Arene exchange accelerated by a pendant ligand

Several novel aspects of arene chromium chemistry are described. Selective double substitution by amine nucleophiles on (difluorobenzene)Cr(CO) ₃ is reported. Replacement of one of the carbonyl ligands by an (N-methylbenzylamine)dipyrrolyl-phosphine retards the nucleophilic aromatic substitution reaction only to a small extent. The phosphine ligand can be generated on the solid phase to allow attachment of the arene via an activating chromium unit as a traceless linker.; Modification of the tri(pyrrolyl)phosphine with a properly designed pendant side chain allows arene exchange in (arene)Cr(CO)₂(phosphine) to occur at room temperature. Temporary coordination of the side chain in the partially dissociated arene intermediate is proposed to be responsible for the mild reaction conditions. A series of (2-substituted-pyrrolyl)dipyrrolylphosphines was synthesized and the effect of the side chain on the arene exchange rate was systematically evaluated. Arene exchange in (arene)Cr(CO)₂((2-methylcarboxypyrrolyl)dipyrrolylphosphine) was investigated for a variety of incoming and leaving arenes. An interchange mechanism is proposed where the transition state occurs on the way from η 2-arene chromium complex to η2-arene′ chromium complex, where arene′ is an incoming arene. The mechanism was substantiated computationally by calculating the Hartree-Fock energies (3-21G(*) basis set) of the intermediates involved.