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)-C(2)B(10)H(10) with R(3)SiC[triple bond, length as m-dash]CBr in ClCH(2)CH(2)Cl gives 4-(R(3)SiC[triple bond, length as m-dash]C)-2-R(1)-o-C(2)B(10)H(10) in moderate to high yields. This reaction is compatible with alkynes possessing sterically bulky silyl groups such as (i)Pr(3)Si or (t)BuMe(2)Si. Meanwhile, another catalytic system of Pd(OAc)(2)/AgOAc/K(2)HPO(4) can catalyze the direct B(4)-alkynylation of 1-COOH-2-R(1)-C(2)B(10)H(10) with terminal alkynes R(2)C[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-R(1)-o-C(2)B(10)H(10) 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.