Mechanistic Insight into Ketone α-Alkylation with Unactivated Olefins via C–H Activation Promoted by Metal–Organic Cooperative Catalysis (MOCC): Enriching the MOCC Chemistry

Mechanistic Investigation on the Regioselectivity of Electrochemical Co(II)-Catalyzed [2 + 2 + 2] Cycloaddition of Terminal Acetylenes

In this paper, the regioselectivity of electrochemical Co(II)-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes was investigated using density functional theory. We explored in detail the energy profiles for both 1,2,4- and 1,3,5-regioselectivity pathways and revealed the origin of the regioselectivity. Two kinds of conformational isomers derived from the different coordination modes of alkynes with cobaltacyclopentadiene have been found, which were formed through electrochemically mediated redox processes. The regioselectivity of the reaction depends on the two coordination modes. When the Co(II) center attacks α-C of the third alkyne, while β2-C in cyclopentadiene bonds to β-C of the alkyne, the reaction favors the formation of 1,2,4-products. In contrast, when the Co(II) center connects to β-C of the alkyne, it forms only the 1,3,5-products via [4 + 2] cycloaddition because of the steric repulsion between the bulky ligand on Co(II) and the phenyl group in the alkyne.

Mechanism of iron complexes catalyzed in the N-formylation of amines with CO2 and H2: the superior performance of N-H ligand methylated complexes

CO2 hydrogenation into value-added chemicals not only offer an economically beneficial outlet but also help reduce the emission of greenhouse gases. Herein, the density functional theory (DFT) studies have been carried out on CO2 hydrogenation reaction for formamide production catalyzed by two different N-H ligand types of PNP iron catalysts. The results suggest that the whole mechanistic pathway has three parts: (i) precatalyst activation, (ii) hydrogenation of CO2 to generate formic acid (HCOOH), and (iii) amine thermal condensation to formamide with HCOOH. The lower turnover number (TON) of a bifunctional catalyst system in hydrogenating CO2 may attribute to the facile side-reaction between CO2 and bifunctional catalyst, which inhibits the generation of active species. Regarding the bifunctional catalyst system addressed in this work, we proposed a ligand participated mechanism due to the low pKa of the ligand N-H functional in the associated stage in the catalytic cycle. Remarkably, catalysts without the N-H ligand exhibit the significant transfer hydrogenation through the metal centered mechanism. Due to the excellent catalytic nature of the N-H ligand methylated catalyst, the N-H bond was not necessary for stabilizing the intermediate. Therefore, we confirmed that N-H ligand methylated catalysts allow for an efficient CO2 hydrogenation reaction compared to the bifunctional catalysts. Furthermore, the influence of Lewis acid and strong base on catalytic N-formylation were considered. Both significantly impact the catalytic performance. Moreover, the catalytic activity of PNMeP-based Mn, Fe and Ru complexes for CO2 hydrogenation to formamides was explored as well. The energetic span of Fe and Mn catalysts are much closer to the precious metal Ru, which indicates that such non-precious metal catalysts have potentially valuable applications.

N-Heterocyclic Carbene-Catalyzed Asymmetric C-O Bond Construction Between Benzoic Acid and o-Phthalaldehyde: Mechanism and Origin of Stereoselectivity

A computational study was contributed to explore the origin of stereoselectivity of NHC-mediated cyclization reaction between benzoic acid and o-phthalaldehyde for asymmetric construction of phthalidyl ester. The most energetically favorable pathway mainly includes the following steps: (1) nucleophilic attack on carbonyl carbon of o-phthalaldehyde by catalyst NHC, (2) formation of Breslow intermediate, (3) oxidation by DQ, (4) asymmetric formation of dual C-O bonds, and (5) dissociation of catalyst with the product. The C-O bond formation was testified as the stereoselectivity-determining step, the R-configurational pathway is more energetically favorable than the S-configurational one. The non-covalent interaction (NCI) and atom-in-molecule (AIM) analyses were performed to reveal that the O-H ⋅⋅⋅ O and C-H ⋅⋅⋅ O hydrogen-bond interactions are the key factors for controlling the stereoselectivity. The detailed mechanism and origin of stereoselectivity give useful insights for understanding organocatalytic reactions for asymmetric construction of C-O bond.

