No abstract

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 -∆ 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 -∆ 3 -butenolide complexes were decomplexed with appropriate reagents to liberate 2,5-dihydrofurans, furans, and ∆ 3 -and ∆ 2 -butenolides in reasonable yields.

Cycloaddition of tungsten−η1-3-furyl compounds with a variety of organic alkenes and alkynes proceeds smmothly at ambient conditions, yielding tungsten−η1-oxabicyclo[2.2.1]heptene and −η1-oxabicyclo[2.2.1]heptadiene compounds in high regioselectivities and stereoselectivities. The cycloaddition is proposed to proceed via a mechanism involving a zwitterion intermediate. Cleavage of oxygen bridges of η1-oxabicyclo[2.2.1]heptenes and η1-oxabicyclo[2.2.1]heptadienes occurs on treatment of CF3SO3H and Lewis acid, leading to atypical carbon−carbon bond scission and deoxygenation reactions to give highly substituted benzene and furan derivatives, respectively. The role of the tunsten fragment in these demetalation reactions is discussed in detail.

The syntheses of a series of CpW(CO) 2 (π-γ-lactonyl) complexes bearing a tethered aldehyde are described. These π-allyl complexes are prepared as either syn-or anti-stereoisomers. Treatment of these dicarbonyl complexes with NOBF 4 and NaI in CH 3 CN effects an intramolecular allylation of the tethered aldehyde, yielding bicyclic R-methylene butyrolactones comprising a homoallylic alcohol. Both syn-and anti-isomers of tungsten-π-allyl compounds produce the same R-methylene butyrolactones. The cyclizations proceed with high diastereoselectivities to give only the cis-fused bicyclic γ-lactones. The preference for cis-fused stereoselection can be rationalized based on a tricyclic transition state mechanism.