Transition‐Metal‐Catalyzed Regioselective Functionalization of Monophosphino‐ o‐ Carboranes

Couple-close construction of non-classical boron cluster-phosphonium conjugates

Heteropolycyclic molecular systems, which are essential components in the fields of materials and pharmacology, frequently consist of 2D extended organic aromatic rings. Here, we introduce a type of inorganic-organic hybrid 3D conjugates by merging an aromatic boron cluster with a phosphine and a π-conjugated unit. To achieve this, a couple-close synthetic strategy via B-H activation of nido-carboranes with alkynes has been developed, which leads to diverse boron cluster-extended phosphoniums in a twisted structure with high yields under mild conditions. Experimental and theoretical results reveal that the fusion between the boron cluster and the formed borophosphonium heterocycle facilitates electron delocalization throughout the structure. The unusual framework demonstrates distinct properties from bare boron clusters and pure aromatic ring-extended counterparts, such as improved thermal/chemical stability and photophysical properties. Thus, the boron cluster-based 3D conjugates expand the library of aromatic-based heterocyclics, showcasing great potential in functional materials.

Breaking the Base Barrier: Cu(II)-Mediated C-H Heteroarylation of o-Carboranes with Base-Sensitive Heteroaryl Halides

While cage C-arylation reactions using strong bases are among the most frequently used transformations in carborane chemistry, there has been no general solution to allow for the use of weak bases in the reaction. Moreover, base-metal-promoted C-H heteroarylation with base-sensitive heteroaryl halides remained elusive. Herein, copper-mediated cage C-H (hetero)arylation has been achieved without the need for strong bases, leading to the facile synthesis of a wide range of C-(hetero)arylated carboranes in good to excellent yields with a broad substrate scope and good functional group compatibility.

Palladium-Catalyzed Regioselective B(3,5)-Dialkenylation and B(4)-Alkenylation of o-Carboranes

Picolyl group directed B(3,5)-dialkenylation and B(4)-monoalkenylation of o-carboranes has been developed with a very low palladium catalyst loading. The degree of substitution is determined by the cage C(2)-substituents due to steric reasons. On the basis of experimental results, a plausible mechanism is proposed including electrophilic palladation and alkyne insertion followed by protonation.

Ir-Catalyzed B(3)-Amination of o-Carboranes with Amines via Acceptorless Dehydrogenative BH/NH Cross-Coupling

An efficient and convenient strategy for Ir-catalyzed selective B(3)-amination of o-carboranes with amines via acceptorless BH/NH dehydrocoupling was developed, affording a series of B(3)-aminated-o-carboranes in moderate to high isolated yields with H2 gas as a sole by-product. Such an oxidant-free system endues the protocol sustainability, atom-economy and environmental friendliness. A reaction mechanism via an Ir(I)-Ir(III)-Ir(I) catalytic cycle involving oxidative addition, dehydrogenation and reductive elimination was proposed.

Rhodium(i)-catalyzed cascade C(sp2)-H bond alkylation - amidation of anilines: phosphorus as traceless directing group

We introduce a versatile Rh(i)-catalyzed cascade reaction, combining C(sp2)-H bond functionalization and amidation between N-arylphosphanamines and acrylates. This innovative approach enables the rapid synthesis of dihydroquinolinone scaffolds, a common heterocycle found in various pharmaceuticals. Notably, the presence of the phosphorus atom facilitates the aniline ortho-C(sp2)-H bond activation prior to N-P bond hydrolysis, streamlining one-pot intramolecular amidation. Moreover, we demonstrate the applicability of this reaction by synthesizing an antipsychotic drug. Detailed mechanistic investigations revealed the involvement of a Rh-H intermediate, with substrate inhibition through catalyst saturation.

Transition metal catalyzed selective B(3)-H or B(4)-H amination of o-carboranes via dehydrogenative BH/NH cross-coupling

A unique approach to vertex-selective catalytic B-H amination at either the B(3)- or B(4)-position in o-carboranes has been developed. Using different transition metal catalysts, dehydrogenative BH/NH cross-coupling of o-carboranes and free amines has been achieved, leading to a wide variety of cage B(3)- or B(4)-aminated o-carboranes in moderate to high yields with excellent regioselectivity, where carboranyl carboxylic acids and amines can serve as competent coupling partners without any pre-functionalization. The isolation and structural identification of a key intermediate provide an insight into the reaction mechanism in the catalytic B(4)-H amination.

Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

Over the past two decades, visible-light-induced transformations have been regarded as being among the most environmentally benign and powerful strategies for constructing complex molecules and diverse synthetic building blocks in organic synthesis. However, the development of efficient photochemical processes for assembling enantiomerically pure molecules remains a significant challenge. Herein, we describe a simple and efficient visible-light-induced C-P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97-99% ee). Even in the absence of transition metal or photoredox catalysts, a variety of P-chiral heteroaryl phosphine oxides, including chiral diphosphine oxide 41, have been directly obtained under air conditions. Density functional theory (DFT) calculations have shown that the reaction involves intersystem crossing and single electron transfer to give a diradical intermediate under visible light irradiation.

