Allylation of Aldehydes, Aldimines and Ring Opening of Terminal Aromatic Epoxides by Scandium Triflate using Polyethylene Glycol (PEG) as an ­Efficient Recyclable Medium

Abstract

Scandium triflate catalyzed allylation of a variety of aldehydes and aldimines using polyethylene glycol (PEG) as a recyclable reaction medium proceeds smoothly and affords corresponding homoallylic alcohols and amines in good yields. ­Furthermore this catalyst catalyzes total regioselective ring opening of terminal 1,2-epoxides to bishomoallylic alcohols in good yields. The PEG-scandium triflate catalyst system can be reused for a ­number of cycles without significant loss of activity.

Key words

allylation - aldehydes - aldimines - ring opening of ­epoxides - scandium triflate - polyethylene glycol

References

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10

Typical Procedure for the Catalytic Allylation of Aldehydes: A mixture of p-nitrobenzaldehyde (1 mmol), allyltributylstannane (1.2 mmol), benzoic acid (1 mmol), PEG (2 g) and scandium triflate (5 mol%) was placed in a 25 mL round bottomed flask. The mixture was stirred at r.t. for 3 h (monitored by TLC). The reaction mixture was extracted with dry Et₂O and the crude product was purified by column chromatography to obtain homoallylic alcohol in excellent yields. The products were characterized by ¹H NMR data.
¹H NMR (200 MHz, CDCl₃): δ = 2.01 (d, 1 H, OH), 2.52 (m, 2 H, CH₂), 4.80 (t, 1 H, CH), 5.14-5.20 (m, 2 H, 2 × vinyl), 5.82 (m, 1 H, vinyl), 7.50 (d, 2 H, aromatic), 8.20 (d, 2 H, aromatic).

12

Typical Procedure for the One-Pot Catalytic Allylation of Aldimines: A mixture of p-nitrobenzaldehyde (1 mmol), p-nitroaniline (1 mmol) allyltributylstannane (1.2 mmol), benzoic acid (1 mmol), PEG (2 g) and scandium triflate 20 mol% was placed in a 25 mL round bottomed flask. The mixture was stirred at r.t. for 4 h (monitored by TLC). The reaction mixture was extracted with Et₂O and the crude product was purified by column chromatography to obtain homoallylic amine in good yields. The products were characterized by ¹H NMR data.
¹H NMR (200 MHz, CDCl₃): δ = 2.52 (m, 2 H, CH₂), 4.60 (t, 1 H, CH), 5.20 (m, 2 H, 2 × vinyl), 5.85 (m, 1 H, vinyl), 7.55 (d, 4 H, aromatic), 8.25 (d, 4 H, aromatic).

17

Typical Procedure for Regioselective Catalytic Allylation of Terminal Aromatic Epoxides: A mixture of styrene epoxide (1 mmol), allytributylstannane (1.2 mmol), and scandium triflate 5 mol%, PEG (2 g) was placed in a 25 mL round bottomed flask. The mixture was stirred at 55-60 °C for 24 h. The reaction mixture was cooled and was extracted by Et₂O and the crude product was purified by column chromatography to obtain bishomoallylic alcohol in good yields. The products were characterized by ¹H NMR data.
¹H NMR (200 MHz, CDCl₃): δ = 2.05-2.35 (m, 2 H), 2.70 (m, 2 H), 3.75 (m, 1 H), 5.01-5.10 (m, 2 H), 5.80 (m, 1 H), 7.12-7.30 (m, 5 H).

18

Extraction with dry Et₂O is essential for consistency in the activity of the reaction medium in the study of recycling.