A new ruthenium-catalyzed direct and selective synthesis of semisaturated bicyclic pyrimidines, from α-aminopyridyl alcohols and nitriles, has been demonstrated. The synthesis proceeds with an easily available catalyst system, broad substrate scope, excellent functional tolerance, and no need for high pressure H2 gas. Control experiments indicate that the reaction proceeds via successive dehydrogenative annulation and transfer hydrogenation of the less electrophilic pyridyl nucleus, and the density functional theory (DFT) study reveals the origin of such a unique selectivity.

Metal ligand cooperation (MLC) plays an important role in the development of homogeneous catalysts. Two major MLC modes have generally been proposed, known as the M-L bond mode and the (de)aromatization mode. To reveal the role of the dual potential functional sites on the MLC process, we present a detailed mechanistic study on a novel-designed Ru-PNNH complex possessing dual potential MLC functional sites for the M-L bond mode and the (de)aromatization mode, respectively. Our results indicate that the Ru-PNNH complex prefers the M-L mode exclusively during different stages of the catalytic cycle. The unusual double deprotonation process and the mechanistic preference are rationalized. The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the amido group. The origin of the unexpected exclusive mechanistic preference on the M-L bond mechanism is due to the conjugation effect of the amido group, which stabilizes the dearomatized complex and diminishes the driving force of the (de)aromatization mode. This study highlights the pivotal role of the ligand’s electronic effect on the MLC mechanism and should provide valuable information for the development of highly efficient bifunctional catalysts.Keywords: bifunctional; conjugation effect; metal ligand cooperation; ruthenium; steric effect;

The development of efficient, robust and economical water oxidation catalysts (WOCs) remains a key challenge for water splitting. Herein, three macrocyclic nickel(II) complexes with four, six and eight methyl groups in the ligands have been utilized as homogeneous electrocatalysts for water oxidation in aqueous phosphate buffer at pH 7.0, in which the catalyst with eight methyl groups exhibits the highest catalytic activity, with a large current density of 1.0 mA cm−2 at 1.55 V vs. NHE (750 mV overpotential) in long-term electrolysis. The results of electrochemistry, UV-vis spectroelectrochemistry and DFT calculations suggest that the axially oriented methyl groups in the macrocyclic ligands with eight and six methyl groups can impose a steric effect on the axial position of the NiIII center, which not only results in higher NiIII/II oxidation potentials but also suppresses the axial coordination of phosphate anions with the NiIII center to achieve better catalytic performance. Such a steric effect in homogeneous WOCs has not been reported so far.

Lewis acid–transition metal (LA–TM) complexes have been emerging as a novel type of bifunctional catalyst for H2 activation. The crucial role of the auxiliary ligand in the mechanism and reactivity of H2 activation were theoretically studied with (tris(phosphino)borane)Fe(L) (L = N2, CNtBu, and CO) as model LA–TM complexes. The axial auxiliary ligand is found to play an important role in the complex electronic structures, via significantly tuning the energy levels of dxz, dyz, and of the Fe–B bond. We systematically evaluated both the ligand dissociative mechanism and the associative mechanism with the binding auxiliary ligand. In the ligand dissociative mechanism, the reaction starts with the dissociation of the axial ligand. Then, H2 coordinates to the iron center either at the axial position or on the equatorial plane. The axial coordinated H2 cleaves via oxidative addition to a metastable octahedral dihydride intermediate, which further isomerizes to a more stable five-coordinated trigonal bipyramidal intermediate with a bridging hydride stabilized by the iron center and the Lewis center, boron. On the other hand, the equatorial coordinated H2 is cleaved by the cooperation of the Fe–B bond, directly to the trigonal bipyramidal dihydride intermediate. In comparison, in the ligand associative mechanism, the H2 molecule splits up upon approaching the iron center via an octahedral transition state, with the auxiliary ligand remaining at the axial position. Our results suggest that the triplet state axial reaction pathway of the L-dissociative mechanism is the most favorable one for H2 activation. The isomerization of hydride instead of H2 cleavage may be the rate-determining step. H2 activation occurs homolytically on the metal center without LA assistance, which is significantly different from other tetradentate LA–TM systems that activate H2 in a synergetic heterolytic mode. The obtained tendency of the dissociation free energies for (tris(phosphino)borane)Fe(L) (L = N2, CNtBu, and CO) (2.3, 8.1, and 15.7 kcal mol−1, respectively) and the activation free energies of the rate-determining step (20.3, 26.0, and 33.7 kcal mol−1, respectively) explains well their activity trend. The extraordinary effect of the auxiliary ligand on the mechanism and reactivity should provide new information for future development of LA–TM bifunctional catalysts.

