A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot sequential strategy is also disclosed to access Heck-type products that are apparently difficult to make directly from arene carboxylic acids. With the increasing awareness of green and sustainable chemistry principles, devising straightforward catalytic protocols to access high-value products with enriched molecular complexity is highly significant in the organic chemistry community. [1] In this scenario, the transition-metal-catalyzed CÀ H bond activation concept that accounts direct utilization of otherwise inert CÀ H bond of organic molecules as a synthetic handle turned out very promising. [2] It renovates the synthetic policies as a greener alternative to traditional cross-coupling reactions by avoiding the need of pre-functionalized substrates and thereby improving overall step-and atom-economy. [3] Specifically, the cross-coupling reaction between two different C(sp 2)À H bonds represents a very powerful CÀ C bond-forming technology as it constitutes a twofold CÀ H functionalization manifold. [4] However, such oxidative cross-dehydrogenative couplings very often demand the employment of stoichiometric amounts of metal-based terminal oxidants such as Cu(II) or Ag (I) salts, generating significant amount of metallic wastes in conflict of the green chemistry principles. [5] Employment of abundant molecular oxygen in lieu of these metal-based oxidants would be a green asset, where water is the sole byproduct. [6] Further, a simple and cost-effective setup is expected if air can be directly engaged for the same purpose. Another critical environmental issue arises from the fact that these transformations, in general, consider a huge amount of organic solvent as compared to the other reagents and thus produce a bulk quantity of chemical waste. On the other hand, solvents play crucial roles in most of the organic transformations by controlling the reaction equilibrium and rate of the reaction. [7] Given the environmentally benign portfolio of water, a prompt [a] A
An efficient weak carboxylate-assisted oxidative cross-dehydrogenative C−H/C−H coupling (CDC) of heteroarenes with readily available olefins has been devised employing water as green solvent under ruthenium(II) catalysis. The reaction is operationally simple, accommodates a large variety of heteroaromatic carboxylic acids as well as olefins, and facilitates a diverse array of high-value olefin-tethered heteroarenes in high yields (up to 87%). The potential of this ortho-C−H bond activation strategy has also been exploited toward tunable synthesis of densely functionalized heteroarenes through challenging unsymmetrical bis-olefination process in a one-pot sequential fashion. Mechanistic investigation demonstrates a reversible ruthenation process and C−H metalation step might not be involved in the rate-determining step.
An enol-assisted regioselective arene C–H alkylation with maleimides is developed under redox-neutral ruthenium(II) catalysis, offering a wide variety of valuable 3-aryl succinimides including amino acid embedded frameworks in good to excellent yields. The products were also aromatized to produce synthetically useful resorcinol-based biaryls. Mechanistic studies support an organometallic pathway with a reversible C–H metalation step for this reaction.
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