Synlett 2016; 27(04): 591-594
DOI: 10.1055/s-0035-1560771
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© Georg Thieme Verlag Stuttgart · New York

An Approach to Highly Hindered BINOL Phosphates

Mattia R. Monaco
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, 45470, Germany   eMail: list@kofo.mpg.de
,
Roberta Properzi
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, 45470, Germany   eMail: list@kofo.mpg.de
,
Benjamin List*
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, 45470, Germany   eMail: list@kofo.mpg.de
› Institutsangaben
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Publikationsverlauf

Received: 02. September 2015

Accepted after revision: 30. September 2015

Publikationsdatum:
20. Oktober 2015 (online)


Abstract

The synthesis of 3-3′-substituted BINOL-derived chiral phosphoric acid catalysts is still largely limited by the limitations of current cross-coupling methodologies. For this reason, despite the importance of sterically demanding catalysts in Brønsted acid catalysis, highly hindered congeners are still unprecedented. Exploiting the aryne addition reaction as key step, we report herein the development of a novel synthetic route to access this unexplored class of catalysts.

Supporting Information

 
  • References and Notes

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  • 22 Key Experimental Procedures (±)-2,2-Diiodo-3,3-bis(2,4,6-tri-tert-butylphenyl)-1,1-binaphthalene (7) A flame-dried, two-neck round-bottom flask was charged with a solution of 2,4,6-tri(tert-butyl)bromobenzene (2 g, 6.15 mmol, 5 equiv) in dry THF (10.5 mL) under an argon atmosphere. The solution was cooled down to –78 °C and BuLi (2.5 M in hexane, 2.45 mmol, 5.1 equiv) was added dropwise. The solution was then stirred at 0 °C for 1 h. Next the reaction was cooled at –78 °C, and a solution of 5 in THF (814 mg, 1.23 mmol, 1 equiv) was added dropwise. The temperature was raised to r.t., and the reaction was vigorously stirred for 12 h. CH2Cl2 was added, and the organic phase washed three times with H2O. After anhydrification over Na2SO4, the solvent was evaporated in vacuo. Purification by column chromatography on silica gel (eluent: hexane–CH2Cl2, 10:1) gave the desired compound in 39% yield. 1H NMR (500 MHz, CD2Cl2): δ = 7.99 (s, 2 H), 7.83 (d, 2 H), 7.52–7.40 (m, 6 H), 7.18 (t, 2 H), 7.05 (d, 2 H), 1.29 (s, 18 H), 1.14 (s, 36 H). 13C NMR (125 MHz, CD2Cl2): δ = 149.5, 147.8, 147.5, 146.1, 145.8, 140.2, 132.6, 132.3, 131.8, 128.7, 127.2, 127.1, 127.0, 124.1, 123.9, 114.5, 38.9, 38.8, 35.2, 35.0, 34.1, 31.6. HRMS: m/z calcd for C56H68I2 [M]: 994.3410; found: 994.3405. (±)-3,3-Bis(2,4,6-tri-tert-butylphenyl)-[1,1-binaphthalene]-2,2-diol (8) In a flame-dried, two-neck round-bottom flask, a 0.007 M solution of 7 in dry Et2O was added under an argon atmosphere. The stirred solution was cooled down to –78 °C, BuLi (2.5 M in hexanes, 4 equiv) was added dropwise, and the reaction was left at –78 °C for 1 h before being cooled down to –95 °C. Next a 2.8 M solution of nitrobenzene in Et2O was added dropwise. After 30 min MeOH was added (1:1 with the reaction solvent), and the temperature was raised to r.t. and stirred for 2 additional hours. CH2Cl2 was added, and the organic phase was washed with H2O. After anhydrification over Na2SO4, the solvent was evaporated in vacuo. Purification by column chromatography on silica gel (eluent: mixtures hexane–CH2Cl2) gave the desired compound in 43% yield. 1H NMR (500 MHz, CD2Cl2): δ = 7.82–7.75 (m, 4 H), 7.58–7.48 (m, 4 H), 7.29 (t, 2 H), 7.20 (t, 2 H), 7.10 (d, 2 H), 4.89 (s, 2 H), 1.29 (s, 18 H), 1.14 (s, 18 H), 1.05 (s, 18 H). 13C NMR (125 MHz, CDCl3): δ = 152.8, 149.5, 149.5, 134.1, 133.5, 132.4, 130.4, 128.3, 127.8, 126.6, 124.7, 123.7, 123.3, 123.3, 113.1, 38.1, 38.0, 35.1, 33.1, 33.0, 31.4. HRMS: m/z calcd for C56H70O2 [M + Na]: 797.5268; found: 797.5269. 3,3-Bis(2,4,6-tri-tert-butylphenyl)-1,1-binaphthyl-2,2-diyl Hydrogenphosphate (2) In a flame-dried, two-neck round-bottom flask, equipped with a reflux condenser, a 0.025 M solution of 8 in dry pyridine was added under Ar atmosphere. Then the stirred solution was cooled down to 0 °C, and 10 equiv of POCl3 were added. The reaction mixture was then heated up to 95 °C, and 10 additional equivalents of POCl3 were added after 24 h. After 4 d full consumption of starting material was observed (TLC eluent: hexane–CH2Cl2, 1:1). Then the reaction was cooled to 0 °C, and H2O (2.5 mL) were added dropwise [careful: exothermic reaction] before raising the temperature to 100 °C. After 4 h the reaction was cooled down to r.t., CH2Cl2 was added, and the organic phase was sequentially washed with a 3 M HCl (aq) solution, H2O, and brine. Then the organic phase was dried over anhydrous Na2SO4, and the solvent was evaporated in vacuo. Purification was accomplished by column chromatography (eluent: mixtures hexane–EtOAc). The isolated compound was subjected to preparative HPLC on chiral stationary phase [Chiralpak QN-AX, 5 μm, 150 × 29 mm; eluent: MeOH–NH4OAc (0.1 M, aq), 80:20] to achieve separation of the enantiomers. Both enantiomers were eventually dissolved in CH2Cl2 and subjected to an acidic wash with 6 M HCl (aq) solution (2: 36% yield; ent-2: 36% yield). 1H NMR (500 MHz, CD2Cl2): δ = 7.83 (s, 2 H), 7.74 (d, J = 8.2 Hz, 2 H), 7.40–7.33 (m, 6 H), 7.09 (t, J = 8.2 Hz, 2 H), 6.87 (d, J = 8.2 Hz, 2 H), 6.32 (br s, 1 H), 1.21 (s, 18 H), 0.97 (s, 18 H), 0.88 (s, 18 H). 13C NMR (125 MHz, CD2Cl2) [overlapping signals]: δ = 149.3, 149.0, 148.7, 148.5, 148.4, 135.7, 135.7, 135.3, 133.2, 130.4, 130.3, 128.4, 127.6, 126.4, 125.9, 124.3, 123.5, 121.5, 38.9, 38.5, 35.1, 34.3, 33.6, 31.6. 31P NMR (202 MHz, CD2Cl): δ = –0.02 (s). HRMS: m/z calcd for C56H69O4P [M – H]: 835.4861; found: 835.4861.