Mild Photochemical Tethering of [RuCl2(η6‑arene)P*] Complexes with P‑Stereogenic 2‑Biphenylylphosphines

The synthesis of enantiopure P-stereogenic diarylphosphines PArPhR (Ar = 9-phenanthryl (a), 1-naphthyl (b), and 2-biphenylyl (c)) with an aryl group at the β (3; R = −CH2CH2­(2-naphthyl)) or γ position (2; R = −CH2Si­(Me2)­CH2Ph)) is described. Treatment of 2a with [RuCl2­(η6-p-cymene)]2 (Dim1) has led to complex [RuCl2­(η6-p-cymene)­(κP-2a)] (4a). Attempts to displace the p-cymene substituent by the pendant aryl group of the phosphine have failed. Reaction of the ligands with [RuCl2(η6-methyl benzoate)]2 (Dim2) has furnished complexes [RuCl2­(η6-methyl benzoate)­(κP-P*)] 6 and 7 (6a–c, P* = 2a–c; 7a–b, P* = 3a–b). The methyl benzoate of 6a and 6b has been successfully replaced by the phenyl ring of the arm of the phosphine, yielding complexes [RuCl2­(κP-η6-phenyl)-P*] (5a and 5b). Unexpectedly, complex 6c has led to tethered [RuCl2­(κP-η6-phenyl)-P­(2-biphenylyl)­PhR] complex (9c), where the coordinated phenyl ring belongs to the 2-biphenylyl group. Reaction of P­(2-biphenylyl)­PhR (R = OMe, Me, i-Pr, and Fc (ferrocenyl)) with Dim2 has given complexes [RuCl2(η6-methyl benzoate)­(κP-P­(2-biphenylyl)­PhR)] (10c–13c). The replacement of methyl benzoate by the phenyl ring of the 2-biphenylyl of the phosphine can be performed at room temperature in the presence of light, giving tethered complexes [RuCl2(κP-η6-phenyl)-P­(2-biphenylyl)­PhR] (14c–17c). Phosphines bearing a substituted 2-biphenylyl group (PArPhR; d (Ar = 2-p-terphenylyl; R = OMe, Me, i-Pr), e (Ar = 2-m-terphenylyl; R = OMe, i-Pr), f (Ar = 2-(1′-naphthyl)­phenyl; R = OMe, i-Pr), and g (Ar = 2-(2′-naphthyl)­phenyl; R = OMe, i-Pr) have been prepared. Phosphines f are present as a mixture of atropisomers. The corresponding Ru complexes have been prepared and subjected to tethering. Tethered complexes with phosphines d are single species in solution, whereas the rest of the phosphines give diastereomeric mixtures with little stereoselection. All complexes have been evaluated in Ru-catalyzed hydrogen transfer of acetophenone in i-PrOH as a solvent obtaining full conversions and up to 47% ee using precursor [RuCl2(κP-η6-phenyl)-P*] (26f, P* = P­(2-(1′-naphthyl)­phenyl)­(OMe)­Ph.