Mechanism of nickel-catalyzed direct carbonyl-Heck coupling reaction: the crucial role of second-sphere interactions

We present a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates and aldehydes to afford ketones. The precatalyst Ni(COD)2 is activated with the phosphine (phos) ligand, followed by coordination of the substrate PhOTf, to form [Ni(phos)(PhOTf)] for intramolecular PhOTf to Ni(0) oxidative addition. The ensuing phenyl-Ni(ii) triflate complex substitutes benzaldehyde for triflate by an interchange mechanism, leaving the triflate anion in the second coordination sphere held by Coulomb attraction. The Ni(ii) complex cation undergoes benzaldehyde C[double bond, length as m-dash]O insertion into the Ni-Ph bond, followed by β-hydride elimination, to produce Ni(ii)-bound benzophenone, which is released by interchange with triflate. The resulting neutral Ni(ii) hydride complex leads to regeneration of the active catalyst following base-mediated deprotonation/reduction. The benzaldehyde C[double bond, length as m-dash]O insertion is the rate-determining step. The triflate anion, while remaining in the second sphere, engages in electrostatic interactions with the first sphere, thereby stabilizing the intermediate/transition state and enabling the desired reactivity. This is the first time that such second-sphere interaction and its impact on cross-coupling reactivity has been elucidated. The new insights gained from this study can help better understand and improve Heck-type reactions.

Catalytic addition of C-H bonds across C-C unsaturated systems promoted by iridium(i) and its group IX congeners

Transition metal-catalyzed hydrocarbonations of unsaturated substrates have emerged as powerful synthetic tools for increasing molecular complexity in an atom-economical manner. Although this field was traditionally dominated by low valent rhodium and ruthenium catalysts, in recent years, there have been many reports based on the use of iridium complexes. In many cases, these reactions have a different course from those of their rhodium homologs, and even allow performing otherwise inviable transformations. In this review we aim to provide an informative journey, from the early pioneering examples in the field, most of them based on other metals than iridium, to the most recent transformations catalyzed by designed Ir(i) complexes. The review is organized by the type of C-H bond that is activated (with C sp², sp or sp³), as well as by the C-C unsaturated partner that is used as a hydrocarbonation partner (alkyne, allene or alkene). Importantly, we discuss the mechanistic foundations of the methods highlighting the differences from those previously proposed for processes catalyzed by related metals, particularly those of the same group (Co and Rh).

Distinct Roles of Ag(I) and Cu(II) as Cocatalysts in Achieving Positional-Selective C-H Alkenylation of Isoxazoles: A Theoretical Investigation

For C-H alkenylation of aryl-substituted diarylisoxazoles, one mode is N-directed C-H alkenylation and the other is C-H alkenylation in the isoxazole ring. In this study, selective C-H alkenylations of 3,5-diarylisoxazoles have been investigated theoretically with the aid of density functional theory (DFT) calculations. With Cp*Rh^III as the catalyst, the N-directed C-H alkenylation is preferred as a result of the stronger interaction energy caused by the nitrogen-directing effect. With Pd(OAc)₂ as the catalyst and Ag₂CO₃ as the cocatalyst, their combination switches the regioselectivity to the C-H alkenylation in the isoxazole ring. The strong structural distortion involved in the competing N-directed olefin insertion transition state was found to suppress N-directed C-H alkenylation. With Pd(OAc)₂ as the catalyst and Cu(OTf)₂ as the cocatalyst, the N-directed C-H alkenylation becomes preferred due to the strong coordination of the nitrogen atom to the copper center. In particular, the structural and mechanistic information involved in the above two heterodimetallic Pd/Ag and Pd/Cu catalytic systems will help toward understanding and designing novel relevant heterodimetallic-catalyzed reactions.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Mechanistic Study of Unprecedented Highly Regioselective Hydrocyanation of Terminal Alkynes: Insight into the Origins of the Regioselectivity and Ligand Effects

