Photochemically generated arylnitrenium ions:substituent effects on reactivity studied by laser flash photolysis

摘要

Laser flash photolysis is used to examine raeaction 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 show that the nitrenium ions react with weak nucleophiles in the order 4-Cl~4-Me>4-Ph>4-OMe.Generally,amines react with the arylnitreniumions at or near the diffusion-limited rate.Only in cases wwheree the least reactive nitremium ion (4-OMe)reacts with sterically hindered amines is any deviation from this generalizatin observed.The addition of a 2-acetyl substituent to these structures produces and interestigng effect.In the case fo the 4-l and 4-Ph analogs,the electron-withdraing acetyl group decreases the rate of nucleopilic 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.