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Green Organic Solvent-Free Oxidation of Alkylarenes with tert-Butyl Hydroperoxide Catalyzed by Water-Soluble Copper Complex

  • Abdelaziz Nait Ajjou EMAIL logo and Ateeq Rahman
Published/Copyright: March 24, 2020
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Open Chemistry
From the journal Open Chemistry Volume 18 Issue 1

Abstract

Different benzylic compounds were efficiently oxidized to the corresponding ketones with aqueous 70% tert-butyl hydroperoxide (TBHP) and the catalytic system composed of CuCl2.2H2O and 2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt (BQC). The catalytic system CuCl2/BQC/TBHP allows obtaining high yields at room temperature under organic solvent-free conditions. The interest of this system lies in its cost effectiveness and its benign nature towards the environment. Benzylic tertbutylperoxy ethers and benzylic alcohols were observed and suggested as the reaction intermediates. Analysis of organic products by atomic absorption did not show any contamination with copper metal. In terms of efficiency, CuCl2/BQC system is comparable or superior to the most of the catalytic systems described in the literature and which are based on toxic organic solvent.

Graphical Abstract

Keywords: Oxidation; water-soluble catalyst; copper chloride; tert-butyl hydroperoxide; alkylarenes; 2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt

1 Introduction

Ketones are one of the most important classes of organic compounds. They are used as solvents and precursors to a variety of polymers and biologically active compounds such as pharmaceuticals, flavors, agrochemicals, and fragrances. Furthermore, the occurrence of ketone moiety is widespread among natural products. Ketones are frequently prepared by different methods including, Friedel-Crafts acylation of aromatics, ozonolysis, hydration of alkynes, Weinreb-Nahn synthesis [1], Kornblum-DeLaMare rearrangement [2], cross-coupling between dialkylcuprate and acyl chlorides, reaction of organolithium and Grignard with nitriles, oxidation of alcohols including Oppenauer-type reaction [3], and oxidation of methylenes.

The oxidation of hydroxyl and methylene groups to the corresponding carbonyl moieties remains one of the most fundamental and indispensable reactions in organic synthesis [4, 5, 6]. For such processes, the utilization of at least stoichiometric amounts of often toxic oxidants, especially chromium (VI) reagents, remains widespread [7]. Safety hazards associated with these oxidants and their toxic by-products, and the difficulty to work-up the reaction mixtures are the major problems of such processes. As a consequence for the increasing demand for cleaner, efficient, and environmentally friendly oxidations, different catalytic methods using small amounts of metallic derivatives and clean oxidants have been developed [4,8, 9]. Thanks to its price relatively low and its reduced form which can be recycled, tert-butyl hydroperoxide (TBHP) has been studied for benzylic oxidation in combination with different transition-metal catalysts such as Cr, Co, Mn, Fe, Ru, Rh, Au, and Cu [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28]. Despite the economic and environmental benefits of copper-based catalysts compare to the other transition metals that are either toxic or very expensive, their use for benzylic oxidations is not abundant [29, 30]. With all these metals, the vast majority of catalytic processes are, unfortunately, performed in costly and toxic organic solvents. Furthermore, in the homogeneous processes, the separation of the catalysts from the reaction products and their quantitative recovery in an active form are cumbersome. Very few organic solvent-free processes have been reported for benzylic oxidation with TBHP [27, 28]. However, these processes are based on high temperatures and are limited to very few substrates. Aqueous organometallic catalysis that was emerged as an active field of research in green chemistry is an excellent approach to overcome these drawbacks [31, 32, 33, 34]. The use of water as solvent is important for economical, safety, and environmental reasons. The water-soluble catalyst which operates and resides in water is easily separated from the reaction products by simple decantation. In addition, the products are not contaminated with traces of metal catalyst, and the use of organic solvents, such as benzene and chlorinated hydrocarbons is circumvented. Despite the evident ecological and economical advantages of aqueous phase catalysis, to the best of our knowledge there are no reports concerning selective oxidation of alkylarenes to benzylic ketones in water at room temperature and based on copper catalysts. In the past years, we developed different catalytic transformations in water including the hydration of nitriles to the corresponding amides catalyzed by [Rh(COD)Cl]2/P(m-C6H4SO3Na)3 (TPPTS) [35], the transfer hydrogenation of aldehydes and ketones with isopropanol catalyzed by water-soluble rhodium complexes [36], Oppenauer-type oxidation of alcohols [37, 38], and reductive amination of aldehydes [39]. We disclosed also the water-soluble CuCl2/BQC as a highly effective catalyst for the oxidation of secondary benzylic, allylic, propargylic, and 1-heteroaryl alcohols with 70% TBHP [40, 41], and for the oxidation of alkynes to the corresponding ynones [42]. The catalytic system is very cheap, stable and can be recycled several times without significant loss of activity. The above mentioned advantages of CuCl2/BQC coupled with those of 70% TBHP prompted us to investigate the catalytic activity of CuCl2/ BQC/TBHP system for the oxidation of benzylic methylenes to the corresponding carbonyl moieties. It is important to note that aqueous TBHP (70% in water) is much safer to use compared to the explosive anhydrous TBHP that has serious safety problems regarding handling, storage and shipping [43]. In this paper we are pleased to report our results regarding this unprecedented, general, and highly efficient organic solvent-free catalytic oxidation of alkylarenes (Scheme 1).

