Energetic Span Research Articles

Theoretical Study on the Mechanism of Ru(II)-Catalyzed Intermolecular [3 + 2] Annulation between o-Toluic Acid and 3,5-Bis(trifluoromethyl)benzaldehyde: Octahedral vs Trigonal Bipyramidal.

Density functional theory was utilized to investigate the mechanism of Ru(II)-catalyzed aromatic C-H activation and addition of aromatic aldehydes. The proposed catalytic cycle consists of C-H bond activation, aldehyde carbonyl insertion for C-C coupling, lactonization for the formation of the final product, product separation, and catalyst recovery. Our calculations suggest that Ru(OAc)2(PCy3) (referred to as CAT) is the most favorable active catalyst, facilitating the C-H bond activation to form a five-membered ring cycloruthenium intermediate (INT2). Subsequently, the aromatic aldehyde reactant 2a enters the Ru coordination sphere, accelerating the C-C coupling and lactonization for the formation of the final product. The involvement of acetate assists in the final product separation, while INT1 re-enters the Ru coordination sphere to initiate a new catalytic cycle. Utilizing the energetic span model, the apparent activation free energy barrier was computed to be 34.3 kcal mol-1 at 443 K. Furthermore, exploration of the reaction mechanism in the absence of phosphine ligands identified Ru(OAc)2(p-cymene) as the most favorable active catalyst. The derived apparent activation free energy barrier offers a comprehensive explanation for the experimentally observed yields. Additionally, we have examined the disparities between the octahedral and trigonal bipyramidal structures of the catalysts concerning their effects on the reaction mechanisms and apparent activation free energy barriers.

Density-based quantification of steric effects: validation by Taft steric parameters from acid-catalyzed hydrolysis of esters.

The steric effect is one of the most widely used concepts for chemical understanding in publications and textbooks, yet a well-accepted formulation of this effect is still elusive. Experimentally, this concept was quantified by the acid-catalyzed hydrolysis of esters, yielding the so-called Taft steric parameter. Theoretically, we recently proposed a density-based scheme to quantify the effect from density functional theory. In this work, we directly compare these two schemes, one from theory and the other from experiment. To this end, we first establish the ester hydrolysis mechanism with multiple water molecules explicitly considered and then apply the energetic span model to represent the hydrolysis barrier height between the two schemes. Our results show that the barrier height of the reaction series is strongly correlated with both Taft steric parameters from experiment and steric quantification from theory. We also obtained strong correlations with steric potential, steric force, and steric charge from our theoretical scheme. Strong correlations with a few information-theoretic quantities are additionally unveiled. To the best of our knowledge, this is the first time in the literature that such a direct comparison between theoretical and experimental results is made. These results also suggest that our proposed two-water three-step mechanism for ester hydrolysis is effective, and our theoretical quantification of the steric effect is valid, robust, and experimentally comparable. In our view, this work should have satisfactorily addressed the issue of how the steric effect can be formulated and quantified, and thus it lays the groundwork for future applications.

Unactivated Pd-catalyzed oxidative cross-coupling reactions of arenes with benzene: a theoretical and experimental study

Oxidative C–H bond activated palladium-catalyzed aromatic cross-coupling reactions offer advantages of fewer required synthetic steps but disadvantages of loss of inherent regioselectivity and selectivity. In one proposed catalytic cycle, each arene adds to a palladium carboxylate catalyst according to a concerted metalation deprotonation mechanism, followed by reductive elimination to couple the arenes and the replenishment of the catalyst by an oxidant. Using this proposed mechanism, molecular structures and free energy profiles were calculated for coupling of benzene with a range of heteroarenes containing various substituents and with several Pd-carboxylate catalysts. Reaction kinetics were analysed using the energetic span model, predicting turnover frequencies (TOFs) of heterocoupling (HB and BH) and homocoupling (HH and BB) pathways and assessing degrees of turnover frequency control (XTOFs). More electron-withdrawing carboxylate ligands and more electron-donating heteroarene substituents decrease activation and reaction free energies of addition steps, with C-2 more favourable than C-3. When heteroarene and catalyst choice are particularly favourable, the first association can become exergonic, causing a significant change in catalytic kinetics. When not exergonic, free energy for addition of benzene is almost always higher, leading to large energetic spans and slower TOFs (BB < HB < BH < HH). Non-exergonic pathways are also faster with more electron-withdrawing ligands and more electron-donating heteroarene substituents because of lower energy addition steps and a smaller energetic span. For exergonic associations, the low-energy arene–catalyst intermediate creates a larger energetic span, slowing down the HH and HB pathways. More electron-withdrawing ligands and more electron-donating substituents increase the relative TOF for BH, and the reaction requires only small excesses of benzene to induce heterocoupling.

