N-Heterocyclic Carbene-Catalyzed Facile Synthesis of Phthalidyl Sulfonohydrazones: Density Functional Theory Mechanistic Insights and Docking Interactions

N-heterocyclic carbene catalysis reaction protocol is disclosed for the synthesis of phthalidyl sulfonohydrazones. A broad range of N-tosyl hydrazones react effectively with phthalaldehyde derivatives under open-air conditions, enabling the formation of a new C–N bond via an oxidative path. The reaction proceeds under mild reaction conditions with broad substrate scopes, wide functional group tolerance, and good to excellent yields. The mechanistic pathway is studied successfully using control experiments, competitive reactions, ESI-MS spectral analyses of the reaction mixture, and computational study by density functional theory. The potential use of one of the phthalidyl sulfonohydrazone derivatives as the inhibitor of β-ketoacyl acyl carrier protein synthase I of Escherichia coli is investigated using molecular docking.


■ INTRODUCTION
Chemical modification on medicinally important natural products or drug molecules is an established strategy to develop chemical entities and prodrugs with enhanced performance or introducing different clinical applications.Sulfonamide-based organic molecules find profound applications in the field of medicine and pharmaceutical areas. 1 Hydrazones are a significant class of organic molecules for drug design and synthesis.Of late, these functional compounds have been developed with numerous biological activities.Some hydrazone derivatives have proven to possess antimicrobial, 2 antimalarial, 3 antiviral, 4 vasodilator, 5a anti-inflammatory, 5b anticancer, 5c and antituberculosis 6 activities (A−C, Figure 1).Tosyl hydrazone compounds are found as antibacterial, antifungal, and anticancer agents (D−G, Figure 1). 7Moreover, tosyl hydrazones are treated as versatile and useful partners in organic synthesis. 8In particular, under basic conditions, the tosyl hydrazone moieties are easily converted into diazo compounds, 9 which can undergo insertion reactions, leading to the construction of various chemical bonds like C−C, C−N, C−Si, etc.In addition, hemiaminal esters constitute crucial building blocks for several biological systems.In general, these compounds are obtained via a dynamic kinetic resolution process using various substrates. 10Among the various types of sulfonamides, phthalidyl sulfonamide promoters were found with impressive success.Phthalidyl sulfonamides derivatives are an important class of heterocycles having potent biological and photophysical properties.For instance, hydrochlorothiazide is a diuretic medication for the treatments of high blood pressure, congestive heart failure, diabetes insipidus, and renal tubular acidosis. 11Saccharin is an artificial sweetener and food additive.Sulfamethoxazoles are known as antibiotics used for bacterial infections such as urinary tract infections, bronchitis, and prostatitis. 12 Sustainable N-heterocyclic carbene (NHC) organocatalysis is established as a powerful tool in modern organic synthesis to access functional molecules. 13Chi and co-workers reported an enantioselective method for the synthesis of chiral phthalidyl ester via NHC-catalyzed acetalization of carboxylic acids using a stoichiometric amount of oxidant (eq i, Scheme 1).14a Very recently, the same group has disclosed the reaction of N-aryl sulfonamides with phthalaldehydes producing optically enriched phthalidyl sulfonamides under NHC organocatalysis and subsequent oxidation (eq ii, Scheme 1).However, the reaction did not proceed without the use of 3,3′,5,5′-tetra-tertbutyldiphenoquinone as an externally supplied oxidant (1 equiv).14b Aerial oxidation in NHC catalysis is well documented in the literature to access aromatic esters or carboxylic acids from aromatic aldehydes with alcohols or nonactivated aldehydes, respectively. 15Thus, we examined the NHC-catalyzed C−O/ C−N coupled reaction using molecular oxygen from air (eq iii, Scheme 1).
Inspired by the very interesting results, we assume that the sulfonohydrazide moiety-containing phthalidyl scaffold could be an appropriate modification of existing phthalidyl sulfonamide derivatives, which were found having diverse bioactivity.To the best of our knowledge, until date, there is no report regarding the bioactivity assay of phthalidyl sulfonohydrazone derivatives, as their synthesis is unknown in the literature.Our aim was to synthesize organic molecules, which in turn looked like as sulfonamides, hemiaminal ester, and phthalidyl sulfonohydrazone.In this context, we disclosed NHC access to bioactive phthalidyl sulfonohydrazone from phthalaldehyde and N-tosyl hydrazones in good to excellent yield under aerobic oxidation (eq iii, Scheme 1).The mechanistic pathways are explored in detail using the density functional theory (DFT) computational study.Further, molecular docking is performed to see the potential bioactivity of one of the reported compounds.

