The epimerization of chiral half sandwich 2-phenylpyridine-based ruthenacycle

The reaction of a chiral terpenic 2-phenylpyridine with mercury (II) acetate afforded a new ortho-chloromercurated substrate, which was treated with [(η6-cymene)RuCl2]2 to afford two diastereomers of the corresponding half-sandwich cyclochlororuthenated terpenic ligand. The latter two complexes were successfully separated by conventional chromatography and their configurational stability investigated by 1H NMR and CD spectroscopy. It was found that both isolated diastereomers displayed a fair configurational stability in CH2Cl2 while when dissolved in MeOH they epimerize. It was shown that the rate of epimerization is rather slow in pure MeOH, whereas in the presence of an excess of Lewis acid such as HgCl2 it was notably accelerated suggesting that the decoordination of the chloride from the Ru(II) center is a key step in the epimerization process.