Selective Catalytic B-H Arylation of o-Carboranyl Aldehydes by a Transient Directing Strategy.

Carboranyl aldehydes are among the most useful synthons in derivatization of carboranes. However, compared to the utilization of carboranyl carboxylic acids in selective B-H bond functionalizations, the synthetic application of carboranyl aldehydes is limited due to the weakly coordinating nature of the aldehyde group. Herein, the direct arylation of o-carboranyl aldehydes has been developed via Pd-catalyzed cage B-H bond functionalization. With the help of glycine to generate a directing group (DG) in situ, a series of cage B(4,5)-diarylated- and B(4)-monoarylated-o-carboranyl aldehydes were obtained in good to excellent yields with high selectivity. A wide range of functional groups are tolerated. The aldehyde group in the B-H arylated products could be readily removed or transformed into o-carboranyl methanol. A plausible catalytic cycle for B-H arylation was proposed based on control experiments and stoichiometric reactions, including the isolation of a key bicyclic palladium complex.