Heck Reaction on 1-Alkoxy-1,3-dienes in Ionic Liquids: A Superior Medium for the Regioselective Arylation of the Conjugated Dienic System

Abstract

α,β-Unsaturated acetals afford, in the presence of the LIC-KOR superbase and of a suitable electrophile, 1-functionalized-1-alkoxybuta-1,3-dienes. These substrates cross couple with aryl derivatives in the presence of a Pd catalyst (Heck conditions) in a regio- and stereoselective mode using different ionic liquids as a solvent.

Key words

Heck reaction - ionic liquid - unsaturated acetals - regio­selectivity - stereoselectivity

References and Notes

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22

For similar considerations about regioselectivity in palladium hydride elimination from the carbopalladate intermediate in the Heck arylation of acrolein diethyl acetal, see ref. 16a and 16b.

23

Typical Procedure.
A heavy-wall tube containing a stirring bar was dried, Ar-filled, and then charged with tetrabutylammonium bromide (TBAB, 1.0 g, 3.1 mmol) and Pd(OAc)₂ (0.0011 g, 0.005 mmol). The mixture was melted heating the vessel at 110 °C in an oil bath for 10 min, then cooled to r.t. Iodobenzene (0.20 g, 1.0 mmol), (E)-4-ethoxyocta-1,3-diene (0.31 g, 2.0 mmol) and NaOAc (0.082 g 1.0 mmol,) were subsequently added and the tube was closed. The reaction mixture was stirred for 16 h at 110 °C. Samples were periodically taken and partitioned between Et₂O and H₂O. The organic layer was analyzed by GC and TLC analysis. After complete consumption of the reagents H₂O was added (10 mL). The reaction was worked up by extraction with Et₂O (3 × 10 mL), dried (K₂CO₃) and filtered. After removal of the solvent the crude reaction mixture was purified by column chromatography on SiO₂ deactivated with Et₃N (1%) (eluent: PE-Et₂O, 98:2) to afford (1E,3E)-4-ethoxy-1-phenyl-octa-1,3-diene (0.184 g, 80%) as a colorless oil: ¹H NMR (200 MHz, CDCl₃): δ = 0.95 (t, J = 7.1 Hz, 3 H), 1.56 (m, 7 H), 2.38 (t, J = 7.1 Hz, 2 H), 3.82 (q, J = 7.1 Hz, 2 H), 5.42 (d, J = 10.8 Hz, 1 H), 6.34 (d, J = 15.6 Hz, 1 H), 6.90 (dd, J = 15.6, 10.8 Hz, 1 H), 7.27 (m, 5 H). ¹³C NMR (50 MHz, CDCl3): δ = 13.78, 14.39, 22.29, 29.98, 30.61, 62.28, 100.05, 125.30, 125.49, 125.66, 125.98, 128.31, 138.44, 161.03. IR (neat): 3062, 3030, 1628, 1599, 750, 700 cm^-1. MS (EI, 70 eV): m/z (%) = 230 (92) [M⁺], 115 (50), 91 (41), 85 (83), 57 (100). Anal. Calcd (%) for C₁₆H₂₂O (230.35): C, 83.43; H, 9.63. Found: C, 83.39; H 9.65.