A general mechanism for H2 activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (PhDPBPh)Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (ΔG‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (ΔG‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (ΔG‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future.Keywords: density functional theory; H2 activation; heterolytic; homolytic; hydrogenation; Lewis acid; Lewis base; mechanism; nickel; synergetic; transition metal

An efficient approach for direct carbamoylation of terminal alkynes with formamides affording propiolamides has been developed by copper-catalyzed oxidative cross coupling of C(sp)-H and C(sp2)-H bonds in the presence of a pincer ligand with two imidazolyl groups. The catalytic reaction is compatible with diverse functional groups but sensitive to the electronic effect of terminal alkyne and the steric effect of formamides. KIE study indicates the cleavage of the carbamoyl C–H bond affording formamide radical is the rate-determining step.Keywords: aminocarbonylation; carbonylation; copper-catalyzed; cross-dehydrogenative coupling; propiolamide

Metal ligand cooperation (MLC) catalysis is a popular strategy to design highly efficient transition metal catalysts. In this presented theoretical study, we describe the key governing factor in the MLC mechanism, with the Szymczak’s NNN-Ru and the Milstein’s PNN-Ru complexes as two representative catalysts. Both the outer-sphere and inner-sphere mechanisms were investigated and compared. Our calculated result indicates that the PNN-Ru pincer catalyst will be restored to aromatic state during the catalytic cycle, which can be considered as the driving force to promote the MLC process. On the contrary, for the NNN-Ru catalyst, the MLC mechanism leads to an unfavored tautomerization in the pincer ligand, which explains the failure of the MLC mechanism in this system. Therefore, the strength of the driving force provided by the pincer ligand actually represents a prerequisite factor for MLC. Spectator ligands such as CO, PPh3, and hydride are important to ensure the catalyst follow a certain mechanism as well. We also evaluate the driving force of various bifunctional ligands by computational methods. Some proposed pincer ligands may have the potential to be the new pincer catalysts candidates. The presented study is expected to offer new insights for MLC catalysis and provide useful guideline for future catalyst design.

Gold catalyzed rearrangement of propargylic esters can undergo 1,3-acyloxy migration to form allenes, or undergo 1,2-acyloxy migration to access gold-carbenoids. The variation in migration leads to different reactivities and diverse cascade transformations. The effect of terminal substituents is very important for the rearrangement. However, it remains ambiguous how terminal substituents govern the selectivity of the rearrangement. This study presents a theoretical model based on the resonance structure of gold activated propargylic ester complexes to rationalize the rearrangement selectivity. Substrates with a major resonance contributor A prefer 5-exo-dig cyclization (1,2-migration), while those with a major resonance contributor B prefer 6-endo-dig cyclization (1,3-migration). This concise model would be helpful in understanding and tuning the selectivity of the metal catalyzed rearrangement of propargylic esters.

Dirhodium-contained catalysts mediated aliphatic C–H bond amination of aryl azides were studied using BPW91 functional. Calculations show the reactions with Rh2(esp)2 (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropionic acid) and the model compound Rh2(OCHO)4 as catalysts take place via similar mechanisms. Firstly, the dirhodium metal complex coordinates with the substrate and releases nitrogen gas. This rate-determining step results in the formation of the metal nitrene. Metal nitrene mediated intramolecular C–H bond amination is conducted via two alternative pathways, respectively. The singlet metal nitrene mediated intramolecular C–H bond amination occurs via a concerted and asynchronous pathway involving direct metal nitrene insertion into the C–H bond. The triplet metal nitrene case is a stepwise pathway involving a hydrogen transfer and then a diradical recombination. Our study suggests the triplet H-abstraction is more favorable than the singlet one. The resulted triplet intermediate would not go through the high-barrier diradical recombination process, but across to the singlet pathway via a MECP and form the final singlet product. The tethered esp ligands in Rh2(esp)2 provide steric effects to constrain the substrate-catalyst compound but indicates inconspicuous influence on the mechanisms of dirhodium catalyzed aliphatic C–H bond amination of aryl azides.

A series of sterically encumbered tetraarylimidazolium carbene Pd-PEPPSI complexes were conveniently prepared and fully characterized. These sterically encumbered Pd-PEPPSI complexes act as active precatalysts for the direct arylation of imidazoles with aryl bromides under aerobic conditions. The catalytic performance of Pd-PEPPSI complexes in cross-coupling processes is investigated. Under the optimal protocols, the cross-coupling reactions regioselectively produced C5-arylation products in moderate to excellent yields, which could tolerate a wide range of functional aryl bromides.

