Synlett 2016; 27(07): 1061-1067
DOI: 10.1055/s-0035-1561362
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© Georg Thieme Verlag Stuttgart · New York

Enantioselective Diels–Alder Reaction Induced by Chiral Supramolecular Lewis Acid Catalysts Based on CN···B and PO···B Coordination Bonds

Manabu Hatano
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan   Email: shihara@cc.nagoya-u.ac.jp
,
Kazushi Hayashi
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan   Email: shihara@cc.nagoya-u.ac.jp
,
Tatsuhiro Sakamoto
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan   Email: shihara@cc.nagoya-u.ac.jp
,
Yuma Makino
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan   Email: shihara@cc.nagoya-u.ac.jp
,
Kazuaki Ishihara*
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan   Email: shihara@cc.nagoya-u.ac.jp
› Author Affiliations
Further Information

Publication History

Received: 09 December 2015

Accepted: 12 January 2016

Publication Date:
05 February 2016 (eFirst)

Abstract

Chiral supramolecular boron Lewis acid catalysts were prepared from chiral 3-phosphoryl-1,1′-bi-2-naphthols, (2-cyanophenyl)boronic acids, and tris(pentafluorophenyl)borane, bound through CN···B and PO···B coordination bonds. In particular, the coordinated tris(pentafluorophenyl)boranes increase the Lewis acidity of the active center in the manner of a Lewis acid assisted Lewis acid catalyst system. A possible cavity in these catalysts was highly suitable for several Diels–Alder probe reactions of acroleins with cyclic or acyclic dienes, which gave the corresponding adducts in good to high yields and high enantio­selectivities.

Supporting Information

 
  • References and Notes

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  • 12 In the ESI-MS (positive mode) analysis of catalyst 2c, a peak for [2a + H2O + H]+ was observed. See the Supporting Information.
  • 13 When we compared the catalytic activity of 10, which has a phosphoryl moiety, to that of 11, which has a 3,5-[3,5-(F3C)2C6H3]2C6H3 moiety, 10 showed a higher yield. This tendency should be correlated to the reaction using 2c or 7a in the presence of a competing 5 mol% of B(C6F5)3 (see footnotes a in Figures 2 and 3).
  • 14 The position of the CN group influenced the enantioselectivity. The use of (3-cyanophenyl)boronic acid or (4-cyanophenyl)boronic acid in place of (2-cyanophenyl)boronic acid in catalyst 10 gave exo-(2S)-5a in ~80% yield with 9% and 11% ee, respectively.
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  • 19 The previous supramolecular catalyst 1 was calculated to be in a similar C 1-symmetric syn-conformation for two bulky tris(pentafluorophenyl)boranes.4
  • 20 Other possible transition states are discussed in the Supporting Information, which also gives the results for other supramolecular catalysts.
  • 21 Diels–Alder Reaction of Methacrolein (4a) with Cyclopenta-1,3-diene (3); Typical Procedure A solution of (R)-3-(5,5-dimethyl-2-oxido-1,3,2-dioxaphosphorinan-2-yl)-5,5′,6,6′,7,7′,8,8′-H8-BINOL (22.1 mg, 0.050 mmol) and [2-cyano-5-(trifluoromethyl)phenyl]boronic acid (10.7 mg, 0.050 mmol) in CH2Cl2 (1 mL), THF (0.3 mL), and H2O (9 μL, 0.5 mmol) was stirred at r.t. for 12 h in a Pyrex Schlenk tube under N2. Volatile compounds were removed under reduced pressure, and powdered 4 Å MS (250 mg, used as received from a commercial source) was added. The resulting white solid was heated at 100 °C (bath temperature) and <5 torr for 2 h. The resulting substance was cooled to r.t. under N2 and tris(pentafluorophenyl)borane (51.2 mg, 0.10 mmol) and freshly distilled CH2Cl2 (2 mL) were added under argon in a glove box. The pale-brown mixture was stirred at r.t. for 1 h, then cooled to –78 °C, and methacrolein 4a (95% purity, 43.4 μL, 0.50 mmol) was added. Freshly distilled cyclopenta-1,3-diene (3; 210 μL, 2.5 mmol) was then added over 15 min at –78 °C, and the mixture was stirred at –78 °C for 3 h. To quench the reaction, Et3N (0.5 mL) was poured into the reaction mixture at –78 °C. The product mixture was directly purified by column chromatography [silica gel, pentane–Et2O (100:1 to 8:1)]. Solvents were removed on a rotary evaporator at 15 °C and <200 torr to give 5a; yield: 60.8 mg (89%). 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 5a was determined by 1H NMR (CDCl3) analysis: δ = 9.40 [s, 1 H, CHO (endo-5a)], 9.69 [s, 1 H, CHO (exo-5a)] (see ref. 4a). The enantioselectivity and absolute stereochemistry of 5a were determined by GC analysis according to the reported method (see ref. 4a).