A Sterically Modified (Salen)Chromium(III) Complex - An Efficient Catalyst for High-Pressure Asymmetric Allylation of Aldehydes

Abstract

A novel (salen)chromium(III) catalyst with a modified salen ligand was synthesised in a simple way starting from readily available precursors. High-pressure allylation reaction of aromatic and aliphatic aldehydes with allyltributyltin upon application of 1 mol% of a modified (salen)chromium(III) complex provides ­homoallylic alcohols in good yield and with high enantioselectivity (up to 92%). The catalyst reveals higher enantioselectivity than the classic Jacobsen’s complex.

Key words

aldehydes - allylstannane - asymmetric catalysis - high-pressure technique - homoallylic alcohols - salen(chromium) complexes

References

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24

Analytical data for the modified salen ligand (1R,2R)-7: mp 89-93 °C; [α]_D ²⁹ -329.5 (c 1.0, CHCl₃); IR (KBr): 2963, 2875, 1628, 1597, 1445, 1263, 699 cm^-1; ¹H NMR (200 MHz, CDCl₃): δ = 13.13 (s, OH, 2 H), 8.00 (s, CHN, 2 H), 7.45 (d, J = 1.8 Hz, 2 H), 7.25-7.09 (m, 10 H), 6.92 (d, J = 1.8 Hz, 2 H), 3.13-2.99 (m, 2 H), 2.50-2.25 (m, 4 H), 2.12-1.94 (m, 4 H), 1.86-1.69 (m, 4 H), 1.66-1.44 (m, 2 H), 1.30 (s, 18 H), 0.60 (t, J = 7.2 Hz, 6 H), 0.53 (t, J = 7.2 Hz, 6 H); ¹³C NMR (50 MHz, CDCl₃): δ = 165.5 (2 × CHN), 157.5 (2 × C), 148.5 (2 × C), 139.2 (2 × C), 133.1 (2 × C), 129.2 (2 × CH), 127.2 (4 × CH), 127.0 (4 × CH), 125.8 (2 × CH), 124.7 (2 × CH), 117.5 (2 × C), 72.3 (2 × CH), 49.0 (2 × C), 34.0 (2 × C), 32.9 (2 × CH₂), 31.5 (6 × CH₃), 28.0 (2 × CH₂), 27.1 (2 × CH₂), 24.3 (2 × CH₂), 8.7 (4 × CH₃); Anal. Calcd for C₅₀H₆₆N₂O₂: C, 82.60; H, 9.15; N, 3.85. Found: C, 82.55; H, 9.23; N, 3.83; HRMS: [M + Na]⁺ calcd for C₅₀H₆₆N₂O₂Na: 749.5022, found: 749.5021.

25

Analytical data for the complex (1R,2R)-8: [α]_D ²⁹ -1420
(c 0.01, CHCl₃); IR (KBr): 3429, 2961, 2873, 1622, 1533, 1437, 1258, 700, 546 cm^-1; HRMS: [M - Cl]⁺ calcd for C₅₀H₆₄N₂O₂Cr: 776.4373, found: 776.4392.

26

General Procedure for High-Pressure Allylation: In a 2-mL Teflon ampoule were placed catalyst 1g (8.7 mg, 1 mol%), CH₂Cl₂ (ca. 1 mL), followed by aldehyde (1 mmol) and allyltributyltin (1.1-1.2 equiv). Finally, the ampoule was filled with CH₂Cl₂, closed and placed in a high-pressure vessel, and the pressure was slowly increased to 10 kbar at 20 °C. After the pressure had stabilized, the reaction mixture was kept under these conditions for 24 h. After decompression, the reaction mixture was diluted with wet Et₂O and dried over MgSO₄. After evaporation of solvents, the residue was chromatographed on a silica gel column (hexane-EtOAc).

27

The enantioselectivity of homoallylic alcohols 3a-f was determined by GC employing a capillary chiral β-dex 120 column. Alcohol 3f was analyzed directly, 3a, 3c and 3d as their O-trimethylsilyl derivatives, 3b as an acetate and 3e as a trifluoroacetate.