ACID-BASE PROPERTIES OF ARYLNITRENIUM IONS

This study uses a combination of laser flash photolysis (LFP) and prod uct analysis to show that singlet nitrenes from the irradiation of phe nyl, 4-biphenylyl, and 2-fluorenyl azide can be trapped by protonation in aqueous solutions forming nitrenium ions. With phenyl azide, the p henylnitrenium ion is indicated by the formation of ring-substituted a nilines in yields of up to 50% in 1 M acids. The acidity dependence fu rnishes the ratio k(H):k(exp) = 1.1, where k(H) refers to H+-trapping of singlet phenylnitrene and k(exp) to ring expansion of this species. With k(H) expected to be 2-4 x 10(10) M(-1) s(-1), k(exp) is therefor e estimated as 2-4 x 10(10) s(-1). Protonation by solvent water also o ccurs, but even though the rate constant is of the order of 10(9) s(-1 ), it constitutes a minor pathway in competition with the ring expansi on. LFP studies in acids reveal a transient that is assigned the struc ture of N-protonated 4-hydroxy-2,5-cyclohexadienone imine, the interme diate formed by water addition to the para position of the phenylnitre nium ion. With 4-biphenylyl- and 2-fluorenylnitrene, ring expansion (a nd intersystem crossing) occurs more slowly and protonation by water i s faster, with the consequence that there are substantial yields of ni trenium ion without added acids. These nitrenium ions are detected wit h ns LFP, and their formation from singlet nitrene is observed with ps LFP. Combining the LFP experiments with product analysis furnishes a pK(a) value of 16 for the 4-biphenylylnitrenium ion deprotonating to s inglet nitrene in 20% acetonitrile. Thus singlet 4-biphenylylnitrene f alls close to the category of a strong base in this solution. LFP expe riments in acids show behavior consistent with N-protonation of the ni trenium ion forming an aniline dication. Kinetic analyses furnish pK(a ) values of 0.1 (4-aminobiphenyl dication) and 0.6 (2-aminofluorene di cation) in 20% acetonitrile with 1 M ionic strength. This and other pi eces of evidence are consistent with these arylnitrenium ions being be tter regarded as 6-iminocyclohexadienyl carbocations. Overall, arylnit renium ions (ArNH+) are very weak acids in water in their deprotonatio n to singlet nitrenes. They are also weak bases, accepting a proton to form the aniline dication-(ArN)-Ar-1 reversible arrow (ArNH+)-Ar-1 re versible arrow (ArNH2)(2+).