An Olefinic 1,2‐Boryl‐Migration Enabled by Radical Addition: Construction ofgem‐Bis(boryl)alkanes

Abstract

As eries of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B 2 Pin 2 (bis(pinacolato)diboron) react with diverse alkyl halides by aR up hotocatalyst to give various gem-bis-(boryl)alkanes.A lkyl radicals add efficiently to the alkenyl diboronate complexes,and the adduct radical anions undergo radical-polar crossover,s pecifically,a1,2-boryl-anion shift from boron to the a-carbon sp 2 center.T his transformation shows good functional-group compatibility and can serve as ap owerful synthetic tool for late-stage functionalization in complex compounds.M easurements of the quantum yield reveal that aradical-chain mechanism is operative in whichthe alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Radical Addition Enables 1,2‐Aryl Migration from a Vinyl‐Substituted All‐Carbon Quaternary Center

“…[6] Furthermore, the 1,2‐metalate rearrangement [7] of vinylboronate complexes has been uncovered by using Et 3 B/air, [8] visible light [9] or nickel catalysts [10] (Figure 1 b). Inspired by these results, our group also developed a photoredox olefinic 1,2‐boryl‐migration by radical addition to build gem ‐bis(boryl)alkanes [11] . In addition, the Studer group has recently disclosed a novel 1,2‐boryl‐migration using allylboronic esters to access radical 1,3‐difunctionalization products (Figure 1 c).…”

Photoredox‐Mediated Reaction of gem‐Diborylalkenes: Reactivity Toward Diverse 1,1‐Bisborylalkanes

“…We posit that one efficient way to form gem ‐diboryl radical ( II ) can be achieved by adding carbon‐centered radicals to gem ‐diborylalkenes 2 as described in Scheme F. An analogous alkylation via generating α‐monoboryl carbon‐centered radicals IV using vinyl‐boron species III by adding a variety of carbon‐centered radicals to vinyl‐borons III has been reported by Aggarwal, Zard, Morken, Studer, Shi, Molander, and others (See Scheme E) . Considering the fact that gem ‐diboryalkenes ( 2 ) are more electron deficient than vinyl‐borons III are, they ( 2 ) are expected to be suitable candidates for this type of radical addition scenario.…”

“…One more advantage of this method over the other processes is that carboxylic acids are abundant and can be found in natural products, drugs, and bioactive organic molecules. Thus, carboxylic acids can easily be linked to gem ‐diboryl units; therefore, this method could serve as a powerful tool for late‐stage diversification . In this regard, in 2014, MacMillan reported a related cross reaction with RCOOH and acrylates .…”

Synthesis of 1,3‐Bis‐(boryl)alkanes through Boronic Ester Induced Consecutive Double 1,2‐Migration

“…More recently, we, Aggarwal, and Renaud developed radical polar crossover reactions, in which 1,2‐alkyl/aryl migrations of vinylboron ate complexes are induced by alkyl radical additions. This radical approach was further extended by Shi and co‐workers to the radical‐induced 1,2‐boron migration …”