First principles energetic span analysis of the activity of transition metal catalysts for N2O decomposition: The reversal of descriptor roles in the presence of NO

The utilization of NH3 combustion faces a significant hurdle in mitigating N2O emissions, a potent greenhouse gas. Current deNOx technologies struggle to effectively decompose N2O, especially under practical conditions due to inhibitory effects from exhaust stream components like O2, NOx, and H2O. This study employs first principles investigations to analyze N2O decomposition activity on precious (Ru, Rh), non-precious (Cu, Ni), and hybrid (Ru/Cu, Rh/Cu, Ru/Ni, Rh/Ni) catalysts. Many new Brønsted-Evans-Polanyi relations are established, enabling mechanistic analysis via the Energetic Span Model. The analyses reveal opportunities for enhancing catalytic activity by exploiting weak correlations between intermediates’ free energies. The model captures the inhibitory effects of surface-adsorbed NO on the N2O decomposition activity, aligning well with experimental observations and catalytic trends. The strong binding energy of adsorbed N2/N2O and O adatom is found to be a key descriptor of activity. It is found to aid the catalytic activity in the presence of NO but becomes limiting in its absence.

Unraveling selectivity in non-noble metal-catalyzed hydrogenation of 5-hydroxymethylfurfural (HMF) through mechanistic insights

The development of heterogeneous catalysts for converting abundant biomass feedstocks to higher value products is one of the most challenges these days. 2,5-dihydroxymethylfuran (DHMF) and 2,5-dihydroxymethyltetrahydrofuran (DHMTHF), synthesized from HMF hydrogenation, serve as crucial precursors in various applications. Non-noble metal catalysts are particularly attractive for this reaction, given their affordability and impressive catalytic efficiency. This work unveils the origin of the unique selectivity over Ni and Cu through mechanistic investigation using density functional theory (DFT), thermodynamic and kinetic analyses. The results emphasize that temperature and solvent play a crucial role in altering the energetic stabilities of intermediates, thereby influencing the energetic span (δG), turnover frequency (TOF), and selectivity of the reaction. The theoretical results align well with experimental observations. At 373.15 K, the highest TOF values over Ni and Cu are predicted in the HMF-to-DHMTHF path (1.79 × 103h−1) and the HMF-to-DHMF path (4.01 × 105h−1), respectively, in gas phase—under low dielectric constant ε condition. In contrast, the highest TOF value for Ni is observed in the HMF-to-DHMF path under implicit water condition (ε = 78.4) at 298.15 K. Competitive DHMF desorption and further hydrogenation influence the reaction’s selectivity. These insightful fundamental findings reveal key descriptors essential for designing new heterogeneous catalysts or enhancing existing ones, with the aim of potentially impacting biomass upgrading and other hydrogenation reactions.

A DFT study on Cu-ZSM-5 as a catalyst for NO decomposition: Possible activity of a Cu(I) pair located at the T3 tetrahedral sites

The decomposition of NO catalysed by Cu+ pairs coordinated at the Al-substituted T3 sites of the 10-membered rings of Cu-ZSM-5 was studied by means of DFT/ONIOM calculations. Three mechanisms for NO decomposition were defined on the active site, all based on the formation and further decomposition of an hyponitrite anion, which gives the N2O molecule and a [Cu‒O‒Cu]2+ bridge, where both copper ions are oxidized. N2O can react with the oxidized active site according to two different spin-allowed mechanisms which give immediately the products, N2 + O2. Otherwise, N2O reacts with a new reduced site according to a spin-forbidden mechanism which gives N2 and a second [Cu‒O‒Cu]2+ bridge. When the latter mechanism occurs, reduced sites are restored by further reaction of [Cu‒O‒Cu]2+ with NO and desorption of NO2, which is finally decomposed on a second [Cu‒O‒Cu]2+ site, giving O2 + NO. Turnover frequencies (TOFs) calculated by means of the Energetic Span model showed that one of the spin-allowed mechanisms should be relatively fast, whereas the spin-forbidden one is expected to be extremely slow. Computational results were discussed with reference to experimental data available in the literature.