Copper(I)-catalyzed 5-sulfonation of quinolines via bidentate-chelation assistance has been developed. The reaction is compatible with a wide range of quinoline substrates and arylsulfonyl chlorides. Experimental and theoretical (DFT) investigation implicated that a single-electron-transfer process is involved in this sulfonylation transformation.

The mimic of hydrogenases has unleashed a myriad of bifunctional catalysts, which are widely used in the catalytic hydrogenation of polar multiple bonds. With respect to ancillary ligands, the bifunctional mechanism is generally considered to proceed via the metal–ligand cooperation transition state. Inspired by the interesting study conducted by Hanson et al. (Chem Commun., 2013, 49, 10151), we present a computational study of a distinctive example, where a CoII-PNP catalyst with an ancillary ligand exhibits efficient transfer hydrogenation through a non-bifunctional mechanism. Both the bifunctional and non-bifunctional mechanisms are discussed. The calculated results, which are based on a full model of the catalyst, suggest that the inner-sphere non-bifunctional mechanism is more favorable (by ∼11 kcal mol−1) than the outer-sphere bifunctional mechanism, which is in agreement with the experimental observations. The origin of this mechanistic preference of the CoII-PNP catalyst can be attributed to its preference for the square planar geometry. A traditional bifunctional mechanism is less plausible for CoII-PNP due to the high distortion energy caused by the change in electronic configuration with the varied ligand field. Considering previous studies that focus on the development of ligands more often, this computational study indicates that the catalytic hydrogenation mechanism is controlled not only by the structure of the ligand but also by the electronic configuration of the metal center.

Removing or reducing NO is meaningful for environment protection. Herein, the investigation of the probability of NO reduction on silicene is presented utilizing DFT calculations. Two mechanisms for NO reduction on silicene are provided: a direct dissociation mechanism and a dimer mechanism. The direct dissociation mechanism is characterized as the direct breaking of the N–O bond. The calculated potential energy surfaces show that the total energy barrier in the favored direct dissociation pathway is 0.466 eV. On the other hand, the dimer mechanism is identified to undergo a (NO)2 dimer formation on silicene, which then decomposes into N2O + Oad or N2 + 2Oad. The (NO)2 dimer formation on silicene is found to be feasible both in thermodynamics and kinetics. The formation energy barriers for (NO)2 dimer are lower than 0.231 eV. The calculation results indicate that the (NO)2 dimers can be readily reduced into N2O or N2. The energy barriers in the favored decomposition pathways to produce N2O are quite low (<0.032 eV). The energy barrier for the release of N2 is calculated to be 0.156 eV. The further reduction of N2O to N2 on silicene is also investigated. The results indicate it is easy to reduce N2O to N2 with an energy barrier of only 0.445 eV. NO reduction on silicene hence prefers to generate N2 via the dimer mechanism when compared to the direct dissociation. NO reduction on silicene with silicane as substrate is further proved to proceed via the same reduction mechanism as compared with the free-standing model. Hence, our results presented here suggest that silicene can be a potential material in NO removal, which will reduce NO into environmentally-friendly gases.

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-N═C(R)–C(R)2–OH]PdCl2} (C1, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; C2, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methylphenyl; C3, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methyoxylphenyl; C4, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-chlorophenyl; C5, R = Ph, Ar = 2,6-dimethylphenyl; C6, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes C1 and C2 were determined by X-ray diffraction. These bidentate N,O-palladium complexes were applied for direct arylation under aerobic conditions. The effects of the reaction conditions and ligand substitution on the catalytic activity were evaluated. Upon a low palladium loading of 0.5 mol %, the bulky palladium complex C6 was successfully used to catalyze the cross-coupling of a variety of five-membered heteroarenes and their benzo-condensed derivatives with (hetero)aryl bromides. The mechanistic investigation on the direct arylation supported the involvement of a Pd(0)/Pd(II) CMD process.

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (Z)-1,3-dienes. In contrast, the relative softer Au(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (E)-1,3-dienes, where the E type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard–soft/acid–base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates.