■ RESULTS AND DISCUSSION
At the outset of our studies, we have investigated the reaction employing N-tosyl hydrazone derivative (2b) and phthalaldehyde (1a) as the model substrates (Table 1) with the variation of catalytic amounts of various NHCs in open air.The oxidative NHC-catalyzed reaction was initially attempted using imidazolium salt 3a or thiazolium salt 3b along with various bases and solvents (entries 1−9, Table 1).These reactions were either unsuccessful or only a trace of the desired product (4b) was obtained in these cases.Gratifyingly, the desired product was obtained in 55% yield on the treatment of the substrate with thiazolium salt 3c and base DBU in dichloromethane solvent for 12 h at ambient temperature (entry 10).To our delight, the yield (86%) and reaction rate (the reaction time decreased to 2 h) were drastically improved on the use of Cs 2 CO 3 as a base in acetonitrile (entry 11).The yield of the 4b was not improved at all on changing the reaction medium and base (entries 12−15).On the enhancement of catalyst loading (12 mol %) and reaction time (6 h), the yield was not significantly improved (88%, entry 16).A decrease of catalyst loading (8 and 6%, respectively) led to the lowering of the reaction rate and yield (entries 17 and 18).The reaction did not proceed in the absence of NHC (entry 19).Thus, we found the optimized conditions for this reaction using Cs 2 CO 3 as the base in acetonitrile solvent with NHC 3c as the efficient catalyst in air to produce the Scheme 1. Synthesis of Phthalidyl Esters (eq i), Sulfonamides (eq ii), and Our Approach (eq iii) to Phthalidyl Sulfonohydrazones desired phthalidyl sulfonohydrazone derivative in 2 h with 86% yield (entry 11).Also, we have performed the reaction under inert atmosphere conditions using "kahrasch oxidant" (3,3′,5,5′tetra-tert-butyldiphenoquinone, DPQ) and observed a moderate yield.14b General applicability of the developed reaction conditions (entry 11, Table 1) using various substituted N-tosyl hydrazones (2) and phthalaldehyde (1) to obtain functionalized phthalidyl sulfonohydrazide moieties (4a−s) was framed in Scheme 2. N-tosyl hydrazones were derived from aryl aldehydes in this case.Electron-donating groups in the aromatic ring of N-tosyl hydrazones performed well under these optimized reaction conditions to yield the product (4b−h) in 75−86%.Moderate to good yields (53−73%) were observed for N-tosyl hydrazones having an electron-withdrawing halogen or nitro group (4i−m).The reaction also went well when sterically hindered aldehyde precursors from naphthalene, biphenyl, and pyrene were tested under the reaction conditions to furnish desired products (4n− p) in 65, 63, and 60% yields, respectively.The reaction is also in consistent with heterocyclic N-tosyl hydrazones (4q, r).N-tosyl hydrazine from cinnamaldehyde also tolerated in this reaction protocol to furnish corresponding phthalidyl sulfonohydrazide 4s in 68% yield.In general, all of these allowed installation of a great diversity of substituents in the sulfonohydrazone template.
We further studied the N-tosyl hydrazones derived from aryl ketones as substrates to react with phthalaldehydes under the optimized reaction conditions (Scheme 3).N-tosyl hydrazones were well tolerated in the reaction protocol irrespective of the major electronic effects of the substituents in the aryl part of the hydrazones and furnished the desired products 4t−v in good yields (60−70%).N-tosyl hydrazones with a heteroaryl moiety in the structure reacted well and furnished the product (4w) in 64% yield.Regioselectivity studies of unsymmetrical phthalaldehyde were investigated, which afford 4x as a single isomer with good yield (60%).A gram-scale synthesis of 4b under the optimized reaction conditions was also verified to afford the desired product in 84% yield (entry 20).The structure of the all unknown compounds (4a−x) was determined unambiguously by recording NMR, FT-IR, and HRMS spectra and single-crystal XRD data analyses of compound 4m (Supporting Information).
