Synlett 2016; 27(04): 564-570
DOI: 10.1055/s-0035-1560369
© Georg Thieme Verlag Stuttgart · New York

Remote Tris(pentafluorophenyl)borane-Assisted Chiral Phosphoric Acid Catalysts for the Enantioselective Diels–Alder Reaction

Manabu Hatano
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 4648603, Japan   Email:
Hideyuki Ishihara
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 4648603, Japan   Email:
Yuta Goto
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 4648603, Japan   Email:
Kazuaki Ishihara*
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 4648603, Japan   Email:
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Further Information

Publication History

Received: 28 August 2015

Accepted after revision: 21 October 2015

Publication Date:
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.

Supporting Information

  • References and Notes

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  • 9 For 2B(C6F5)3–(R)-3a/3b/3c, the aging time (0.5–2 h) at r.t. in advance showed no significant differences in the Diels–Alder reaction.
  • 10 Lower enantioselectivities (ca. 60% ee) were observed when a solution of the catalyst 2B(C6F5)3–(R)-3b or 2B(C6F5)3–(R)-3c at r.t. was added to the solution of 4 and 5a at –78 °C.
  • 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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  • 15 We examined Me3Al, Et3Al, i-Bu2AlH (DIBAL-H), Me2AlNTf2, allyltrimethylsilane, pinacolborane, 9-borabicyclo[3.3.1]nonane (9-BBN), etc. However, the combined use of these achiral Lewis acid sources to 2B(C6F5)3–(R)-3c showed low reactivities (0–15% yields), and the sole exception was catecholborane. In this regard, the combined use of a stoichiometric amount of catecholborane with chiral phosphoric acid catalyst in the enantio­selective reduction of ketones was reported by Antilla. See: Zhang Z, Jain P, Antilla JC. Angew. Chem. Int. Ed. 2011; 50: 10961
  • 16 We examined the reactions of acroleins 5ae 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