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H2 and the hydride transfer. The heterolytic cleavage of H2 is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H2 heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H2. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO2. This ligand assisted pathway is more favorable than the direct attack of hydride to CO2. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide.Keywords: carbon dioxide; catalytic mechanism; cobalt; density functional theory; hydrogenation; iridium; metal effect; rhodium

Direct asymmetric Suzuki coupling between arylboronic acids and 2-diarylphosphinyl-1-naphthyl bromides was successfully developed for the first time with the use of Pd–L1 or Pd–(Cy-MOP) as the catalyst. A variety of axially chiral 2-functionalized-2′-diarylphosphinyl-1,1′-biaryls were afforded in 34–99% yields with up to 94% ee. This methodology provides a highly efficient and practical strategy for the synthesis of novel axially chiral biaryl monophosphine oxides and the corresponding phosphines. The existence of an ortho formyl group in arylboronic acids greatly improves the coupling efficiency and permits further versatile transformations in organic synthesis. Density functional calculations were used to determine the origin of stereoselectivity during the reductive elimination step of the closely related coupling of 2-formylphenylboronic acid with naphthylphosphonate bromide. These studies indicate that both the significant transition metal hydrogen bond between the H atom of the formyl group and palladium(II) and the weak interaction between the Pd center and the phosphoryl oxygen atom in the transition state are crucial for high enantioselectivity of the coupling products.Keywords: asymmetric catalysis; biaryls; computational chemistry; phosphine oxides; Suzuki coupling

Newly emerged xanthone derivatives have attracted considerable interests as a novel class of potent α-glucosidase inhibitors. To provide insights into the inhibitory and binding mechanisms of xanthone-based inhibitors toward α-glucosidase, we carried out experimental and theoretical studies on two typical xanthone derivatives, i.e., 1,3,7-trihydroxyxanthone and 1,3-dihydroxybenzoxanthone. The results indicate that these two xanthone derivatives belong to noncompetitive inhibitors and induce a loss in the α-helix content of the secondary structure of α-glucosidase. Docking simulation revealed the existence of multiple binding modes, in which polyhydroxyl groups and expanded aromatic rings acted as two key pharmacophores to form H-bonding and π–π stacking interactions with α-glucosidase. The fact that 1,3,7-tridroxyxanthone and 1,3-dihydroxybenzoxanthone exhibited significant synergetic inhibition to α-glucosidase strongly suggests that both xanthone derivatives simultaneously bind to the distinct noncompetitive sites of yeast’s α-glucosidase. On the basis of the plausible binding clues, synergetic inhibition can be developed to be a promising strategy to achieve enhanced inhibitory activities.

Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar–NC(R)–C(R)N–Ar]PdCl2, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph). The structures of these palladium complexes were well characterized, while C1 and C3 were further characterized by X-ray diffraction. The catalytic performances of the precatalysts were screened for direct C–H bond arylation of heteroarenes. The bidentate N,N-palladium complex C3 with both a backbone and N-aryl bulkiness was found to be a highly efficient precatalyst under aerobic conditions. With a low palladium loading of 0.5–0.1 mol%, a variety of heteroarenes with challenging bulky steric aryl bromides as well as heteroaryl bromides are all applicable for this cross-coupling reaction.

Gold catalyzed rearrangement of propargylic esters can undergo 1,3-acyloxy migration to form allenes, or undergo 1,2-acyloxy migration to access gold-carbenoids. The variation in migration leads to different reactivities and diverse cascade transformations. The effect of terminal substituents is very important for the rearrangement. However, it remains ambiguous how terminal substituents govern the selectivity of the rearrangement. This study presents a theoretical model based on the resonance structure of gold activated propargylic ester complexes to rationalize the rearrangement selectivity. Substrates with a major resonance contributor A prefer 5-exo-dig cyclization (1,2-migration), while those with a major resonance contributor B prefer 6-endo-dig cyclization (1,3-migration). This concise model would be helpful in understanding and tuning the selectivity of the metal catalyzed rearrangement of propargylic esters.

The mimic of hydrogenases has unleashed a myriad of bifunctional catalysts, which are widely used in the catalytic hydrogenation of polar multiple bonds. With respect to ancillary ligands, the bifunctional mechanism is generally considered to proceed via the metal–ligand cooperation transition state. Inspired by the interesting study conducted by Hanson et al. (Chem Commun., 2013, 49, 10151), we present a computational study of a distinctive example, where a CoII-PNP catalyst with an ancillary ligand exhibits efficient transfer hydrogenation through a non-bifunctional mechanism. Both the bifunctional and non-bifunctional mechanisms are discussed. The calculated results, which are based on a full model of the catalyst, suggest that the inner-sphere non-bifunctional mechanism is more favorable (by ∼11 kcal mol−1) than the outer-sphere bifunctional mechanism, which is in agreement with the experimental observations. The origin of this mechanistic preference of the CoII-PNP catalyst can be attributed to its preference for the square planar geometry. A traditional bifunctional mechanism is less plausible for CoII-PNP due to the high distortion energy caused by the change in electronic configuration with the varied ligand field. Considering previous studies that focus on the development of ligands more often, this computational study indicates that the catalytic hydrogenation mechanism is controlled not only by the structure of the ligand but also by the electronic configuration of the metal center.