Tungsten η<sup>1</sup>-Five-Membered Oxygenated Heterocycles Derived from Tungsten η<sup>1</sup>-Propargyl Compounds: Systematic Syntheses, Structural Rearrangement, Electrophilic Alkylations, and Oxidative Demetalations
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In the presence of BF3·Et2O, tungsten η1-propargyl complexes reacted smoothly with aldehydes to give tungsten η1-2,5-dihydro-3-furyl complexes 1−3. Treatment of 1−3 with Ph3CBF4 in CH2Cl2 (−60 °C, 1 h) delivered tungsten η1-3-furylidene complexes 4−6 which underwent skeletal rearrangement to η1-2-furylidene 13−15 isomers when warmed to 5 °C. Treatment of 4−6 with Et3N at −60 °C produced tungsten η1-3-furyl complexes 7−9 which were isomerized to η1-2-furyl isomers 10−12 by a strong Bronsted acid. Protonation of 10−12 by CF3CO2H yielded η1-2-furylidene complexes 13−15. The two exceptional isomerizations were examined by in situ 1H NMR studies. Further oxidation of η1-2-furyl compounds 10−12 with m-chloroperbenzoic acid gave tungsten η1-Δ3-butenolides 16−18. Tungsten η1-3-furyl complexes 7 and 9 underwent alkylation with trimethoxymethane and aldehydes at −60 °C to yield η1-3-furylidene cationic precipitates that were subsequently reduced with NaBH3CN to yield complex η1-2,5-dihydro-3-furyl compounds in good diastereoselectivity; the stereochemical courses were distinct for 7 and 9. Tungsten η1-2,5-dihydro-3-furyl, η1-2-furyl, η1-3-furyl, and η1-Δ3-butenolide complexes were decomplexed with appropriate reagents to liberate 2,5-dihydrofurans, furans, and Δ3- and Δ2-butenolides in reasonable yields.
