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Stereodivergent Access to Trisubstituted Alkenylboronate Esters through Alkene Isomerization

Lucas Segura, Itai Massad, Masamichi Ogasawara and Ilan Marek*

*Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Haifa 3200009, Israel, Email: chilanmtechnion.ac.il

L. Segura, I. Massad, M. Ogasawara, I. Marek, Org. Lett., 2021, 23, 9194-9198.

DOI: 10.1021/acs.orglett.1c03513


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Abstract

Efficient alkene isomerizations of readily available 1,1-disubstituted alkenylboronate esters provide synthetically valuable trisubstituted alkenylboronates. Either stereoisomer of the target alkenylboronate motif can be obtained by employing different isomerization catalysts.

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proposed mechanisms



General procedure for the Iridium catalyzed alkenylboronic ester mono-isomerization

Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (10 mg, 0.011 mmol, 5.6 mol%) is placed in a flame-dried Schlenk tube under argon flow. PCy3 (8.41 mg, 0.03 mmol, 15 mol%) and [Ir(cod)Cl]2 (3.36 mg, 0.005 mmol, 2.5 mol%) are subsequently added under a flow of argon and the solids stirred for 1 minute at room temperature before DCM (1 mL) is added. Upon complete dissolution of the solids (resulting in an orange-reddish solution), hydrogen gas is bubbled through a needle for 1 minute, adding DCM at the end of this hydrogenation step can be necessary to conserve the desired volume of solvent. Subsequently, the reaction is degassed by performing 3 freeze-pump-thaw cycles. The substrate (0.2 mmol) is added neat and the resulting mixture is allowed to stir at 40 ºC. Upon consumption of the starting material (usually 1 hour, conversion was monitored by TLC: petroleum ether/diethyl ether eluent system (20/1), visualized with anisaldehyde. The isomerized boronic esters are typically 0.05 units higher in Rf than the starting material). The mixture is then allowed to cool down to room temperature, DCM is removed in vacuo, and the resulting crude product is purified by column chromatography (Et2O/petroleum ether).

Note: the quality of the Ir-precatalyst often affects the reaction rate, with the best reactivity obtained with [Ir(cod)Cl]2 purchased from Strem.


Key Words

alkene isomerization, vinyl boronates


ID: J54-Y2021