Synthesis and characterization of chiral rhenium σ-ketone complexes. Binding properties of symmetrical ketones and alkylcyclohexanones
Reactions of the dichloromethane complex [Re(η5-C5H5)(NO)(PPh3)(ClCH2Cl)]BF41 with cyclohexanone (a), cyclobutanone (b), diphenylcyclopropenone (c), and tropone (d) give the symmetrical σ-ketone complexes [Re(η5-C5H5)(NO)(PPh3)(σ-OCR2)] BF47a–7d in 50–85% yields after work-up. Similar reactions of 1 with pentan-3-one (e) and 2,4-dimethylpentan-3-one (f) give the analogous ketone complexes 7e and 7f in good spectroscopic yields, but low isolated yields. However, 7e can be isolated in 66% yield from the reaction of the chlorobenzene complex [Re(η5-C5H5)(NO)(PPh3)(ClC6H5)] BF4 and pentan-3-one. Reactions of 1 with 3-methylcyclohexanone (h) and trans-1-decalone (j) give complexes 7h and 7j as mixtures of Re/C configurational diastereomers (40–71%). Under most conditions, the symmetrical ketone complexes 7a–7e exhibit a single set of 1H and 13C NMR resonances for the CO substituents that are Z and E to the rhenium. Low-temperature 1H NMR experiments show that the Z/E ethyl groups in 7e exchange with ΔG‡151 K= 7.0 kcal mol–1.