CaSH Organocatalysis: Enantioselective
Friedel-Crafts Alkylation of Indoles with α,β-Unsaturated
Aldehydes

Tian Tian; Bao-Jian Pei; Qing-Hua Li; Hao He; Ling-Yan Chen; Xiang Zhou; Wing-Hong Chan; Albert W. M. Lee

doi:10.1055/s-0029-1217552

LETTER

CaSH Organocatalysis: Enantioselective Friedel-Crafts Alkylation of Indoles with α,β-Unsaturated Aldehydes

Tian Tian^a,b, Bao-Jian Pei^a, Qing-Hua Li^a, Hao He^a, Ling-Yan Chen^a, Xiang Zhou^b, Wing-Hong Chan^a, Albert W. M. Lee*^a
^a Department of Chemistry, Hong Kong Baptist University, Hong Kong, P. R. of China
Fax: +85234117438; e-Mail: alee@hkbu.edu.hk;
^b College of Chemistry and Molecular Sciences, Wuhan University, Hubei, P. R. of China

Abstract

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Abstract

Enantioselective Friedel-Crafts alkylation of indole with α,β-unsaturated aldehyde was catalyzed by camphor sulfonyl hydrazine (CaSH) with good enantioselectivity (81-88%).

Key words

camphor sulfonyl hydrazine (CaSH) - organocatalysis - indole alkylation - α,β-unsaturated aldehydes

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References

References and Notes

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14

Aldehyde 3 (R^¹ = Me): ^¹H NMR (400 MHz, CDCl₃): δ = 9.75 (s, 1 H), 7.63 (d, J = 8.0 Hz, 1 H), 7.31 (m, 1 H), 7.26 (m, 1 H), 7.14 (m, 1 H), 6.84 (s, 1 H), 3.75 (s, 3 H), 3.68 (m, 1 H), 2.87 (m, 1 H), 2.71 (m, 1 H), 1.43 (d, J = 6.8 Hz, 3 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 203.1, 137.4, 126.8, 125.4, 121.9, 119.3, 119.0, 109.6, 51.2, 32.9, 26.1, 21.9 ppm.
Alcohol 4 was obtained by NaBH₄ reduction. Alcohol 4 (R^¹ = Me): ^¹H NMR (400 MHz, CDCl₃): δ = 7.64 (dd, J = 8.0, 0.8 Hz, 1 H), 7.30 (dd, J = 7.2, 0.8 Hz, 1 H), 7.23 (m, 1 H), 7.10 (m, 1 H), 6.85 (s, 1 H), 3.75 (s, 3 H), 3.66 (m, 1 H), 3.22 (m, 1 H), 2.06 (m, 1 H), 1.96 (m, 1 H), 1.40 (d, J = 6.8 Hz, 3 H) ppm.

15

General Experimental Procedure for CaSH 1 Catalyzed Friedel-Crafts Reaction of Indoles with α,β-Unsaturated Aldehydes
TFA (0.15 mmol) was added to a solution of CaSH 1 (0.15 mmol) in toluene (1 mL). The solution was stirred for 20 min and then cooled to -40 ˚C. The α,β-unsaturated aldehyde (1.5 mmol) was then added. After stirring for another 20 min, the N-substituted indole (0.5 mmol) was added. The reaction was stirred until complete consumption of the indoles as determined by TLC. MeOH (2 mL) was added to the reaction mixture followed by NaBH₄ (3.0 mmol). The mixture was warmed to 0 ˚C and stirred for 20 min. The reaction was quenched by H₂O and extracted with EtOAc. The organic solution was dried over anhyd Na₂SO₄. The product 6 was purified by silica gel chromatography (PE-EtOAc, 4:1). The ee was determined by chiral HPLC (Chiracel AD-H) of the alcohol 6 (5% i-PrOH in hexane
as eluent, 1 mL min^-¹).

