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
Journal
Journal of the American Chemical Society
Journal Volume
119
Journal Issue
19
Pages
4404-4412
Date Issued
1997
Author(s)
Abstract
Treatment of tungsten-η1-α,δ- and -η1-α,ε-alkynols 4-6 with RCHO/BF3 · Et2O (R = alkyl, aryl) in cold diethyl ether effected cycloalkenation reaction, yielding tungsten-η1-furylidene and -η1- 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 H20, NaBH3CN, 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 η1-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 13C- and 2H-labeling experiments were performed to elucidate the reaction mechanism.
SDGs
Type
journal article