Regioselective Nitration and/or Halogenation of Iridabenzofurans through Electrophilic Substitution

Abstract

Regioselective electrophilic substitution reactions of the iridabenzofurans [Ir(C 7 H 5 O{OMe-7})(CO)(PPh 3 ) 2 ]-[OTf] (1) and IrCl(C 7 H 5 O{OMe-7})(PPh 3 ) 2 (2) provide a convenient route to mononitro-, dinitro-, and mixed nitro-/halosubstituted derivatives. Treatment of cationic 1 with copper(II) nitrate in acetic anhydride ("Menke" nitration conditions) gives the mononitrated iridabenzofuran [Ir(C 7 H 4 O{NO 2 -2}{OMe-7})(CO)(PPh 3 ) 2 ][O 3 SCF 3 ] (3). Under the same conditions neutral 2 undergoes dinitration to form IrCl(C 7 H 3 O{NO 2 -2}{NO 2 -6}{OMe-7})(PPh 3 ) 2 (5). Simple substitution of the carbonyl ligand in 3 with chloride gives the neutral mononitro derivative IrCl(C 7 H 4 O{NO 2 -2}{OMe-7})(PPh 3 ) 2 (4). Depending on the conditions employed, treatment of the iridabenzofurans 1 and 2 with Cu(NO 3 ) 2 and either lithium chloride or lithium bromide in acetic anhydride gives either the mixed nitro-/halo-substituted iridabenzofurans IrCl(C 7 H 3 O{NO 2 -2}{Cl-6}{OMe-7})(PPh 3 ) 2 (6) and IrCl(C 7 H 2 O{NO 2 -2}{NO 2 -4}{Cl-6}{OMe-7})(PPh 3 ) 2 (7) or the simple halo-substituted iridabenzofurans [Ir(C 7 H 4 O-{Cl-6}{OMe-7})(CO)(PPh 3 ) 2 ][OTf] (8), [Ir(C 7 H 4 O{Br-6}{OMe-7})(CO)(PPh 3 ) 2 ][OTf] (9), and IrBr(C 7 H 3 O{Br-2}{Br-6}{OMe-7})(PPh 3 ) 2 (10). Bromination of 4 with pyridinium tribromide gives IrCl(C 7 H 3 O{NO 2 -2}{Br-6}{OMe-7})(PPh 3 ) 2 (11). The molecular structures of 3−7 and 11 have been obtained by X-ray crystallography.

Syntheses of Amino‐Substituted Iridabenzofurans and Subsequent Selective N‐Functionalisation

“…In the 13 C{ 1 H} NMR spectra, the metal‐bound carbon atoms C1 or C5 resonate at downfield positions, 118.37 or 196.29 ppm, respectively for 3 , and 134.56 or 190.18 ppm, respectively, for 4 . This is in line with the chemical shifts observed for the corresponding atoms in related iridabenzofurans …”

“…In the 1 H NMR spectra of 3 or 4 , the chemical shifts for H1 are found at 8.10 or 9.12 ppm, respectively. These values are notably up‐field from the shifts for H1 in the nitro‐substituted precursors 1 (9.74 ppm) or 2 (12.08 ppm), but still within the range reported for other iridabenzofurans . The chemical shifts of the remaining ring protons of 3 (H3 5.50, H4 5.67, and H6 5.60 ppm) or 4 (H3 5.41, H4 4.88, and H6 5.00 ppm) are much closer to those of 1 or 2 .…”

“…Our efforts to reduce the nitro‐substituents of the 2‐nitroiridabenzofurans, 1 or 2 , initially focussed on relatively mild procedures that we expected would be less likely to disrupt the iridabenzofuran framework in these compounds. Numerous approaches were investigated including addition of tin(II) chloride, sodium borohydride, hydrazine/catalyst, or hydrogen/catalyst.…”

“…We recently discovered that the cationic nitro‐substituted iridabenzofuran, [Ir(C 7 H 4 {NO 2 ‐2}{OMe‐7})(CO)(PPh 3 ) 2 ][O 3 SCF 3 ] ( 1 ) can be conveniently prepared in high yield through nitration of the parent cationic iridabenzofuran, [Ir(C 7 H 4 {NO 2 ‐2}{OMe‐7})(CO)(PPh 3 ) 2 ][O 3 SCF 3 ] . The neutral analogue, Ir(C 7 H 4 O{NO 2 ‐2}{OMe‐7})Cl(PPh 3 ) 2 ( 2 ), can be obtained by a simple ligand substitution reaction (Scheme ).…”

A Metallaanthracene and Derived Metallaanthraquinone

“…H4 is detected at 6.08 ppm and the protons on the terminal benzene ring give rise to four multiplets between 7.65 and 8.27 ppm. [13,15] Although these fused rings are essentially co-planar,t he terminal benzene bends at the two carbonyl groups forming an angle of 298 8 to this plane. Them olecular structure of 7 ( Figure 3) has been determined by single-crystal Xray diffraction.…”