Reaction of aromatic peroxyl radicals with alkynes: a mass spectrometric and computational study using the distonic radical ion approach
The reaction of the aromatic distonic peroxyl radical cations N-methyl pyridinium-4-peroxyl (PyrOO.+) and 4-(N,N,N-trimethyl ammonium)-phenyl peroxyl (AnOO.+), with symmetrical dialkyl alkynes 10 a-c was studied in the gas phase by mass spectrometry. PyrOO.+ and AnOO.+ were produced through reaction of the respective distonic aryl radical cations Pyr.+ and An.+ with oxygen, O 2. For the reaction of Pyr.+ with O2 an absolute rate coefficient of k1=7.1×10-12 cm 3 molecule-1 s-1 and a collision efficiency of 1.2 % was determined at 298 K. The strongly electrophilic PyrOO.+ reacts with 3-hexyne and 4-octyne with absolute rate coefficients of k hexyne=1.5×10-10 cm3 molecule -1 s-1 and koctyne=2.8×10-10 cm3 molecule-1 s-1, respectively, at 298 K. The reaction of both PyrOO.+ and AnOO.+ proceeds by radical addition to the alkyne, whereas propargylic hydrogen abstraction was observed as a very minor pathway only in the reactions involving PyrOO.+. A major reaction pathway of the vinyl radicals 11 formed upon PyrOO.+ addition to the alkynes involves γ-fragmentation of the peroxy O-O bond and formation of PyrO.+. The PyrO.+ is rapidly trapped by intermolecular hydrogen abstraction, presumably from a propargylic methylene group in the alkyne. The reaction of the less electrophilic AnOO.+ with alkynes is considerably slower and resulted in formation of AnO .+ as the only charged product. These findings suggest that electrophilic aromatic peroxyl radicals act as oxygen atom donors, which can be used to generate α-oxo carbenes 13 (or isomeric species) from alkynes in a single step. Besides γ-fragmentation, a number of competing unimolecular dissociative reactions also occur in vinyl radicals 11. The potential energy diagrams of these reactions were explored with density functional theory and ab initio methods, which enabled identification of the chemical structures of the most important products.