Density functional theory investigations on the mechanisms of homogeneous Sm complex catalyzed CO[formula omitted] cycloaddition with epoxides

Converting excess CO2 into chemically valuable products is a promising routine for sustainable development. Catalyzing epoxides and CO2 cycloaddition reactions homogeneously with rare-earth compounds is an emerging approach. The reaction efficiency can be understood by mechanism study. In this work, we report a detailed density functional theory investigation on a well-designed experimental Sm catalyst. We calibrate the ωB97XD exchange–correlation functional as the best candidate functional for this specific catalytic system. The energetic span of catalytic cycle is approximately 25 kcal mol−1. The rate-determining transition state is the one for ring-closure to generate the product. Two competing paths are revealed. Through both ionic and covalent metal–ligand interactions, the lowering of the epoxide substrate ring-opening step barrier is significant compared with the Sm compound free case. This work demonstrate the values for utilizing modern density functional theory to complement the understandings on the homogeneous rare-earth catalytic processes.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH.

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Mechanistic insights highlight regioselective Cobalt-catalyzed [3+3] annulation of anilides with benzylallenes

C-H annulation of anilide with benzylallene under the catalysis of Co(III)-complex has been studied theoretically by employing density functional theory (DFT) to find out the plausible mechanistic path. Our findings reveal that the C-H activation prefers the formation of a four-membered cobalta benzoazetidine intermediate rather than six-membered 1,2,4-cobalta benzoxazine intermediate to generate the product. This fact reveals a new mechanism in contrast to the mechanism proposed earlier. The metal mediated annulation of various anilides include six steps - coordination of substrate with catalyst, N-H metalation, C-H metalation, insertion of benzylallene, proton abstraction and migration insertion. Moreover, it reveals regioselectivity of cobalt for allene insertion. The effect of substituents on the mechanism has also been studied and the high energetic span obtained for this catalytic cycle implies that very high temperature is required for this reaction. It is consistent with the experimental result.

Rh4 cluster supported on the In2O3(111) surface for enhancing the turnover frequency of CO2 hydrogenation to methanol: The application of energetic span model

The turnover frequency of CO2 hydrogenation to methanol on the surface of Rh4 cluster supported In2O3(111) is investigated using the first-principles calculation. The Rh4 cluster and H2 promote oxygen vacancy formation, which facilitates the interaction between the Rh4 cluster and the In2O3(111) carrier and improves the stability of catalyst. The adsorption behavior of CO2 and H2 on catalyst is predicted by electrostatic potential and Fukui(−) index. The loading of Rh4 cluster promotes the adsorption of CO2 and allows easier dissociation of H2 compared to the In2O3(111)_D surface. Afterwards, Gibbs free energy diagrams of three hydrogenation pathways are established and the turnover frequencies of all possible routes are calculated based on the energetic span model. The results reveal that the HCOO pathway is regarded as the optimum hydrogenation mechanism due to its greatest turnover frequency (3.02 × 10−5 s−1), higher than that of In2O3(111)_D and other reported catalysts. The specific process is CO2(g) + 6H → HCOO* + 5H → HCOOH* + 4H → H2COOH* + 3H → H2CO* + 2H + H2O(g) → H2COH* + H + H2O(g) → CH3OH(g) + H2O(g), while the RWGS pathway and by-products are effectively inhibited, thus enhancing the selectivity of methanol. The improvement in catalytic activity may result from the loaded Rh4 cluster facilitating the adsorption of CO2 and the dissociation of H2.

Theoretical Investigations of Palladium-Catalyzed [3+2] Annulation via Benzylic and meta C-H Bond Activation.

The palladium-catalyzed reaction of aromatic amides with maleimides results in the formation of a double C-H bond activation product, which occurs at both the benzylic and meta positions. Computational chemistry studies suggest that the first C-H bond activation unfolds via a six-membered palladacycle, maleimide insertion, protonation of the Pd-N bond, and then activation of the meta C-H bond. The process concludes with reductive elimination, producing an annulation product. The energy decomposition analysis (EDA) showed that the deformation energy favors the ortho C-H bond activation process. However, this route is non-productive. The interaction energy controls the site where the maleimide is inserted into the Pd-C(sp3 ) bond, which determines its site selectivity. The energetic span model indicates that the meta C-H bond activation step is the one that determines the turnover frequency. Regarding the directing group, it has been concluded that the strong Pd-S bonding and the destabilizing effect of the deformation energy allow the 2-thiomethylphenyl to function effectively as a directing group.

