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Issue 9, 2016
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Palladium catalyzed regioselective B–C(sp) coupling via direct cage B–H activation: synthesis of B(4)-alkynylated o-carboranes

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Abstract

Pd-catalyzed carboxylic acid guided regioselective alkynylation of cage B(4)–H bonds in o-carboranes has been achieved for the first time using two different catalytic systems. In the presence of 5 mol% Pd(OAc)2 and 3 equiv. of AgOAc, the reaction of 1-COOH-2-R1-C2B10H10 with R3SiC[triple bond, length as m-dash]CBr in ClCH2CH2Cl gives 4-(R3SiC[triple bond, length as m-dash]C)-2-R1-o-C2B10H10 in moderate to high yields. This reaction is compatible with alkynes possessing sterically bulky silyl groups such as iPr3Si or tBuMe2Si. Meanwhile, another catalytic system of Pd(OAc)2/AgOAc/K2HPO4 can catalyze the direct B(4)-alkynylation of 1-COOH-2-R1-C2B10H10 with terminal alkynes R2C[triple bond, length as m-dash]CH in moderate to high yields. The latter has a broader substrate scope from bulky silyl to aromatic to carboranyl substituents. Desilylation of the resultant products affords carboranyl acetylene 4-(HC[triple bond, length as m-dash]C)-2-R1-o-C2B10H10 which can undergo further transformations such as Sonogashira coupling, dimerization and click reactions. It is suggested that the above two catalytic systems may proceed via Pd(II)–Pd(IV)–Pd(II) and Pd(II)–Pd(0)–Pd(II) catalytic cycles, respectively. In addition, the silver salt is found to promote the decarboxylation reaction and thereby controls the mono-selectivity.

Graphical abstract: Palladium catalyzed regioselective B–C(sp) coupling via direct cage B–H activation: synthesis of B(4)-alkynylated o-carboranes

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Publication details

The article was received on 26 Feb 2016, accepted on 12 May 2016 and first published on 13 May 2016


Article type: Edge Article
DOI: 10.1039/C6SC00901H
Citation: Chem. Sci., 2016,7, 5838-5845
  • Open access: Creative Commons BY license
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    Palladium catalyzed regioselective B–C(sp) coupling via direct cage B–H activation: synthesis of B(4)-alkynylated o-carboranes

    Y. Quan, C. Tang and Z. Xie, Chem. Sci., 2016, 7, 5838
    DOI: 10.1039/C6SC00901H

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