Tungsten(II)-carbene complex functions as a dicationic synthon: Efficient constructions of furan and pyran frameworks from readily available alpha,delta- and alpha,epsilon-alkynols

Treatment of tungsten-eta(1)-alpha,delta- and -eta(1)-alpha,epsilon-alkynols 4-6 with RCHO/BF3 . Et2O (R = alkyl, aryl) in cold diethyl ether effected cycloalkenation reaction, yielding tungsten-eta(1)-furylidene and -eta(1)-pyrylidene salts in excellent yields (>95%). The structures of these oxacarbeniums were elucidated through X-ray diffraction studies of the representative compounds 7 and 8 in addition to standard NMR and IR spectral data. In contrast with conventional metal carbeniums, these tungsten oxacarbeniums reacted with two molecules of nucleophiles such as H2O, NaBH3CN, and Grignard reagents, resulting in alpha,alpha-double addition reactions to afford furan and pyran derivatives iq good yields. In the hydride case, unsymmetric alpha,alpha-double addition of eta(1)-furylidenium salts was achieved via treatment with NaBH4/MeOH. Organocuprates also effected double alkylations of these salts but in a distinct 1,3-addition pathway. The reactions of these oxacarbeniums with CH2N2 were examined; the outcome depends on their vinyl substituents. When the substituent is an aliphatic group, the carbenium species undergo highly diastereoselective cyclopropanation with CH2N2. For an aryl substituent, the reaction with CH2N2 yielded a new tungsten oxacarbenium with a significantly altered structure; in this case C-13- and H-2-labeling experiments were performed to elucidate the reaction mechanism.