Tungsten eta(1)-five-membered oxygenated heterocycles derived from tungsten eta(1)-propargyl compounds: Systematic syntheses, structural rearrangement, electrophilic alkylations, and oxidative demetalations

In the presence of BF3 . Et(2)O, tungsten eta(1)-propargyl complexes reacted smoothly with aldehydes to give tungsten eta-2,5-dihydro-3-furyl complexes 1-3. Treatment of 1-3 with Ph(3)CBF(4) in CH2Cl2 (-60 degrees C, 1 h) delivered tungsten eta(1)-3-furylidene complexes 4-6 which underwent skeletal rearrangement to eta(1)-2-furylidene 13-15 isomers when warmed to 5 degrees C. Treatment of 4-6 with Et(3)N at -60 degrees C produced tungsten eta(1)-3-furyl complexes 7-9 which were isomerized to eta(1)-2-furyl isomers 10-12 by a strong Bronsted acid. Protonation of 10-12 by CF3CO2H yielded eta(1)-furylidene complexes 13-15. The two exceptional isomerizations were examined by in situ H-1 NMR studies. Further oxidation of eta(1)-2furyl compounds 10-12 with m-chloroperbenzoic acid gave tungsten eta(1)-Delta(3)-butenolides 16-18. Tungsten eta(1)-3-furyl complexes 7 and 9 underwent alkylation with trimethoxymethane and aldehydes at -60 degrees C to yield eta(1)-3-furylidene cationic precipitates that were subsequently reduced with NaBH3CN to yield complex eta(1)-2,5-dihydro-3-furyl compounds in good diastereoselectivity; the stereochemical courses were distinct for 7 and 9. Tungsten eta(1)-2,5-dihydro-3-furyl, eta(1)-2-furyl, eta(1)-3-furyl, and eta(1)-Delta(3)-butenolide complexes were decomplexed with appropriate reagents to liberate 2,5-dihydrofurans, furans, and Delta(3)- and Delta(2)-butenolides in reasonable yields.