Cycloaddition Reactions of Dihapto-Coordinated Furans

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Abstract
Complexes of the type [TpRe(CO)(L)(η2-furan)], where Tp = hydridotris(pyrazolyl)borate and L = PMe3 (1) or tBuNC (2), undergo dipolar cycloadditions with TCNE (tetracyanoethylene) to afford 7-oxabicycloheptene complexes 3 and 4, respectively. The corresponding 2-methylfuran complexes (5 and 7) for these L ligands give similar methyloxabicycloheptene complexes (6 and 8), with a diastereomer ratio >20:1 for 8. For L = N-methylimidazole (MeIm, 9), TCNE oxidizes the complex, but cycloadditions can be achieved with DMAD (dimethyl acetylenedicarboxylate) as the electrophile. Three complexes are observed: one in which DMAD undergoes a cycloaddition with the carbonyl ylide form of the complex (10C), and two complexes that are coordination diastereomers where DMAD has undergone a formal [2+2] cycloaddition with the uncoordinated double bond of the 4,5-η2-furan ligand (10B and 10A). With the imidazole complex of 2-methylfuran (11), only the [2+2] products (12B and 12A) are observed. In the case of the 2,5-dimethylfuran complex (L = MeIm, 13), which is formed as a single coordination diastereomer, only one [2+2] product is observed (14), the structure of which was confirmed by X-ray crystallography. Oxidative decomplexation of 14 results in liberation of the free oxabicyclo[3.2.0]heptadiene 15, which can be thermally converted to the corresponding oxepin 16 in 70% yield.