IR photofragmentation of the phenyl cation: spectroscopy and fragmentation pathways

We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio (C6H5+) and perdeutero (C6D5+) forms, in the 260–1925 cm−1 (5.2–38 μm) spectral range, and investigate the observed photofragmentation.

Effects of Ultrasounds on Neat nitrobenzene

Neat nitrobenzene was continuously irradiated at two ultrasonic frequencies: 40 and 200 kHz, under air and argon atmosphere, respectively. Samples taken at intervals of 1, 5, 10 and 24 h were analyzed by GC-MS and decomposition products were identified. Possible reaction mechanisms are discussed. Presence of air as dissolved gas leads to oxygenated compounds such as 1,4-benzoquinone, 2,4-dinitrophenol, m-dinitrobenzene while argon inhibits the decomposition of nitrobenzene, especially at sonication times under 5 h. Based on the nature of the compounds identified we advanced a mechanism, involving a divergent splitting of unstable radical cation of NB in air and argon respectively. Thus, under air, the phenyl cation formation is preferred leading to 1,4-benzoquinone nitro-biphenyls and dinitrobenzene, while under argon, the phenyl radical formation seems to be favored, leading to phenol and diphenyl ether. The oxygenated compounds detected under argon clearly are a consequence of the nitro group splitting.

The mechanochemical production of phenyl cations through heterolytic bond scission

High mechanical forces applied to polymeric materials typically induce unselective chain scission. For the last decade, mechanoresponsive molecules, mechanophores, have been designed to harness the mechanical energy applied to polymers and provide a productive chemical response. The selective homolysis of chemical bonds was achieved by incorporating peroxide and azo mechanophores into polymer backbones. However, selective heterolysis in polymer mechanochemistry is still mostly unachieved. We hypothesized that highly polarized bonds in ionic species are likely to undergo heterolytic bond scission. To test this, we examined a triarylsulfonium salt (TAS) as a mechanophore. Poly(methyl acrylate) possessing TAS at the center of the chain (PMA-TAS) is synthesized by a single electron transfer living radical polymerization (SET-LRP) method. Computational and experimental studies in solution reveal the mechanochemical production of phenyl cations from PMA-TAS. Interestingly, the generated phenyl cation reacts with its counter-anion (trifluoromethanesulfonate) to produce a terminal trifluoromethyl benzene structure that, to the best of our knowledge, is not observed in the photolysis of TAS. Moreover, the phenyl cation can be trapped by the addition of a nucleophile. These findings emphasize the interesting reaction pathways that become available by mechanical activation.

Smooth photogeneration of α,n-didehydrotoluenes (DHTs)

Irradiation of the three isomeric (chlorobenzyl)trimethylsilanes in methanol–water generates the corresponding didehydrotoluenes (DHTs). The process involves expulsion of a chloride ion to give the triplet phenyl cation and ensuing elimination of the trimethylsilyl cation. This straightforward generation of DHT intermediates overcomes a shortcoming of previous methodology (cycloaromatization of enyne-allenes), which is limited to the meta-isomer, and opens the path for understanding the chemistry (and possibly the biological action) of these unusual intermediates.