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

Abstract

Pd-catalyzed carboxylic acid guided regioselective alkynylation of cage B(4)-H bonds in <i>o</i>-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-<i>o</i>-C₂B₁₀H₁₀ in moderate to high yields. This reaction is compatible with alkynes possessing sterically bulky silyl groups such as ⁱPr₃Si or ^<i>t</i> BuMe₂Si. 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-<i>o</i>-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 <i>via</i> 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.