Iridium-catalyzed electrochemical C-H activation was accomplished within a cooperative catalysis manifold, setting the stage for electrooxidative C-H alkenylations through weak O-coordination. The iridium-electrocatalyzed C-H activation featured high functional-group tolerance through assistance of a metal-free redox mediator through indirect electrolysis. Detailed mechanistic insights provided strong support for an organometallic C-H cleavage and a synergistic iridium(III/I)/redox catalyst regime, enabling the use of sustainable electricity as the terminal oxidant with improved selectivity features.
Despite major advances, organometallic CÀH transformations are dominated by precious 5d and4dt ransition metals,s uch as iridium,p alladium and rhodium. In contrast, the unique potentialo fl ess toxic Earth-abundant 3d metals has been underexplored. While iron is the mostn aturally abundant transition metal, its use in oxidative, organometallic CÀHa ctivationh as faced major limitations due to the need for superstoichiometric amountso fc orrosive, cost-intensiveD CIB as the sacrificial oxidant. To fully address these restrictions, we describe herein the unprecedented merger of electrosynthesis with iron-catalyzed CÀHa ctivation through oxidation-inducedr eductive elimination. Thus, ferra-and manganaelectro-catalyzed CÀHa rylations werea ccomplished at mild reaction temperatures with ample scope by the action of sustainable iron catalysts, employing electricity as ab enign oxidant.CÀHa ctivation has surfaced as an increasingly powerful tool for molecular engineering, [1] with transformative applications throughout the materials ciences, [2] natural product syntheses, [3] late-stage diversification, [4] and have also been used on pharmaceutical industrials cales. [5] In particular, arylationso f otherwisei nert CÀHb onds have proven instrumental as a step-economical alternative to the Nobel Prize winning palladium-catalyzed cross-couplings, [6] avoiding lengthyp refunctionalizationp rotocols and therebyp reventing undesired waste formation. [7] While theseC ÀHa ctivationsh ave thus far been dominated by rare and toxic 4d transitionm etals (Figure 1a), considerable recent impetus was gainedb yi dentifying viable catalysts based on Earth-abundant [8] 3d base metals. [9] In particular,i nexpensive iron catalysis has gained considerable recent momentum due to its non-toxic nature (Figure 1b), [10] with major potentialf or translationala pplications on scale, particularly when considering trace metal impurities. Despite these major advances,a ll documented iron-catalyzed CÀHa rylations continue to be strongly limited by the need for superstoichiometricq uantities of dichloroisobutane (DCIB)a st he sacrificial oxidant,w hile simplev icinal dihalides or other chemical oxidants are generally not effective in iron-catalyzed CÀHa ctivations. [10c] Unfortunately,D CIB [11] is elusive on commercial scale, features considerables afety hazards, generates overstoichiometrica mounts of corrosive by-products,w hich overall signifi-Figure 1. Strategies for CÀHa rylation. (a) Preciousmetal catalyzed CÀHa rylation. (b) Iron-catalyzed CÀHa ctivationwith DCIB as oxidant. (c) Cost of goods:D CIB is as cost-intensive as is Pd(OAc) 2 .( d) Computed oxidation potential. (e) Thisreport: Electricity as oxidant for iron-catalyzed CÀHa ctivation.
Electrochemical selenation in undivided electrochemical cells allows the preparation of selenium‐containing naphthoquinones. The rapid, green and efficient protocol avoids chemical oxidants and enables the synthesis of target molecules in a fast and reliable way. This strategy provides and efficient and general method for the synthesis of quinoidal compounds with activity against five cancer cell lines and Trypanosoma cruzi, the parasite that causes Chagas disease.
Chromones represent a privileged scaffold in medicinal chemistry and are an omnipresent structural motif in natural products. Chemically encoded non-natural peptidomimetics feature improved stability towards enzymatic degradation, cell permeability and binding affinity, translating into a considerable impact on pharmaceutical industry. Herein, a strategy for the sustainable assembly of chromones via electro-formyl C–H activation is presented. The rational design of the rhodaelectro-catalysis is guided by detailed mechanistic insights and provides versatile access to tyrosine-based fluorogenic peptidomimetics.
Rhodium(III) catalysis has set the stage for a plethora of oxidative C-H functionalizations over the last decade, which have predominantly employed stoichiometric amounts of toxic and expensive metal oxidants, such as silver(I) salts. In the meantime, electrosynthesis has emerged as an increasingly viable alternative for expensive and toxic oxidants. Recently, significant momentum has been achieved with the merger of electrocatalysis with organometallic C-H activation. However, user-friendly and robust rhodaelectro-catalysis has until very recently proven elusive for oxidative C-H activations. This minireview highlights the current knowledge and recent advances of electrooxidation in rhodium-catalyzed C-H or C-C activations, with a topical focus on contributions from the Ackermann group through July 2020.
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