Density functional theory (DFT) calculations were performed to gain insight into the mechanism of the nickel-catalyzed hydrocyanation of terminal alkynes with Zn(CN)₂ and water to exclusively generate the branched nitrile with excellent Markovnikov selectivity. After precatalyst activation to give the LNi(0) active species, the transformation proceeds via the following steps: (1) oxidative addition of H₂ O to the LNi(0) provides the intermediate LNi(II)H(OH); (2) ligand exchange of LNi(II)H(OH) with Zn(CN)₂ gives the intermediate LNi(II)H(CN); (3) alkyne insertion to the LNi(II)H(CN) forms the alkenyl nickel complex, followed by the reductive elimination step reaching the final product. This mechanism is kinetically and thermodynamically more favorable than that of the experimental proposed ones. On the basis of the experimental observations, more water molecules cannot further improve the reaction as it has also been rationalized. Furthermore, the origin of the high regioselectivity of the product, the variable effectiveness of the metal mediator as function of ligands, as well as the high yield of the alkyl-substituted alkynes substrates, is analyzed in detail. © 2019 Wiley Periodicals, Inc.

Nickel-catalyzed hydroalkenylation of styrene with phenylpropanal: theoretical studies on the mechanism, regioselectivity, and role of phenylboronic acid

The effects of different solvents, Brønsted acids, and ligands on reactivity are clarified through our calculations.

Ligands and Bases Mediate Switching between Aminocarbonylations and Alkoxycarbonylations in Coupling of Aminophenols with Iodoarenes

The mechanisms of aminocarbonylations and alkoxycarbonylations in coupling of aminophenols with iodoarenes catalyzed by the bidentate phosphorus ligand Pd complexes were explored with theoretical calculations. The origins of chemoselective carbonylation mediated by ligands and bases were disclosed. According to our calculations, the bifurcation points of reaction pathways caused by different ligands and bases combinations are ^L1/L2Int5, a [DPPP/DIBPP]benzoylpalladium(II)iodide complex. The affinity of ^L1/L2Int5 and adducts (K₂CO₃ and DBU), as well as the substrate itself, are the predominant factors of switching from aminocarbonylation to alkoxycarbonylation. The results reveal that K₂CO₃ directly exchanges iodine with ^L1Int5 and assists in hydrogen transfer in the DPPP-K₂CO₃ combination, in which alkoxycarbonylation is more favorable than aminocarbonylation, while for the DIBPP-DBU combination, iodine exchange is achieved by means of the hydrogen bond formed between the carbonyl group on ^L2Int5 and the substrate amino H due to the influence of the ligand, and then iodine exchange occurs; subsequently DBU-assisted amino H transfers to complete the aminocarbonylation. The proton transfer is the step that determines the chemoselectivity in the DPPP-K₂CO₃ combination. The iodine exchange determines the chemoselectivity between aminocarbonylation and alkoxycarbonylation in the DIBPP-DBU one. These results would be helpful to deeply understand the roles of each component in a chemoselective reaction in a multicomponent complex system.

Branched-Selective Direct α-Alkylation of Cyclic Ketones with Simple Alkenes

Herein, we describe an intermolecular direct branched-selective α-alkylation of cyclic ketones with simple alkenes as the alkylation agents. Through an enamine-transition metal cooperative catalysis mode, the α-alkylation is realized in an atom- and step-economic manner with excellent branched selectivity for preparing β-branched ketones. Employment of a pair of bulky Brønsted acid and base as additives is responsible for enhanced efficiency. Promising enantioselectivity (74 % ee) has been obtained. Experimental and computational mechanistic studies suggest that a pathway through alkene migratory insertion into the Ir-C bond followed by C-H reductive elimination is involved for the high branched selectivity.

A DFT study of Co(i) and Ni(ii) pincer complex-catalyzed hydrogenation of ketones: intriguing mechanism dichotomy by ligand field variation

Ligand field variation governs the mechanism dichotomy for isoelectronic catalysts.

