Arene C–H borylation strategy enabled by a non-classical boron cluster-based electrophile

Abstract

Introducing a tri-coordinate boron-based functional group (e.g., boronic ester) into an unactivated C–H bond in the absence of directing groups is an ongoing challenge in synthetic chemistry. Despite previous developments in transition metal-catalyzed and -free approaches, C–H borylation of sterically hindered arenes remains a largely unsolved problem to date. Here, we report a synthetic strategy of a two-step, precious metal-free electrophilic C–H borylation of sterically hindered alkyl- and haloarenes to generate aryl boronic esters. The first step relies on electrophilic aromatic substitution (EAS) induced by cage-opening of Cs2[closo-B10H10], forming a 6-Ar-nido-B10H13 product containing a B–C bond, followed by a cage deconstruction of arylated decaboranes promoted by diols. The combination of these two steps allows for the preparation of aryl boronic esters that are hardly accessible by current direct C–H borylation approaches. This reaction does not require any precious metals, highly-engineered ligands, pre-functionalized boron reagents, or inert conditions. In addition, the unique properties of a non-classical boron cluster electrophile intermediate, B10H13+, afford a regioselectivity with unique steric and electronic control without the undesirable side reactions.

Redox-Active Boron Clusters

“…Various alkylbenzenes and halobenzenes were examined with optimized conditions, and we determined that the resulting selectivity was highly dependent on the functional groups of the arene compounds. A few examples of our overall observations are that monosubstituted benzene compounds gave the corresponding aryl boronic esters with high para selectivity following the electronic control commonly observed in EAS reactions 49 (Figure 10B). Sterically bulkier alkyl groups also showed higher para selectivity in general.…”

“…(A)–(B) Electrophilic borylation approach using closo -[B 10 H 10 ] 2– as a synthetic reagent. Panel (A) adapted from ref with permission. Copyright 2023 Springer Nature.…”

“…Interestingly, a catalyst-free thermal arylation 50 used previously for closo-[B 12 H 12 ] 2− can be employed for direct arylation of closo-[B 10 H 10 ] 2− , resulting in the formation of polyarylated clusters. 49 Oxidative deconstruction of these species leads to the isolation of mononuclear boronate ester molecules. It is worth noting that during the oxidation, a transient polyarylated cluster compound featuring a deep blue color and a diagnostic EPR signal consistent with the presence of a boron-centered radical can be observed.…”

“…Interestingly, a catalyst-free thermal arylation used previously for closo- [B 12 H 12 ] 2– can be employed for direct arylation of closo- [B 10 H 10 ] 2– , resulting in the formation of polyarylated clusters . Oxidative deconstruction of these species leads to the isolation of mononuclear boronate ester molecules.…”

Decaborane: From Alfred Stock and Rocket Fuel Projects to Nowadays

“…Hydrogen atoms of organic substituents in the left structure are omitted for clarity. The reactions of Cs2[closo-B10H10] with triflic acid or (NH4)2[closo-B10H10] with sulfuric acid in the presence of aromatic hydrocarbons produce the corresponding 6-aryl derivatives of decaborane [nido-6-Ar-B10H13] (Ar = Ph, C6H4-4-Me, C6H3-3,5-Me2, C6H2-2,4,6-Me3, C6H2-2,4,6i Pr3, C6H4Cl, C6H4CF3)[202,203,321,340]. The solid state structures of [6-Ph-B10H13], [6-p-Tol-B10H13], and [6-(2′,4′,6′i Pr3-C6H2-B10H13] were determined by singlecrystal X-ray diffraction (Figure38)[202,203,340].…”