Binuclear Aromatic C−H Bond Activation at a Dirhenium Site

Mechanistic Insights into Rhenium-Catalyzed Regioselective C-Alkenylation of Phenols with Internal Alkynes

A (μ-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel-Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon-carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes.

Multiple C-H Bond Activations in Corannulene by a Dirhenium Complex

The reaction of Re₂ (CO)₈ (μ-C₆ H₅ )(μ-H), 1 with corannulene (C₂₀ H₁₀ ) yielded the product Re₂ (CO)₈ (μ-H)(μ-η² -1,2-C₂₀ H₉ ), 2 (65 % yield) containing a Re₂ metalated corannulene ligand formed by loss of benzene from 1 and the activation of one of the CH bonds of the nonplanar corannulene molecule by an oxidative-addition to 1. The corannulenyl ligand has adopted a bridging η² -σ+π coordination to the Re₂ (CO)₈ grouping. Compound 2 reacts with a second equivalent of 1 to yield three isomeric doubly metalated corannulene products: Re₂ (CO)₈ (μ-H)(μ-η² -1,2-μ-η² -10,11-C₂₀ H₈ )Re₂ (CO)₈ (μ-H), 3 (35 % yield), Re₂ (CO)₈ (μ-H)(μ-η² -2,1-μ-η² -10,11-C₂₀ H₈ )Re₂ (CO)₈ (μ-H), 4 (12 % yield), and Re₂ (CO)₈ (μ-H)(μ-η² -1,2-μ-η² -11,10-C₂₀ H₈ )Re₂ (CO)₈ (μ-H), 5 (12 % yield), by a second CH activation on a second rim double bond on the corannulene molecule. The isomers differ by the relative orientations of the coordinated Re₂ (CO)₈ (μ-H) groupings. All new products were characterized structurally by single crystal X-ray diffraction analysis.

Contrasting Synergistic Heterobimetallic (Na-Mg) and Homometallic (Na or Mg) Bases in Metallation Reactions of Dialkylphenylphosphines and Dialkylanilines: Lateral versus Ring Selectivities

A series of dialkylphenylphosphines and their analogous aniline substrates have been metallated with the synergistic mixed-metal base [(TMEDA)Na(TMP)(CH₂ SiMe₃ )Mg(TMP)] 1. Different metallation regioselectivities for the substrates were observed, with predominately lateral or meta-magnesiated products isolated from solution. Three novel heterobimetallic complexes [(TMEDA)Na(TMP)(CH₂ PCH₃ Ph)Mg(TMP)] 2, [(TMEDA)Na(TMP)(m-C₆ H₄ PiPr₂ )Mg(TMP)] 3 and [(TMEDA)Na(TMP)(m-C₆ H₄ NEt₂ )Mg(TMP)]  4 and two homometallic complexes [{(TMEDA)Na(EtNC₆ H₅ )}₂ ] 5 and [(TMEDA)Na₂ (TMP)(C₆ H₅ PEt)]₂  6 derived from homometallic metallation have been crystallographically characterised. Complex 6 is an unprecedented sodium-amide, sodium-phosphide hybrid with a rare (NaNNaP)₂ ladder motif. These products reveal contrasting heterobimetallic deprotonation with homometallic induced ethene elimination reactivity. Solution studies of metallation mixtures and electrophilic iodine quenching reactions confirmed the metallation sites. In an attempt to rationalise the regioselectivity of the magnesiation reactions the C-H acidities of the six substrates were determined in THF solution using DFT calculations employing the M06-2X functional and cc-pVTZ Dunning's basis set.