Mechanism of the Visible-Light-Mediated Copper-Catalyzed Coupling Reaction of Phenols and Alkynes

A recent experimental study reported a visible-light-mediated aerobic oxidative coupling reaction of phenol with alkynes that produces hydroxyl-functionalized aryl ketones using inexpensive CuCl as catalyst under mild conditions. Here we apply the complete active space self-consistent field (CASSCF) method and multistate second-order perturbation (MS-CASPT2) theory in combination with density functional theory (DFT) to systematically explore the entire photocatalytic reaction between phenol and phenylacetylene in acetonitrile solution in the presence of molecular oxygen and CuCl. Our main findings are as follows: (1) The visible-light-driven conversion of phenylacetylene to PhCCCu(I) occurs thermally because of efficient excited-state deactivation to the S₀ state. (2) The single electron transfer from PhCCCu(I) to molecular oxygen that leads to the PhCCCu(II) cation takes place in the T₁ state after an efficient S₁ → T₁ intersystem crossing. (3) During the initial oxidation of phenol, molecular oxygen prefers to attack the para position of the phenol radical intermediate to produce 1,4-benzoquinone, which further reacts with PhCCCu(II) to generate para-hydroxyl-substituted aryl ketones; this is the origin of the experimentally observed regioselectivity. (4) The C≡C bond of the phenylacetylene moiety is not activated by the triplet-state single electron transfer from PhCCCu(I) to molecular oxygen but is cleaved at a later stage, in the [2+2] cycloaddition between PhCCCu(II) and 1,4-benzoquinone. (5) The substrate phenol plays an active role in several hydrogen transfer and decarboxylation reactions; the barriers to these phenol-assisted reactions are lower than those for the corresponding direct or water-assisted reactions, which explains the experimental finding that adding water does not enhance the photocatalytic reaction yield. In summary, while supporting the general features of the experimentally proposed mechanism, our computational study provides detailed mechanistic insights that should be useful for understanding and further improving visible-light-induced copper-catalyzed coupling reactions.

A theoretical study on the oxidation of alkenes to aldehydes catalyzed by ruthenium porphyrins using O2 as the sole oxidant

Density functional theory (DFT) calculations were used to study the ruthenium porphyrin-catalyzed oxidation of styrene to generate an aldehyde. The results indicate that two reactive oxidants, dioxoruthenium and monooxoruthenium-superoxo porphyrins, participate in the catalytic oxidation. In the mechanism, the resultant monooxoruthenium porphyrin acts in the tandem epoxide isomerization (E-I) to selectively yield an aldehyde and generate a dioxoruthenium porphyrin, thereby triggering new oxidation reaction cycles. In this calculation, several key elements responsible for the observed oxidative ability have been established by using Frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, etc., which include the reaction energy, the spin exchange effect, the spin-state conversion process, and the energy level of the lowest unoccupied molecular orbitals (LUMOs) of the reactive oxidants. The comparative oxidative abilities of the ruthenium-oxo/superoxo compounds with different axial ligands are also investigated. The results suggest that the ruthenium-oxo/superoxo species featuring a chlorine axial ligand is more reactive than that substituted with oxygen. This tuneable reactivity can be understood when considering the different electronic characters of the two ligands and the effective atomic number rule (EAN).

Multiple pathways for C–H cleavage in cationic Cp*Rh(iii)-catalyzed C–H activation without carboxylate assistance: a computational study

Multiple pathways for C–H cleavage was uncovered in cationic Cp*Rh(iii)-catalyzed C–H functionalization with different heteroatom-containing species as possible proton acceptors.

Theoretical investigation of the impact of ligands on the regiodivergent Rh-catalyzed hydrothiolation of allyl amines

The impacts of ligands on regioselectivity were rationalized by using density functional theory calculations.

Mechanism, selectivity, and reactivity of iridium- and rhodium-catalyzed intermolecular ketone α-alkylation with unactivated olefinsviaan enamide directing strategy

DFT calculations were performed to investigate the title reaction, focusing on detailed reaction mechanism and origins of selectivity and reactivity.

Mechanism and origins of the directing group-controlled endo- versus exo-selectivity of iridium-catalysed intramolecular hydroalkenylation of 1,1-disubstituted alkenes

The reaction mechanism and the important role of directing groups in determining selectivity have been investigated by means of DFT calculations.

Mechanistic insight into the C7-selective C–H functionalization of N-acyl indole catalyzed by a rhodium complex: a theoretical study

The role of the additive AgNTf₂ and the origins of the reaction are clarified through our calculations.

