Electrochemical β‐Selective Hydrocarboxylation of Styrene Using CO₂ and Water

Abstract

in enediolate intermediates. [1] Recently, a similar mechanism of CO 2 insertion at the unsaturated carbon bond has been adopted in the synthesis of carboxylic acids employing alkynes, [2] α-olefins [3] and internal alkenes as substrates. [4] These methods enabled CO 2 to be harnessed as a renewable one-carbon building block; however, the valorization of CO 2 is still challenging because the gas is thermodynamically and kinetically stable. [5] Consequently, numerous advances in chemical carboxylation using CO 2 have relied on highly reactive organometallic nucleophiles to facilitate the reaction. [5,6] Recent reports have demonstrated that the elaborate design of organometallic nucleophiles is the primary requisite for modulation of site-selectivity and extension of substrates in carboxylation. [6,7] Recently, Martin and co-workers developed an elegant protocol for site-selectivity tunable carboxylation via nickel hydride or nickelalactone formation for the extensive scope of unsaturated hydrocarbons, such as styrenes, alkenes and alkynes. [8] As an alternative approach, heterogeneously catalyzed electrochemical carboxylation has gained increasing attention. This reactions are mainly driven by reductive electrical potential on a cathode electrode. [9] Therefore, the reduction reaction takes place in the absence of a reducing agent and the reducing power can be easily controlled by the applied potentials. Among the unsaturated hydrocarbon feedstocks, this study focused on the electrochemical carboxylation of styrene as a representative model.In the carboxylation of styrene using CO 2 , hydrocarboxylation of the αor β-position and dicarboxylation at both positions are feasible. Vianello and co-workers first pioneered the electrochemical dicarboxylation of styrene to form 2-phenylsuccinic acid (1). [10] Since then, several succeeding works on the electrochemical carboxylation of styrene have reported dicarboxylation as a primary reaction both in the presence [11] and absence [12] of homogeneous catalysts. These studies proposed the electrochemical formation of β-carboxylate radical anions as a key intermediate, followed by additional CO 2 insertion to the benzylic position. [12a,c] It has been hypothesized that the electrochemical carboxylation of styrene is mostly carried out by the dicarboxylation pathway, whereas addition of water as protic agent can change reaction pathway to The carboxylation of hydrocarbons using CO 2 as a one-carbon building block is an attractive route for the synthesis of carboxylic acids and their derivatives. Until now, chemical carboxylation catalyzed by organometallic nucleophiles and reductants has been generally adopted particularly for the precise selectivity control of carboxylation sites. As another approach, electrochemical carboxylation has been attempted but these carboxylation reactions are limited to only a few pathways. In the case of styrene, dicarboxylation at the αand β-positions is mostly observed with electrochemical carboxylation while site-selective hydrocarboxy...

Visible-Light-Driven Anti-Markovnikov Hydrocarboxylation of Acrylates and Styrenes with CO 2

“…As 3-arylpropionic acids also are valuable compounds (Figure 1a), 35,[43][44][45] we further investigated the visible-lightdriven photocatalyst-free anti-Markovnikov hydrocarboxylation of styrenes (Figure 2). Under slightly modified reaction conditions (Condition B: p-t BuC 6 H 4 SH is used instead of 2,4,6-triisopropylthiophenol in Condition A), diverse styrenes bearing either electron-neutral (3a-3c, 3i, and 3m) or electron-donating (3d, 3f, and 3j) groups on the arene moiety all reacted smoothly to afford the desired products 4 in moderate to good yields.…”

“…17,18,22,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Notably, Iwasawa pioneered this strategy by realizing the hydrocarboxylation of alkenes with Markovnikov regioselectivity via visible-light photoredox/Rh dual catalysis. 27,41 Besides Markovnikov selectivity, König et al 35 also realized a novel hydrocarboxylation of styrenes with anti-Markovnikov regioselectivity 26,[43][44][45] via photoredox/ nickel dual catalysis. In these cases, both photocatalysts and transition-metal catalysts are indispensable.…”

Electrocatalytic Reactions for Converting CO2 to Value‐Added Products

“…[69] The activation of CO 2 and olefin can also occur simultaneously, as reported in other works. [70][71][72] In general, the reduction of alkenes and olefins is the preferred approach. Buckley and coworkers reported an electrosynthetic approach using stainless steel as cathode and triethanolamine c) Reproduced with permission.…”

“…c) Reaction mechanism of electrochemical β-selective hydrocarboxylation of styrene. Reproduced with permission [70]. Copyright 2019, Wiley.…”

Recent Advances in the Synthesis of Ibuprofen and Naproxen

“…Iron-catalyzed hydrocarboxylation, as reported by Thomas et al, is shown in Scheme 4 [ 27 ]. Although hydrocarboxylation of styrene was expected to introduce the ibuprofen skeleton, the regioselectivity hampered it [ 28 ]. However, the highly selective addition of CO 2 was possible by employing an iron catalyst and pyridine ligand 15 .…”