Phosphorodiamidic Acid as a Novel Structural Motif of Brønsted Acid Catalysts for Direct Mannich Reaction of N-Acyl Imines with 1,3-Dicarbonyl Compounds

Abstract

Phosphorodiamidic acid 1 was developed as an efficient Brønsted acid catalyst for the direct Mannich reaction of N-acyl imines with 1,3-dicarbonyl compounds. Phosphorodiamidic acids were proposed as a novel structural motif of enantioselective ­Brønsted acid catalysts because they possess unique features, including the capability of introducing various substituents to the nitrogen atoms and the preparation from readily available chiral diamines in a short step. We demonstrated that chiral phosphorodiamidic acid 1b derived from binaphthalene bis(sulfonamide) functioned as an enantioselective catalyst to give the Mannich product in an optically active form.

organocatalyst - Mannich reaction - green chemistry - asymmetric catalysis - imines

References and Notes

1

Present address: Sagami Chemical Research Center, Ayase 252-1193, Japan.

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11

Preparation of Phosphorodiamidic Acid ( 1a).
N,N′-Ditosylbenzene-1,2-diamine (208.3 mg, 0.5 mmol), prepared according to the literature procedure, [23] was dissolved into pyridine (1 mL) under nitrogen atmosphere. To the resulting solution was added phosphorus oxychloride (115.0 mg, 0.75 mmol) at r.t. After being stirred for 12 h at ambient temperature, H₂O (1 mL) was poured into the reaction mixture. The resulting suspension was stirred for additional 30 min. Then, EtOAc was added and all pyridine was removed by reverse extraction with 1 N HCl. The organic phase was dried over Na₂SO₄. After being concentrated, the residue was purified by column chromatography. Compound 1a was isolated as a white solid in 90% yield. ¹H NMR (270 MHz, DMSO-d ₆,): δ = 2.30 (6 H, s), 6.79-6.86 (2 H, m), 7.17-7.24 (2 H, m), 7.31 (4 H, d, J = 8.4 Hz), 8.02 (4 H, d, J = 8.4 Hz). ¹³C NMR (67.8 MHz, DMSO-d ₆): δ = 21.0, 113.1 (d, J _P-C = 5.4 Hz), 122.9, 126.8 (t, J _P-C = 10.3 Hz), 127.9, 129.5, 135.7, 144.2. IR (KBr): 3425, 3072, 1375, 1175, 1119 cm^-1. HRMS (ESI): m/z calcd for C₂₀H₁₉N₂O₆PS₂ [M - H]^-: 477.0349. Found: 477.0351.

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12c

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16

The distribution of keto-enol tautomers of 3 in CDCl₃ was measured by ¹H NMR. The percentages of the enol form are listed as follows: 3a: 72%; 3b: 61%; 3c: >98%; 3d: 6%; 3e: 6%; 3f: 21%; 3g: <2%.

17

Although in the case of unsymmetrical 1,3-dicarbonyl compounds, keto esters 3d,e and keto amide 3f, the diastereomeric mixtures were obtained, their ratios were changed during the course of experiment.

For binaphthalene bis(sulfonamide) derivatives as a chiral ligand for metal-based catalysts, see:

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20

Compound (R)-1b: [α]_D 150 (c 0.99, CHCl₃). ¹H NMR (270 MHz, DMSO-d ₆): δ = 3.33 (6 H, s), 6.23 (4 H, d, J = 8.1 Hz), 6.74 (2 H, d, J = 7.6 Hz), 7.01 (4 H, d, J = 8.1 Hz), 7.10 (2 H, t, J = 7.6 Hz), 7.37 (2 H, t, J = 7.6 Hz), 7.43 (2 H, d, J = 7.6 Hz), 7.73 (2 H, d, J = 7.6 Hz), 7.75 (2 H, d, J = 7.6 Hz). ¹³C NMR (67.8 MHz, DMSO-d ₆): δ = 20.9, 125.3, 125.4, 125.8, 127.1, 127.4 (d, J _P-C = 1.0 Hz), 127.8, 128.1 (t, J _P-C = 1.5 Hz), 130.8 (t, J _P-C = 1.5 Hz), 131.1 (d, J _P-C = 1.0 Hz), 131.5 (d, J _P-C = 1.0 Hz), 132.1, 134.5-134.6 (m), 137.6-137.8 (m), 140.6 (d, J _P-C = 4.9 Hz). IR (KBr): 3458, 3055, 2910, 1344, 1171, 1113 cm^-1. HRMS (ESI): m/z calcd for C₃₄H₂₇N₂O₆PS₂ [M - H]^-: 653.0975. Found 653.0975.

21

Compound 4e: white solid; R _f = 0.20 (hexane-EtOAc, 2:1). HPLC analysis Chiralpak AD-H (hexane-EtOH, 80:20, 1.0 mL/min, 254 nm, 10 °C) 19.1 (S), 27.3 (R) min. ¹H NMR (270 MHz, CDCl₃): δ = 2.12 (3 H, s), 2.32 (3 H, s), 4.41 (1 H, d, J = 4.9 Hz), 6.06 (1 H, dd, J = 9.2, 4.9 Hz), 7.22-7.54 (8 H, m), 7.77-7.86 (2 H, m), 7.97 (1 H, br d, J = 9.2 Hz). ¹³C NMR (67.8 MHz, CDCl₃): δ = 29.8, 31.6, 52.3, 70.0, 126.3, 127.1, 127.8, 128.6, 128.8, 131.8, 133.7, 139.2, 166.8, 202.6, 205.9. IR (KBr): 3369, 3032, 2918, 1724, 1639, 1522 cm^-1. HRMS (ESI): m/z calcd for C₁₉H₁₉NaNO₃ [M + Na]⁺: 332.1257. Found: 332.1260.

22

The absolute configuration of Bz-product 4e was determined after transformation into the stereochemically known N-benzoylphenylglycine methyl ester. The stereochemical determination of 4a and the experimental procedure for derivatization of 4 to N-protected glycine methyl ester were described in ref. 8a.

23 Dubey PK. Kulkarni SM. Kumar RV. Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem.  2002,  41:  1305