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

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-R-C2B10H10 with R3SiC^CBr in ClCH2CH2Cl gives 4(R3SiC^C)-2-R -o-C2B10H10 in moderate to high yields. This reaction is compatible with alkynes possessing sterically bulky silyl groups such as Pr3Si or BuMe2Si. Meanwhile, another catalytic system of Pd(OAc)2/AgOAc/K2HPO4 can catalyze the direct B(4)-alkynylation of 1-COOH-2-R -C2B10H10 with terminal alkynes RC^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^C)-2-R-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.