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.
Still elusive: The reduction of dimethylimidazol‐2‐ylidene dichloroborane by sodium naphthalenide has been suggested to provide a borylene that cycloadds to naphthalene to make a borirane. Evidence has been provided that this borirane instead arises from coupling of a boryl radical and sodium naphthalenide. In contrast, reduction of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene dichloroborane and the related dimesityl derivative provide novel CH insertion products that may have arisen from a borylene.
Borenium-catalyzed hydroboration reactions of a stable, readily available N-heterocyclic carbene−borane with allyl-, alkenyl-, and alkynylsilane substrates provides either standard 1,2-hydroboration products or rearranged 1,1-hydroboration products, depending on the structure of the substrate. A competent catalyst can be generated in situ by addition of bis(trifluoromethane)sulfonimide or diiodine. In a typical 1,2hydroboration, reaction of 1,3-dimethylimidazol-2-ylidine−borane (diMe-Imd-BH 3 ) with 1,2-bis(trimethylsilyl)ethene provides 1,3-dimethylimidazol-2-ylidine−(1,2-bis(trimethylsilyl)ethyl)borane (diMe-Imd-BH 2 CH-(TMS)CH 2 TMS)) as a stable product. In a typical 1,1-hydroboration, the reaction of diMe-Imd-BH 3 with bis(trimethylsilyl)ethyne provides 1,3-dimethylimidazol-2-ylidine−bis(2,2-bis(trimethylsilyl)ethenyl)borane (diMe-Imd-BH(CHC(TMS) 2 ) 2 again as a stable product.
The hydroboration of alkenes of diverse structural types by assorted N-heterocyclic carbene boranes can be accomplished by addition of 5-10% diiodine. For example, reaction of 1,3-dimethylimidazol-2-ylidene borane (diMe-Imd-BH3) with 10% I2 followed by addition of 2,3-dimethyl-2-butene provided the corresponding thexyl NHC-borane (diMe-Imd-BH2thexyl) in 75% yield. This and related monohydroboration products are stable to chromatography and storage. The scope of the new reaction is described, and the mechanism is probed by (11)B NMR experiments.
An efficient preparation of γ-lactols and methylene-γ-lactols is described. Highly acid-sensitive lactols are prepared in a concise manner by using a radical cyclization of aluminum acetals. The precursors for the radical reactions are readily prepared from allyl or propargyl alcohols and α-bromo acids. Functionalization of the resulting γ-lactols and methylene-γ-lactols can be achieved following isolation, leading to synthetically useful building blocks, such as 1,4-diols, 1,4-dienes, γ-lactones, and polysubstituted tetrahydrofurans.
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