Palladium-Catalyzed Regioselective B(3,4)-H Acyloxylation of o-Carboranes

We disclose herein an efficient regioselective B(3,4)-H activation via a ligand strategy, affording B(3)-monoacyloxylated and B(3,4)-diacyloxylated o-carboranes. The identification of amino acid and phosphoric acid ligands is crucial for the success of B(3)-mono- and B(3,4)-diacyloxylation, respectively. This ligand approach is compatible with a broad range of carboxylic acids. The functionalization of complex drug molecules is demonstrated. Other acyloxyl sources, including sodium benzoate, benzoic anhydride, and iodobenzene diacetate, are also tolerated.

Rhodium-catalyzed sequential B(3)-, B(4)-, and B(5)-trifunctionalization of o-carboranes with three different substituents

A rhodium-catalyzed one-pot trifunctionalization of o-carboranes with three different substituents via a carboxy group directed sequential B(5)-alkenylation, B(4)-alkyne annulation and B(3)-acyloxylation has been developed for the first time, leading to the synthesis of a new class of B(3,4,5)-trisubstituted o-carborane derivatives. Treatment of 1-COOH-2-CH₃-o-C₂B₁₀H₁₀ with ArCCAr in the presence of a [Cp*RhCl₂]₂ catalyst and a Cu(OPiv)₂ oxidant gave 1,4-[COOC(Ar)C(Ar)]-2-Me-3-OPiv-5-[C(Ar)CH(Ar)-o-C₂B₁₀H₇ in good to high yields. This protocol represents a new strategy for the catalytic selective polyfunctionalization of carboranes with different substituents.

Metal-catalyzed B-H acylmethylation of pyridylcarboranes: access to carborane-fused indoliziniums and quinoliziniums

Metal-catalyzed mono-acylmethylation of pyridylcarboranes has been realized using α-carbonyl sulfoxonium ylides as a coupling partner. The reaction features high efficiency, excellent site-selectivity and good functional group tolerance. In the presence of pyridyl and enolizable acylmethyl groups, a post-coordination mode has been proposed and validated by in situ high resolution mass spectroscopy (HRMS) to rationalize the unique mono-substitution. Post-functionalization at the newly incorporated alkyl site provides additional utility of this method, including the construction of carborane-fused indoliziniums and quinoliziniums. We believe that these mono-alkylated carboranes, together with their post-functionalized derivatives, may find applications in luminescent materials and drug discovery in the near future.

A Strategy for Selective Catalytic B-H Functionalization of o-Carboranes

Carboranes are a class of polyhedral carbon-boron molecular clusters featuring three-dimensional aromaticity, which are often considered as 3D analogues of benzene. Their unique structural and electronic properties make them invaluable building blocks for applications ranging from functional materials to versatile ligands to pharmaceuticals. Thus, selective functionalization of carboranes has received tremendous research interest. In earlier days, the vast majority of the works in this area were focused on cage carbon functionalization via facile deprotonation of cage CH, followed by reaction with electrophiles. On the contrary, cage B-H activation is very challenging since the 10 B-H bonds on o-carborane are very similar, and how to achieve the desired transformation at specific boron vertex is a long-standing issue.As carbon is considered more electronegative than boron, this property results in different vertex charges on the o-carborane cage, which follow the order B(3,6)-H ≪ B(4,5,7,11)-H < B(8,10)-H < B(9,12)-H. We thought that this difference may trigger the favorite interaction of a proper transition metal complex with a specific B-H bond of carborane, which could be utilized to solve the selectivity issue. Accordingly, our strategy is described as follows: (1) electron-rich transition metal catalysts are good for the activation of the most electron-deficient B(3,6)-H bonds (connected to both cage C-H vertices); (2) electron-deficient transition metal catalysts are good for the activation of the relatively electron-rich B(8,9,10,12)-H bonds (with no bonding to either cage C-H vertices); and (3) directing-group-assisted transition metal catalysis is appropriate for the activation of the B(4,5,7,11)-H bonds (connected to only one cage C-H vertex), whose vertex charges lie in the middle of the range for the 10 B-H bonds. This strategy has been successfully applied by our laboratory and other groups in the development of a series of synthetic routes for catalytic selective activation of B-H bonds of the carborane cage, resulting in the synthesis of a large number of cage-boron-functionalized carborane derivatives in a regioselective and catalytic fashion. Subsequently, significant progress in this emerging area has been made.In 2013 we reported the selective tetrafluorination of o-carboranes at the B(8,9,10,12)-H bonds using an electron-deficient Pd(II) salt, [Pd(MeCN)₄][BF₄], as the catalyst. In 2014 we disclosed the first example of carboxy-directed alkenylation of o-carboranes at the B(4) vertex promoted by an Ir(III) catalyst. Subsequently, in 2017 we presented an electron-rich Ir(I)-catalyzed diborylation of o-carboranes at the B(3,6)-H bonds. We also uncovered the first example of Pd-catalyzed asymmetric synthesis of chiral-at-cage o-carboranes in 2018. These proof-of-principle studies have greatly stimulated research activities in selective B-H activation of carboranes and boron clusters enabled by transition metal catalysts. We have so far developed a toolbox of synthetic methods for selective catalytic cage B-olefination, -arylation, -alkenylation, -alkynylation, -oxygenation, -sulfenylation, -borylation, -halogenation, and -amination. We have recently expanded our research to base metal catalysis. As the field progresses, we expect that other methods for regioselective cage B-H activation will be invented, and the results detailed in this Account will promote these efforts.

