Incorporation of an n-Butylsulfonate Functionality To Induce Aqueous Solubility on Ruthenium(II) η⁶-Arene Complexes

The reaction of the new building block sodium n-butyl sulfonate cyclohexadiene 3 with RuCl3·xH2O leads to the formation of different dimeric halide-bridged ruthenium complexes, depending on the reaction conditions. Isolation of pure dimeric complexes from these mixtures was not successful. Changing the cyclohexadiene preligand in the reaction to its neutral isobutyl-protected analogue 2 did allow for the synthesis and isolation of neutral halide-bridged dimeric complexes 5 (chloride bridged) and 6 (iodide bridged) in good to excellent yields. Upon deprotection of the isobutyl sulfonate groups in 6 using sodium iodide, the anionic, water-soluble dimeric complex 7 was obtained in near-quantitative yield. Complexes 6 and 7 are starting materials for the synthesis of water-soluble heteroleptic Ru arene complexes, as was demonstrated by the reaction of 6 with (S,S)-TsDPEN, followed by a reaction with NaI, to yield the mononuclear water-soluble Ru arene TsDPEN complex 8. In preliminary experiments 8 was found to be an effective catalyst in the asymmetric transfer hydrogenation (ATH) of acetophenone to quantitatively provide (S)-phenylethanol in 94% ee.