16

Spectroscopic Data of Products 6 (Table 2) Compound 6 (R^² = Me): ^¹H NMR (400 MHz, CDCl₃): δ = 7.74 (dd, J = 7.6, 0.8 Hz, 1 H), 7.33 (m, 4 H), 7.21 (m, 4 H), 6.96 (s, 1 H), 5.29 (s, 2 H), 3.30 (m, 2 H), 3.27 (m, 1 H), 2.08 (m, 1 H), 1.95 (m, 1H), 1.44 (d, J = 7.2 Hz, 3 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 138.0, 137.1, 129.0, 127.7, 127.6, 126.9, 124.6, 122.0, 121.1, 119.8, 119.1, 110.0, 61.7, 50.1, 40.6, 27.9, 22.1 ppm. HRMS (MALDI-TOF): m/z calcd for C₁₉H₂₂NO [M + H]⁺: 280.1696; found: 280.1695.
Compound 6 (R^² = Et): ^¹H NMR (400 MHz, CDCl₃): δ = 7.64 (d, J = 7.6, Hz, 1 H), 7.25 (m, 4 H), 7.14 (m, 1 H), 7.06 (m, 3 H) 6.88 (s, 1 H), 5.24 (s, 2 H), 3.56 (m, 2 H), 2.93 (m, 1 H), 1.98 (m, 2 H), 1.74 (m, 2H), 0.83 (t, J = 7.0 Hz, 3 H) ppm.
Compound 6 (R^² = Pr): ^¹H NMR (400 MHz, CDCl₃): δ = 7.69 (d, J = 8.0 Hz, 1 H), 7.33-7.24 (m, 4 H), 7.18 (m, 1 H), 7.17-7.06 (m, 3 H) 6.93 (s, 1 H), 5.29 (s, 2 H), 3.62 (m, 2 H), 3.05 (m, 1 H), 2.03 (m, 2 H), 1.18 (m, 2 H), 1.30 (m, 2 H), 0.88 (t, J = 7.2 Hz, 3 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 138.0, 137.1, 128.9, 127.8, 127.7, 126.7, 125.6, 121.8, 119.9, 119.0, 118.9, 110.0, 61.9, 50.0, 39.0, 38.9, 33.6, 21.0, 14.4 ppm.
Compound 6 (R^² = Bu): ^¹H NMR (400 MHz, CDCl₃): δ = 7.66 (d, J = 7.6 Hz, 1 H), 7.30-7.22 (m, 4 H), 7.14 (t, J = 7.6 Hz, 1 H), 7.09-7.03 (m, 3 H), 6.90 (s, 1 H), 5.27 (s, 2 H), 3.59 (m, 2 H), 3.01 (m, 1 H), 2.00 (m, 2 H), 1.78 (m, 2 H), 1.23 (m, 4 H), 0.83 (t, J = 7.2 Hz, 3 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 138.0, 137.1, 128.9, 127.8, 127.7, 126.7, 125.6, 121.8, 119.9, 119.0, 110.0, 61.9, 50.0, 39.0, 36.3, 33.8, 30.2, 23.0, 14.3 ppm.
Compound 6 (R^² = Ph): ^¹H NMR (400 MHz, CDCl₃): δ = 7.52 (m, 1 H), 7.38-7.07 (m, 12 H), 7.03 (m, 2 H), 5.30 (s, 2 H), 4.44 (t, J = 7.6 Hz, 1 H), 3.68 (m, 2 H), 2.48 (m, 1 H), 2.29 (m, 1 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 145.1, 138.0, 237.2, 129.0, 128.7, 128.1, 127.9, 127.8, 126.9, 126.4, 125.6, 122.1, 120.0, 119.3, 119.1, 109.9, 61.5, 50.2, 39.4, 39.0 ppm.
Compound 6 (R^² = 4-ClC₆H₄): ^¹H NMR (400 MHz, CDCl₃): δ = 7.41 (d, J = 8.0 Hz, 1 H), 7.33-7.22 (m, 8 H), 7.15 (m, 3 H), 7.01 (m, 2 H), 5.29 (s, 2 H), 4.40 (t, J = 7.6 Hz, 1 H), 3.64 (m, 2 H), 2.43 (m, 1 H), 2.22 (m, 1 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 143.6, 137.8, 137.2, 131.9, 129.4, 129.0, 128.7, 127.8, 127.6, 126.8, 125.5, 122.2, 119.8, 119.4, 118.5, 110.0, 61.2, 50.2, 38.8, 38.7 ppm.
Compound 6 (R^² = 4-BrC₆H₄): ^¹H NMR (400 MHz, CDCl₃): δ = 7.40 (m, 3 H), 7.37-7.18 (m, 6 H), 7.15-7.07 (m, 3 H), 6.99 (m, 2 H), 5.28 (s, 2 H), 4.38 (t, J = 8.0 Hz, 1 H), 3.62 (m, 2 H), 2.43 (m, 1 H), 2.22 (m, 1 H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 144.1, 137.8, 137.2, 131.7, 129.8, 129.0, 127.8, 127.6, 126.8, 125.5, 122.2, 120.0, 119.8, 119.4, 118.4, 110.0, 61.2, 50.2, 38.8, 38.7 ppm.

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