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Remote Tris(pentafluorophenyl)borane-Assisted Chiral Phosphoric Acid Catalysts for the Enantioselective Diels–Alder Reaction
Received: 28 August 2015
Accepted after revision: 21 October 2015
12 November 2015 (online)
Tris(pentafluorophenyl)borane-assisted chiral supramolecular phosphoric acid catalysts were developed for the model Diels–Alder reaction of α-substituted acroleins with cyclopentadiene. Two remotely coordinated tris(pentafluorophenyl)boranes should help to increase the Brønsted acidity of the active center in the supramolecular catalyst and create effective bulkiness for the chiral cavity. The prepared supramolecular catalysts acted as not only conjugated Brønsted acid–Brønsted base catalysts but also bifunctional Lewis acid–Brønsted base catalysts with the addition of a central achiral Lewis acid source such as catecholborane.
Key wordsDiels–Alder reaction - phosphoric acid - Brønsted acid - Lewis acid - supramolecular catalyst - chiral cavity - molecular recognition
References and Notes
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- 11 Compound 5a is too reactive to evaluate meaningful differences in the catalytic activity between 2B(C6F5)3–(R)-3c and free B(C6F5)3. However, the catalytic activity of 2B(C6F5)3–(R)-3c was much higher than that of free B(C6F5)3. See Scheme 3 and the SI.
- 12 To confirm whether or not the coordination of the P=O moiety to B(C6F5)3 would occur, we used (R)-3,3′-Ph2BINOL-derived phosphoric acid, which may avoid competitive coordinations. In 31P NMR (CD2Cl2) analysis at r.t., a singlet peak at δ = +1.7 ppm changed to δ = –1.0 ppm with a small upfield shift, which suggests the coordination of the P=O moiety to B(C6F5)3. Next, as with 2B(C6F5)3–(R)-3c, almost the same shifted peaks at δ = –137.0, –158.8, and –165.8 ppm were observed in 19F NMR (CD2Cl2) at r.t.
- 13 31P NMR (CD2Cl2) analysis of (R)-3b and (R)-3c at –78 °C showed a peak at δ = 6.1 ppm and 5.0 ppm, respectively, although solubility of them at –78 °C was low. See the SI.
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- 16 We examined the reactions of acroleins 5a–e with cyclopentadiene 4 with the use of a supramolecular catalyst, which was prepared from (R)-3c, B(C6F5)3, and catecholborane, However, better enantioselectivities were not observed compared with 2B(C6F5)3–(R)-3c as shown in Table 2 and Scheme 2. The results are summarized in the SI.
- 17 Typical Procedure for the Diels–Alder Reaction: To a mixture of (R)-3c (31.9 mg, 0.050 mmol) and powdered MS 4Å (200 mg) in a Schlenk tube under a nitrogen atmosphere, tris(pentafluorophenyl)borane (51.2 mg, 0.10 mmol) and freshly distilled CH2Cl2 (2 mL) were added via a cannula, and this suspension was stirred at r.t. for 1 h. Next, the mixture was cooled to –78 °C, and as soon as possible (within 5 min) after cooling to –78 °C, methacrolein 5a (95% purity, 43.4 μL, 0.50 mmol) and freshly distilled cyclopentadiene 4 (203 μL, 2.5 mmol) were added at –78 °C. After that, the resultant mixture was stirred at –78 °C for 1 h. To quench the reaction, Et3N (0.2 mL) was poured into the reaction mixture at –78 °C. The product mixture was warmed to r.t. and directly purified by silica gel column chromatography (eluent: pentane–Et2O, 9:1). Solvents were removed under 200 Torr at 20 °C by a rotary evaporator, and the product 6a was obtained (68.2 mg, >99% yield). 1H NMR (400 MHz, CDCl3): δ = 0.76 (d, J = 12.0 Hz, 1 H), 1.01 (s, 3 H), 1.39 (m, 2 H), 2.25 (dd, J = 12.0, 3.9 Hz, 1 H), 2.82 (br s, 1 H), 2.90 (br s, 1 H), 6.11 (dd, J = 6.0, 3.0 Hz, 1 H), 6.30 (dd, J = 6.0, 3.0 Hz, 1 H), 9.69 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ = 20.1, 34.6, 43.2, 47.6, 48.5, 53.9, 133.1, 139.6, 205.9. HRMS (EI): m/z [M]+ calcd for C9H12O: 136.0888; found: 136.0893. The endo/exo ratio of 6a was determined by NMR analysis. 1H NMR (CDCl3): δ = 9.40 [s, 1 H, CHO (endo-6a)], 9.69 [s, 1 H, CHO (exo-6a)]; see ref 3a. The enantioselectivity and absolute stereochemistry of 6a were determined by GC analysis according to the literature (see ref. 3a).
- 18 We just recently reported boron tribromide assisted chiral phosphoric acid catalyst for a highly enantioselective Diels–Alder reaction of 1,2-dihydropyridines. See: Hatano M, Goto Y, Izumiseki A, Akakura M, Ishihara K. J. Am. Chem. Soc. 2015; 137: 13472
For recent reviews on supramolecular catalysis, see:
For reviews on the Diels–Alder reaction, see:
In general, endo/exo-selectivity in the Diels–Alder reaction depends on the substrates, see:
Yamamoto developed the pioneering concept of combined acid catalysis. See reviews:
For reviews on chiral phosphoric acids, see:
Reviews on acid–base combination chemistry, see:
For anomalous exo-selective Diels–Alder reactions of α-nonsubstituted acroleins, see: