DFT Mechanistic Study Research Articles

Ethylene–Butadiene Copolymerization by Neodymocene Complexes: A Ligand Structure/Activity/Polymer Microstructure Relationship Based on DFT Calculations

Ethylene/butadiene copolymerization can be performed by neodymocene catalysts in the presence of an alkylating/chain transfer agent. A variety of polymerization activities and copolymer microstructures can be obtained depending on the neodymocene ligands. For a set of four catalysts, namely (C5Me5)2NdR, [Me2Si(3-Me3SiC5H3)2]NdR, [Me2Si(C5H4)(C13H8)]NdR and [Me2Si(C13H8)2]NdR, we report a DFT mechanistic study of this copolymerization reaction performed in the presence of dialkylmagnesium. Based on the modeling strategy developed for the ethylene homopolymerization catalyzed by (C5Me5)2NdR in the presence of MgR2, our model is able to account for the following: (i) the formation of Nd/Mg heterobimetallic complexes as intermediates, (ii) the overall differential activity of the catalysts, (iii) the copolymerization reactivity indexes, and (iv) the specific microstructure of the resulting copolymers, including branching and cyclization. The analysis of the reaction mechanisms and the energy profiles thus rel...

DFT Mechanistic Study of Functionalizations of ω-Ene-Cyclopropanes and Alkylidenecyclopropanes via Allylic C–H and C–C Bond Cleavage Facilitated by a Zirconocene Complex

A DFT mechanistic study has been performed to understand the [Zr]C4H8 (Zr = ZrCp2)-mediated transformations of ω-ene-cyclopropane (ω-ene-CP) and alkylidenecyclopropane (ACP) to acyclic compounds. The transformations proceed via allylic C–H bond activation, hydride transfer, C–C bond cleavage of the three-membered ring, and additions of electrophiles. The energetic results indicate that, among the possible pathways, the one leading to the experimental products is most energetically favorable, rationalizing the selectivity of the reactions. The Zr-walk takes place via allylic C–H bond activation followed by hydride transfer, completing a 1,3-hydrogen transfer. In comparison, the Pd-walk involved in the Pd-catalyzed Heck-type relay coupling reactions proceeds via migratory insertion followed by β-H elimination, resulting in a 1,2-hydrogen transfer. The difference is due to the fact that the [Zr] active species does not have a Zr–H or Zr–C bond for C═C bond migratory insertion, while the Pd–H or Pd–C bond in ...

Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c–4e Bonding: A DFT Mechanistic Study

A DFT study has been performed to gain insight into the formic-acid-catalyzed depolymerization of the oxidized lignin model (1ox) to monoaromatics, developed by Stahl et al. (Nature 2014, 515, 249–252). The conversion proceeds sequentially via formylation, elimination, and hydrolysis. Intriguingly, the elimination process exploits an unconventional mechanism different from the known ones such as E2 and E1cb. The new mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c–4e bond (HCOO⊖···H⊕···⊖O═Cα) and by shifting the 3c–4e bond to the 3c–4e HCOO⊖···H⊕···⊖OOCH bond in the joint leaving group that is originally a regular H-bond (HCOO–H···OOCH−). According to these characteristics, as well as the important role of the original HCOO–H···OOCH– bond, we term the mechanism as E1H-3c4e elimination. The root-cause of the E1H-3c4e elimination is that the poor leaving formate group is less competitive in stabilizing the negative charge resulted from Hβ abstraction by the HCOO...

A polar nature of benzoic acids extrusion from nitroalkyl benzoates: DFT mechanistic study.

Using DFT calculations at various theory levels, quantum-chemical simulations of decomposition paths were performed for a series of nitroalkyl benzoates. It was discovered, that these reactions proceed via polar, but one-step mechanism. It turned out that depending on the nature of the substituent in the ester molecule and on medium polarity, the studied reactions may take place via transition states with varied synchronicity — from E1-like structures, to E1cb-like structures. A purely ionic, two-stage mechanism was not identified in any of the cases. Graphical Benzoic acids extrusion from nitroalkyl benzoates

Intermolecular proton transfer in cyclic carbonate synthesis from epoxide and carbon dioxide catalyzed by azaphosphatranes: a DFT mechanistic study

The activity of azaphosphatranes, novel types of non-metal and solvent-free catalysts for the synthesis of cyclic carbonates from epoxides and CO2, is unraveled by DFT calculations.

Nitrenium ions and trivalent boron ligands as analogues of N-heterocyclic carbenes in olefin metathesis: a computational study.

We used the density functional theory to evaluate the suitability of nitrenium ions and trivalent boron ligands as analogues of N-heterocyclic carbenes in ruthenium-based metathesis catalysts. We demonstrate that these analogues induce only minor structural changes in Hoveyda-Grubbs-like precatalysts, but have major impact on precatalyst initiation. Nitrenium ion-modified precatalysts are characterized by a weak Ru-N bond resulting in a relatively strong Ru-O bond and large free energy barriers for initiation, making them good candidates for efficient latent Ru-based catalysts. On the other hand the trivalent boron ligand, bearing a formal -1 charge, binds strongly to the ruthenium ion, weakening the Ru-O bond and facilitating its dissociation, to promote fast reaction initiation. We show that the calculated bond dissociation energy of the Ru-C/N/B bond may serve as an accurate indicator of the Ru-O bond strength and the rate of metathesis initiation.

DFT mechanistic study of the H2-assisted chain transfer copolymerization of propylene andp-methylstyrene catalyzed by zirconocene complex

DFT computations have been performed to investigate the mechanism of H2-assisted chain transfer strategy to functionalize polypropylene via Zr-catalyzed copolymerization of propylene and p-methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2-insertion over 2,1-insertion both kinetically and thermodynamically, explaining the observed 1,2-insertion regioselectivity for propylene insertion. (ii) The 2,1-inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2-insertion. The observation of 2,1-insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H2 can react with the deactivated [Zr]−pMS−PPn facilely, which displace functionalized pMS−PPn chain and regenerate [Zr]H active catalyst to continue copolymerization. The effects of counterions are also discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 576–585

DFT Mechanistic Study of RuII-Catalyzed Amide Synthesis from Alcohol and Nitrile Unveils a Different Mechanism for Borrowing Hydrogen

Using RuII complex as a mediator, Hong and co-workers recently developed a redox-neutral synthetic strategy to produce amide from primary alcohol and nitrile with complete atom economy. Intrigued by the novel strategy, we performed DFT computations to unravel the catalytic mechanism of the system. The transformation is catalyzed by RuIIH2(CO)(PPh3)(IiPr) (IiPr = 1,2-diisopropylimidazol-2-ylidene) via four stages including nitrile reduction, alcohol dehydrogenation, C–N coupling, and amide production. Generally, alcohol dehydrogenation in dehydrogenative coupling (DHC) or borrowing hydrogen methodology (BHM) takes place separately, transferring the Hα and hydroxyl HOH atoms of alcohol to the catalyst to form the catalyst-H2 hydride. Differently, the alcohol dehydrogenation in the present system couples with nitrile hydrogenation; alcohol plays a reductant role to aid nitrile reduction by transferring its HOH to nitrile N atom directly and Hα to the catalyst and meanwhile becomes partially oxidized. In our ...

Ring‐Expansion Reaction of Isatins with Ethyl Diazoacetate Catalyzed by Dirhodium(II)/DBU Metal‐Organic System: En Route to Viridicatin Alkaloids

AbstractWe present here the NHC‐dirhodium(II)/DBU‐catalyzed ring expansion reaction of isatins with ethyl diazoacetate. This new one‐pot protocol yields the ethyl 3‐hydroxy‐2(1H)‐oxoquinoline‐4‐carboxylate core, regioselectively and in good to excellent yields. A DFT mechanistic study indicates metallocarbene formation between the 3‐hydroxyindole‐diazo intermediate and the dirhodium(II) complex to be the rate‐limiting step of the reaction. The synthesized ethyl 3‐hydroxy‐2(1H)‐oxoquinoline‐4‐carboxylate core could be readily converted to viridicatin alkaloids, in yields up to 80 % by a microwave‐assisted Suzuki–Miyaura cross coupling of the 3‐hydroxy‐4‐bromoquinolin‐2(1H)‐one with arylboronic acid.

Trivalent Uranium Complex As a Catalyst to Promote the Functionalization of Carbon Dioxide and Carbon Disulfide: A Computational Mechanistic Study.

We report our recent DFT mechanistic study on the functionalization of CO2 and CS2 promoted by a trivalent uranium complex (Tp*)2UCH2Ph. In the calculations, the uranium atom is described by a quasi-relativistic 5f-in-core ECP basis set (LPP) developed for the trivalent uranium cation, which was qualified by the calculations with a quasi-relativistic small core ECP basis set (SPP) for uranium. According to our calculations, the functionalization proceeds in a stepwise manner, and the CO2 or CS2 does not interact with the central uranium atom to form a stable complex prior to the reaction due to the steric hindrance from the bulky ligands but directly cleaves the U-C (benzyl) bond by forming a C-C covalent bond. The released coordination site of uranium is concomitantly occupied by one chalcogen atom of the incoming molecule and gives an intermediate with the uranium atom interacting with the functionalized CO2 or CS2 in an η(1) fasion. This step is followed by a reorientation of the (dithio)carboxylate side chain of the newly formed PhCH2CE2(-) (E = O, S) ligand to give the corresponding product. Energetically, the first step is characterized as the rate-determining step with a barrier of 9.5 (CO2) or 25.0 (CS2) kcal/mol, and during the reaction, the chalcogen atoms are reduced, while the methylene of the benzyl group is oxidized. Comparison of the results from SPP and LPP calculations indicates that our calculations qualify the use of an LPP treatment of the uranium atom toward a reasonable description of the model systems in the present study.

ChemInform Abstract: Unusual Formal [4 + 2] Cycloaddition of Ethyl Allenoate with Arylidenoxindoles: Synthesis of Dihydropyran‐Fused Indoles.

An unusual DABCO-catalyzed formal [4 + 2] cycloaddition of ethyl allenoate, as a surrogate of a “1,2-dipole”, with various arylidenoxindoles has been developed for the synthesis of dihydropyran-fused indoles. The DFT mechanistic study indicates that the cycloaddition takes place stepwise and the essential role of the catalyst is to raise the HOMO of allenoate.

Substrate Hydroxylation by the Oxido–Iron Intermediate in Aromatic Amino Acid Hydroxylases: A DFT Mechanistic Study

AbstractSubstrate hydroxylation by an FeIV=O cluster model of the active center in aromatic amino acid hydroxylases (AAHs) has been investigated by means of DFT calculations. Whereas benzene was used as a model for the aromatic amino acid substrate, the water‐free FeIV=O cluster model has been used in previous studies of enzyme activation and formation of the hydroxylating intermediate. This cluster model also has the pterin cofactor placed in the first coordination sphere of the iron atom and differs substantially from models used in previous computational studies of AAH‐catalyzed hydroxylations. The formation of the (Fe)O–C(benzene) bond is associated with a free‐energy barrier (12.6 kcal mol–1) that is slightly lower than that calculated previously for the formation of the FeIV=O species. The subsequent steps, the NIH shift and the tautomerization leading to the phenol product are both associated with lower energy barriers. The substrate hydroxylation is followed by protonation of the oxidized iron‐bound cofactor to give the pterin‐4a‐carbinolamine product. The latter is subsequently dissociated upon rebinding of water molecules to produce the hexacoordinate iron complex of the enzyme resting state. Consequently, no product is released before the oxidation of both the substrate and the cofactor has been completed. Finally, the current study completes the catalytic cycle and regenerates the catalyst, with a barrier energy comparable to that of the (Fe)O–C(benzene) bond formation.

Chemoselective Sulfide and Sulfoxide Oxidations by CpMo(CO)3Cl/HOOR: a DFT Mechanistic Study

The mechanism of sulfide and sulfoxide oxidation with peroxides (ROOH, R = H, Me), catalyzed by Mo(VI) complexes, was investigated by means of DFT/PBE1PBE calculations. Two different catalytic systems were considered: the first is based on the dioxocyclopentadienyl (Cp) complex CpMoO2Cl (Cp = η5-C5H5), also active as a catalyst for olefin epoxidation, and the second based on MoO2Cl2. The most favorable mechanism in the Cp system is initiated by the O−H activation of the HOOR oxidant, which in the presence of CpMoO2Cl leads to formation of CpMoO(OH)(OOR)Cl. Although this is the active species for olefin epoxidation, an alternative pathway with lower energy is available. With the crucial H-bond assistance of another oxidant molecule, the oxoperoxo complex CpMoO(O2)Cl forms, with release of alcohol ROH as byproduct and a calculated energy barrier below 25 kcal mol−1. The mechanisms unveiled for sulfide to sulfoxide oxidation and for sulfoxide to sulfone oxidation are equivalent in their general features and ...

Unusual Formal [4 + 2] Cycloaddition of Ethyl Allenoate with Arylidenoxindoles: Synthesis of Dihydropyran-Fused Indoles

An unusual DABCO-catalyzed formal [4 + 2] cycloaddition of ethyl allenoate, as a surrogate of a "1,2-dipole", with various arylidenoxindoles has been developed for the synthesis of dihydropyran-fused indoles. The DFT mechanistic study indicates that the cycloaddition takes place stepwise and the essential role of the catalyst is to raise the HOMO of allenoate.

A nickel–manganese catalyst as a biomimic of the active site of NiFe hydrogenases: a combined electrocatalytical and DFT mechanistic study

The dinuclear nickel–manganese complex [Ni(xbsms)Mn(CO)3(H2O)]+ (H2xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) is reported as a bio-inspired mimic of the active site of NiFe hydrogenases catalyzing hydrogen evolution from trifluoroacetic acid in DMF with an overpotential requirement of 860 mV. This is higher than that displayed by Ni–Ru systems [Canaguier et al., Chem.–Eur. J., 2009, 15, 9350–9364] but similar to that found for related noble metal free Ni–Fe mimics [Canaguier et al., Chem. Commun. 2010, 46, 5876–5878]. A combined electrochemical and theoretical (DFT) study suggests a heterolytic mechanism for hydrogen evolution from a hydride derivative. The structure of the active intermediate, with a bridging hydride ligand between Ni and Mn, resembles that of the Ni–C active state of NiFe hydrogenases.

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium-based precatalysts (H and NO(2)(4)) in diene ring-closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs-Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru...O interaction strength. In contrast, precatalysts with lower alkoxy-dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1-C2 bond length (i.e., lower pi character of the C-C bond that exists between the metal-carbene (Ru=C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release-return mechanism occurs or not, is also mainly determined by the pi delocalization. Therefore, future Grubbs-Hoveyda-type catalyst development should be based on fine-tuning the pi-electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru...O interaction.

The fine balance between one cross-coupling and two β-hydride elimination pathways: a DFT mechanistic study of Ni(π-allyl)2-catalyzed cross-coupling of alkyl halides and alkyl Grignard reagents

The model cross-coupling of EtBr with EtMgCl mediated by Ni(pi-allyl)(2) proceeds by a catalytic cycle commencing with transmetalation establishing a novel Mg-allyl interaction, followed by Mg-assisted oxidative addition (the rate-determining step), followed by reductive elimination having much lower barrier than 2 competing beta-hydride elimination pathways, and completed by departure of MgBrCl.

The role of cyclopentadienyl versus indenyl in Mo(II) spirodiene complexes reactivity: A DFT mechanistic study

Spiro[2,4]hepta-4,6-diene and spiro[4,4]nona-1,3-diene react with [Mo(CO) 2Cp’(NCCH 3) 2][BF 4] (Cp’ = Cp, Ind; Ind = η 5-C 9H 7) to afford the corresponding diene complex [Mo(diene)(CO) 2Cp′] +. When Cp’ = Ind, the reaction proceeded forward leading to ring opening in the case of the small spiro ring. Although this and another product resulting from migration of the side arm to the carbonyl were detected when Cp’ = Cp, they did not form from the diene complex. A DFT/PBE1PBE study was carried out and showed a kinetically controlled reaction pathway leading from the [Mo(diene)(CO) 2Ind] + to the reaction product, with an activation barrier of 21.3 kcal mol −1. The thermodynamic preferred species was the non-observed complex (insertion), and its formation required higher barriers. In the presence of Cp, all the barriers increased significantly, explaining the inertness of the initial diene complex. The interpretation of this behaviour is associated with the ease of the η 5 → η 3 haptotropic rearrangement of the indenyl, which helps to lower some relevant barriers. This route is not available for the Cp analogue.

Direct and Remarkably Efficient Conversion of Methane into Acetic Acid Catalyzed by Amavadine and Related Vanadium Complexes. A Synthetic and a Theoretical DFT Mechanistic Study

Vanadium(IV or V) complexes with N,O- or O,O-ligands, i.e., [VO{N(CH2CH2O)3}], Ca[V(HIDPA)2] (synthetic amavadine), Ca[V(HIDA)2], or [Bu4N]2[V(HIDA)2] [HIDPA, HIDA = basic form of 2,2'-(hydroxyimino)dipropionic or -diacetic acid, respectively], [VO(CF3SO3)2], Ba[VO(nta)(H2O)]2 (nta = nitrilotriacetate), [VO(ada)(H2O)] (ada = N-2-acetamidoiminodiacetate), [VO(Hheida)(H2O)] (Hheida = 2-hydroxyethyliminodiacetate), [VO(bicine)] [bicine = basic form of N,N-bis(2-hydroxyethyl)glycine], and [VO(dipic)(OCH2CH3)] (dipic = pyridine-2,6-dicarboxylate), are catalyst precursors for the efficient single-pot conversion of methane into acetic acid, in trifluoroacetic acid (TFA) under moderate conditions, using peroxodisulfate as oxidant. Effects on the yields and TONs of various factors are reported. TFA acts as a carbonylating agent and CO is an inhibitor for some systems, although for others there is an optimum CO pressure. The most effective catalysts (as amavadine) bear triethanolaminate or (hydroxyimino)dicarboxylates and lead, in a single batch, to CH3COOH yields > 50% (based on CH4) or remarkably high TONs up to 5.6 x 103. The catalyst can remain active upon multiple recycling of its solution. Carboxylation proceeds via free radical mechanisms (CH3* can be trapped by CBrCl3), and theoretical calculations disclose a particularly favorable process involving the sequential formation of CH3*, CH3CO*, and CH3COO* which, upon H-abstraction (from TFA or CH4), yields acetic acid. The CH3COO* radical is formed by oxygenation of CH3CO* by a peroxo-V complex via a V{eta1-OOC(O)CH3} intermediate. Less favorable processes involve the oxidation of CH3CO* by the protonated (hydroperoxo) form of that peroxo-V complex or by peroxodisulfate. The calculations also indicate that (i) peroxodisulfate behaves as a source of sulfate radicals which are methane H-abstractors, as a peroxidative and oxidizing agent for vanadium, and as an oxidizing and coupling agent for CH3CO* and that (ii) TFA is involved in the formation of CH3COOH (by carbonylating CH3*, acting as an H-source to CH3COO*, and enhancing on protonation the oxidizing power of a peroxo-VV complex) and of CF3COOCH3 (minor product in the absence of CO).

Olefin epoxidation with tert-butyl hydroperoxide catalyzed by MoO2X2L complexes: a DFT mechanistic study

DFT calculations suggest that the catalytic epoxidation of olefins by Mo(vi) complexes, modeled by MoO2Br2(MeN=C(H)-C(H)=NMe), in the presence of MeOOH, the model for tert-butyl hydroperoxide, starts with a hydrogen transfer from the peroxide to one of the terminal Mo=O oxygen atoms and the remaining MeOO anion binds as a seventh ligand, forming a five-membered Mo-O(alpha)-O(beta)(Me)...H-O-Mo ring held together by a hydrogen bond. In the second step, a concerted approach of ethylene to the Mo-O(alpha) bond gives rise to an intermediate containing a seven-membered Mo-C-C-O(alpha)-O(beta)(Me)...H-O-Mo ring. In the final step, decomposition of the intermediate leads to the starting complex, alcohol and the epoxide. The activation energy for the addition of the olefin (second step) is the highest one, in agreement with available kinetic studies showing that the catalyst formation is not always a rate-limiting step. DFT calculations also show that the alcohol by-product (MeOH) can react with the starting complex, competing with ROOH and hence leading to the progressive catalyst poisoning, which has been observed experimentally.