Scheme 1 Oxidation of alkylarenes to ketones, t-butylperoxy ethers and benzylic alcohols.
Scheme 1

Oxidation of alkylarenes to ketones, t-butylperoxy ethers and benzylic alcohols.

2 Experimental

2.1 Materials and Instruments

All the substrates, Copper chloride dihydrate, 2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt (BQC), tetrabutylammonium chloride (TBAC), sodium carbonate, and aqueous 70% tert-butyl hydroperoxide were purchased from Aldrich Chemical Co. and used without further purification.

Routine NMR measurements were performed on a Bruker AC-200 spectrometer at 200 MHz and 50 MHz, respectively, for 1H and 13C, using TMS as internal standard and CDCl3 as solvent.

2.2 Typical procedure for the oxidation of alkylarenes

Into an open 25 mL round-bottom flask charged with distilled water (5 mL), CuCl2.2H2O (0.02 mmol), Na2CO3 (0.14 mmol), and BQC (0.02 mmol), was added TBAC (0.06 mmol). The green-blue solution was stirred for 5 minutes then the substrate (2 mmol) was introduced followed by aqueous 70% TBHP (4 to 8 mmol). The purple mixture was allowed to react for 17 hours at room temperature. At the end of the reaction, the mixture is still purple. The products and substrate, which are not soluble in water, were extracted three times with ethyl acetate (20 mL). The combined organic layers were dried (MgSO4), evaporated to dryness, and then analyzed by thin layer chromatography, 1H NMR and 13C NMR. Conversions and yields were determined after the reaction mixtures were purified using column chromatography (silica gel) with a gradient of petroleum ether / ethyl acetate (100 to 95/5) as the eluant.

Ethical approval: The conducted research is not related to either human or animal use.

3 Results and Discussion

We first investigated the oxidation of indane (1) chosen as a model reaction, under different conditions. No reaction was observed in the absence of aqueous 70% t-butyl hydroperoxide (TBHP) or CuCl2. When indane (2 mmol) was oxidized with TBHP (2 equivalents, 4 mmol) in the presence of CuCl2 (0.02 mmol), BQC (0.02 mmol) and Na2CO3 (0.14 mmol) in distilled water, 1-indanone was obtained with 61% yield and 74% conversion. The other product obtained is 1-tert-butylperoxy-indane (13%) (Table 1, entry 1). When the oxidation of indane was repeated in the presence of tetrabutylammonium chloride (TBAC, 0.06 mmol) as the phase transfer catalyst, full conversion of indane was observed and 1-indanone was obtained with 92% yield along with 1-tert-butylperoxyindane (3%) and 1-indanol (2%) (Table 1, entry 2). The positive influence of TBAC on the reaction conversion and 1-indanone yield is expected since phase transfer catalysts, such as surfactants, co-solvents and tenside ligands, have been successfully used in order to increase the reaction rates in aqueous-phase organometallic catalysis [40,42,44]. Also, in our previously reported oxidation of alcohols [40] and alkynes [42] with TBHP/ CuCl2/BQC, we demonstrated that the addition of water-soluble co-solvents such as methanol or acetonitrile, phase-transfer catalysts such as cyclodextrins or ammonium salts, increase reaction rates, and the best results were obtained with tetrabutylammonium chloride. A decrease in the oxidation conversion (100% to 66%) and in 1-indanone yield (92% to 30%) was observed when the reaction was performed without BQC (Table 1, entries 2-3). Sasson and al. demonstrated the formation of Cu(OH)Cl from CuCl2 under their biphasic conditions for benzylic oxidations with TBHP/CuCl2 system [29]. We believe that under our conditions, (BQC)Cu(OH)Cl may be formed and leads to more active water-soluble catalytic species. Sodium carbonate plays a crucial role in this catalytic process as it keeps the ligand BQC in its basic form that makes it water-soluble [40]. In distilled water, without Na2CO3, CuCl2 generates an acidic solution where BQC is transformed by protonation to water insoluble 2,2’-biquinoline-4,4’-dicarboxylic acid (BQCH) that results in green water insoluble copper complex (Cu/ BQCH). Under these conditions, probably there is not sufficient contact between the catalyst and the substrate, which leads to the lower yields obtained (Table 1, entries 4-5). The oxidation performed with 3 equivalents of TBHP led to an excellent yield of 1-indanone with no traces of over oxidation products such as 1,3-indandione (Table 1, entry 6).

