2-Aminophenanthroline Ligands Enable Mild, Undirected, Iridium-Catalyzed Borylation of Alkyl C–H Bonds

Abstract

The catalytic, undirected borylation of alkyl C−H bonds typically occurs at high reaction temperatures or with excess substrate, or both, because of the low reactivity of alkyl C−H bonds. Here we report a new iridium system comprising 2-anilino-1,10-phenanthroline as the ligand that catalyzes the borylation of alkyl C−H bonds with little to no induction period and with high reaction rates. This superior activation and reactivity profile of 2-aminophenanthroline-ligated catalysts leads to broader reaction scope, including reactions of sensitive substrates, such as epoxides and glycosidic acetals, enhanced diastereoselectivity, and higher yields of borylated products. These catalysts also enable the borylation of alkanes, amines, and ethers at room temperature for the first time. Mechanistic studies imply that facile N-borylation occurs under the reaction conditions and that iridium complexes containing N-boryl aminophenanthrolines are competent precatalysts for the reaction.

Do we really need ligands in Ir-catalyzed C–H borylation?

“…The latest developments in this area include the use of air stable pre-catalysts, [4] improved regioselectivity by the use of sterically encumbered ligands, [5] use of predictive models, [6] and progress towards C(sp 3 )-H borylation. [7] More recently, functional group directed C-H borylations have emerged, in which a coordinating motif (directing group DG) governs the regioselectivity of the C-B coupling. [2c, 8] Generally this is achieved by a clever ligand design allowing for an outer sphere attractive interaction between the ligand and a suitable directing group (DG) of the starting material (Figure 1b) or by an inner sphere chelation of the directing group (DG) to the iridium metal center (Figure 1c).…”

Boronyl-Group-Assisted Decatungstate-Catalyzed Benzylic C(sp3)–H Alkylation

“…Boronic acid derivatives are essential molecules for synthetic organic chemistry because their carbon–boron bonds can be easily converted to carbon–carbon and carbon–heteroatom bonds despite their high bench-stability . In addition, boronates diversify out of simple synthetic intermediates and contribute significantly in modern pharmaceutical and materials sciences. − The general synthetic route to boronic acid derivatives involves the introduction of boronyl or boronate groups through transmetalation, hydroboration, catalytic borylation of a carbon–halogen bond, and C–H borylation (Scheme A). ,, In contrast, the functionalization of boronic acids with the retention of their boronyl groups has received limited attention (Scheme B). Although boronates have often been used as substrates in recent chemical reaction developments, the preparation of functionalized boronates with the aid of boronyl groups has been less explored.…”