Work-Up-Free Deprotection of Borane Complexes of Phosphines, Phosphites, and Phosphinites with Polymer-Supported Amines

Abstract

Borane complexes of phosphorus compounds, a very common oxidation-free relay for catalytic ligands (phosphines, phosphites, and phosphinites), can be easily deprotected by treatment with polymer-supported piperazine or N-methylpiperazine. Deprotection conditions have been optimized for the different types of phosphorus compounds, and the resulting solutions can be used without any intermediate work-up or purification process.

deprotection - ligands - phosphorus - boron - supported reagents

References and Notes

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8

Anchoring N-Methylpiperazine onto a Merrifield Resin: The amine (1 mmol) was added to a mixture of (chloro-methyl)polystyrene (10 mmol) and Cs₂CO₃ (2.5 mmol) in DMF (10 mL). The mixture was heated for 24 h at 50 °C. After cooling the suspension was filtered and the resin was washed with DMF (4 × 10 mL), H₂O (4 × 10 mL), H₂O-MeOH (1:1; 2 × 10 mL), MeOH (4 × 10 mL), toluene (4 × 10 mL), and CH₂Cl₂ (4 × 10 mL). The solid was dried in vacuo for 24 h at 40 °C to constant weight.

9

Resin 2 (1% DVB, f _max = 2.15) from (chloromethyl)poly-styrene (1% DVB, f _o = 2.0-2.5 mmol/g). ¹³C gel-phase NMR (100 MHz, CDCl₃): δ = 40.6, 46.1, 52.9, 55.2, 62.8, 127.7. Anal. Found: N, 5.41; C, 84.82; H, 10.47; f = 1.93 mmol/g.

10 Vidal-Ferran A. Bampos N. Moyano A. Pericàs MA. Riera A. Sanders JKM. J. Org. Chem.  1998,  63:  6309 

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12

(S)-2-[2-(Diphenylphosphino)phenyl]-4-phenyl-4,5-dihydrooxazole Diborane Complex (4): White solid; yield: 98%; mp 95-96 °C; [α]_D ²⁷ +26.6 (c 1.00, CHCl₃). IR (ATR): 3064, 2966, 2864, 2385-2262, 1650, 1478, 1434, 1387, 1321, 1168, 1062, 928, 742, 695 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.95-1.98 (br s, 6 H), 3.86 (dd, J = 10.5, 9.3 Hz, 1 H), 4.17 (dd, J = 9.2, 6.7 Hz, 1 H), 4.58 (dd, J = 11.0, 6.7 Hz, 1 H), 7.18-7.21 (m, 2 H), 7.32-7.68 (m, 12 H), 7.70-7.76 (m, 2 H), 7.78-7.86 (m, 2 H), 7.94-7.99 (m, 1 H). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ = 26.9 (br m). ¹¹B NMR (128 MHz, CDCl₃): δ = -36.5 (br m, PBH₃), -19.42 (br s, NBH₃). HRMS (ESI+): m/z calcd for C₂₇H₂₈B₂NOPNa (M + Na): 458.1992; found: 458.2012.

13

Diphenyl[(1S,2R)-2-phenylcyclohexyloxy]phosphine-Borane Complex (5): [α]_D ²⁷ +87.9 (c 0.70, CH₂Cl₂). IR (ATR): 3057, 3027, 2931, 2856, 2378, 1739, 1600, 1492, 1437, 1361, 1114, 1063, 1014, 975, 737, 698 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.55-1.65 (br m, 3 H), 1.20-1.71 (m, 4 H), 1.68-1.83 (m, 2 H), 1.88-1.97 (m, 1 H), 2.13-2.25 (m, 1 H), 2.74 (ddd, J = 12.4, 10.5, 3.8 Hz, 1 H), 4.57-4.66 (m, 1 H), 6.78-6.83 (m, 2 H), 7.04-7.09 (m, 2 H), 7.14-7.28 (m, 6 H), 7.38-7.51 (m, 3 H), 7.64-7.70 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 24.8, 25.7, 34.3, 34.9, 52.0 (d, J = 6.4 Hz), 82.2 (d, J = 2.5 Hz), 126.5, 127.9, 128.0, 128.3, 128.4, 130.7, 130.8, 131.3, 131.4, 131.5 (d, J = 2.9 Hz), 133.6, 143.5. ³¹P{¹H} NMR (162 MHz, CDCl₃): δ = 105.5 (br m). ¹¹B NMR (128 MHz, CDCl₃): δ = -39.7 (m, BH₃). HRMS (ESI+): m/z calcd for C₂₄H₂₈BOPNa (M + Na): 397.1869; found: 397.1886.

14

(S)-2,2′-Bis(diphenylphosphinooxy)-1,1′-binaphthyl-Diborane Complex (6): White solid; mp 190-191 °C; [α]_D ²⁶ -2.5 (c 0.83, THF). IR (ATR): 3051, 2372, 2333, 1585, 1500, 1455, 1227, 1114, 975, 830, 730, 664 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.7-1.6 (br s, 3 H), 6.96-7.02 (m, 4 H), 7.05-7.15 (m, 6 H), 7.16-7.27 (m, 8 H), 7.30-7.41 (m, 4 H), 7.42-7.52 (m, 4 H), 7.55 (d, J = 8.9 Hz, 2 H), 7.81 (t, J = 9.6 Hz, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 120.3 (d, J = 4.0 Hz), 122.8 (d, J = 5.1 Hz), 125.2, 126.4, 127.0, 127.9, 128.2 (d, J = 10.5 Hz), 128.7 (d, J = 10.5 Hz), 129.6, 130.7, 131.0 (d, J = 4.4 Hz), 131.1 (d, J = 4.4 Hz), 131.7 (d, J = 2.0 Hz), 131.8 (d, J = 2.0 Hz), 131.9, 132.5, 133.7, 148.7 (d, J = 4.2 Hz). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ = 111 (br m). ¹¹B NMR (128 MHz, CDCl₃): δ = -39 (m, BH₃). HRMS (ESI+): m/z calcd for C₄₄H₃₈B₂O₂P₂Na (M + Na): 705.2459; found: 705.2431.

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16

P-B Bond Cleavage; General Procedure: A solution of the phosphorus-borane complex (0.025 mmol) in anhyd toluene (0.5 mL) was added to a smoothly stirred suspension of the resin (see Table [1] , f = 1.93-0.86 mmol/g) in anhyd toluene (1 mL) placed in a schlenk tube under an argon atmosphere at 60 °C. The mixture was heated at 60-63 °C to complete deprotection (see Table [1] ) and then the solution was filtered under an argon atmosphere and the resin was washed with toluene (2 × 1 mL). Removal of the solvent in vacuo provided the pure phosphorus compound without further purification.

17a Verdaguer X. Pericàs MA. Riera A. Maestro MA. Mahia J. Organometallics  2003,  22:  1868 

17b Verdaguer X. Lledo A. Lopez-Mosquera C. Maestro MA. Pericàs MA. Riera A. J. Org. Chem.  2004,  69:  8053