Investigation Of The Influence Of Substitution On The Photochemical Reaction Pathway Of Arylnitrenes And Arylnitrenium Ions

Arylnitrenes and arylnitrenium ions are both short-lived intermediates and have pretty high reactivities. Arylnitrenes have been widely used in lithography, photoaffinity labeling, and covalent modification of polymer surfaces. Arylnitrenium ions have been revealed to be produced during biological metabolism process of the N-containing chemical contaminants. And arylnitrenium ions are able to react with guanine derivatives to form the carcinogenic C8 adducts.Most aryl azides have advantages such as stable at room temperature, easily to be synthesized and preserved compared to other precursors for generating arylnitrenes and arylnitrenium ions. So in laboratory, aryl azides are often used to generate nitrenes and nitrenium ions. By photolysis of the aryl azide in solution, singlet arylnitrene can be obtained, and it undergoes intersystem crossing(ISC) or ring expansion to produce the triplet nitrene or/and ketenimine in aprotic solvent. Triplet nitrene is not stable under ambient conditions and forms azobenzene. Ketenimine is easily trapped by aliphatic amines, or forms dimer without the presence of trapping agents. While in protic solvent, some singlet nitrenes can be protonated to produce nitrenium ions which can be quenched by nucleophiles such as the azide anion, 2′-deoxyguanosine at encounter limit(～5×109 M-1?s-1). Substituent has significant influence on the reactivity of the singlet nitrene. For example, the electron-donating substituent accelerates the ISC rate, but has little influence on the activation barrier of cyclization. The reactivity of the nitrenium ion towards nucleophiles can be decreased by strong π donors, and accelerated by proper electron-withdrawing substituents.Time-resolved transient absorption, time-resolved infrared and time-resolved Raman spectroscopies have been successfully used in laboratory for studying the reactivities and properties of arylnitrenes and arylnitrenium ions. With the aid of steady state measurement methods such as high performance liquid chromatography(HPLC) and “azide-clock”, and combined with the density functional theory(DFT) calculation, the structural and electronic characteristics of the arylnitrenes and arylnitrenium ions in aprotic solvents or protic solvents were specified. And the effects of substituents to the corresponding singlet nitrene, nitrenium ions were also analyzed.In this work, aryl azides such as 4’-nitro-4-biphenyl azide(NBiPhN3), 4-phenoxyphenyl azide(PhOPhN3), 4-methoxy-2-nitrophenyl azide(MNPhN3) were synthesized as precursors for generating the corresponding singlet nitrene upon photolysis. Nanosecond transient absorption and transient resonance Raman spectroscopic measurements combined with the DFT computation were used to detect and indentify the intermediates generated from the singlet nitrenes in aprotic and aqueous solutions. The results are as following :In aprotic solvents(such as acetonitrile), the singlet 4′-nitro-4-biphenylnitrene underwent ISC to the triplet nitrene. In contrast, in protic solvents(such as the mixed aqueous solution or formic acid solution), this singlet nitrene can be protonated to produce the nitrenium ion. The 4′-nitro-4-biphenylnitrenium ion can be quenched by NaN3 and 2′-deoxyguanosine(dG) with the rate constants of 5.28×109 and 3.58×109 M-1?s-1 respectively. Compared with its un-substituted counterpart, the nitro substitution had little influence on the ISC reaction pathway of the singlet 4-biphenylnitrene. With regard to the un-substituted nitrenium ion, the nitro group decreased its reactivities towards water and azide anion, while accelerated its reaction rate towards 2′-deoxyguanosine based on the different quench reaction rates between the nitrenium ion and azide anion/2′-deoxyguanosine.Singlet 4-phenoxyphenylnitrene underwent ISC to the triplet nitrene with a time constant of 346 ps, a little slower than the counterpart of the methoxy one(108 ps). In protic solvent, singlet phenoxyphenylnitrene was protonated with a constant of 37 ps, also slower than the methoxy one(t=5.4 ps). Single pulse transient Raman spectrum combined with the DFT calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion had comparable quinoidal character and charge distribution to those of methoxy- and ethoxy-phenylnitrenium ions. Compared to the 4-methoxyphenylnitrene, phenoxy substitution had a slight influence on the structure of phenylnitrenium ion, but has some impact on the reactivity of phenylnitrene.Singlet 4-methoxy-2-nitrophenylnitrene underwent ISC and ring expansion to generate triplet nitrene and ketenimine in aprotic solvent. In mixed formic acid aqueous solution, the singlet nitrene were protonated to produce the nitrenium ion, which had a lifetime of only ～64ns. The nitro substitution had a huge impact on 4-methoxy-2-nitrophenylnitrene, as decreasing the activation barrier of cyclization, improving the rate for cyclization in aprotic solution, and depressing the protonation rate of singlet nitrene.