Transition metal catalyzed regioselective amination of the cage B(4)-H bond in o-carboranes has been achieved for the first time using O-benzoyl hydroxylamines or organic azides as the amination reagents, leading to the preparation of a series of tertiary and secondary carboranyl amines. Both amination reactions proceeded under mild conditions without the addition of any external oxidants. Hydrogenolysis of the resultant product 4-N(CHPh)-o-carborane afforded the primary carboranyl amine, 4-amino-o-carborane, in quantitative yield.
Mild methods to cleave the carbon-oxygen (C−O) bond in alkyl ethers could
simplify chemical syntheses through the elaboration of these robust, readily
available precursors. Here we report that dibromoboranes react with alkyl ethers
in the presence of a nickel catalyst and zinc reductant to insert boron into the
C−O bond. Subsequent reactivity can effect oxygen-to-nitrogen substitution or
one-carbon homologation of cyclic ethers and more broadly streamline preparation
of bioactive compounds. Mechanistic studies reveal a cleavage-then-rebound pathway
via zinc/nickel tandem catalysis.
Palladium-catalyzed direct dialkenylation of cage B(4,5)-H bonds in o-carboranes has been achieved with the help of a carboxylic acid directing group, leading to the preparation of a series of 4,5-[trans-(ArCH=CH)]2-ocarboranes in high yields with excellent regioselectivity. The traceless directing group, eliminated during the course of the reaction, is responsible for controlling regioselectivity and dialkenylation. A possible catalytic cycle is proposed, involving a tandem sequence of Pd(II) -initiated cage B-H activation, alkene insertion, β-H elimination, reductive elimination, and decarboxylation.
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