A DFT study on the copper-single-atom modified 2D electride Ca2N monolayer for ethanol dehydrogenation

Two-dimensional (2D) electrides, with electrons confined in interlayer spaces instead of at atomic proximities, have received considerable attention due to their exotic properties. However, the current exploration of electrides in the field of catalysis urgently needs to be expanded. Here, we report novel thermodynamically stable Cu single-atom-modified 2D electride Ca2N monolayer catalysts for ethanol dehydrogenation using density functional theory calculations. The interaction and synergistic catalytic mechanism of copper single atoms and Ca2N monolayers were thoroughly revealed. The coordination environment of copper greatly affects the reaction sites for ethanol dehydrogenation. Furthermore, it was found that the strong electron transfer ability of Ca2N monolayer promoted the adsorption and activation of ethanol molecules on the catalyst, resulting in a cleavage energy barrier of the O–H bond as low as 0.29 eV on Cu1/Ca2N. Furthermore, turnover frequency (TOF) analysis demonstrates that Cu1/Ca2N exhibits significantly higher activity compared to Ca2N with lower energetic span (δE) of 2.52 eV at 298.15 K for the cycle of ethanol dehydrogenation to acetaldehyde. Our work demonstrates a new avenue to the discovery of 2D electride-based catalysts and provides new insights into the mechanism of ethanol dehydrogenation.

Understanding the Mechanism and Selectivity of 1,1-Diborylalkanes from Alkenes Catalyzed by a Zirconium Complex.

The synthesis of 1,1-diborylalkanes from readily available alkenes is an appealing method. The density functional theory (DFT) method was employed to investigate the reaction mechanism of 1,1-diborylalkanes, which was synthesized from alkenes and a borane, and the reaction was catalyzed by a zirconium complex Cp2ZrCl2. The entire reaction is divided into two cycles: dehydrogenative boration to form vinyl boronate esters (VBEs) and hydroboration of VBEs. This article focuses on the hydroboration cycle and elaborates on the role of the reducing reagents in the equilibrium of self-contradictory reactivity (dehydrogenative boration and hydroboration). The H2 and HBpin pathways were investigated as the reducing reagents in the hydroboration process. The calculated results showed that it is more advantageous to use H2 as a reducing agent (path A). Furthermore, the σ-bond metathesis is the rate-determining step (RDS) with an energetic span of 21.4 kcal/mol. This is consistent with the self-contradictory reactivity balance proposed in the experiment. The reaction modes of the hydroboration process were also discussed. These analyses revealed the origin of selectivity in this boration reaction, in which the σ-bond metathesis of HBpin needs to overcome the strong interaction between HBpin and the Zr metal. Meanwhile, the origin of the selectivity of different positions of H2 is the interaction between the σ(H1-H2) → σ*(Zr1-C1) overlap and these findings have implications for catalyst design and application.

Enhancing Kinetics of Carbonyl Sulfide Hydrolysis Using Pt-Supported Al2O3 Catalysts: First-Principles-Informed Energetic Span Analysis

Catalytic hydrolysis is considered an effective strategy for treating carbonyl sulfide (COS)─a toxic sulfur-containing gas that causes problems to the environment and petrochemical industries. Al2O3-based materials are commonly used as catalysts for COS hydrolysis owing to their stability and cost effectiveness. However, they still suffer from sulfur poisoning leading to partial COS conversion after long time use. To improve their catalytic performances, herein, we computationally designed and studied the catalytic activity of the Pt-supported Al2O3 catalysts by means of density functional calculations. We mechanistically explored the COS hydrolysis on both bare and Pt-decorated surfaces to reveal the role of Pt catalysts in the reaction kinetics. We find that bare Al2O3 suffers from difficult C–S bond breaking as its barrier is relatively high. Pt facilitates C–S bond breaking where its barrier is reduced by more than half (1.34 to 0.60 eV). However, the Pt–Al2O3 catalyst could encounter sulfur poisoning as the sulfur-containing intermediates are rather stable. Such issues could be remedied by increasing the operating temperature to destabilize the intermediates and promote product desorption. The energetic span models reveal that the important states of bare and Pt–Al2O3 are indeed C–S bond breaking and product desorption corresponding to an energy span of 2.72 and 1.67 eV at 773 K, respectively─suggesting that Pt dramatically enhances the catalytic activity of Al2O3-based catalysts toward COS hydrolysis. The suggested operating temperatures are above 673 K to avoid sulfur poisoning. Our findings will be useful for the development of more efficient Al2O3-based catalysts for treating COS.

