Cyclometalation and Bicyclometalation Reactions of Trimethylphosphine Supported Iron, Cobalt and Nickel Compounds via C-H Activation with Imine and Carbonyl Anchoring Groups
Since the decisive breakthrough by Murai et al., who achieved a highly efficient ruthenium-catalyzed addition of aromatic C-H bonds to unsaturated substrates this reaction is recognized as a new category of chemistry. The compounds which were synthesized and presented in this thesis serve as stable...
| Summary: | Since the decisive breakthrough by Murai et al., who achieved a highly efficient ruthenium-catalyzed addition of aromatic C-H bonds to unsaturated substrates this reaction is recognized as a new category of chemistry. The compounds which were synthesized and presented in this thesis serve as stable models of reactive intermediates, proposed in catalytic transformations with the ruthenium counterparts. It is also general interest to study the reactivity of trimethylphosphine stabilized iron, cobalt and nickel compounds toward aromatic imines and ketones. Other than some cyclomanganated products, there are no examples of cyclometalated products of 3d row transition metals with imines. In this section, a review of reactions in the activation and functionalization of C-H bonds by solution-phase transition metal-based systems are presented, with an emphasis on the activation of aromatic C-H bonds. Phenyl ketimines react smoothly under mild contions with low–valent iron (0) and methyl-cobalt(I) adducts to form five-membered metallacycles. Stable cobalt(I) complexes are formed through reductive elimination of methane, and hydrido-iron(II) compounds arise from formal insertion of iron into the aromatic C-H bond. Benzylic imines react with Fe(CH3)2(PMe3)4 by elimination of methane to afford hexacoordinate methyl-iron(II) complexes. These are the first examples of ortho-metalated iron and cobalt complexes with imine anchoring groups which are fully characterized.An imine function is the nitrogen analogue of a carbonyl group. Following the isolecetronic principle, CoCH3(PMe3)4 with ketones affords cyclometalated, diamagnetic cobalt(I) compounds through methane elimination under same reaction conditions. Even though C-F bonds are about 30 kcal/mol stronger than C-H bonds, partially fluorinated 2,3,4,5,6-pentafluorobenzophenone reacts with CoCH3(PMe3)4 via C-F activation by elimination of fluoromethane. Upon combining a dimethyliron compound with certain diarylated imines, a reaction sequence is observed consisting of double metalation at the same metal complex center leading to a metallabicycle. Fe(CH3)2(PMe3)4 affords bicyclo-metalated iron(II) compounds with N-(1-naphthylmethylene)-aniline and N-Benzylbenzylideneimine. Co(CH3)(PMe3)4 forms hydrido-cobalt(III) complexes. |
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