Efficient One-Pot Synthesis of Mono- and Bis[di(2-pyridyl)phosphine Oxides] from Tris(2-pyridyl)phosphine

Abstract

An efficient one-pot access to di(2-pyridyl)phosphine oxides Py₂P(R)=O and bis[di(2-pyridyl)phosphine oxides] Py₂P(O)–Z–P(O)Py₂ has been developed based on the reaction of available tris(2-pyridyl)phosphine with various organic halides, followed by treatment of the resulting phosphonium salts with alkali in situ. The isolated yields of the phosphine oxides were in the range 40–96%.

Key words

dipyridylphosphine oxides - bisphosphine oxides - phosphonium salts - ligands - halo compounds - chemoselectivity

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• 13 Phosphine Oxides 3a–f; General Procedure A mixture of tris(2-pyridyl)phosphine (1; 133 mg, 0.5 mmol) and the appropriate organic halide (5.0 mmol) was stirred at 24–140 °C for 0.5–1.5 h (see Table 1). When the reaction was complete (³¹P NMR), the mixture was cooled to 20–25 °C and powdered KОН·0.5 H₂O (36 mg, 0.55 mmol) was added. The mixture was stirred for another 30 min at this temperature until the ³¹P NMR resonance from the phosphonium salts 2a–g disappeared. CHCl₃ (5 mL) was then added to the resulting mixture, the undissolved residue was removed by filtration, and the filtrate was concentrated. The residue was mixed with anhyd Et₂O (3 mL) and the mixture was filtered. The volatiles were evaporated from the filtrate and the residue was dried in vacuo (1 Torr). Methylbis(2-pyridyl)phosphine Oxide (3a) Yellow-brown oil; yield: 105 mg (96%). FT-IR (film): 3048, 2992, 2918, 2854, 1661, 1575, 1455, 1426, 1292, 1197, 1133, 1084, 1045, 990, 885, 761, 706, 618, 513 cm^–1. ¹H NMR (400.13 MHz, CDCl₃): δ = 2.20 (d, ² J _PH = 13.6 Hz, 3 H, Me), 7.34–7.38 (m, 2 H, H-3 in Py), 7.76–7.81 (m, 2 H, H-4 in Py), 8.04–8.07 (m, 2 H, H-5 in Py), 8.77 (d, ³ J _6–5 = 4.5 Hz, 2 H, H-6 in Py). ¹³C NMR (100.62 MHz, CDCl₃): δ = 13.57 (d, ² J _PC = 76.6 Hz, Me), 125.42 (d, ⁴ J _PC = 1.9 Hz C-5 in Py), 127.12 (d, ² J _PC = 19.8 Hz, C-3 in Py), 136.11 (d, ³ J _PC = 8.5 Hz, C-4 in Py), 150.33 (d, ³ J _PC = 19.1 Hz, C-6 in Py), 156.55 (d, ¹ J _PC = 127.6 Hz, C-2 in Py). ³¹P NMR (161.98 MHz, CDCl₃): δ = 32.28; Anal. Calcd for C₁₁H₁₁N₂OP: С, 60.55; Н, 5.08; N, 12.84. Found: С, 60.47; Н, 5.16; N, 12.65.
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• 16 Details of this X-ray analysis are provided in the Supporting Information. CCDC 1477937 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.