Preparation of (E)-1-Alkenylboronic Acid Pinacol Esters via Transfer of Alkenyl Group from Boron to Boron

Abstract

Two synthetic routes to (E)-1-alkenylboronic acid pinacol esters 3 were investigated. Hydroboration of 1-alkynes 1 with 1,3,2-benzodioxaborole (catecholborane), in situ generated by the reaction of BH₃ in THF with catechol, proceeded in the presence of a catalytic amount of dicyclohexylborane in THF at room temperature to give the corresponding (E)-1-alkenylboronic acid catechol esters 2. Treatment of the resultant esters 2 with 2,3-dimethyl-2,3-butanediol (pinacol) easily afforded the desired products 3, which are insensitive to air, moisture and chromatography, in good to high overall yields. The sequential reaction is a highly efficient route to 3 from BH₃ in THF in a one-pot manner. Alternatively, hydroboration of 1 with 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pinacolborane) was achieved in the presence of a catalytic amount of dicyclohexylborane at room temperature under neat conditions to afford the corresponding products 3 directly in good to excellent yields. This route is extremely efficient and environmentally benign from the viewpoints of making good use of pinacolborane and of using no solvent, and is capable of using a variety of alkynes 1 including functionalized ones such as HCCCH₂Cl and HCCCH₂OTHP.

(E)-1-alkenylboronic acid pinacol ester - 1-alkyne - dicyclohexylborane - catecholborane - pinacolborane

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There is a characteristic odor of dimethyl sulfide in the syringe used for removing the pinacolborane.

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We were aware of the fact that both Me₂S-free and Me₂S-containing pinacolborane were commercially available. This prompted us to examine the dicyclohexylborane-mediated hydroboration of 1b with Me₂S-free pinacolborane, and thus 3b was obtained in 77% yield under the same reaction conditions, except for the reaction time (18 h).

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Dicyclohexylborane was used as a suspension in THF, and thus THF must be removed under reduced pressure in order to get dicyclohexylborane neat. If the residue, obtained by removal of THF, is a white solid, the hydroboration with pinacolborane will proceed without any trouble. But if the residue is viscous liquid, the hydroboration will scarcely proceed.

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The other alkenyl carbon atom was not observed. For example, see: refs. [5] [10]