Mechanistic Insights into the Stepwise (4+2) Cycloaddition toward Chiral Fused Uracil Derivatives

AbstractThe mechanism of the previously reported stereoselective (4+2) cycloaddition of N‐protected 6‐methyluracil‐5‐carbaldehydes and (E)‐β‐nitrostyrenes catalyzed by Takemoto's tertiary amine/thiourea organocatalyst was explored, using density functional theory (DFT) calculations on a model representative reaction. The cyclization reaction, which afforded notable enantioenriched carbocycle‐fused uracils embedding three contiguous stereocenters, was here proven to be the result of a four‐step sequence comprising a key stereo‐defining Michael addition, followed by a completely diastereoselective intramolecular Henry reaction. Going beyond the hitherto reported activation modes, a complex and unprecedented network of hydrogen‐bonding interactions between the chiral catalyst and the reaction partners has been disclosed, in which the protonated tertiary amine and the thiourea moiety of the catalyst simultaneously activate both the electrophile and the nucleophile components. By applying the Energetic Span Model (ESM) to four competitive energetic profiles, we unveiled the most plausible reaction pathways best fitting the experimental data, with close correlation with the observed enantiomeric ratio of the product.magnified image

Computational Study on a Transfer Hydrogenation Catalysed by a Ru(II) Bis‐Pyrazolyl Pyridine Complex

AbstractDensity functional calculations were performed to investigate the Ru‐catalysed transfer hydrogenation of acetophenone to 1‐phenylethanol using isopropanol as the reducing agent. The catalyst is [Ru(bpp)Cl2(PPh3)], where bpp is a tridentate bis‐pyrazolyl pyridine ligand. We studied an inner sphere monohydridic mechanism where the active Ru centre binds, in addition to bpp, one of the original ligands as a spectator, which can be either chloride or triphenylphosphine. In both cases, catalyst initiation appears to be feasible (with triphenylphosphine being the preferred spectator ligand) and the catalytic cycle is efficient with a small energetic span. For which one of the cases the calculated energetic span is smaller depends on the treatment of entropy in solution. Since pathways involving dissociation of charged ligands are not preferred, the product is formed by a fast proton transfer within the Ru coordination sphere, from a coordinated isopropanol to the product alcoholate. Subsequently, both product release and catalyst regeneration can be accomplished by the dissociation of the charge neutral product alcohol. The β‐hydride transfer from a coordinated isopropanolate anion to ruthenium is slightly different depending on whether there is a chloride or phosphine present trans to the alcoholate. In both cases, this step contains the transition state with the highest Gibbs energy.

A DFT study on the reaction mechanism of the gold(I)‐catalyzed cycloisomerization of alkynylhydroxyallylamides to 4‐Oxa‐6‐azatricyclo[3.3.0.02,8]octane and 3‐Acyl‐4‐alkenylpyrrolidine

AbstractIn this study, we use density functional theory calculations to investigate the discrepancy between two experimental results of Au(I)‐catalyzed cycloisomerization reactions of alkynylhydroxyallyl tosylamide under similar reaction conditions, with the only variations being reaction temperature and time. The experimental results reported by Yeh and Chung groups, respectively, showed that 3‐acyl‐4‐alkenylpyrrolidines are produced dominantly at ambient temperature, while 4‐aza‐6‐oxatricyclo[3.3.0.02,8]octanes are produced in higher yield at elevated temperature. Using (Z)‐4‐([3‐phenylprop‐2‐yn‐1‐yl]amino)but‐2‐en‐1‐ol and [Au(PPh3)]+ as the model starting material and active catalyst species, respectively, we identified two major pathways leading to 4‐aza‐6‐oxatricyclo[3.3.0.02,8]octane (pathway I) and 3‐acyl‐4‐alkenylpyrrolidine (pathway II). The overall free energy barrier (ΔG‡max) and the energetic span (ΔG‡span) of each pathway were 38.3 and 48.4 kcal/mol for pathway I and 29.0 and 37.1 kcal/mol for pathway II. Our analysis shows that the disparate outcomes observed in the experiments by two separate groups mainly originate in the reaction kinetics, with both the overall activation barrier and energetic span being the important factor.