Table 1

Oxidation of indane (1) with aqueous 70%TBHP catalyzed by CuCl2.2H2O/BQC.[a]

EntryTBHP (equiv.)Conversion (%)Yield (%)
1[b]2746113Traces
221009232
3[c]26630306
4[d]27550214
5[d]394641512
6310098TracesTraces

We also investigated the oxidation of ethylbenzene (2). Thus, the oxidation of 2 (2 mmol) with 70% TBHP (2 equivalents) in the presence of CuCl2.2H2O (0.02 mmol), BQC (0.02 mmol), tetrabutylammonium chloride (0.06 mmol), and Na2CO3 (0.14 mmol), proceeds smoothly at room temperature, will full conversion, affording acetophenone (79%), 1-tert-butylperoxy-ethylbenzene (11%), and 1-phenylethanol with 10% yield (Table 2, entry 1). The catalytic activity and yields were not affected by scaling up the oxidation to a gram-scale of substrate (Table 2, entry 2). When the reaction was performed using 3 and 4 equivalents of TBHP, the amounts of acetophenone collected increased in detriment of 1-tert-butylperoxy-ethylbenzene and 1-phenylethanol which decreased markedly, and 98% yield was achieved with 4 equivalents of TBHP (Table 2, entries 3-4). Traces of benzoic acid as an over oxidation product were not detected. Since we demonstrated in our previous works that the catalytic system can be recycled efficiently for the oxidation of alcohols [40] and propargylic methylenes [42], no recycling experiments have been attempted in this project. The analysis of organic products by atomic absorption, however, did not show any contamination of acetophenone with copper.

Table 2

Oxidation of ethylbenzene (2) with aqueous 70%TBHP catalyzed by CuCl2.2H2O/BQC.[a]

EntryTBHP (equiv.)Conversion (%)Yield (%)
12100791110
2[b]2100761210
331008486
4410098TracesTraces

To evaluate the synthetic potential of CuCl2/BQC system, other alkylarenes were subjected to the oxidation with two to four equivalents of aqueous 70% TBHP. Thus, tetralin (3), fluorene (4), diphenylmethane (5), deoxybenzoin (6), 6-methoxytetralin (7), and phthalan (8) were studied, and the best results are summarized in Table 3. The oxidation of benzylic methylenes proceeded smoothly to carbonyl moieties with excellent yields in most cases. As illustrated in Tables 1 to 3, ketones were obtained along with small amounts of either benzylic tert-butylperoxy ethers, benzylic alcohols, or both, except in the case of deoxybenzoin (Table 3, entry 9). Such compounds have been proven as reaction intermediates for benzylic oxidations of alkylarenes with TBHP [8, 9, 12,17,26,29,45, 46]. Tetralin afforded, with 4 equivalents of TBHP, α-tetralone in 77% yield, tert-butylperoxy ether (10%), and 6% of 2,3-dihydro-1,4-naphthoquinone. The latter, as an over-oxidation product, was not formed with stoichiometric amount (2 equivalents) of TBHP (Table 3, entries 3-4). This dione was also observed when tetralin was oxidized with anhydrous TBHP and catalyzed by Rh2(cap)4 [17]. 1,4-naphtoquinone that was formed with different chromium catalysts [9], was not detected with our system. While 3 equivalents of TBHP are sufficient to lead to 100% fluorenone from fluorene, 4 equivalents of the oxidant are needed to reach excellent yields in the cases of benzophenone and benzil, respectively from diphenylmethane and deoxybenzoin (Table 3, entries 5-9). 6-Methoxytetralin behaves almost in same manner as tetralin. Using 2 equivalents of TBHP, mono ketones 6-methoxy-1-tetralone and 7-methoxy-1-tetralone were isolated respectively in 39 and 17% yields, along with the corresponding peroxides and alcohols, and no over-oxidation products were detected (Table 3, entry 10). Mixture of mono ketones with different ratios have been reported with CrO3/TBHP [12, 13] and Rh2(cap)4/ anhydrous TBHP [17] systems. By increasing the amount of TBHP to 4 equivalents, yields of mono ketones did not increase significantly. However, mono ketones ratio changed to 50/8 and 6-methoxy-2,3-dihydro-1,4-naphthoquinone was isolated in 12% yield (Table 3, entry 11). The oxidation of phthalan with 2 equivalents of TBHP afforded phthalide in 70% yield, with full conversion (Table 3, entry 12). The other products according to the analysis of the crude reaction mixture by H1 NMR, are probably reaction intermediates mainly tert-butylperoxy ether. A similar intermediate was obtained in the case of the oxidation of isochroman with Rh2(cap)4/anhydrous TBHP [17]. No increase in the yields of phthalide was observed with longer reaction time, more TBHP, or by increasing reaction temperature.

Table 3

Oxidation of various alkylarenes with aqueous 70% TBHP catalyzed by CuCl2.2H2O/BQC.[a]

EntrySubstrateTBHP (equiv.)Conversion (%)Yield (%)
1410098TracesTraces
2310098TracesTraces
3280561210
449477[b]10Traces
52898450
6310010000
73857950
84938670
94949400
1027856 (39/17)[c]14[d]8[e]
1149858 (50/8)[c],[f]20[d]0
12[g]210070--

The synthetic utility of our system is illustrated in Table 4 where our data are compared with those of other homogeneous or heterogeneous catalytic systems described in the literature. The comparison is limited to the oxidations based on the use of an excess of TBHP (at least two equivalents to substrate), and only yields of mono ketones are indicated. In terms of efficiency, our system is comparable or superior to the most of the catalytic systems described. For example, oxidation of deoxybenzoin (6) with our system led to benzil with 94% yield, while Cr-PILC [10] system is completely inactive (Table 4, entries 1-2). All these catalytic systems require the use of one or more of the following conditions: toxic organic solvent, high temperature, dangerous anhydrous TBHP, or catalyst that may be toxic, expensive or difficult to synthesize. With regard to economic and environmental issues, it is largely beneficial to use our system based on CuCl2.2H2O and BQC which are available commercially, cheap cost, and relatively benign. Our system is even more interesting since the ligand BQC (2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt) is cheaper than the corresponding parent ligand 2,2’-biquinoline. From Aldrich Chemical Co., BQC and 2,2’-biquinoline cost 8,485 and 12,994 Canadian dollars per mole, respectively.

