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

Abstract

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 words

Diels–Alder reaction - phosphoric acid - Brønsted acid - ­Lewis acid - supramolecular catalyst - chiral cavity - molecular recognition

• References and Notes

• 1 Lehn JM. Science 1985; 227: 849

For recent reviews on supramolecular catalysis, see:
• 2a Ma L, Abney C, Lin W. Chem. Soc. Rev. 2009; 38: 1248
• 2b Corma A, García H, Llabrés i Xamena FX. Chem. Rev. 2010; 110: 4606
• 2c Meeuwissen J, Reek JN. H. Nature Chem. 2010; 2: 615
• 2d Raynal M, Ballester P, Vidal-Ferran A, van Leeuwena PW. N. M. Chem. Soc. Rev. 2014; 43: 1660
• 2e Raynal M, Ballester P, Vidal-Ferran A, van Leeuwena PW. N. M. Chem. Soc. Rev. 2014; 43: 1734
• 2f Brown CJ, Toste FD, Bergman RG, Raymond KN. Chem. Rev. 2015; 115: 3012
• 3a Hatano M, Mizuno T, Izumiseki A, Usami R, Asai T, Akakura M, Ishihara K. Angew. Chem. Int. Ed. 2011; 50: 12189
• 3b Hatano M, Ishihara K. Chem. Commun. 2012; 48: 4273

For reviews on the Diels–Alder reaction, see:
• 4a Kagan HB, Riant O. Chem. Rev. 1992; 92: 1007
• 4b Du H, Ding K In Handbook of Cyclization Reactions . Ma S. Wiley-VCH; Stuttgart: 2010: 1-57
• 4c Ishihara K, Sakakura A In Science of Synthesis, Stereoselective Synthesis . Vol. 3. Evans PA. Thieme; Stuttgart: 2011: 67-123

In general, endo/exo-selectivity in the Diels–Alder reaction depends on the substrates, see:
• 5a Hoffmann R, Woodward RB. J. Am. Chem. Soc. 1965; 87: 4388
• 5b García JI, Mayoral JA, Salvatella L. Acc. Chem. Res. 2000; 33: 658
• 5c Barba C, Carmona D, García JI, Lamata MP, Mayoral JA, Salvatella L, Viguri F. J. Org. Chem. 2006; 71: 9831
• 5d Wannere CS, Paul A, Herges R, Houk KN, Schaefer HF. III, von Ragué Schleyer P. J. Comput. Chem. 2007; 28: 344

Yamamoto developed the pioneering concept of combined acid catalysis. See reviews:
• 6a Ishibashi H, Ishihara K, Yamamoto H. Chem. Rec. 2002; 2: 177
• 6b Yamamoto H, Futatsugi K. Angew. Chem. Int. Ed. 2005; 44: 1924
• 6c Yamamoto H, Futatsugi K In Acid Catalysis in Modern Organic Synthesis . Vol. 1. Yamamoto H, Ishihara K. Wiley-VCH; Weinheim: 2008: 1-34

For reviews on chiral phosphoric acids, see:
• 7a Akiyama T. Chem. Rev. 2007; 107: 5744
• 7b Terada M. Synthesis 2010; 1929
• 7c Parmar D, Sugiono E, Raja S, Rueping M. Chem. Rev. 2014; 114: 9047

Reviews on acid–base combination chemistry, see:
• 8a Kanai M, Kato N, Ichikawa E, Shibasaki M. Synlett 2005; 1491
• 8b Ishihara K, Sakakura A, Hatano M. Synlett 2007; 686
• 8c Ishihara K. Proc. Jpn. Acad. Ser. B 2009; 85: 290
• 8d Shibasaki M, Kanai M, Matsunaga S, Kumagai N. Acc. Chem. Res. 2009; 42: 1117
• 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.

For anomalous exo-selective Diels–Alder reactions of α-nonsubstituted acroleins, see:
• 14a Maruoka K, Imoto H, Yamamoto H. J. Am. Chem. Soc. 1994; 116: 12115
• 14b Kündig EP, Saudan CM, Alezra V, Viton F, Bernardinelli G. Angew. Chem. Int. Ed. 2001; 40: 4481
• 14c Kano T, Tanaka Y, Maruoka K. Org. Lett. 2006; 8: 2687
• 14d Hayashi Y, Samanta S, Gotoh H, Ishikawa H. Angew. Chem. Int. Ed. 2008; 47: 6634. Also see ref 3
• 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 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