Reactions of Diarylnitrenium Ions with Electron Rich Alkenes: An Experimental and Theoretical Study

Photolysis of N-(diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate (1a) and N-[bis(4-methylphenyl)amino]-2,4,6-trimethylpyridinium salt (1b) gives products attributable to diarylnitrenium ion (Ar₂N⁺, 2). The major products of these reactions include products from nucleophilic addition of various π-nucleophiles (e.g. electron rich alkenes) to the ortho and para positions of one of the phenyl rings. Nanosecond and EPR spectroscopy show that radicals also form. These radicals are thought to give rise to the diarylamines isolated as minor products from the photolysis of la and 1b. In addition to the para addition products and Ph₂NH, N-phenylindoles and N-phenylindolinones are isolated when silyl enol ethers and silyl ketene acetals are used as trapping agents, respectively. The indoles and indolinones are generated from initial addition of the nucleophile to the ortho position on 2 followed by cyclization of the resulting intermediate. A product resulting from N addition of the nucleophile to 2 is isolated only when silyl ketene acetals are used. A number of electronic sturcture calculations at different levels of molecular orbital and density functional theory were carried out on Ph₂N⁺. There do not seem to be effects associated with either the charge distribution or the LUMO that would strongly influence ortho/para/N selectivity in nucleophilic trapping. Laser flash photolysis on la provides absolute rate constants for the nucleophilic addition of various alkenes to Ph₂N⁺. These fall in the range of 10⁹-10¹⁰ M^-1 s^-1 and correlate with the oxidation potential of the alkene. From these data it is clear that the more easily oxidized the alkene the faster it will react with Ph₂N⁺.