Rhenium(I) η²-Coordinated Furan Complexes: Converting Furan into a 1,3-Carbon Dipole

TpRe(CO)(MeIm)(η2-2-methylfuran) (1; Tp = hydridotris(pyrazolyl)borate and MeIm = N-methylimidazole) and TpRe(CO)(MeIm)(η2-2,5-dimethylfuran) (9) undergo Lewis acid promoted cyclopentannulation reactions with enones and enals to generate 3-acetylcyclopentene complexes. During the reaction, a rearrangement occurs such that the α- and β-carbons of the enone or enal are incorporated into the new carbocycle. The reactions are completely regioselective. Yields vary from 26 to 78%, depending on reaction conditions. Diastereomer ratios vary from 50:50 to >95:5. In a similar fashion, these furan complexes undergo reactions with aldehydes to form acetyldihydrofuran complexes, where a new dihydrofuran ring is generated from the aldehyde and C3−C5 of the original furan. These dihydrofuran complexes are readily converted into the corresponding trisubstituted furans via oxidative demetalation. Attempts to synthesize TpRe(CO)(MeIm)(η2-2-methoxyfuran) for the synthesis of cyclopentene esters resulted in the formation of a novel rhenium carbene.