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