Substituent Effects on Nitrosation of 1, 3-Diarylureas with Nitrosyl Chloride, Dinitrogen Trioxide, and Dinitrogen Tetroxide

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Twenty-eight para-substituted and eleven ortho-substituted 1, 3-diarylureas were prepared.Their proton nuclear magnetic resonance (¹H-NMR) spectra were measured to evaluate the acidity of the ureido protons in the para-substituted ureas, and to estimate the conformations of ortho-substituted molecules. The chemical shifts and acidities of two ureido protons were additively controlled by the two para-substituents in the 1, 3-diarylureas. The sixteen para-substitued diarylureas were nitrosated with gaseous nitrosating reagents (dinitrogen trioxide, dinitrogen tetroxide or nitroxyl chloride) to give unstable isomeric mixtures of 1, 3-diaryl-1-nitrosoureas [R¹-C₆H₄-N(NO)-CO-NH-C₆H₄-R²] and 1, 3-diaryl-3-nitrosoureas [R¹-C₆H₄-NH-CO-N(NO)-C₆H₄-R²]. The isomer ratio of each nitrosated urea was correlated with the difference between the chemical shifts (d[NH]) of the two ureido protons in the ¹H-NMR spectrum of the starting urea. When d[NH] was below 0.25 ppm (with NOCl) or 0.45 ppm (with N₂O₃), good linearity was obtained (r(NOCl)=0.943, n=11; r(N₂O₃)=0.869, n=15). When d[NH] was over 0.25 ppm (NOCl) or 0.45 ppm (N₂O₃), the 1-nitrosated isomer [R¹-C₆H₄-N(NO)-CO-NH-C₆H₄-R²] was the sole product. These principles ould not be extended to dinitrogen tetroxide.Thirteen para-substituted diarylureas gave 1- or 3-nitrosated ureido derivatives as sole products and only four gave nitrosated isomeric mixtures.The ¹H-NMR spectra of 5 types of ortho-substituted 1, 3-diarylureas were measured for comformational analysis. Their conformations seem to be different from those of para-substituted diarylureas. Eleven ortho-substituted 1, 3-diarylureas were nitrosated with nitrosyl chloride and dinitrogen tetroxide. In many cases, the nitroso group was predominantly introduced at the ureido nitrogen, which is less hindered sterically, and isomeric mixtures of 1- and 3-nitrosated ureas were obtained.