To elucidate the reaction pathway, ESI-MS and DFT calculations were performed.We assume that a nucleophilic NHC-aldehyde adduct (Breslow intermediate) (I) is formed are formed when the HOO− reacts at the electrophilic carbon center through a barrierless process.Since the II-s is energetically more stable than the corresponding triplet state (II-t), an intersystem crossing process containing a transition from triplet to singlet occurs.During the reaction, there is an elimination of a HOO − ion from II and III being formed.However, the dissociation of zwitterionic prerequisites a significant electrostatic effort.Moreover, the HOO − ion is a   very strong base and nucleophile.Thus, there is a requirement of receiver species that can capture this leaving anion to assist the process.The second aldehyde or solvent molecule could act as a receiver.15d Finally, the oxidative product (III) is formed and is exergonic by 2.30 kcal/mol.
Intramolecular Annulation.As proposed, the nucleophilic addition of N-tosyl hydrazone to the oxidized Breslow intermediate (III) starts with a vdW complex, Int 5-s (Figure 4).Before the nucleophilic addition of N-tosyl hydrazine, it should be deprotonated at the N center.In this study, we have considered Cs 2 CO 3 to remove the hydrogen in N-tosyl hydrazone.The production of cesium salt (Int 5-s) is energetically favored.Once it is deprotonated, the nucleophilic attack of N-tosyl hydrazine occurs at the carbonyl group in III.Two stereoisomeric channels associated with the nucleophilic attack of the re or si face of the carbonyl carbon are feasible.The lengths of the C−N bonds are 1.866 Å (Re-TS3) and 1.981 Å (Si-TS3) (Figure S34, Supporting Information).Thus, the transition state TS3 is the stereoselectivity determining step responsible for the R or S configuration of III.Thus, the reaction proceeds through O−C coupling to generate intermediate IV.
Finally, there is extrusion of the NHC catalyst from intermediate IV and it produces the final product ( 4).The final step in the reaction follows a pathway through transition state TS4, where the activation free energy barrier is 10.86 kcal/mol higher than that of the intermediate IV.A free energy difference of 3.83 kcal/ mol between (S)-TS4 and (R)-TS4 can be attributed to the fact that (R)-TS4 is sterically less crowded compared to (S)-TS4.At (R)-TS4, the distance between NHC carbene C and ester C is measured to be 2.084 Å, whereas at (S)-TS4, this distance increases to 2.129 Å (Figure S34, Supporting Information).From the separated reactants, the formation of the final product is strongly exergonic by −23.63 kcal/mol.
In this section, we will investigate the potential of one of the phthalidyl sulfonohydrazone derivatives (4f) as the inhibitor of β-ketoacyl acyl carrier protein synthase I (KAS I) of Escherichia coli.Escherichia coli, commonly known as E. coli, is a prevalent bacterium responsible for various bacterial infections in humans (Figure 5).These infections encompass a wide range of conditions, including cholecystitis, bacteremia, cholangitis, urinary tract infections, and traveler's diarrhea, as well as clinical infections like neonatal meningitis and pneumonia.Certainly, three types of β-ketoacyl acyl carrier protein synthase (KAS) enzymes play a crucial role in addressing bacterial resistance issues.Thus, targeting these enzymes can be an effective strategy in tackling antibiotic resistance.Disruption of KAS I can impede the synthesis of essential fatty acids, crucial for bacterial membrane formation and cell growth.Thiolactomycin (TLM), a unique thiolactone molecule comprising natural products, inhibits bacterial cell growth by impeding the βketoacyl-ACP synthase activity.Thus, it becomes possible to compare the binding activity of TLM with the final product 4f (R/S).The docking studies are performed with the protein (1FJ4) 17 with a resolution factor of 2.35 Å.The active site of KAS I procedures a catalytic triad hole consisting of His−His− Cys. 18,19The docking study shows that TLM binds at the active site and forms a hydrogen bond with His333, contributing to the stabilization of the protein−inhibitor complex.The noncovalent interactions of TLM with the amino acids at the active site are elucidated in Figure S35 (Supporting Information).The calculated binding energy of reference TLM is −6.15 kcal/ mol, whereas (R)-4 shows a favorable binding energy of −6.87 kcal/mol.(S)-4 has a slightly weak binding interaction (−5.02 kcal/mol) at the active site compared to reference TLM.Thr-300 exhibits strong hydrogen bonding interactions with both compounds (Figure 5 and Figure S36, Supporting Information).Based on the phthalidyl sulfonohydrazone−receptor interactions, it is suggested that these leads have the potential to be developed into effective antimicrobial drugs targeting Gramnegative E. coli.

