Tungsten η1-Five-Membered Oxygenated Heterocycles Derived from Tungsten η1-Propargyl Compounds: Systematic Syntheses, Structural Rearrangement, Electrophilic Alkylations, and Oxidative Demetalations

In the presence of BF 3 ·Et 2 O, tungsten η 1 -propargyl complexes reacted smoothly with aldehydes to give tungsten η 1 -2,5-dihydro-3-furyl complexes 1 − 3 . Treatment of 1 − 3 with Ph 3 CBF 4 in CH 2 Cl 2 (−60 °C, 1 h) delivered tungsten η 1 -3-furylidene complexes 4 − 6 which underwent skeletal rearrangement to η 1 -2-furylidene 13 − 15 isomers when warmed to 5 °C. Treatment of 4 − 6 with Et 3 N at −60 °C produced tungsten η 1 -3-furyl complexes 7 − 9 which were isomerized to η 1 -2-furyl isomers 10 − 12 by a strong Bronsted acid. Protonation of 10 − 12 by CF 3 CO 2 H yielded η 1 -2-furylidene complexes 13 − 15 . The two exceptional isomerizations were examined by in situ 1 H NMR studies. Further oxidation of η 1 -2-furyl compounds 10 − 12 with m -chloroperbenzoic acid gave tungsten η 1 -Δ 3 -butenolides 16 − 18 . Tungsten η 1 -3-furyl complexes 7 and 9 underwent alkylation with trimethoxymethane and aldehydes at −60 °C to yield η 1 -3-furylidene cationic precipitates that were subsequently reduced with NaBH 3 CN to yield complex η 1 -2,5-dihydro-3-furyl compounds in good diastereoselectivity; the stereochemical courses were distinct for 7 and 9 . Tungsten η 1 -2,5-dihydro-3-furyl, η 1 -2-furyl, η 1 -3-furyl, and η 1 -Δ 3 -butenolide complexes were decomplexed with appropriate reagents to liberate 2,5-dihydrofurans, furans, and Δ 3 - and Δ 2 -butenolides in reasonable yields.