Natural-Product-Directed Catalytic Stereoselective Synthesis of Functionalized Fused Borane Cluster-Oxazoles for the Discovery of Bactericidal Agents.

The identification of an alternative chemical space in order to address the global challenge posed by emerging antimicrobial resistance is very much needed for the discovery of novel antimicrobial lead compounds. Boron clusters are currently being explored in drug discovery due to their unique steric and electronic properties. However, the challenges associated with the synthesis and derivatization techniques of these compounds have limited their utility in the rapid construction of a library of molecules for screening against various biological targets as an alternative molecular platform. Herein, we report a transition-metal-catalyzed regioselective direct B-H alkylation-annulation of the closo-dodecaborate anion with natural products such as menthol and camphor as the directing groups. This method allowed the rapid construction of a library of 1,2,3-trisubstituted clusters, which were evaluated in terms of their antibacterial activity against WHO priority pathogens. Several of the synthesized dodecaborate derivatives displayed medium- to high-level bactericidal activity against Gram-positive and Gram-negative bacteria.

A 3D Analogue of Phenyllithium: Solution-Phase, Solid-State, and Computational Study of the Lithiacarborane [Li-CB11 H11 ].

The lithiacarborane [Li-CB11 H11 ]- plays a central role in carborane chemistry, as it is a key intermediate to achieve the selective functionalization of the monocarba-closo-dodecaborate [CB11 H12 ]- for applications in various fields. Also, it is an organometallic species of fundamental interest because it represents a 3D analogue of phenyllithium featuring an exo C-Li bond in addition to the delocalized negative endo charge of the spherical cluster. For the first time, the elusive and highly reactive endo/exo formal dianion [CB11 H11 ]2- has been isolated as its lithiate as well as zincate in pure form and fully characterized. DFT calculations corroborate the experimental findings and underscore the remarkably high reactivity of the lithiacarborane. Subsequent derivatizations demonstrate the relevance of its initial clean formation.

The 12-ethynylmonocarba-closo-dodecaborate anion as a versatile ligand for Cu(i) alkyne and heterobimetallic Cu(i)/M(ii) (M = Pd, Pt) alkynide complexes.

The anionic monocarborane alkyne [12-(HC[triple bond, length as m-dash]C)-CB11H11]- was employed as a ligand towards Cu(i) to form terminal alkyne complexes. Spectroscopic methods and X-ray crystallography allowed for a detailed structural analysis of complexes with nitrogen ligands, which are the first examples featuring carborane-C[triple bond, length as m-dash]CH → metal π coordination. Addition of phosphines to compound 2 afforded homoleptic Cu(i) complexes [Cu(PR3)n]+ (4), in which case the carborane becomes a non-coordinating anion. Polymeric acetylide 5 was obtained in almost quantitative yield from 2 and proved to be a suitable precursor to heterobimetallic Cu(i)/M(ii) (M = Pd, Pt) alkynide complexes with side-on and end-on coordination to the metal centers.

Highly selective palladium-catalyzed one-pot, five-fold B-H/C-H cross coupling of monocarboranes with alkenes.

Palladium-catalyzed dehydrogenative B-H/C-H cross coupling of monocarborane anions with alkenes is reported, allowing for the first time the isolation of selectively penta-alkenylated boron clusters. The reaction cascade is regioselective for the cage positions, leading directly to B2-6 functionalization. Under mild and convenient conditions, styrenes, benzylic alkenes and aliphatic alkenes are demonstrated to be viable coupling partners with exclusive vinyl-type B-C bond formation. Multiple subsequent transformations provide access to directing group-free products, chiral derivatives and penta-alkylated cages. The five-fold coupling, combined with the latter reactions, represents a powerful methodology for the straightforward synthesis of new classes of boron clusters.

Efficient access to amides of the carborane carboxylic acid [1-(COOH)-CB11H11].

The preparation of the carborane acid chloride [1-(COCl)-CB11H11]- from the carboxylic acid [1-(COOH)-CB11H11]- is reported. This acid chloride exhibits remarkable inertness towards moisture and can be stored under ambient conditions for several months. Reaction with amines affords secondary and tertiary carborane amides [1-(CONR1R2)-CB11H11]- in moderate to high yields under mild conditions. Two of the amide products were characterized by X-ray crystallography in addition to spectroscopic analysis. Preliminary studies show that the amides can be reduced to the corresponding amines and that the acid chloride has the potential to serve as a starting material for carborane ester formation.