■ CONCLUSIONS
In conclusion, this study offers an excellent methodology for the synthesis of new phthalidyl sulfonohydrazone compounds using NHC-catalyzed reaction conditions under open air.The reaction mechanism proceeds through the formation of the Breslow intermediate, followed by aerial oxidation.Ultimately, phthalidyl sulfonohydrazones are formed via intramolecular annulation.The full mechanism is supported by both experimental and computational methods.Finally, the phthalidyl sulfonohydrazide-β-ketoacyl acyl carrier protein synthase I interactions are studied successfully using molecular docking, which suggests the potential of phthalidyl sulfonohydrazones to be effective as an antimicrobial drug targeting Gram-negative E. coli.

■ EXPERIMENTAL SECTION
All reagents were purchased from commercial suppliers and used without further purification, unless otherwise specified.Commercially supplied ethyl acetate and petroleum ether were distilled before use.All solvents were dried through usual methods.The petroleum ether used in our experiments has a boiling range of 60−80 °C.Analytical thin-layer chromatography was performed on 0.25 mm extra-hard silica gel plates with a UV254 fluorescent indicator.The reported melting points are uncorrected.The 1 H NMR and 13 C NMR spectra were recorded at ambient temperature using both 300 MHz spectrometers (300 MHz for 1 H and 75 MHz for 13 C).Chemical shifts are reported in ppm with respect to tetramethylsilane internal reference, and coupling constants are reported in Hz.Proton multiplicities are represented as s (singlet), d (doublet), dd (double doublet), t (triplet), q (quartet), and m (multiplet).The infrared spectra were recorded on an FT-IR spectrometer in thin films.HRMS data were recorded on a Q-tof-micro quadruple mass spectrophotometer.
General Procedure for the Synthesis of Phthalidyl Sulfonohydrazide Derivatives (4, GP-1).In a 25 mL roundbottom flask, phthalaldehyde (1, 1.0 mmol) and N-tosyl hydrazones 1 (2, 1.0 mmol) were added in CH 3 CN (5 mL) in the presence of thiazolium bromide 3c (10 mol %) in open-air conditions, and Cs 2 CO 3 (20 mol %) was added to this reaction mixture and stirred at room temperature for 2−5 h.Upon completion (monitored through TLC), the reaction mixture was filtered through the Celite bed and evaporated in a rotary evaporator under reduced pressure and then extracted with CH 2 Cl 2 (2 × 15 mL).The combined organic layer was washed with water (3 × 10 mL) and dried over anhydrous Na 2 SO 4 , filtered, and evaporated in a rotary evaporator under reduced pressure at room temperature.The residue was chromatographed on a silica gel column (60−120 mesh) using ethyl acetate−petroleum ether (9 to 20%, v/v) as an eluent, which afforded the corresponding hemiaminal phthalidyl ester derivatives (4).
Computational Methods.All the geometries considered in this study have been fully optimized using the dispersioncorrected PBE0-D3 20,21 functional with the def2-TZVPP basis set. 22The solvent effects were considered via the COSMO solvation model 23 with acetonitrile solvent medium.The vibrational frequencies of each stationary point were carried out at the same level of theory to classify the stationary points either as real minima (with no imaginary frequencies) or as transition state with only one imaginary frequency.All the calculations were performed using Gaussian 16. 24  Molecular docking was performed using the Schrodinger Suite molecular modeling package (version 2021-3) using the default parameters.Co-crystal structures of thiolactomycin with β-ketoacyl-[acyl carrier protein] 2 synthase (PDB: 1FJ4), 17 with a resolution of 2.35 Å, were used as templates and were prepared using the Protein Preparation Wizard.In this step, force field atom types and bond orders were assigned, missing atoms were added, tautomer/ionization states were assigned, and the tautomers of ionizable residues (Asn, Gln, and His residues) were adjusted to optimize the hydrogen bond network.Hydrogen-constrained energy minimization was then performed.Glide SP docking was used to grant full flexibility of ligands into the active site. 25,26A postdocking minimization, in which only the ligands were flexible, was performed on the output complexes.The binding energies were calculated for each ligand.

Figure 3 .
Figure 3. Free energy diagram of the aerial oxidation pathway to generate the acyl azolium intermediate (III).Orange color, triplet pathway; violet color, singlet pathway.

Figure 4 .
Figure 4. Free energy diagram for forming the final product hemiaminal ester along with the regeneration of the active catalyst.Red color, proceeds through the Re face; violet color, proceeds through the Si face.

Figure 5 .
Figure 5. Glide molecular docking interactions of the receptor (PDB ID: 1FJ4) with (R)-4f.(a) Protein−ligand schematic interaction diagram of the protein and (R)-4f complex.(b) Binding pose of (R)-4f in the active site of the receptor.