Tungsten(II)−Carbene Complex Functions as a Dicationic Synthon: Efficient Constructions of Furan and Pyran Frameworks from Readily Available α,δ- and α,ε-Alkynols

Treatment of tungsten−η 1 -α,δ- and −η 1 -α,ε-alkynols 4 − 6 with RCHO/BF 3 ·Et 2 O (R = alkyl, aryl) in cold diethyl ether effected cycloalkenation reaction, yielding tungsten−η 1 -furylidene and −η 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 H 2 O, NaBH 3 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 1 -furylidenium salts was achieved via treatment with NaBH 4 /MeOH. Organocuprates also effected double alkylations of these salts but in a distinct 1,3-addition pathway. The reactions of these oxacarbeniums with CH 2 N 2 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 2 N 2 . For an aryl substituent, the reaction with CH 2 N 2 yielded a new tungsten oxacarbenium with a significantly altered structure; in this case 13 C- and 2 H-labeling experiments were performed to elucidate the reaction mechanism.