Rhodium-Catalyzed, Phosphorus(III)-Directed Hydroarylation of Internal Alkynes: Facile and Efficient Access to New Phosphine Ligands

Abstract

Organophosphines are an important class of ligands widely used in organic chemistry. Although great progress has recently been made in the rapid construction of new phosphines through Rh- or Ru-catalyzed C–H bond functionalizations, synthetic access to more diverse phosphines remains a challenge. We describe an efficient process for the rhodium-catalyzed phosphorus(III)-directed hydroarylation of internal alkynes to generate various alkenylated and 2′,6′-dialkenylated biarylphosphines with high selectivity. A range of diverse alkynes and phosphines were effectively prepared with broad functional-group compatibility under the optimized conditions. In addition, the developed protocol can be extended to modify chiral phosphine ligands, providing enantioenriched alkenylated phosphines without erosion of the enantiomeric excess.

Key words

rhodium catalysis - hydroarylation - alkynes - phosphines - C–H activation - directing group

Publication History

Received: 14 October 2020

Accepted after revision: 18 November 2020

Accepted Manuscript online:
18 November 2020

Article published online:
08 January 2021

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• 12 (2′-Alkenylbiaryl-2-yl)(diphenyl)phosphines 3aa–da; General Procedure A In an oven-dried 25-mL Schlenk tube with a Teflon screw cap, the appropriate diarylphosphine 1 (0.20 mmol, 1.0 equiv), alkyne 2 (0.20 mmol, 1.0 equiv), [Rh(COD)Cl]₂ (4.9 mg, 0.01 mmol, 5 mol%), and NaOPiv (49.6 mg, 0.40 mmol, 2.0 equiv) were dissolved in CH₂Cl₂ (2.0 mL) under N₂. The mixture was stirred at 100 °C for 12 h until the starting material was completely consumed (TLC), then cooled to r.t. The solvent was removed and the residue was directly purified by column chromatography (silica gel). {2′-[(1E)-1-Ethylbut-1-en-1-yl]biphenyl-2-yl}(diphenyl)phosphine(3aa). Prepared according to General Procedure A, from biphenyl-2-yl(diphenyl)phosphine (1a; 67.6 mg, 0.20 mmol, 1.0 equiv), hex-3-yne (2a; 16.4 mg, 0.20 mmol, 1.0 equiv), [Rh(COD)Cl]₂ (2.5 mg, 0.01 mmol, 5 mol%), and PivONa (49.6 mg, 0.40 mmol, 2.0 equiv) in CH₂Cl₂ (2.0 mL) at 100 °C under N₂ for 12 h, and purified by column chromatography [silica gel, PE–CH₂Cl₂ (20:1)] as a colorless oil; yield: 58.8 mg (70%). ATR-FTIR: 3054, 1434, 1264, 734, 702, 449, 418 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.36–7.32 (m, 3 H), 7.32–7.26 (m, 6 H), 7.257.20 (m, 3 H), 7.19–7.08 (m, 4 H), 7.05–6.99 (m, 1 H), 6.79–6.72 (m, 1 H), 5.39 (t, J = 7.3 Hz, 1 H), 2.12–1.82 (m, 4 H), 0.90–0.81 (m, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 148.5 (d, J = 32.8 Hz), 143.0, 141.6, 134.6 (d, J = 2.3 Hz), 134.0, 133.8, 133.54, 133.47, 133.3, 133.1, 132.9, 131.1 (d, J = 5.0 Hz), 130.7 (d, J = 5.8 Hz), 129.7, 128.4 (d, J = 3.0 Hz), 128.3 (d, J = 7.1 Hz), 128.2, 128.1, 128.0, 127.3, 127.0, 125.2, 23.7, 21.3, 14.2, 13.5 (d, J = 2.0 Hz). ³¹P NMR (162 MHz, CDCl₃): δ = –15.0. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₀H₃₀P: 421.2080; found: 421.2082.
• 13 CCDC 2018128, 2018129, 2018130, and 20181301 contains the supplementary crystallographic data for compound 3ah, 3ai, 4af, and 4ag and respectively. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures
• 14 (2′-Alkenylbiaryl-2-yl)(diphenyl)phosphines 3ea–ja and (2′6′-Dialkenylbiaryl-2-yl)(diphenyl)phosphines 4ab–bd: General Procedure B In an oven-dried 25-mL Schlenk tube with a Teflon screw cap, the appropriate diarylphosphine 1 (0.20 mmol, 1.0 equiv), alkyne 2 (0.60 mmol, 3.0 equiv), [Rh(COD)Cl]₂ (4.9 mg, 0.01 mmol, 5 mol%), and K₂CO₃ (55.2 mg, 0.40 mmol, 2.0 equiv) were dissolved in toluene (2.0 mL) under N₂. The mixture was stirred at 120 °C for 12 h until the reaction was complete (TLC), then cooled at r.t. The solvent was removed, and the residue was directly purified by column chromatography (silica gel, PE–CH₂Cl₂). {2′,6′-Bis[(1E)-1-ethylbut-1-en-1-yl]biphenyl-2-yl}(diphenyl)phosphine (4aa) Prepared according to General Procedure B, from biphenyl-2-yl(diphenyl)phosphine (1a; 67.6 mg, 0.20 mmol, 1.0 equiv), hex-3-yne (2a; 49.3 mg, 0.6 mmol, 3.0 equiv), [Rh(COD)Cl]₂ (2.5 mg, 0.01 mmol, 5 mol%), and K₂CO₃ (55.3 mg, 0.40 mmol, 2.0 equiv) in toluene (2.0 mL) at 120 °C under N₂ for 12 h, and purified by column chromatography [silica gel, PE–CH₂Cl₂ (20:1)] as a yellow oil; yield: 82.7 mg (82%). ATR-FTIR: 3054, 1413, 1246, 896, 731, 703 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.49–7.42 (m, 1 H), 7.32–7.25 (m, 4 H), 7.26–7.20 (m, 6 H), 7.22–7.15 (m, 4 H), 7.13 (s, 1 H), 7.11 (s, 1 H), 5.17 (dd, J = 8.1, 6.2 Hz, 2 H), 1.90–1.79 (m, 2 H), 1.78–1.67 (m, 2 H), 1.63–1.53 (m, 4 H), 0.71 (t, J = 7.5 Hz, 6 H), 0.66 (t, J = 7.5 Hz, 6 H). ¹³C NMR (126 MHz, CDCl₃): δ = 148.0 (d, J = 35.4 Hz), 144.1 (d, J = 1.5 Hz), 142.2, 139.8 (d, J = 15.1 Hz), 137.2 (d, J = 5.5 Hz), 136.2 (d, J = 13.0 Hz), 135.9 (d, J = 3.0 Hz), 133.5 (d, J = 1.5 Hz), 133.4, 133.3, 132.2 (d, J = 7.0 Hz), 128.8, 127.93, 127.87 (d, J = 6.1 Hz), 127.8, 127.7, 126.7 (d, J = 4.0 Hz), 23.7, 21.1, 14.0, 13.4. ³¹P NMR (202 MHz, CDCl₃): δ = –15.0. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₆H₄₀P: 503.2862; found: 503.2862.