Unexpected formation of 1-diethylaminobutadiene in photosensitized oxidation of triethylamine induced by 2,3-dihydro-oxoisoaporphine dyes. A ¹H NMR and isotopic exchange study

Photoreduction of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the N-hydrogen oxoisoaporphine anion. When triethylamine, TEA, was used as the electron donor in anaerobic conditions, 1-diethylaminobutadiene, DEAB, was one of the oxidation products of TEA, among diethylamine and acetaldehyde. DEAB was identified by ¹H NMR and GC-MS experiments by comparison with the authentic 1- diethylaminobutadiene. This is the first report of a butadienyl derivative formed in the dye-sensitized photooxidation of TEA. In addition, isotopic exchange experiments with TEA-d₁₅ and D₂O show that the hydrogens at carbon-2 and carbon-4 of the butadienyl moiety are exchangeable. The observed isotopic exchange pattern could be explained by the head-to-tail coupling of an N,N-diethylvinylamine intermediate that exchanges hydrogens at the C-β via the enammonium ion.