Reaction of the methyl(pyridine)tungsten complex Cp∗(CO)2W(py)Me with borane-NHC (NHC = N-heterocyclic carbene; MeIMe, MeIiPr) (MeIMe = 1,3,4,5-tetramethylimidazol-2-ylidene, MeIiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) adducts gave the NHC-stabilized hydrido(hydroborylene)tungsten complexes Cp∗ (CO)2W(H)(BH·NHC) (1a: NHC = MeIMe, 1b: NHC = MeIiPr) as dark brown crystals in high yields accompanied by liberation of pyridine and methane. X-ray crystal structure analysis revealed that the W-B bond of 1b is bridged by a hydrido ligand. Reactions of 1a,b with phenylacetylene at room temperature afforded NHC-stabilized η3-boraallyl complexes 2a,b, respectively, through a new type of hydroboration of the alkyne triple bond: i.e., addition of a hydrogen and a borylene. Reaction of 1a with 3-butyn-2-one also led to hydroboration of the triple bond, not the C=O double bond, to give η3-boraallyl complex 3. In the reaction of 1a with (trimethylsilyl)acetylene, 1-oxo-4-borabutenyl complex 4 was formed as a primary product through carboboration of the alkyne triple bond. The subsequent slow reaction of 4 at ambient temperature resulted in the formation of η3-boraallyl complex 5 and carbyne complex 6 in the ratio of 5:4. In all of these reactions, regioselective boration of the terminal carbon of alkynes was observed.
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