Electrooxidative o-carborane chalcogenations without directing groups: cage activation by copper catalysis at room temperature

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position- and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the late-stage diversification of o-carboranes.

Iridium-Catalyzed Annulation of o-Carboranyl Carboxylic Acids with Alkynes: Synthesis of Carborano-Isocoumarins

An efficient iridium-catalyzed formal [4+2] annulation of carboranyl carboxylic acids with alkynes is developed, resulting in the facile synthesis of a new class of carborano-isocoumarin derivatives. The carboxyl group not only serves as a directing group to control the regioselectivity but also ingeniously becomes a part of the final products. The reaction mechanism involves sequential carboxyl-directed B(4)-H metalation, alkyne insertion, and reductive elimination.

Pd-catalyzed selective tetrafunctionalization of diiodo-o-carboranes

A palladium-catalyzed highly selective tetrafunctionalization of 3,6-I2-o-carborane and 4,7-I2-o-carborane has been developed, leading to the preparation of 3,6-dialkenyl-4,11-R2-o-carboranes and 4,7-dialkenyl-5,11-R2-o-carboranes (R = alkyl, allyl and aryl) in moderate to excellent yields. This represents a new strategy for selective synthesis of polyfunctionalized o-carborane derivatives via a one-pot process.

Palladium-catalyzed selective B(3)-H arylation of o-carboranes with arylboronic acids at room temperature

A palladium-catalyzed selective B(3)-H arylation of o-carboranes at room temperature has been developed using readily available arylboronic acids as the aryl source, and the corresponding 3-aryl-o-carboranes were obtained in good to excellent yields. This method provides a powerful synthetic route for constructing polysubstituted o-carborane derivatives.

Transition metal-mediated B(4)-H hydroxylation/halogenation of o-carboranes bearing a 2-pyridylsulfenyl ligand

The introduction of the 2-pyridylsulfenyl directing group to o-carboranes allowed either B(3)-Ir or B(4)-Ir bond formation using a steric effect strategy. Moreover, the reactivity of the B(4)-Rh o-carborane complexes with small molecules was probed by reactions with N-bromosuccinimide, N-iodosuccinimide and O₂. Rhodium-mediated B(4)-hydroxylation and B(4)-halogenation which are seldom reported have been achieved under practical and mild conditions.

Rh-Catalyzed Decarbonylative Cross-Coupling between o-Carboranes and Twisted Amides: A Regioselective, Additive-Free, and Concise Late-Stage Carboranylation

The convenient cross-coupling of sp² or sp³ carbons with a specific boron vertex on carborane cage represents significant synthetic values and insurmountable challenges. In this work, we report an Rh-catalyzed reaction between o-carborane and N-acyl-glutarimides to construct various B_cage -C bonds. Under the optimized condition, the removable imine directing group (DG) leads to B(3)- or B(3,6)-C couplings, while the pyridyl DG leads to B(3,5)-Ar coupling. In particular, an unexpected rearrangement of amide reagent is observed in pyridyl directed B(4)-C(sp³ ) formation. This scalable protocol has many advantages, including easy access, the use of cheap and widely available coupling agents, no requirement of an external ligand, base or oxidant, high efficiency, and a broad substrate scope. Leveraging the Rh^I dimer and twisted amides, this method enables straightforward access to diversely substituted and therapeutically important carborane derivatives at boron site, and provides a highly valuable vista for carborane-based drug screening.

Pd-Catalyzed sequential B(3)–I/B(4)–H bond activation for the synthesis of 3,4-benzo-o-carboranes

A Pd-catalyzed sequential B(3)–I and B(4)–H bond activation was developed for the synthesis of 3,4-benzo-o-carboranes via a formal [2 + 2 + 2] cycloaddition.

8-Aminoquinoline as a bidentate traceless directing group for Cu-catalyzed selective B(4,5)–H disulfenylation of o-carboranes

A traceless bidentate directing group guided copper catalyzed cage B(4,5)–H disulfenylation of o-carboranes has been achieved, where the in situ departure of 8-aminoquinoline circumvents additional process for directing group removal.