Yangjian Quan

Iridium Catalyzed Regioselective Cage Boron Alkenylation of o-Carboranes via Direct Cage B–H Activation

Iridium catalyzed alkyne hydroboration with o-carborane cage B-H has been achieved, leading to the formation of a series of 4-B-alkenylated-o-carborane derivatives in high yields with excellent regioselectivity via direct B-H bond activation. In this reaction the carboxy group is used as a traceless directing group, which is removed during a one-pot process. After the confirmation of a key intermediate, a possible mechanism is proposed, involving a tandem sequence of Ir-mediated B-H activation, alkyne insertion, protonation, and decarboxylation.

Palladium-Catalyzed Regioselective Intramolecular Coupling of o-Carborane with Aromatics via Direct Cage B–H Activation

Palladium-catalyzed intramolecular coupling of o-carborane with aromatics via direct cage B-H bond activation has been achieved, leading to the synthesis of a series of o-carborane-functionalized aromatics in high yields with excellent regioselectivity. In addition, the site selectivity can also be tuned by the substituents on cage carbon atom.

Palladium-Catalyzed Direct Dialkenylation of Cage BH Bonds in o-Carboranes through Cross-Coupling Reactions

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.

Palladium-Catalyzed Regioselective Diarylation of o-Carboranes By Direct Cage B-H Activation.

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 Pd(II)-Pd(IV)-Pd(II) catalytic cycle is proposed to be responsible for the stepwise arylation.

Transition-Metal-Mediated Three-Component Cascade Cyclization: Selective Cage B–C(sp2) Coupling of Carborane with Aromatics and Synthesis of Carborane-Fused Tricyclics

Zirconium/nickel comediated one-pot three-component cascade cyclization of carboryne, alkene, and 2-bromophenyltrimethylsilylacetylene has been achieved, leading to the formation of a series of C,C,B-substituted carborane-fused tricyclics. On the basis of experimental results, a plausible mechanism is proposed including [2 + 2 + 1] cross-cyclotrimerization followed by intramolecular direct selective cage B-C(sp(2)) coupling. This represents the first example of direct cage B-C(phenyl) coupling via cage B-H activation.

Palladium-Catalyzed Selective Fluorination of o-Carboranes

A Pd(II)-catalyzed direct selective fluorination reaction of carboranes using a F(+) reagent has been developed, leading to a series of polyfluorocarboranes in high isolated yields. The mechanism involving electrophilic B-H activation, oxidation of Pd(II) by F(+) species, and reductive elimination is proposed.

Rhodium-Catalyzed Regioselective Hydroxylation of Cage B−H Bonds of o-Carboranes with O2 or Air

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.

Three-component [2+2+1] cross-cyclotrimerization of carboryne, unactivated alkene, and trimethylsilylalkyne co-mediated by Zr and Ni.

A three-component [2+2+1] cross-cyclotrimerization of carboryne, alkene, and trimethylsilylalkyne has been achieved under the cooperative action of zirconium and nickel, leading to the synthesis of a series of dihydrofulvenocarboranes. The bulkiness of the alkyne and phosphine ligand plays a key role in the selective formation of the products.

Transition Metal Catalyzed Selective Cage B-H Functionalization of o-Carboranes

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.

Transition Metal Catalyzed, Regioselective B(4)-Halogenation and B(4,5)-Diiodination of Cage B−H Bonds in o-Carboranes

Transition metal catalyzed, regioselective carborane-cage B(4)-H iodination, bromination, and chlorination as well as B(4,5)-H diiodination were achieved by using NXS (X=I, Br), FeCl3 , or IOAc as the halogenating agent, respectively. A series of previously inaccessible B(4)-halogenated o-carboranes were synthesized in a simple one-pot process, and proved to be valuable synthons for the functionalization of carboranes. Mono- and di-selectivity can be controlled by in situ removal of the carboxy directing group. The resultant 4-I-o-C2 B10 H11 can serve as a versatile feedstock for the construction of cage B-C(sp2 ), B-C(sp), B-O, and B-N bonds.