Highly Acidic BINOL-Derived Phosphoramidimidates and their Application in the Brønsted Acid Catalyzed Synthesis of α-Tocopherol

Dedicated to Professor Steven V. Ley on the occasion of his 70^th birthday

Abstract

The design and synthesis of highly acidic BINOL-derived N,N′-bistriflylphosphoramidimidate and N,N′-bisarylsulfonylphosphoramidimidate Brønsted acid catalysts are reported.

Key words

N,N′-bistriflylphosphoramidimidate - N,N′-bisarylsulfonylphosphoramidimidate - BINOL-derived - chiral Brønsted acid - C ₂-symmetric anions

• References and Notes

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• 17 General Procedure In a flame-dried flask under Ar the corresponding (S)- or (R)-BINOL derivative (1.0 equiv) was dissolved in freshly distilled pyridine (0.10 M), PCl₅ (2.0 equiv) was added, and the mixture was heated to 85 °C until full consumption of the starting material was observed (TLC). The reaction mixture was cooled to r.t. and CF₃SO₂NH₂ (12 equiv) and N,N-dimethylpyridin-4-amine (DMAP, 0.5 equiv) were added, and the mixture was heated to 85 °C until full consumption was observed (TLC). The reaction mixture was cooled to r.t., and concentrated under reduced pressure. HCl (1.0 M) was added, and the aqueous layer was extracted with CH₂Cl₂. The combined organic layers were washed with brine, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. Purification by column chromatography (silica gel), acidification in CH₂Cl₂ with HCl (6.0 M) followed by drying under reduced pressure with toluene afforded the phosphoramidimidates.
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• 21 Experimental Details A mixture of TMHQ (5 mg, 0.03 mmol, 1.0 equiv), isophytol (9 mg, 0.03 mmol, 1.0 equiv), and catalyst 8 (2 mg, 1.5 μmol, 0.05 equiv) in CH₂Cl₂ was stirred at 23 °C for 23 h. Et₃N (3 μL) was added, and the mixture was filtered through silica gel and concentrated under reduced pressure. Conversions were determined by ¹H NMR relative to TMHQ followed by purification on silica gel. Diastereomeric ratios (d.r.) were determined by HPLC with a chiral stationary phase (Daicel Chiracel Cellucoat RP column, 1 mL/min, 90:10 MeCN–H₂O, 220 nm, t _R = 4.00 min, t _R = 4.34 min).