Planar chirality in tethered η6:η1-(phosphinophenylenearene-P)ruthenium(II) complexes and their potential use as asymmetric catalysts

Article abstract of DOI:10.1021/om030080q

Reaction of 2-dicyclohexylphosphinobiphenyl with [(η⁶-benzene)RuCl₂]₂ yielded the tethered complex [Ru(η⁶:η¹-dicyclohexylphosphinobiphenyl-P)Cl₂ ], 1, rather than a bridge-splitting product, [(η⁶-benzene)Ru(L)Cl₂], that is often observed. Treatment of [(η⁶-benzene)RuCl₂]₂ with 2-dicyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl yielded the planar chiral, tethered complex [Ru(η⁶:η¹-2-dicyclohexylphosphino-2-′-(N,N-d imethylamino)biphenyl-P)Cl₂], 2. Abstraction of a chloride from 2 with AgSbF₆ and treatment with PPh₃ selectively gave the chiral-at-metal complex [anti-Ru(η⁶:η¹-2-dicyclohexylphosphino-2′-(N ,N-dimethylamino)-biphenyl-P) (PPh₃)Cl]SbF₆, 3a, which underwent spontaneous resolution upon crystallization. The Me₂N group is coplanar with the η⁶-phenyl ring in the cations and directs attack at the metal center, as well as determining the thermodynamic stability of anti versus syn epimers. The dication derived from enantiopure 3a catalyzed the Diels-Alder reaction of methacrolein and cyclopentadiene with modest (19-23%) enantioselectivity. Analogues of 2 and 3a containing 2-(dicyclohexylphosphino)-2′-methylbiphenyl were also prepared. We have found that epimerization at the metal center is slow in these compounds. Averaged NMR spectra at ambient temperatures are observed, however, due to rapid conformational interconversions that can be slowed at low temperature.