Table 4

Oxidation of alkylarenes with TBHP: comparative data from various catalytic systems.

EntryCatalytic method[a]RefYield of ketone (%)
12345678_
1[b]CuCl2.2H2O (1%)/BQC/TBHP[c], RT, water989877100869439/1770
2Cr-PILC (2.5%)/anh. TBHP[d] (2)/RT/CH2Cl21083929188900
3(Ph3SiO)2CrO2 (5%)/TBHP[c] (4)/RT/CH2Cl21186889879
4CrO3 (5%)/ TBHP[c] (4)/RT/BTF12928377999357/12
5CrO3 (5%)/ TBHP[c] (7)/RT/CH2Cl2136043945336/17
6CrE-ZSM-5 (0.2%)/ TBHP[c] (4)/80oC/PhCH3149385858789
7CrSBA-15/TBHP (2)/80-120oC/PhCl15,1676[e]95[e]41[e]58[f]
8Rh2(cap)4 (1%)/anh. TBHP (5)/RT/DCE178420[g]60995530/30
9RuCl2(PPh3)3 (1%)/anh. TBHP (4)/RT/PhH1851538771
10Bi (20%)/ TBHP[c] (6)/ 100oC/pyridine, AcOH1965779195
11FeCl3 (2%)/ TBHP[c] (3)/82oC/pyridine20611741>9945
12Fe(BTC)/ TBHP[h] /70oC/CH3CN215144[i]72
13KAuCl4,0.5H2O (5%)/anh. TBHP (2)/90oC/pyridine2258>999965
14ANMnO2 (10%)/ TBHP[c] (3)/80oC/CH3CN23897410092
15Co-SiO2 (1.25%)/anh. TBHP (6)/90oC/isooctane246558
16CoEPS3 (6.8%)/anh. TBHP (6)/50oC/CH3CN2597[g]91
17Cu(AMP)Cl2 (1%)/ TBHP[j] (5)/50oC/CH3CN2663
18LaCrO3 (10%)/ TBHP[c] (2)/90oC/no solvent2785849195
19Mn(TMCPP)/ TBHP[h] (3)/150oC/no solvent2891

As we mentioned benzylic tert-butylperoxy ethers and benzylic alcohols have been proven as reaction intermediates for benzylic oxidations of alkylarenes with TBHP [8, 9,12,17,26,29,45, 46]. The oxidation mechanism that we propose in Scheme 2 accounts for the formation of these intermediates, and is inspired by previously reported benzylic oxidations with TBHP and catalyzed by Cu((II) [26,29]. Initially CuCl2 reacts with the hydrophilic ligand BQC to form the water-soluble compound (BQC) CuCl2 (equation 1). (BQC)CuCl2 reacts with 2 equivalents of TBHP to generate (BQC)Cu(OH)Cl and t-butyloxy and t-butylperoxy radicals (equations 2 and 3). This later complex leads to (BQC)Cu(Cl)(OOt-Bu) after a reaction with TBHP (equation 4). Benzylic radical is then generated by the abstraction of the benzylic hydrogen of the substrate by t-butyloxy radical [8,9,26,29,47,48] or t-butylperoxy radical [8,21,47,48,49,50] (equations 5 and 6). Equations 7 and 8 explain the formation of tert-butylperoxy ethers and benzylic alcohols from the benzylic radical and respectively (BQC)Cu(Cl)(OOt-Bu) [26,29] and (BQC)Cu(OH)Cl [8,18,28,51]. The formation of benzylic alcohols may also be explained by the reaction of the benzylic radical with the coordinated TBHP to the copper complex (Scheme 3). This mechanism is prompted by the mechanism proposed by Muzart for chromium(VI)-catalyzed benzylic oxidation [8,9]. tert-Butylperoxy ethers (Ar)(R)CH(OOt-Bu) undergo homolytic cleavage of C-H bond, leading to (Ar)(R) (OOt-Bu)C radicals that lead to aromatic ketones through a mechanism similar to that proposed for chromium-catalyzed homolytic scission of organic peroxides to ketones with TBHP [45].

Scheme 2 Plausible mechanisms for the formation of t-Butylperoxy ethers and benzylic alcohols.
Scheme 2

Plausible mechanisms for the formation of t-Butylperoxy ethers and benzylic alcohols.

Scheme 3 Formation of benzylic alcohols via coordinated TBHP.
Scheme 3

Formation of benzylic alcohols via coordinated TBHP.

