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

The reaction of Re-2(CO)(8)(mu-C6H5)(mu-H) (1) with furan in CH2Cl2 at 40 degrees C yielded two new isomeric dirhenium compounds, Re-2(CO)(8)(mu-eta(2)-2,3-C4H3O)(mu-H) (2) and Re-2(CO)(8)(mu-eta(2)-3,2-C4H3O)(mu-H) (3), which contain a bridging (sigma + pi)-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 degrees C for 7 days. The reaction of compound 2 with a second 1 equiv of 1 in CH2Cl2 at 40 degrees C yielded the doubly metalated furanbis[Re-2(CO)(8)(mu-H)] product [Re-2(CO)(8)(mu-H)](2)(mu-eta(2)-2,3-mu-eta(2)-5,4-C4H2O) (4) containing a 2,5-furdiyl ligand that is (sigma + pi). coordinated to two Re-2(CO)(8)(mu-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 Re-2(CO)(8)(mu-H)[mu-eta(2)-C(H)=C(H)Bu-n] in toluene to reflux. An isomer of 4, [Re-2(CO)(8)(mu-H)](2)(mu-eta(2)-2,3-mu-eta(2)-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 degrees C for 24 h. The reaction of 1 with 2,5-dimethylfuran (DMFUR) in CH2Cl2 at 40 degrees C yielded the new compound [Re-2(CO)(8)(mu-eta(2)-3,2-(CH3)(2)C4H2O)](mu-H) (6), which contains a bridging (sigma + pi)-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.