Borenium ion catalyzed hydroboration of alkenes with N-heterocyclic carbene-boranes.
Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene (NHC)-derived borane 2 and catalytic HNTf(2) (Tf = trifluoromethanesulfonyl (CF(3)SO(2))) effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C(3) of the hexyl group to C(2) was observed over a time scale of minutes to hours. Oxidative workup gave a mixture of alcohols containing 2-hexanol as the major product. A similar preference for the C(2) alcohol was observed after oxidative workup of the 3-octene and 1-cyclohexyl-1-butene hydroborations. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations, and the C(2) selective migrations are attributed to the four-center interconversion of borenium cations with cationic NHC-borane-olefin π-complexes.
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, USA.
Aleksandrs
A
Prokofjevs
Anne
A
Boussonnière
Linfeng
L
Li
Hélène
H
Bonin
Emmanuel
E
Lacôte
Dennis P
DP
Curran
Edwin
E
Vedejs
D000475:Alkenes / Q000737:chemistry*; D001880:Boranes / Q000737:chemistry*; D001895:Boron / Q000737:chemistry*; D002384:Catalysis; D007477:Ions / Q000737:chemistry; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry
D000475:Alkenes; D001880:Boranes; D007477:Ions; C030011:carbene; C117224:1-hexene; D001895:Boron; D008697:Methane