Multiple aromatic C-H bond activations by a dirhenium carbonyl complex

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.

Multiple Aromatic C-H Bond Activations by an Unsaturated Dirhenium Carbonyl Complex

Reactions of Re₂(CO)₈(μ-C₆H₅)(μ-H), 1, with naphthalene and anthracene have yielded the first multiply-CH activated arene products through the reductive elimination of benzene from 1 and multiple oxidative-additions of the dirhenium octacarbonyl grouping to these polycyclic aromatic compounds under very mild conditions. In addition, compound 1 was found to react with itself to yield the bis-Re₂-metalated C₆H₄ bridged compound Re₂(CO)₈(μ-H)(μ-1,μ-3-C₆H₄)Re₂(CO)₈(μ-H), 3. Reaction of 1 with naphthalene yielded two doubly CH activated isomers, Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-3,4-C₁₀H₆)Re₂(CO)₈(μ-H), 4, 41% yield, and Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-5,6-C₁₀H₆)Re₂(CO)₈(μ-H), 5, via the mono CH activated intermediate Re₂(CO)₈(μ-η²-C₁₀H₇)(μ-H), 2. Compound 4 contains two Re₂(CO)₈(μ-H) groups on one C₆ ring formed by CH activations at the 2- and 4-positions. Compound 5 contains two Re₂(CO)₈(μ-H) groups; one formed by CH activation at the 2-position on one C₆ ring and the other formed by CH activation at the 6-position (or centrosymmetrically related 2'-position) on the second C₆ ring. The Re₂(CO)₈(μ-H) groups are coordinated to the C₆ rings by binuclear σ + π coordination to two adjacent carbon atoms in the rings. Compound 1 reacts with anthracene to yield the mono-CH activated compound Re₂(CO)₈(μ-η²-1,2-C₁₄H₉)(μ-H), 6, and two doubly CH activated compounds, Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-3,4-C₁₄H₈)Re₂(CO)₈(μ-H), 7, and Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-5,6-C₁₄H₈)Re₂(CO)₈(μ-H), 8. Compounds 7 and 8 are isomers that are structurally similar to 4 and 5. Compounds 7 and 8 can also be obtained in good yields from the reaction of 6 with 1. In the presence of a 5/1 ratio of 1/anthracene, a small amount (5% yield) of the tetra-substituted anthracene product [Re₂(CO)₈(μ-H)]₄(μ-η²-1,2-μ-η²-3,4-μ-η²-5,6-μ-η²-7,8-C₁₄H₆), 9, was formed. Compound 9 contains four σ + π coordinated Re₂(CO)₈(μ-H) groups formed by oxidative additions of the CH bonds of anthracene to the Re₂(CO)₈ groups at the 2, 4, 6, and 8 positions of the three ring system. Molecular orbital calculations have been performed for all new compounds in order to develop an understanding of the bonding of the ring systems to the Re₂(CO)₈(μ-H) groups. All new compounds were characterized by single-crystal X-ray diffraction analyses.

Multiple activations of CH bonds in arenes and heteroarenes

Re₂(CO)₈(μ-C₆H₅)(μ-H) reacts with anthracene four times to yield the quadruply CH activated complex [Re₂(CO)₈(μ-H)]₄(μ-η²-1,2-μ-η²-3,4-μ-η²-5,6-μ-η²-7,8-C₁₄H₆).

Multiple C-H Bond Activations and Ring-Opening C-S Bond Cleavage of Thiophene by Dirhenium Carbonyl Complexes

The reaction of Re₂(CO)₈(μ-C₆H₅)(μ-H) (1) with thiophene in CH₂Cl₂ at 40 °C yielded the new compound Re₂(CO)₈(μ-η²-SC₄H₃)(μ-H) (2), which contains a bridging σ-π-coordinated thienyl ligand formed by the activation of the C-H bond at the 2 position of the thiophene. Compound 2 exhibits dynamical activity on the NMR time scale involving rearrangements of the bridging thienyl ligand. The reaction of compound 2 with a second 1 equiv of 1 at 45 °C yielded the doubly metalated product [Re₂(CO)₈(μ-H)]₂(μ-η²-2,3-μ-η²-4,5-C₄H₂S) (3), formed by the activation of the C-H bond at the 5 position of the thienyl ligand in 2. Heating 3 in a hexane solvent to reflux transformed it into the ring-opened compound Re(CO)₄[μ-η⁵-η²-SCC(H)C(H)C(H)][Re(CO)₃][Re₂(CO)₈(μ-H)] (4) by the loss of one CO ligand. Compound 4 contains a doubly metalated 1-thiapentadienyl ligand formed by the cleavage of one of the C-S bonds. When heated to reflux (125 °C) in an octane solvent in the presence of H₂O, the new compound Re(CO)₄[η⁵-μ-η²-SC(H)C(H)C(H)C(H)]Re(CO)₃ (5) was obtained by cleavage of the Re₂(CO)₈(μ-H) group from 4 with formation of the known coproduct [Re(CO)₃(μ₃-OH)]₄. All new products were characterized by single-crystal X-ray diffraction analyses.