Regioselective Nitration and/or Halogenation of Iridabenzofurans through Electrophilic Substitution
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Regioselective electrophilic substitution reactions of the iridabenzofurans [Ir(C7H5O{OMe-7})(CO)(PPh3)2][OTf] (1) and IrCl(C7H5O{OMe-7})(PPh3)2 (2) provide a convenient route to mononitro-, dinitro-, and mixed nitro-/halo-substituted derivatives. Treatment of cationic 1 with copper(II) nitrate in acetic anhydride (“Menke” nitration conditions) gives the mononitrated iridabenzofuran [Ir(C7H4O{NO2-2}{OMe-7})(CO)(PPh3)2][O3SCF3] (3). Under the same conditions neutral 2 undergoes dinitration to form IrCl(C7H3O{NO2-2}{NO2-6}{OMe-7})(PPh3)2 (5). Simple substitution of the carbonyl ligand in 3 with chloride gives the neutral mononitro derivative IrCl(C7H4O{NO2-2}{OMe-7})(PPh3)2 (4). Depending on the conditions employed, treatment of the iridabenzofurans 1 and 2 with Cu(NO3)2 and either lithium chloride or lithium bromide in acetic anhydride gives either the mixed nitro-/halo-substituted iridabenzofurans IrCl(C7H3O{NO2-2}{Cl-6}{OMe-7})(PPh3)2 (6) and IrCl(C7H2O{NO2-2}{NO2-4}{Cl-6}{OMe-7})(PPh3)2 (7) or the simple halo-substituted iridabenzofurans [Ir(C7H4O{Cl-6}{OMe-7})(CO)(PPh3)2][OTf] (8), [Ir(C7H4O{Br-6}{OMe-7})(CO)(PPh3)2][OTf] (9), and IrBr(C7H3O{Br-2}{Br-6}{OMe-7})(PPh3)2 (10). Bromination of 4 with pyridinium tribromide gives IrCl(C7H3O{NO2-2}{Br-6}{OMe-7})(PPh3)2 (11). The molecular structures of 3–7 and 11 have been obtained by X-ray crystallography.
