Regioselective Mono-, Di-, and Trifunctionalization of Iridabenzofurans through Electrophilic Substitution Reactions

Abstract

Reaction between the cationic iridacyclopentadiene complex [Ir(C4H4)(NCMe)(CO)(PPh3)2][CF3SO3] (1) and methylpropiolate produces the cationic iridabenzofuran [Ir(C7H5O{OMe-7})(CO)(PPh3)2][CF3SO3] (2) in high yield. On treatment of 2 with chloride, the carbonyl ligand is displaced and the corresponding neutral iridabenzofuran Ir(C7H5O{OMe-7})Cl(PPh3)2 (3) is formed. The fused metallacyclic rings of the iridabenzofurans 2 and 3 bear only one substituent (OMe), and therefore these compounds are well suited for studies of electrophilic aromatic substitution reactions. Bromination of cationic 2 with pyridinium tribromide proceeds to give the monobrominated iridabenzofuran [Ir(C7H5O{OMe-7}{Br-6})(CO)(PPh3)2][CF3SO3] (4) exclusively. Bromination of neutral 3 with the same reagent gives the dibrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-2})Br(PPh3)2 (5) exclusively. Treatment of compound 3 with mercury(II) trifluoroacetate followed by excess bromide (to displace coordinated trifluoroacetate) produces the trimercurated iridabenzofuran Ir(C7H5O{OMe-7}{HgBr-6}{HgBr-4}{HgBr-2})Br(PPh3)2 (6). The three Hg−C bonds in 6 are readily cleaved on addition of pyridinium tribromide, and the resulting product is the tribrominated iridabenzofuran Ir(C7H5O{OMe-7}{Br-6}{Br-4}{Br-2})Br(PPh3)2 (7). These regioselective mono-, di-, and trifunctionalization reactions of iridabenzofurans have been studied by DFT calculations, and the derived condensed Fukui functions have been used to rationalize the preferred sites for electrophilic attack. The crystal structures of 2−7 have been obtained.