Previously we demonstrated that the rate-limiting step for the oxidation of alcohols to ketones with BQC/CuCl2/ TBHP system involves heterolytic C-H bond cleavage [40], in agreement with Sasson and al. who, in addition, suggest Cu(IV)=O as catalytic active species [29]. High-valent species have been reported when catalytic precursors react with TBHP [8,18,48,52,53] (equation 9). We thus, propose the oxidation of (BQC)Cu(OH)Cl with TBHP to (BQC)(Cl)(OH)Cu=O (equation 10). Then we rationalize the formation of benzylic ketones with the mechanism in Scheme 4 that involves heterolytic dehydrogenation of benzylic alcohols via six-membered cyclic transition state proposed by Muzart for chromium complexes [8,9].

Scheme 4 Mechanism for the oxidation of benzylic alcohols to aromatic ketones.
Scheme 4

Mechanism for the oxidation of benzylic alcohols to aromatic ketones.

4 Conclusion

In conclusion, the catalytic system composed of CuCl2.2H2O and 2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt (BQC), was found to be highly efficient for the selective oxidation of alkyarenes to the corresponding benzylic ketones, with aqueous 70% tert-butyl hydroperoxide at room temperature, under organic solvent-free conditions. This very simple catalytic system is cheap and environmentally friendly.

Acknowledgements

We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) and to the Faculté des Études Supérieures et de la Recherche (FESR) of the Université de Moncton for financial support of this research.

  1. Conflict of interest: Authors declare no conflict of interest.

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Received: 2019-04-04
Accepted: 2019-11-25
Published Online: 2020-03-24

© 2020 Abdelaziz Nait Ajjou, Ateeq Rahman, published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  6. Molecular dynamics simulation of sI methane hydrate under compression and tension
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  8. Amino-functionalized graphene oxide for Cr(VI), Cu(II), Pb(II) and Cd(II) removal from industrial wastewater
  9. Chemical composition and in vitro activity of Origanum vulgare L., Satureja hortensis L., Thymus serpyllum L. and Thymus vulgaris L. essential oils towards oral isolates of Candida albicans and Candida glabrata
  10. Effect of excess Fluoride consumption on Urine-Serum Fluorides, Dental state and Thyroid Hormones among children in “Talab Sarai” Punjab Pakistan
  11. Design, Synthesis and Characterization of Novel Isoxazole Tagged Indole Hybrid Compounds
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https://doi.org/10.1515/chem-2020-0018
Keywords for this article
Oxidation; water-soluble catalyst; copper chloride; tert-butyl hydroperoxide; alkylarenes; 2,2’-biquinoline-4,4’-dicarboxylic acid dipotassium salt
Creative Commons
BY 4.0

