Synthesis of (2-Arylethylidene)cyclobutanes by Palladium-Catalyzed Reactions of Aryl Halides with Homoallyl Alcohols Bearing a Trimethylene Group at the Allylic Position

Abstract

Treatment of aryl bromides with homoallyl alcohols bearing a trimethylene group at the allylic position in the presence of cesium carbonate under palladium catalysis affords (2-aryl­ethylidene)cyclobutanes selectively. The selective formation of the alkylidenecyclobutane skeleton results from regiospecific retro-­allylation of the homoallyl alcohols, which accompanies the transposition of the double bonds.

Key words

palladium - methylenecyclobutanes - homoallyl alcohols - cleavage reactions - allylation

References and Notes

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9

Optimized at the B3LYP/6-31G* level. For the optimized structures, see Supporting Information.

11

Typical Procedure. Cs₂CO₃ (0.12 g, 0.36 mmol) was placed in a 20 mL two-necked reaction flask equipped with a Dimroth condenser. The Cs₂CO₃ was dried in vacuo with heating with a hair dryer for 2 min. Pd(OAc)₂ (2.8 mg, 0.0125 mmol) and tri(4-tolyl)phosphine (15 mg, 0.050 mmol) were added to the reaction flask. The flask was then filled with argon using standard Schlenk techniques. Toluene (2.0 mL), homoallyl alcohol 2a (79 mg, 0.30 mmol), and 2-bromonaphthalene (1a; 52 mg, 0.25 mmol) were sequentially added at ambient temperature. The resulting mixture was heated at reflux for 13 h. After the mixture was cooled to room temperature, water (20 mL) was added. The product was extracted with hexane (3 × 20 mL) and the combined organic layer was dried over sodium sulfate, and concentrated in vacuo. Silica gel column purification with hexane as an eluent gave 2-(2-cyclobutyl-idenepropyl)naphthalene (3a; 52.0 mg, 94% yield). Characterization data for 3a: IR (neat): 2826, 1600, 1508 cm^-¹; ^¹H NMR (CDCl₃): δ = 1.44 (s, 3 H), 1.95-2.01 (m, 2 H), 2.68-2.72 (m, 2 H), 2.80-2.82 (m, 2 H), 3.35 (s, 2 H), 7.30-7.32 (m, 1 H), 7.40-7.47 (m, 2 H), 7.60 (s, 1 H), 7.75-7.81 (m, 3 H); ^¹³C NMR (CDCl₃): δ = 15.68, 15.86, 29.26, 29.43, 38.95, 124.83, 125.02, 125.78, 126.69, 127.42, 127.53, 127.58, 127.76, 132.02, 133.59, 134.38, 138.40. Anal. Calcd for C₁₇H₁₈: C, 91.84; H, 8.16. Found: C, 91.57; H, 8.06.