Search

Summary of main observation and conclusion: Ir‐catalyzed cascade dehydrogenative CH/BH and BH/OH cross‐coupling of carboranyl carboxylic acid with readily available benzoic acid has been achieved, leading to the facile synthesis of previously unavailable carborano‐coumarin in a simple one‐pot process. Two cage B—H, one aryl C—H and one O—H bonds are activated to construct efficiently new B—C and B—O bonds. The cascade cyclization can stop at the first B—H/C—H cross‐coupling step by tuning the reaction conditions, resulting in a series of α‐carboranyl benzoic acid and aryl carborane derivatives. Control experiments indicate that B—H/C—H dehydrocoupling proceeds preferentially over B—H/O—H dehydrocoupling, and both directing groups and oxidants are crucial for this reaction. An iridium(V) intermediate is proposed to be involved in the catalytic cycle. [ABSTRACT FROM AUTHOR]

Abstract: Carboranes are a class of carbon–boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o‐carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C−H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B−H activation/functionalization among chemically very similar ten B−H vertices is very challenging. Considering the differences in electron density of ten cage B−H bonds in o‐carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron‐rich transition metal catalysts for the functionalization of the most electron‐deficient B(3,6)−H vertices (bonded to both cage CH vertices); using electron‐deficient transition‐metal catalysts for the functionalization of relatively electron‐rich B(8,9,10,12)−H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)−H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B‐functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B‐functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts. [ABSTRACT FROM AUTHOR]

A rhodium-catalyzed hydroxylation of a cage B4−H bond in o-carboranes with either O2 or air as the oxygen source is described, and serves as a new methodology for the regioselective generation of a series of 4-OH- o-carboranes in a one-pot process. The use of either O2 or air as both the oxidant and the oxygen source makes this protocol very environmentally friendly and practical. [ABSTRACT FROM AUTHOR]

Palladium-catalyzed intermolecular coupling of o-carborane with aromatics by direct cage B−H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)-diarylated- o-carboranes in high yields with excellent regioselectivity. Traceless directing group -COOH plays a crucial role for site- and di-selectivity of such intermolecular coupling reaction. A PdII-PdIV-PdII catalytic cycle is proposed to be responsible for the stepwise arylation. [ABSTRACT FROM AUTHOR]

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 PdII-initiated cage BH activation, alkene insertion, β-H elimination, reductive elimination, and decarboxylation. [ABSTRACT FROM AUTHOR]