Articles in the same Issue

  1. Regular Articles
  2. Electrochemical antioxidant screening and evaluation based on guanine and chitosan immobilized MoS2 nanosheet modified glassy carbon electrode (guanine/CS/MoS2/GCE)
  3. Kinetic models of the extraction of vanillic acid from pumpkin seeds
  4. On the maximum ABC index of bipartite graphs without pendent vertices
  5. Estimation of the total antioxidant potential in the meat samples using thin-layer chromatography
  6. Molecular dynamics simulation of sI methane hydrate under compression and tension
  7. Spatial distribution and potential ecological risk assessment of some trace elements in sediments and grey mangrove (Avicennia marina) along the Arabian Gulf coast, Saudi Arabia
  8. Amino-functionalized graphene oxide for Cr(VI), Cu(II), Pb(II) and Cd(II) removal from industrial wastewater
  9. Chemical composition and in vitro activity of Origanum vulgare L., Satureja hortensis L., Thymus serpyllum L. and Thymus vulgaris L. essential oils towards oral isolates of Candida albicans and Candida glabrata
  10. Effect of excess Fluoride consumption on Urine-Serum Fluorides, Dental state and Thyroid Hormones among children in “Talab Sarai” Punjab Pakistan
  11. Design, Synthesis and Characterization of Novel Isoxazole Tagged Indole Hybrid Compounds
  12. Comparison of kinetic and enzymatic properties of intracellular phosphoserine aminotransferases from alkaliphilic and neutralophilic bacteria
  13. Green Organic Solvent-Free Oxidation of Alkylarenes with tert-Butyl Hydroperoxide Catalyzed by Water-Soluble Copper Complex
  14. Ducrosia ismaelis Asch. essential oil: chemical composition profile and anticancer, antimicrobial and antioxidant potential assessment
  15. DFT calculations as an efficient tool for prediction of Raman and infra-red spectra and activities of newly synthesized cathinones
  16. Influence of Chemical Osmosis on Solute Transport and Fluid Velocity in Clay Soils
  17. A New fatty acid and some triterpenoids from propolis of Nkambe (North-West Region, Cameroon) and evaluation of the antiradical scavenging activity of their extracts
  18. Antiplasmodial Activity of Stigmastane Steroids from Dryobalanops oblongifolia Stem Bark
  19. Rapid identification of direct-acting pancreatic protectants from Cyclocarya paliurus leaves tea by the method of serum pharmacochemistry combined with target cell extraction
  20. Immobilization of Pseudomonas aeruginosa static biomass on eggshell powder for on-line preconcentration and determination of Cr (VI)
  21. Assessment of methyl 2-({[(4,6-dimethoxypyrimidin-2-yl)carbamoyl] sulfamoyl}methyl)benzoate through biotic and abiotic degradation modes
  22. Stability of natural polyphenol fisetin in eye drops Stability of fisetin in eye drops
  23. Production of a bioflocculant by using activated sludge and its application in Pb(II) removal from aqueous solution
  24. Molecular Properties of Carbon Crystal Cubic Structures
  25. Synthesis and characterization of calcium carbonate whisker from yellow phosphorus slag
  26. Study on the interaction between catechin and cholesterol by the density functional theory
  27. Analysis of some pharmaceuticals in the presence of their synthetic impurities by applying hybrid micelle liquid chromatography
  28. Two mixed-ligand coordination polymers based on 2,5-thiophenedicarboxylic acid and flexible N-donor ligands: the protective effect on periodontitis via reducing the release of IL-1β and TNF-α
  29. Incorporation of silver stearate nanoparticles in methacrylate polymeric monoliths for hemeprotein isolation
  30. Development of ultrasound-assisted dispersive solid-phase microextraction based on mesoporous carbon coated with silica@iron oxide nanocomposite for preconcentration of Te and Tl in natural water systems
  31. N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes: Isolation, spectral characterization, morphology, antibacterial and cytotoxicity against leukemia cells
  32. Determination of the content of selected trace elements in Polish commercial fruit juices and health risk assessment
  33. Diorganotin(iv) benzyldithiocarbamate complexes: synthesis, characterization, and thermal and cytotoxicity study
  34. Keratin 17 is induced in prurigo nodularis lesions
  35. Anticancer, antioxidant, and acute toxicity studies of a Saudi polyherbal formulation, PHF5
  36. LaCoO3 perovskite-type catalysts in syngas conversion
  37. Comparative studies of two vegetal extracts from Stokesia laevis and Geranium pratense: polyphenol profile, cytotoxic effect and antiproliferative activity
  38. Fragmentation pattern of certain isatin–indole antiproliferative conjugates with application to identify their in vitro metabolic profiles in rat liver microsomes by liquid chromatography tandem mass spectrometry
  39. Investigation of polyphenol profile, antioxidant activity and hepatoprotective potential of Aconogonon alpinum (All.) Schur roots
  40. Lead discovery of a guanidinyl tryptophan derivative on amyloid cascade inhibition
  41. Physicochemical evaluation of the fruit pulp of Opuntia spp growing in the Mediterranean area under hard climate conditions
  42. Electronic structural properties of amino/hydroxyl functionalized imidazolium-based bromide ionic liquids
  43. New Schiff bases of 2-(quinolin-8-yloxy)acetohydrazide and their Cu(ii), and Zn(ii) metal complexes: their in vitro antimicrobial potentials and in silico physicochemical and pharmacokinetics properties
  44. Treatment of adhesions after Achilles tendon injury using focused ultrasound with targeted bFGF plasmid-loaded cationic microbubbles
  45. Synthesis of orotic acid derivatives and their effects on stem cell proliferation
  46. Chirality of β2-agonists. An overview of pharmacological activity, stereoselective analysis, and synthesis
  47. Fe3O4@urea/HITh-SO3H as an efficient and reusable catalyst for the solvent-free synthesis of 7-aryl-8H-benzo[h]indeno[1,2-b]quinoline-8-one and indeno[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives
  48. Adsorption kinetic characteristics of molybdenum in yellow-brown soil in response to pH and phosphate
