Ketones and alkynes join together: The catalytic title reaction proceeds rapidly at 120 °C and requires only 1 hour for completion (see scheme). The reaction mechanism likely involves chelation‐assisted ortho CH activation, insertion of the alkyne moiety, carbocyclization, and protonation. Cp*=pentamethylcyclopentadienyl.

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A green atom‐economical method for the synthesis of highly functionalized 1‐amino and 1‐carbon substituted isoquinolines from the reaction of N′‐hydroxybenzimidamides and aryl ketoximes, respectively, with alkynes via pentamethylcyclopentadienylcobalt(III)‐catalyzed CH/NO bond activation is described. The external oxidant‐free annulation reaction uses the =NOH moiety in N′‐hydroxybenzimidamides or N‐aromatic ketone oximes as the directing group and internal oxidant. This first row transition metal‐catalyzed annulation serves as an efficient alternative for the synthesis of isoquinolines, as water is the only by‐product and expensive noble metals such as rhodium(III), iridium(III), palladium(II), and ruthenium(II) are not required. The reaction proceeds via CH activation, alkyne insertion, reductive elimination, and NO activation.magnified image

Visible-light induced, palladium catalyzeda lkylations of a,b-unsaturated acids with unactivated alkyl bromides are described. Av ariety of primary,s econdary,a nd tertiary alkylb romides are activated by the photoexcited palladium metal catalyst to provide aseries of olefins at room temperature under mild reaction conditions.Mechanistic investigations and density functional theory (DFT) studies suggest that ap hotoinduced inner-sphere mechanism is operative in whichabarrierless,single-electron transfer oxidative addition of the alkyl halide to Pd 0 is key for the efficient transformation.Cross-coupling reactions are one of the most important synthesis tools for the formation of C À Co rC-heteroatom bonds.C onventionally,t hree key steps are involved in the cross-coupling reaction:o xidative addition, transmetalation, and reductive elimination. Often challenges arise in the individual steps,d epending on the coupling partners,m etal catalysts or reaction conditions employed. In recent decades, numerous catalysts,aswell as ligands,have been developed in order to reduce the energy barrier in the bond-forming and breaking steps.More recently,ithas been shown that visiblelight enhances cross-couplings employing radicals. [1] This methodology allows single-electron transfer (SET) to replace the conventional two electron transfer process by using photoredox and transition metal dual-catalytic systems,t hus significantly reducing the barrier for each step in crosscoupling reactions. [2] Despite developments made in crosscoupling chemistry,t he conventional oxidative addition of alkyl electrophiles to Pd 0 remains sluggish, even at elevated temperature,o wing to the electron-rich nature of the alkyl halide bond. [3] In fact, computational studies have revealed that the alkyl bromide oxidative addition to Pd 0 is endothermic with ah igh energy barrier of 41.6 kcal mol À1 .F urther, alkyl-palladium(II) species that are formed through at wo-electron transfer are considerably less stable because of the lack of p-electrons interacting with the empty d-orbitals of the metal. This results in fast b-hydride (b-H) elimination from salkyl-palladium(II) intermediates which outcompetes the required olefin insertion in decarboxylative alkylation crosscoupling reactions. [3,4] In contrast to the photoredox dual-catalysis strategy which promotes cross-coupling by lowering the energy barrier, we now report the use of am echanistically distinct, SET barrierless oxidative addition strategy for alkylation of vinylic acids,using Pd complexes as both photosensitizer and cross-coupling catalyst. [5] This lowers the oxidative addition barrier resulting in an alkyl radical and aP d I intermediate (Figure 1) which can add to an a,b-unsaturated acids. Subsequent carbon dioxide elimination provides the akylated olefin and the regenerated catalyst. To the best of our knowledge,avisible light-induced palladium catalyzed decarboxylative C(sp 3 ) À C(sp 2 )c ross-coupling alkylation has not been reported.Vinylic acids are readily available and stable,...

Four cobalt-catalyzed oxidative annulation reactions of nitrogen-containing arenes with alkynes proceeds by C-H activation, thus leading to biologically useful quaternary ammonium salts, including pyridoisoquinolinium, cinnolinium, isoquinolinium, and quinolizinium salts, in high yields. The results are comparable to those reactions catalyzed by rhodium and ruthenium complexes. The transformation of the salts into various N-heterocycles has also been demonstrated.

A new cobalt-catalyzed phenolic OH-assisted C–H functionalization of 2-vinylphenols with allenes to give various 2H-chromenes is described. It is the first time that allenes are used as the coupling partners in the cobalt-catalyzed C–H activation reactions. In most cases, cobalt-catalyzed oxidative annulation of arenes with alkenes or alkynes via C–H activation gave [4 + 2] or [3 + 2] cyclization products, but the present catalytic reaction afforded an oxidative [5 + 1] cyclization products with the allenes acting as a one-carbon coupling partner. The catalytic reaction is proposed to proceed via the C–H activation of the vinyl group, allene insertion, and an unusual intramolecular regioselective phenoxide addition.

An efficient Ru(II)-catalyzed amidation of 2-arylpyridines with isocyanates via C-H bond activation is described.

Various methods that use a photocatalyst for electron transfer between an organic substrate and a transition metal catalyst have been established. While triplet sensitization of organic substrates via energy transfer from photocatalysts has been demonstrated, the sensitization of transition metal catalysts is still in its infancy. Here, we describe the selective alkylation of C(sp3)–H bonds via triplet sensitization of nickel catalytic intermediates with a thorough elucidation of its reaction mechanism. Exergonic Dexter energy transfer from an iridium photosensitizer promotes the nickel catalyst to the triplet state, thus enabling C–H functionalization via the release of bromine radical. Computational studies and transient absorption experiments support that the reaction proceeds via the formation of triplet states of the organometallic nickel catalyst by energy transfer.