Theoretical Study on the Catalytic CO2 Hydrogenation over the MOF-808-Encapsulated Single-Atom Metal Catalysts

The search for new catalytic agents for reducing excess CO2 in the atmosphere is a challenging but essential task. Due to the well-defined porous structures and unique physicochemical properties, metal–organic frameworks (MOFs) have been regarded as one of the promising materials in the catalytic conversion of CO2 into valuable platform chemicals. In particular, introducing the second metal (M) atom to form the MII–O–Zr4+ single-atom metal sites on the Zr nodes of MOF-808 would further greatly improve the catalytic performance. Herein, CO2 hydrogenation reaction mechanisms and kinetics over a series of MOF-808-encapsulated single-atom metal catalysts, i.e., MII–MOF-808 (MII = CuII, FeII, PtII, NiII, and PdII), were systematically studied using density functional theory calculations. First, it has been found that the stability for the encapsulation of a divalent metal ion follows the trend of PtII > NiII > PdII > CuII > FeII, while they all possess moderate anchoring stability on the MOF-808 with the Gibbs replacement energies ranging from −233.7 to −310.3 kcal/mol. Two plausible CO2 hydrogenation pathways on CuII–MOF-808 catalysts, i.e., formate and carboxyl routes, were studied. The formate route is more favorable, in which the H2COOH*-to-H2CO* step is kinetically the most relevant step over CuII–MOF-808. Using the energetic span model, the relative turnover frequencies of CO2 hydrogenation to various C1 products over MII–MOF-808 were calculated. The CuII–MOF-808 catalyst is found to be the most active catalyst among five MII–MOF-808 catalysts.

Promoting water dissociation for efficient solar driven CO2 electroreduction via improving hydroxyl adsorption

Exploring efficient electrocatalysts with fundamental understanding of the reaction mechanism is imperative in CO2 electroreduction. However, the impact of sluggish water dissociation as proton source and the surface species in reaction are still unclear. Herein, we report a strategy of promoting protonation in CO2 electroreduction by implementing oxygen vacancy engineering on Bi2O2CO3 over which high Faradaic efficiency of formate (above 90%) and large partial current density (162 mA cm−2) are achieved. Systematic study reveals that the production rate of formate is mainly hampered by water dissociation, while the introduction of oxygen vacancy accelerates water dissociation kinetics by strengthening hydroxyl adsorption and reduces the energetic span of CO2 electroreduction. Moreover, CO3* involved in formate formation as the key surface species is clearly identified by electron spin resonance measurements and designed in situ Raman spectroscopy study combined with isotopic labelling. Coupled with photovoltaic device, the solar to formate energy conversion efficiency reaches as high as 13.3%.

Iodine(I)-based and iodine(III)-based halogen bond catalysis on the Friedel-Crafts reaction: a theoretical study.

Halogen bond catalysis, especially iodine derivatives catalysis, has attracted increasing attention in recent years owing to the advantages of relatively cheap, stable, green, easy to handle, and favorable catalytic activity. To obtain insights into the catalytic mechanism and activity of halogen bond donor catalysts, iodine(I)-based and iodine(III)-based halogen bond catalysis on the Friedel-Crafts reaction were investigated in this study. The entire reaction contains several key steps: carbon-carbon bond coupling, proton transfer, hydroxyl departure, indole addition, and deprotonation process. According to the energetic span model, iodine(III)-based donor catalysts exhibit higher catalytic activity than iodine(I)-based catalysts and double cationic catalysts are more potent than single cationic ones. For halogen bond catalysis, the Gibbs energy barriers have linear relation to the electron density at the halogen bond critical points. Furthermore, the Gibbs energy barriers are also linearly related to the integral charge values of the increased region of electron density outside the oxygen atom of reactants. Therefore, the stronger halogen bond results in lower Gibbs energy barrier, and the stronger polarization further benefits the halogen bond catalysis.