6a
Synthesis of the Ligand Precursor and the Complexes. The starting compound, 1,3-bis[(4S)-4-isopropyl-2-oxazolin-2-yl]benzene was prepared by reaction of isophthaloyl chloride (5.0 mmol), l-valinol (10.0 mmol), and Et3N (75 mmol) in CH2Cl2 (40 mL) for ca 1 h followed by oxazoline formation by addition of MsCl (11 mmol). The mixture was stirred for 5 h. After alkaline treatment and concentration, the desired ligand was obtained in 87% by silica gel chromatography with hexane-EtOAc. For an alternative route with isophthalonitrile, see ref. 6b. The complex 2 was easily prepared by C-H bond activation method. The mixture of RhCl3(H2O)3 (2.4 mmol) and the bis(oxazolinyl)benzene (2.0 mmol) in MeOH (40 mL) was heated at 50 °C for 6 h. The concentrated residue was purified by silica gel chromatography with EtOAc and hexane to give 2 in 64% yield; see ref.
[5a]
[d]
for spectroscopic data. The complex 1 was prepared by reaction of 2 (0.5 mmol) and silver acetate (2.0 mmol) in CH2Cl2 (28 mL) at r.t. The mixture was vigorously stirred for 18 h. The concentrated residue was purified by silica gel chromatography with EtOAc and MeOH to give 1 in 88% yield as yellowish orange solids, mp 235 °C (dec.). 1H NMR (300 MHz, CDCl3): δ = 0.75 (d, J = 7.2 Hz, 6 H), 0.97 (d, J = 7.2 Hz, 6 H), 1.71 (s, 6 H), 2.50 (m, 2 H), 4.38 (m, 2 H), 4.60-4.80 (m, 4 H), 5.08 (b, 2 H), 7.30 (t, J = 7.2 Hz, 1 H), 7.43 (d, J = 7.2 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ = 14.79, 19.13, 23.42, 29.50, 67.72, 71.27, 122.9, 127.5, 131.5, 171.3, 182.3 (d, J
Rh-C = 25.7 Hz). IR (disk): ν = 1615, 1395 cm-1. The structure of 1 was also determined by elemental analysis and X-ray analysis of a single crystal, which will be disclosed elsewhere.
