d6 and d8 Metal Carbonyl Complexes of 7,7-Dimethoxy-5,6-dimethylidenebicyclo[2.2.1]hept-2-ene. Stereoselective Hydroformylation of an [Fe(CO)4(olefin)] Complex

Reaction of 7,7-dimethoxy-5,6-dimethylidenebicyclo2.2.1]hept-2-ene ( 2 ) with various metal carbonyls and their derivatives gave the η²-M(CO)₄ (M = Fe ( 17 ), Ru ( 18 )), η⁴-M(CO)₃ (M = Fe ( 19x, 19n ), Ru ( 20n )), and η²-M(CO)₅ and η⁶-M(CO)₃ (M = Cr, Mo, W) complexes. The trigonal bipyramidal η²-M(CO)₄ complexes present an exceptional C_3v symmetry at the metal with the C,C-double bond in an axial position. In all the η²-complexes, this double bond is stereospecifically coordinated by its exo-vs. endo-η⁴-Fe(CO)₃ configuration was established by chemical correlation (hydrolysis, hydrogenation) with the corresponding complexes ( 24x, 24n ) of 7,7-dimethoxy-2,3-dimethylidenebicyclo2.2.1]heptane ( 5 ). The relative rates of hydrolysis (AcOH/H₂O 2:1, 50°C) of ligands 2 and 5 and of complexes 19x, 19n, 24x , and 24n to the corresponding ketones showed an acceleration effect only when the metal is coordinated to the exo-face. This was attributed to an F-strain effect on the leaving group of the substrate. Compound 17 was further metallated by Fe₂(CO)₉] giving the bimetallic isomers 21xn and 21xx . The endocyclic C,C-double bond of the latter can be stereospecifically hydroformylated (1 atm CO, AcOH/H₂O, 25°C) giving 29x (49%). Hydroformylation of 17 gave the corresponding uncoordinated aldehydes 30x/30n in better yields (76%) but with lower selectivity (3:1). These are the first examples of hydroformylation of an isolated Fe(CO)₄(olefin)] complex.