Treatment of tungsten-η 1 -R,δ-and -η 1 -R, -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 R,R-double addition reactions to afford furan and pyran derivatives in good yields. In the hydride case, unsymmetric R,R-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 1,3addition 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.
A series of tungsten-eta(1)-alkynols tethered with a dimethylacetal, methyl ketone, or trimethoxymethane group are prepared. Treatment of these functionalized tungsten-alkynols with BF(3).Et(2)O leads to intramolecular cycloalkenation, producing bicyclic tungsten-oxacarbeniums in high yields. Air oxidation of these oxacarbenium salts produces unsaturated bicyclic lactones in good yields. The lactone products include delta- and epsilon-lactones fused with five-, six- and seven-membered carbocyclic rings. The preceding bicyclic tungsten-eta(1)-oxacarbeniums are highly reactive toward organocuprates, Grignard reagents, and diazomethane, leading to demetalation to give various derivatives of bicyclic lactones. A short synthesis of (+/-)-mitsugashiwalactone is developed based on this method.
Treatment of several alkynyltungsten(II) compounds with 4.0 mol equiv of CF3SO3H led to
oxidative
carbonylation to yield acyltungsten(IV) compounds;
this
reaction can be applied to syntheses of indanones and
unsaturated carbonyl compounds.
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