Abstract
Treatment of tungsten-η <sup>1</sup> -α,δ- and -η <sup>1</sup> -α,ε-alkynols 4-6 with RCHO/BF <sub>3</sub> · Et <sub>2</sub> O (R = alkyl, aryl) in cold diethyl ether effected cycloalkenation reaction, yielding tungsten-η <sup>1</sup> -furylidene and -η <sup>1</sup> - 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 H <sub>2</sub> 0, NaBH <sub>3</sub> CN, and Grignard reagents, resulting in α,α-double addition reactions to afford furan and pyran derivatives in good yields. In the hydride case, unsymmetric α,α-double addition of η <sup>1</sup> -furylidenium salts was achieved via treatment with NaBH <sub>4</sub> /MeOH. Organocuprates also effected double alkylations of these salts but in a distinct 1,3-addition pathway. The reactions of these oxacarbeniums with CH <sub>2</sub> N <sub>2</sub> were examined; the outcome depends on their vinyl substituents. When the substituent is an aliphatic group, the carbenium species undergo highly diastereoselective cyclopropanation with CH <sub>2</sub> N <sub>2</sub> . For an aryl substituent, the reaction with CH <sub>2</sub> N <sub>2</sub> yielded a new tungsten oxacarbenium with a significantly altered structure; in this case <sup>13</sup> C- and <sup>2</sup> H-labeling experiments were performed to elucidate the reaction mechanism.