Electrochemistry enabled C-H/N-H functionalizations at room temperature by external oxidant-free cobalt catalysis. Thus, the sustainable cobalt electrocatalysis manifold proceeds with excellent levels of chemoselectivity and positional selectivity, and with ample scope, thus allowing electrochemical C-H activation under exceedingly mild reaction conditions at room temperature in water.
Electrocatalysis has been identified as a powerful strategy for organometallic catalysis, and yet electrocatalytic C-H activation is restricted to strongly N-coordinating directing groups. The first example of electrocatalytic C-H activation by weak O-coordination is presented, in which a versatile ruthenium(II) carboxylate catalyst enables electrooxidative C-H/O-H functionalization for alkyne annulations in the absence of metal oxidants; thereby exploiting sustainable electricity as the sole oxidant. Mechanistic insights provide strong support for a facile organometallic C-H ruthenation and an effective electrochemical reoxidation of the key ruthenium(0) intermediate.
Expedient hydroarylations of C=Het bonds (Het=heteroatom) were accomplished by user-friendly organometallic C-H activation in a positional-selective manner. The broadly applicable C-H functionalization platform enabled the step-economical transformation of aldehydes, ketones, and imines under additive-free reaction conditions. In contrast to palladium, rhodium, ruthenium, rhenium, iridium, nickel, and cobalt catalysis, solely manganese(I) complexes outcompeted the innate substrate control, clearly highlighting the unique power of manganese(I) C-H activation catalysis.
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.
The catalytic generation of hypervalent iodine(III) reagents by anodic electrooxidation was orchestrated towards an unprecedented electrocatalytic C−H oxygenation of weakly coordinating aromatic amides and ketones. Thus, catalytic quantities of iodoarenes in concert with catalytic amounts of ruthenium(II) complexes set the stage for versatile C−H activations with ample scope and high functional group tolerance. Detailed mechanistic studies by experiment and computation substantiate the role of the iodoarene as the electrochemically relevant species towards C−H oxygenations with electricity as a sustainable oxidant and molecular hydrogen as the sole by‐product. para‐Selective C−H oxygenations likewise proved viable in the absence of directing groups.
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