Optically active organoiron complexes. Preparation and chemistry

Article abstract of DOI:10.1021/om00091a031

The achiral and optically active Fp(dioxene)BF₄ complexes 2a and 2b (Fp = dicarbonylcyclopentadienyliron) are readily prepared from Fp(1,2-dimethoxyethylene)BF₄ by glycol exchange reactions. The barrier to ring inversion in these compounds is estimated to be 7.6 kcal/mol from low-temperature ¹³C NMR spectral measurements. A crystal structure determination of 2a shows significant distortions due to transannular steric interactions between the Fp carbonyl groups and an axial ring proton. Lateral displacement of the Fp group along the C-C double-bond axis is also suggested by the X-ray data and is supported by ¹³C and ¹H NMR spectral analyses. Both 2a and 2b add a number of carbon nucleophiles. These reactions with 2b are highly regioselective and yield a single enantiomeric product, 4, in which the nucleophile and Fp substituents are diaxial on the dioxane ring. An X-ray structure determination of a cyano adduct, 4c, has been carried out. The methyl adduct 4b has been converted to optically active Fp(cis-propenyl methyl ether)BF₄ and to Fp(propene)BF₄. Ring opening of the phenyl adduct 4d may take place through phenyl rather than Fp assistance and, under those circumstances, leads to loss of optical activity in the product olefin complex. Fp-assisted ring opening of the dioxene complex 4a is shown to be highly dependent on the conformation of the Fp substituent.