Borenium cation derivatives

Zwitterionic borane adducts were reacted with trityl cation salts to provide a series of primary borenium cation derivatives (Scheme 12). For example, the A-heterocyclic carbene (NHC) borane (45) was prepared from the corresponding NHC (44). With the addition of the trityl salt, the dicationic dimer (46) was isolated. Product (46) was characterized by X-ray crystallography and trapped by nucleophiles. Its observation by NMR resonance (5 + 10.6) is in accordance with the GIAO calculated value (5 + 0.8). Similar borenium dications were generated from amine nucleophiles, such as compound (47). The authors considered the question why some borenium ion salts... 

The reactive boronium ion (88) has been prepared from l,8-bis(dimethylamino) naphthalene and has been shown to be effective in the borylation of pyrrole and indole derivatives. Catechol-ligated borenium cations, such as (89), have also been used in the borylation of a range of anilines, thiophenes, and (V-heterocycles. ... 

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-l-butene with the NHC-derived borane and catalytic HNTf2 (Tf=trifluoromethanesulfonyl, CF3SO2) affects hydroboration at room temperature. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations." Different NHC-CuCl complexes catalyse hydroborations of propargylic alcohols and ethers with opposite regioselectivity a 6-NHC-CuCl (23) complex provides a-selectivity, whereas / -selectivity is achieved using a 5-NHC-CuCl (24) complex." ... 

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