Theoretical insight into the zinc(ii)-catalyzed synthesis of 2-indolyltetrahydroquinolines from N-propargylanilines and indoles: cross-dehydrogenative coupling with temporally separated catalytic activity

An abnormal catalytic cascade reaction with temporally separated catalytic activity.

Rh(III)-catalyzed oxidative ortho-C–H alkylation of 2,4-diarylquinazoline with potassium alkyltrifluoroborates

Alkyltrifluoroborates were used for Rh(III)-catalyzed ortho-alkylation of 2,4-disubstituted quinazoline via C–H bond activation. The reaction proceeded well with a broad substrate scope, providing a direct way to access high functional quinazoline core structure derivatives in yields up to 95%.

Stabilization of Two Radicals with One Metal: A Stepwise Coupling Model for Copper-Catalyzed Radical-Radical Cross-Coupling

Transition metal-catalyzed radical-radical cross-coupling reactions provide innovative methods for C-C and C-heteroatom bond construction. A theoretical study was performed to reveal the mechanism and selectivity of the copper-catalyzed C-N radical-radical cross-coupling reaction. The concerted coupling pathway, in which a C-N bond is formed through the direct nucleophilic addition of a carbon radical to the nitrogen atom of the Cu(II)-N species, is demonstrated to be kinetically unfavorable. The stepwise coupling pathway, which involves the combination of a carbon radical with a Cu(II)-N species before C-N bond formation, is shown to be probable. Both the Mulliken atomic spin density distribution and frontier molecular orbital analysis on the Cu(II)-N intermediate show that the Cu site is more reactive than that of N; thus, the carbon radical preferentially react with the metal center. The chemoselectivity of the cross-coupling is also explained by the differences in electron compatibility of the carbon radical, the nitrogen radical and the Cu(II)-N intermediate. The higher activation free energy for N-N radical-radical homo-coupling is attributed to the mismatch of Cu(II)-N species with the nitrogen radical because the electrophilicity for both is strong.

Mechanistic insights into the selective cyclization of indolines with alkynes and alkenes to produce six- and seven-membered 1,7-fused indolines via Rh(iii) catalysis: a theoretical study

The origin of different partners for the coupling reaction determining the different products was rationalized.

Computational studies on the Rh-catalyzed carboxylation of a C(sp2)–H bond using CO2

CO₂ insertion is facilitated by the critical effects of a Lewis acid and an agostic interaction.

Differences between the elimination of early and late transition metals: DFT mechanistic insights into the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes

Early transition metals (TMs), such as titanium, generally resist undergoing reductive elimination to form C-X bonds due to their weak electronegativity. By analyzing the mechanism of the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes, the present study revealed that titanium is able to promote C-N bond formation via an unconventional elimination pathway, passing through a comparatively stable masked Ti^II complex (i.e., IM4) rather than pyrrole directly. The formation of IM4 originates from the bilateral donation and back-donation between Ti and the pyrrole ligand. Formally, it could be considered that the two electrons resulting from the unconventional reductive elimination are temporarily buffered by back-donation to a symmetry-allowed unoccupied π-orbital of the pyrrole ring in IM4 rather than becoming a lone pair on a Ti center as adopted in the catalysis of late TMs. Because of its stability, IM4 requires additional oxidation by diazene to liberate pyrrole. The triplet counterpart (IM4^T ) of IM4 is more stable than IM4, but the elimination is unlikely to reach IM4^T , because the process is spin-forbidden and the spin-orbit coupling is weak. Alternatively, one may consider the forming pyrrole in IM4 as a redox-active ligand, reserving the two electrons resulting from the formal reductive elimination and then releasing the electrons when IM4 is oxidized by diazene. These insights allow us to propose the conditions for early TMs to undergo a similar elimination, whereby the forming product will have symmetry-allowed frontier molecular orbitals to form donation and back-donation bonding with a TM center and a substrate possessing a comparatively strong oxidizing ability to oxidize an IM4-like intermediate for product release. These insights may provide another way of constructing C-X bonds through a similar reductive elimination pathway, using early TM catalysts.

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.