Palladium-Catalysed Cascades Triggered by Dehydrogenation of Secondary or Tertiary Amines: Access to Bridged- and Fused-Ring Heterocycles

Abstract

Palladium-catalysed cascades triggered by dehydrogenation of secondary or tertiary amines and trapping the intermediate imines with cycloadditions and Mannich reactions result in a range of bridged- and fused-ring heterocyclic motifs in moderate to good yields.

Key words

cascade reaction - hydrogen transfer - Mannich reaction - dipolar cycloaddition

References and Notes

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9

1,2-Benzo-4,7-imino-4-methoxycarbonyl- exo -6-benzyl-oxycarbonylcyclohept-1-ene (7)
Methyl 1,2,3,4-tetrahydroisoquinoline-3-carboxylate (1, 0.1 g, 0.5 mmol), benzyl acrylate (0.17 g, 1.0 mmol), and Pd black (0.0055 g, 0.05 mmol) were mixed in toluene (15 mL) and heated at 110 ˚C for 15 h. After removal of the solvent the residue was purified by column chromatography eluting with Et₂O-PE (1:1, v/v) to afford 7 (0.091 g, 50%) as a colourless semisolid and 8 (10%). Anal. Calcd (%) for C₂₁H₂₁NO₄: C, 71.79; H, 5.98; N, 3.99. Found: C, 71.70; H, 6.20; N, 4.05. ^¹H NMR (300 MHz, CDCl₃): δ = 7.15-7.07 (m, 4 H, ArH), 5.12 (s, 2 H, CH₂Ph), 4.57 (s, 1 H, H_a), 3.79 (s, 3 H, CO₂Me), 3.26 (d, 1 H, J = 16.60 Hz, ArCHH), 3.14 (dd, 1 H, J = 8.85, 2.60 Hz H_b), 3.02 (d, 1 H, J = 16.60 Hz, ArCHH), 2.47 (dd, 1 H, J = 13.75, 2.60 Hz, H_c), 2.22 (dd, 1 H, J = 13.75, 8.85 Hz, H_d). MS: m/z (%) = 351 (0.2) [M⁺], 292 (3), 260 (0.4), 189 (100), 157 (18).