diyle

“…Bislang sind derartige Verbindungen jedoch nicht in präparativem Maß-stab synthetisierbar gewesen, da sie Lebenszeiten von deutlich unter einer Sekunde aufweisen. Allerdings konnten Abfangprodukte [9][10][11][12] -Hausane ([2.1.0]Bicyclopentane) nach Ringschluss [4,5,10,[13][14][15][16] oder auch Cyclopentene nach Gruppenumlagerung [11,17] -isoliert werden. Kleine Moleküle der 15.…”