title: Cycloaddition reactions of dihapto-coordinated furans.
abstract: Complexes of the type [TpRe(CO)(L)(eta(2)-furan)], where Tp = hydridotris(pyrazolyl)borate and L = PMe(3) (1) or (t)BuNC (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-eta(2)-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.
authors.affiliation: Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, USA.
authors.forename: Lee A
authors.initials: LA
authors.lastname: Friedman
authors.forename: Michal
authors.initials: M
authors.lastname: Sabat
authors.forename: W Dean
authors.initials: WD
authors.lastname: Harman
mesh_terms: D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D003508:Cycloheptanes / Q000138:chemical synthesis / Q000737:chemistry*; D005663:Furans / Q000737:chemistry*
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D003508:Cycloheptanes; D005663:Furans