Abstract
Laser flash photolysis is used to examine reaction rates of four arylnitrenium ions: N-methyl-N-(4-chlorophenyl)-, -(4-biphenylyl)-, -(4-methylphenyl)-, and -(4-methoxyphenyl)nitrenium ion. These intermediates are generated from photolysis of appropriately substituted N-aminopyridinium ions. Kinetic analysis of trapping rates shows that the nitrenium ions react with weak nucleophiles in the order 4-Cl ∼ 4-Me > 4-Ph > 4-OMe. Generally, amines react with the arylnitrenium ions at or near the diffusion-limited rate. Only in cases where the least reactive nitrenium ion (4-OMe) reacts with sterically hindered amines is any deviation from this generalization observed. The addition of a 2-acetyl substituent to these structures produces an interesting effect. In the case of the 4-Cl and 4-Ph analogs, the electron-withdrawing acetyl group decreases the rate of nucleophilic addition, yet with the 4-OMe and 4-Me compounds, the acetyl group increases the rate of addition. These results are discussed in terms of competing electronic and steric effects.
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This paper is dedicated to Professor Fred Lewis on the event of his 60th birthday.
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Kung, A.C., Chiapperino, D. & Falvey, D.E. Photochemically generated arylnitrenium ions: substituent effects on reactivity studied by laser flash photolysis. Photochem Photobiol Sci 2, 1205–1208 (2003). https://doi.org/10.1039/b307994e
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DOI: https://doi.org/10.1039/b307994e