Transition‐Metal‐Free Radical Borylation Reactions

Abstract

Dedication to Prof. Weisheng Tian on the occasion of his 70th birthdayOrganoboron compounds are versatile synthons in synthetic chemistry, materials science, and medicinal chemistry. The preparation of them often requires either reactive organometallic reagents or transition metals. In recent years, transition-metal-free borylation methodologies using radical chemistry for CÀ B bond formation have attracted much attention and developed rapidly, which circumvents the use of preformed organometallic reagents. Various successful ap-proaches from different radical precursors (such as carboxylic acids, amines and alcohols, which are abundant in nature) through either thermochemical, photochemical or electrochemical pathways have been developed. Therefore, in this review, we hope to provide an overview of recent advances in the discovery, development, and mechanistic study of transitionmetal-free radical borylation processes. Borylation of CÀ N BondsAmines are cheap, widely commercially available, and are abundant in drugs and natural products. Thus, transforming readily available amines and amine derivatives into valuable boronic esters will be of great use.

Copper-Mediated Dehydrogenative C(sp3)–H Borylation of Alkanes

“…This means that the subsequent photoinduced LMCT-mediated borylation becomes a redox-neutral process and therefore catalytic in CuCl 2 . To our knowledge, this represents the first report of bis-boryloxides being used as borylating agents for alkyl radicals and could have useful implications in radical-mediated borylation reactions by providing a polarity reversal from commonly used, nucleophilic diboron reagents. ,, …”

Transition metal- and solvent-free anti-Markovnikov selective protoboration of alkenes with bis(pinacolato)diboron

“…More importantly, transition metal-free and solvent-free approaches are more attractive from the standpoint of environmentally friendly and sustainable chemistry. 3…”

“…11 The synthesis of alkyl boronate esters in the absence of transition metals has gained significant attention recently. 3 In 2011, Fernández et al reported the first example of a transition-metal-free Lewis base Cs 2 CO 3 catalyzed diboration reaction of olefins with B 2 pin 2 . 12 The reaction progressed via the activation of a diboron reagent by in situ generation of an sp 2 boron nucleophile species in the presence of metal alkoxides.…”

“…Recently, there have been reports on the hydroboration of olefins with pinacolborane (HBpin) in the presence of Lewis acids and bases for the synthesis of alkyl boronate esters. 3 Oestreich et al reported the boron Lewis acid tris[3,5-bis(trifluoromethyl)phenyl]borane (BAr 3 F )-catalyzed regioselective 1,2- cis -hydroboration of various alkenes (Scheme 1a). 19 The Thomas group has utilized borane BH 3 ·THF as a catalyst for the anti -Markovnikov hydroboration of terminal alkynes and alkenes with HBpin.…”

“…There are reports on inorganic and organic base-catalyzed protoboration of alkenes using B 2 pin 2 as a boron reagent. 3 In 2016, Song and co-workers reported the hydroboration of arylacetylenes and vinyl arenes with B 2 pin 2 in the presence of 2.5 equiv. of Cs 2 CO 3 (Scheme 1e), and the mechanism involves tandem borylation and protodeboronation for the synthesis of alkyl boronate esters.…”