Borenium Ion Catalyzed Hydroboration of Alkenes with N-Heterocyclic Carbene-Boranes

Abstract

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene-derived borane 2 and catalytic HNTf2 effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C3 of the hexyl group to C2 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 4-center inter-conversion of borenium cations with cationic NHC-borane olefin π-complexes.

Olefin Hydroborations with Diamidocarbene–BH3 Adducts at Room Temperature

“…Over time or upon heating the reaction mixture to 55 • C, the two multiplets converted to a single multiplet resonance at δ 3.67 ppm (see Supplementary Materials, Figure S17). Similar results were reported by Curran and co-workers, who attributed the phenomena to "chain walking" of a carbene-BH 3 adduct [19][20][21][22][23][24][25][26]. Additionally, when cis-or trans-3-hexene was introduced to 1a, compound 10 was obtained as the sole product, as indicated by the presence of a single multiplet at δ 3.67 ppm (C 6 D 6 ).…”

“…In the absence of any external initiators, the NHC-BH3 adducts typically do not react with olefins. However, as shown in Scheme 1, the NHC-BH3 adducts do facilitate the hydroboration of a wide range of organic molecules at elevated temperatures and/or upon the addition of a radical initiator [19][20][21][22][23][24][25][26][27]. Scheme 1.…”

Fundamental and Applied Properties of Borocations

“…In this case, LBH 2 X is therefore a functional equivalent of a borenium rather than a true borenium cation. More recently, this approach was extended by Vedejs, Curran, Lacôte and co-workers to the borenium equivalents (NHC)BH 2 NTf 2 and [78]. In contrast to amine borane adducts, NHCs do not dissociate from boron even under forcing conditions.…”

Catalytic Electrophilic C−H Borylation Using NHC⋅Boranes and Iodine Forms C2‐, not C3‐, Borylated Indoles

“…Building on the seminal studies by Vedejs, Curran and Lacôte, 10 we recently reported that NHC-boranes (NHC = Nheterocyclic carbene) activated with sub-stoichiometric iodine effect alkyne C-H borylation in the absence of exogenous base. 11 The turnover enabling step was proposed to involve the expected by-product from C-H borylation, HI, reacting with another equivalent of NHC⋅BR2(H) to regenerate the active boron electrophile, NHC⋅BR2(I), and H2.…”