  49. Enhancement of thermal properties of bio-based microcapsules intended for textile applications
  50. Exploring the effect of khat (Catha edulis) chewing on the pharmacokinetics of the antiplatelet drug clopidogrel in rats using the newly developed LC-MS/MS technique
  51. A green strategy for obtaining anthraquinones from Rheum tanguticum by subcritical water
  52. Cadmium (Cd) chloride affects the nutrient uptake and Cd-resistant bacterium reduces the adsorption of Cd in muskmelon plants
  53. Removal of H2S by vermicompost biofilter and analysis on bacterial community
  54. Structural cytotoxicity relationship of 2-phenoxy(thiomethyl)pyridotriazolopyrimidines: Quantum chemical calculations and statistical analysis
  55. A self-breaking supramolecular plugging system as lost circulation material in oilfield
  56. Synthesis, characterization, and pharmacological evaluation of thiourea derivatives
  57. Application of drug–metal ion interaction principle in conductometric determination of imatinib, sorafenib, gefitinib and bosutinib
  58. Synthesis and characterization of a novel chitosan-grafted-polyorthoethylaniline biocomposite and utilization for dye removal from water
  59. Optimisation of urine sample preparation for shotgun proteomics
  60. DFT investigations on arylsulphonyl pyrazole derivatives as potential ligands of selected kinases
  61. Treatment of Parkinson’s disease using focused ultrasound with GDNF retrovirus-loaded microbubbles to open the blood–brain barrier
  62. New derivatives of a natural nordentatin
  63. Fluorescence biomarkers of malignant melanoma detectable in urine
  64. Study of the remediation effects of passivation materials on Pb-contaminated soil
  65. Saliva proteomic analysis reveals possible biomarkers of renal cell carcinoma
  66. Withania frutescens: Chemical characterization, analgesic, anti-inflammatory, and healing activities
  67. Design, synthesis and pharmacological profile of (−)-verbenone hydrazones
  68. Synthesis of magnesium carbonate hydrate from natural talc
  69. Stability-indicating HPLC-DAD assay for simultaneous quantification of hydrocortisone 21 acetate, dexamethasone, and fluocinolone acetonide in cosmetics
  70. A novel lactose biosensor based on electrochemically synthesized 3,4-ethylenedioxythiophene/thiophene (EDOT/Th) copolymer
  71. Citrullus colocynthis (L.) Schrad: Chemical characterization, scavenging and cytotoxic activities
  72. Development and validation of a high performance liquid chromatography/diode array detection method for estrogen determination: Application to residual analysis in meat products
  73. PCSK9 concentrations in different stages of subclinical atherosclerosis and their relationship with inflammation
  74. Development of trace analysis for alkyl methanesulfonates in the delgocitinib drug substance using GC-FID and liquid–liquid extraction with ionic liquid
  75. Electrochemical evaluation of the antioxidant capacity of natural compounds on glassy carbon electrode modified with guanine-, polythionine-, and nitrogen-doped graphene
  76. A Dy(iii)–organic framework as a fluorescent probe for highly selective detection of picric acid and treatment activity on human lung cancer cells
  77. A Zn(ii)–organic cage with semirigid ligand for solvent-free cyanosilylation and inhibitory effect on ovarian cancer cell migration and invasion ability via regulating mi-RNA16 expression
  78. Polyphenol content and antioxidant activities of Prunus padus L. and Prunus serotina L. leaves: Electrochemical and spectrophotometric approach and their antimicrobial properties
  79. The combined use of GC, PDSC and FT-IR techniques to characterize fat extracted from commercial complete dry pet food for adult cats
  80. MALDI-TOF MS profiling in the discovery and identification of salivary proteomic patterns of temporomandibular joint disorders
  81. Concentrations of dioxins, furans and dioxin-like PCBs in natural animal feed additives
  82. Structure and some physicochemical and functional properties of water treated under ammonia with low-temperature low-pressure glow plasma of low frequency
  83. Mesoscale nanoparticles encapsulated with emodin for targeting antifibrosis in animal models
  84. Amine-functionalized magnetic activated carbon as an adsorbent for preconcentration and determination of acidic drugs in environmental water samples using HPLC-DAD
  85. Antioxidant activity as a response to cadmium pollution in three durum wheat genotypes differing in salt-tolerance
  86. A promising naphthoquinone [8-hydroxy-2-(2-thienylcarbonyl)naphtho[2,3-b]thiophene-4,9-dione] exerts anti-colorectal cancer activity through ferroptosis and inhibition of MAPK signaling pathway based on RNA sequencing
  87. Synthesis and efficacy of herbicidal ionic liquids with chlorsulfuron as the anion
  88. Effect of isovalent substitution on the crystal structure and properties of two-slab indates BaLa2−xSmxIn2O7
  89. Synthesis, spectral and thermo-kinetics explorations of Schiff-base derived metal complexes
  90. An improved reduction method for phase stability testing in the single-phase region
  91. Comparative analysis of chemical composition of some commercially important fishes with an emphasis on various Malaysian diets
  92. Development of a solventless stir bar sorptive extraction/thermal desorption large volume injection capillary gas chromatographic-mass spectrometric method for ultra-trace determination of pyrethroids pesticides in river and tap water samples
  93. A turbidity sensor development based on NL-PI observers: Experimental application to the control of a Sinaloa’s River Spirulina maxima cultivation
  94. Deep desulfurization of sintering flue gas in iron and steel works based on low-temperature oxidation
  95. Investigations of metallic elements and phenolics in Chinese medicinal plants
  96. Influence of site-classification approach on geochemical background values
  97. Effects of ageing on the surface characteristics and Cu(ii) adsorption behaviour of rice husk biochar in soil
  98. Adsorption and sugarcane-bagasse-derived activated carbon-based mitigation of 1-[2-(2-chloroethoxy)phenyl]sulfonyl-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl) urea-contaminated soils
