Activation of Heteroaromatic C-H Bonds in Furan and 2,5-Dimethylfuran

The reaction of Re₂(CO)₈(μ-C₆H₅)(μ-H) (1) with furan in CH₂Cl₂ at 40 °C yielded two new isomeric dirhenium compounds, Re₂(CO)₈(μ-η²-2,3-C₄H₃O)(μ-H) (2) and Re₂(CO)₈(μ-η²-3,2-C₄H₃O)(μ-H) (3), which contain a bridging (σ + π)-coordinated furyl ligand formed by activation of the C-H bond at the 2 and 3 positions of furan, respectively. Compound 3 is the first example of a compound formed by C-H bond activation at the 3 position of a furan ring. Compound 3 was isomerized to 2 by heating to 80 °C for 7 days. The reaction of compound 2 with a second 1 equiv of 1 in CH₂Cl₂ at 40 °C yielded the doubly metalated furan-bis[Re₂(CO)₈(μ-H)] product [Re₂(CO)₈(μ-H)]₂(μ-η²-2,3-μ-η²-5,4-C₄H₂O) (4) containing a 2,5-furdiyl ligand that is (σ + π)-coordinated to two Re₂(CO)₈(μ-H) groups by activation of the C-H bond at the 5 position of the furyl ligand in 2. Compound 2 can also be formed as the major isomer, together with a small quantity of 4 by heating furan with Re₂(CO)₈(μ-H)[μ-η²-C(H)═C(H)Bu ⁿ] in toluene to reflux. An isomer of 4, [Re₂(CO)₈(μ-H)]₂(μ-η²-2,3-μ-η²-4,5-C₄H₂O) (5), was obtained from the reaction of 3 with 1 through C-H activation at the 4 position of the furyl ring in 3. Compound 4 was also obtained from 5 by heating to 110 °C for 24 h. The reaction of 1 with 2,5-dimethylfuran (DMFUR) in CH₂Cl₂ at 40 °C yielded the new compound [Re₂(CO)₈(μ-η²-3,2-(CH₃)₂C₄H₂O)](μ-H) (6), which contains a bridging (σ + π)-coordinated 2,5-dimethylfuryl ligand formed by activation of the C-H bond at the 3 position of DMFUR. All of the new compounds were characterized structurally by single-crystal X-ray diffraction analysis.