One-Pot, Three-Step Synthesis of Cyclopropylboronic Acid Pinacol Esters from Synthetically Tractable Propargylic
Jonathan A Spencer1,2, Craig Jamieson1, Eric P A Talbot2
1University of Strathclyde , 295 Cathedral Street, Glasgow G1 1XL, U.K.
Abstract:
Simple propargylic silyl ethers can be converted to complex cyclopropylboronic acid pinacol esters in an efficient one-pot procedure. Terminal acetylenes undergo a Schwartz's reagent catalyzed hydroboration; subsequent addition of further Schwartz's reagent and Lewis acid-mediated activation of neighboring silyl ether allows cyclization to access a range of cyclopropylboronic acid pinacol esters. The scope includes aromatic, aliphatic, quaternary, and spiro substituted cyclopropyl rings, which can be transformed via Suzuki coupling into a range of lead-like substituted cyclopropyl aryl products.
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Preparation of Diols and Pinacol Rearrangement
The reaction begins with transferring a proton from the acid catalyst to one of the hydroxyl groups, producing an oxonium ion.
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.


