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.

A Metallaanthracene and Derived Metallaanthraquinone

“…This ring-expansion reaction presumably proceeds via rearrangement of coordinated methyl propiolate to vinylidene,i nsertion of the a-carbon of this ligand into the iridacyclopentadiene ring and coordination of the ester carbonyl oxygen to form the fused iridafuran ring. [13] Thec rystal structure determination of 3 (see Figure 1f or molecular geometry) confirms the fused-ring iridabenzofuran (or 8,15dihydroiridaanthrafuran) formulation. [13,15] In the 13 CNMR spectrum of 3 the iridium-bound carbon atoms of the fused iridabenzofuran ring (C1 and C5) are observed at downfield positions (121.95 and 198.47 ppm, respectively), as is typical for iridabenzofurans of this type.…”

“…[13] In the 1 HNMR spectrum of 3 the resonance for H1 (7.65 ppm), H6 (5.66 ppm), H8 (3.01 ppm), H15 (2.76 ppm) and the protons on the remote fused benzene ring (6.90-7.11 ppm) are all observed in the expected regions. This ring-expansion reaction presumably proceeds via rearrangement of coordinated methyl propiolate to vinylidene,i nsertion of the a-carbon of this ligand into the iridacyclopentadiene ring and coordination of the ester carbonyl oxygen to form the fused iridafuran ring.…”

“…[13] In the 1 HNMR spectrum of 3 the resonance for H1 (7.65 ppm), H6 (5.66 ppm), H8 (3.01 ppm), H15 (2.76 ppm) and the protons on the remote fused benzene ring (6.90-7.11 ppm) are all observed in the expected regions. Thes tructural parameters associated with the iridabenzofuran ring system of 3 are very similar to those of the corresponding unsubstituted iridabenzofuran, [13] for example,I r, C1-C7, and Oa re essentially coplanar and the distances Ir À C1 (2.064(6) ) and Ir À C5 (2.055 (7) )are nearly equal. [13] Thec rystal structure determination of 3 (see Figure 1f or molecular geometry) confirms the fused-ring iridabenzofuran (or 8,15dihydroiridaanthrafuran) formulation.…”

“…[16] Thet wo protons on C6 of the tethering arm are detected at 2.24 ppm which is appropriate for as aturated CH 2 group.I nt he 13 CNMR spectrum the metal-bound carbon atom C1 is observed at the very downfield position of 275.12 ppm, while the other metal-bound carbon, C5 (154.58 ppm), is less strongly downfield shifted. However,our attempts to dehydrogenate 3 or 4 using typical oxidants (e.g.DDQ,CuCl, O 2 ), Pd/C, or simply by heating were not successful.…”

“…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.…”