2183-86-0Relevant articles and documents
Acid-base properties of arylnitrenium ions
McClelland, Robert A.,Kahley, Mary Jo,Davidse, P. Adriaan,Hadzialic, Gordana
, p. 4794 - 4803 (2007/10/03)
This study uses a combination of laser flash photolysis (LFP) and product analysis to show that singlet nitrenes from the irradiation of phenyl, 4-biphenylyl, and 2-fluorenyl azide can be trapped by protonation in aqueous solutions forming nitrenium ions. With phenyl azide, the phenylnitrenium ion is indicated by the formation of ring-substituted anilines in yields of up to 50% in 1 M acids. The acidity dependence furnishes 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 1010 M-1 s-1, k(exp) is therefore estimated as 2-4 x 1010 s-1. Protonation by solvent water also occurs, but even though the rate constant is of the order of 109 s-1, it constitutes a minor pathway in competition with the ring expansion. LFP studies in acids reveal a transient that is assigned the structure of N-protonated 4-hydroxy-2,5-cyclohexadienone imine, the intermediate formed by water addition to the para position of the phenylnitrenium ion. With 4-biphenylyl- and 2-fluorenylnitrene, ring expansion (and intersystem crossing) occurs more slowly and protonation by water is faster, with the consequence that there are substantial yields of nitrenium ion without added acids. These nitrenium ions are detected with 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 singlet nitrene in 20% acetonitrile. Thus singlet 4-biphenylylnitrene falls close to the category of a strong base in this solution. LFP experiments in acids show behavior consistent with N-protonation of the nitrenium ion forming an aniline dication. Kinetic analyses furnish pK(a) values of 0.1 (4-aminobiphenyl dication) and 0.6 (2-aminofluorene dication) in 20% acetonitrile with 1 M ionic strength. This and other pieces of evidence are consistent with these arylnitrenium ions being better regarded as 6-iminocyclohexadienyl carbocations. Overall, arylnitrenium ions (ArNH+) are very weak acids in water in their deprotonation to singlet nitrenes. They are also weak bases, accepting a proton to form the aniline dication - 1ArN ? 1ArNH+ ? (ArNH2)2+.
Photolysis and Thermolysis of Phenyl Azide in Acetic Acid. Trapping of 1-Azacyclohepta-1,2,4,6-tetraene and Nucleophilic Aromatic Substitution
Takeuchi, Hiroshi,Koyama, Kikuhiko
, p. 1269 - 1274 (2007/10/02)
Photolysis and thermolysis of phenyl azide in acetic acid yielded 1H-azepin-2(3H)-one (4), 2-methylbenzoxazole (15), 2- and 4- acetamidophenyl acetates (16) and (12), 2- and 4- acetamidophenols (14) and (13), acetanilide, and azobenzene.Addition of ethanol to the system caused a linear decrease in the yield of (4) and a more remarkable decrease in total yield for the para-products than that for the ortho-products to give 2- and 4- acetamidophenyl ethyl ethers (9) and (8).Photolysis of the azide in ethanol in the presence of phenol afforded 2-phenoxy-3H-azepine (5) and aniline, but no 2- and 4-phenetidines.The yield of compounds (4) and (12)-(16) was independent of the presence of penta-1,3-diene(0.01 M), and somewhat decreased by an addition of bromobenzene.The rate of decomposition of azide in acetic acid was not accelerated as compared with that in 1,4-dioxan, and the activation parameters varied little in the two solvents.The results suggest that 1-azacyclohepta-1,2,4,6-tetraene (1) formed via singlet phenylnitrene or via singlet excited phenyl azide is trapped by acetic acid or phenol to give (4) or (5), and that a resonance-stabilized ion (3) neighbouring acetate anion is formed by an attack of the singlet nitrene on acetic acid to give aromatic nucleophilic substitution.
