Ligand-Driven Advances in Iridium-Catalyzed sp3 C–H Borylation: 2,2′-Dipyridylarylmethane

Metal-Free B-B Dehydrocoupling Reaction of a Simple Borane Adduct: Convenient Access to a Nucleophilic Diborane(4)

The selective formation of homonuclear bonds is of key importance in synthetic chemistry. Especially, dehydrocoupling reactions are attractive as ecologically and economically friendly alternatives to established reductive bond forming reactions, since they do not require the use of stoichiometric amounts of a reducing reagent and produce only valuable dihydrogen as by-product. Here, we report on a metal-free B-B dehydrocoupling reaction that starts directly from a simple, easily accessible BH3 adduct, providing convenient access to a new nucleophilic dihydridodiborane in excellent yield. The dihydridodiborane in turn activates dihydrogen, allowing to obtain quantitatively the dideuteridodiborane from the dihydridodiborane by D2 activation. On the basis of detailed quantum-chemical calculations, the mechanism of this unprecedented reaction is elucidated. Some key points that are essential for metal-free dehydrocoupling are disclosed, paving the way for their systematic evaluation and application.

Catalytic Undirected Meta-Selective C-H Borylation of Metallocenes

Metallocenes are privileged backbones in the fields of synthetic chemistry, catalysis, polymer science, etc. Direct C-H functionalization is undoubtedly the simplest approach for tuning the properties of metallocenes. However, owing to the presence of multiple identical C(sp2 )-H sites, this protocol often suffers from low reactivity and selectivity issues, especially for the regioselective synthesis of 1,3-difunctionalized metallocenes. Herein, an efficient iridium-catalyzed meta-selective C-H borylation of metallocenes is reported. With no need of preinstalled directing groups, this approach enables a rapid synthesis of various boronic esters based on benzoferrocenes, ferrocenes, ruthenocene, and related half sandwich complex. A broad range of electron-deficient and -rich functional groups are all compatible with the process. Notably, C-H borylation of benzoferrocenes takes place exclusively at the benzene ring, which is likely ascribed to the shielding effect of pentamethylcyclopentadiene. The synthetic utility is further demonstrated by easy scalability to gram quantities, the conversion of boron to heteroatoms including N3 , SePh, and OAc, as well as diverse cross-coupling reactions.

Metal-Free Directed Site-Selective Csp3 -H Borylation of Saturated Cyclic Amines

The borylation of Csp3 -H bonds is a challenging transformation that is typically restricted to transition metal catalysis. Herein, we report the site-selective metal-free Csp3 -H borylation of saturated cyclic amines. It is possible to selectively borylate piperidine derivatives at the α or β positions according to the reaction conditions. The mechanism was supported by NMR spectroscopy, calorimetry experiments and density functional theory (DFT) computations. It suggests that the piperidine is dehydrogenated by complexation with BBr3 to produce an enamine intermediate, which is in turn borylated at either the α or β position according to the reaction conditions.