Direct C−H Bond Borylation of (Hetero)Arenes: Evolution from Noble Metal to Metal Free

Abstract

Organoboron compounds are among the most versatile and useful building blocks in modern synthetic chemistry. The past decades have seen impressive developments in the direct C−H bond borylation of (hetero)arenes in which the applied catalysts are shifting from noble metals to metal‐free systems. This Highlight gives a brief summary on this evolution and focuses on recent elegant work in this field.

Transient Imine as a Directing Group for the Metal-Free o-C–H Borylation of Benzaldehydes

“…During the past two decades, remarkable progress has been made in transition-metal-catalyzed directed C–H bond functionalization reactions. − One of the important transformations is borylation to produce organoboron reagents, , which potentially have wide applications in many research areas, including natural product synthesis, drug discovery, and advanced material synthesis . Among various C–H borylation strategies, − one of the commonly known strategies is a metal-catalyzed directed approach, which has been widely explored due to its reliability and for the high regioselectivity of such reactions (Figure A–C). , Initial explorations in this area focused on the use of either an in-built directing group, in which the functional group itself acts as a directing group (Figure A), or a directing group that has been preinstalled on a functional group (Figure B). Although this preinstalled auxiliary-directed, metal-catalyzed C–H borylation approach has enjoyed considerable success, one of the major issues is that this strategy is not step economical because the directing group needs to be first installed and then removed after the borylation.…”

“…7 Among various C−H borylation strategies, 8−11 one of the commonly known strategies is a metal-catalyzed directed approach, which has been widely explored due to its reliability and for the high regioselectivity of such reactions (Figure 1A−C). 12,13 Initial explorations in this area focused on the use of either an in-built directing group, in which the functional group itself acts as a directing group (Figure 1A), or a directing group that has been preinstalled on a functional group (Figure 1B). Although this preinstalled auxiliary-directed, metal-catalyzed C−H borylation approach has enjoyed considerable success, one of the major issues is that this strategy is not step economical because the directing group needs to be first installed and then removed after the borylation.…”

Organophosphorus-catalyzed relay oxidation of H-Bpin: electrophilic C–H borylation of heteroarenes

“…hydroboration with H–Bpin, of diazaphospholene catalysis might be diverted via sequential oxidative transfers, or relay oxidation, to access electrophilic “pinB + ” synthons 15 for delivery to suitable nucleophilic substrates for C–H borylation ( Scheme 1 , bottom). Herein, we realize this vision and achieve a C–H borylation 16 of electron-rich heteroarenes with H–Bpin as the boron donor under the catalytic action of phosphorus triamide 1 and a mild chloroalkane oxidant, establishing a new organophosphorus catalyzed platform for C–H borylation.…”

Application of Mutualism in Organic Synthetic Chemistry: Mutually Promoted C−H Functionalization of Indole and Reduction of Quinoline

“…In recent years, the selective functionalization (i. e. borylation or silylation) of C−H bond in N‐ heterocylic aromatics such as indoles has attracted intense attention due to their wide applications in synthesis, [1–3] medicinal chemistry [4,5] and materials sciences [6,7] . Different synthetic strategies have been developed to accomplish this goal by metal [8–23] or metal‐free catalysts [24–35] . Among them, potent boron Lewis acid tris(pentafluorophenyl)borane, B(C 6 F 5 ) 3 , have demonstrated its powerful capability in the C−H borylation (silylation) of indoles [36–39] .…”