  99. Antimicrobial and antifungal activities of bifunctional cooper(ii) complexes with non-steroidal anti-inflammatory drugs, flufenamic, mefenamic and tolfenamic acids and 1,10-phenanthroline
  100. Application of selenium and silicon to alleviate short-term drought stress in French marigold (Tagetes patula L.) as a model plant species
  101. Screening and analysis of xanthine oxidase inhibitors in jute leaves and their protective effects against hydrogen peroxide-induced oxidative stress in cells
  102. Synthesis and physicochemical studies of a series of mixed-ligand transition metal complexes and their molecular docking investigations against Coronavirus main protease
  103. A study of in vitro metabolism and cytotoxicity of mephedrone and methoxetamine in human and pig liver models using GC/MS and LC/MS analyses
  104. A new phenyl alkyl ester and a new combretin triterpene derivative from Combretum fragrans F. Hoffm (Combretaceae) and antiproliferative activity
  105. Erratum
  106. Erratum to: A one-step incubation ELISA kit for rapid determination of dibutyl phthalate in water, beverage and liquor
  107. Review Articles
  108. Sinoporphyrin sodium, a novel sensitizer for photodynamic and sonodynamic therapy
  109. Natural products isolated from Casimiroa
  110. Plant description, phytochemical constituents and bioactivities of Syzygium genus: A review
  111. Evaluation of elastomeric heat shielding materials as insulators for solid propellant rocket motors: A short review
  112. Special Issue on Applied Biochemistry and Biotechnology 2019
  113. An overview of Monascus fermentation processes for monacolin K production
  114. Study on online soft sensor method of total sugar content in chlorotetracycline fermentation tank
  115. Studies on the Anti-Gouty Arthritis and Anti-hyperuricemia Properties of Astilbin in Animal Models
  116. Effects of organic fertilizer on water use, photosynthetic characteristics, and fruit quality of pear jujube in northern Shaanxi
  117. Characteristics of the root exudate release system of typical plants in plateau lakeside wetland under phosphorus stress conditions
  118. Characterization of soil water by the means of hydrogen and oxygen isotope ratio at dry-wet season under different soil layers in the dry-hot valley of Jinsha River
  119. Composition and diurnal variation of floral scent emission in Rosa rugosa Thunb. and Tulipa gesneriana L.
  120. Preparation of a novel ginkgolide B niosomal composite drug
  121. The degradation, biodegradability and toxicity evaluation of sulfamethazine antibiotics by gamma radiation
  122. Special issue on Monitoring, Risk Assessment and Sustainable Management for the Exposure to Environmental Toxins
  123. Insight into the cadmium and zinc binding potential of humic acids derived from composts by EEM spectra combined with PARAFAC analysis
  124. Source apportionment of soil contamination based on multivariate receptor and robust geostatistics in a typical rural–urban area, Wuhan city, middle China
  125. Special Issue on 13th JCC 2018
  126. The Role of H2C2O4 and Na2CO3 as Precipitating Agents on The Physichochemical Properties and Photocatalytic Activity of Bismuth Oxide
  127. Preparation of magnetite-silica–cetyltrimethylammonium for phenol removal based on adsolubilization
  128. Topical Issue on Agriculture
  129. Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions
  130. The effect of silica-calcite sedimentary rock contained in the chicken broiler diet on the overall quality of chicken muscles
  131. Physicochemical properties of selected herbicidal products containing nicosulfuron as an active ingredient
  132. Lycopene in tomatoes and tomato products
  133. Fluorescence in the assessment of the share of a key component in the mixing of feed
  134. Sulfur application alleviates chromium stress in maize and wheat
  135. Effectiveness of removal of sulphur compounds from the air after 3 years of biofiltration with a mixture of compost soil, peat, coconut fibre and oak bark
  136. Special Issue on the 4th Green Chemistry 2018
  137. Study and fire test of banana fibre reinforced composites with flame retardance properties
  138. Special Issue on the International conference CosCI 2018
  139. Disintegration, In vitro Dissolution, and Drug Release Kinetics Profiles of k-Carrageenan-based Nutraceutical Hard-shell Capsules Containing Salicylamide
  140. Synthesis of amorphous aluminosilicate from impure Indonesian kaolin
  141. Special Issue on the International Conf on Science, Applied Science, Teaching and Education 2019
  142. Functionalization of Congo red dye as a light harvester on solar cell
  143. The effect of nitrite food preservatives added to se’i meat on the expression of wild-type p53 protein
  144. Biocompatibility and osteoconductivity of scaffold porous composite collagen–hydroxyapatite based coral for bone regeneration
  145. Special Issue on the Joint Science Congress of Materials and Polymers (ISCMP 2019)
  146. Effect of natural boron mineral use on the essential oil ratio and components of Musk Sage (Salvia sclarea L.)
  147. A theoretical and experimental study of the adsorptive removal of hexavalent chromium ions using graphene oxide as an adsorbent
  148. A study on the bacterial adhesion of Streptococcus mutans in various dental ceramics: In vitro study
  149. Corrosion study of copper in aqueous sulfuric acid solution in the presence of (2E,5E)-2,5-dibenzylidenecyclopentanone and (2E,5E)-bis[(4-dimethylamino)benzylidene]cyclopentanone: Experimental and theoretical study
  150. Special Issue on Chemistry Today for Tomorrow 2019
  151. Diabetes mellitus type 2: Exploratory data analysis based on clinical reading
  152. Multivariate analysis for the classification of copper–lead and copper–zinc glasses
  153. Special Issue on Advances in Chemistry and Polymers
  154. The spatial and temporal distribution of cationic and anionic radicals in early embryo implantation
  155. Special Issue on 3rd IC3PE 2020
  156. Magnetic iron oxide/clay nanocomposites for adsorption and catalytic oxidation in water treatment applications
  157. Special Issue on IC3PE 2018/2019 Conference
  158. Exergy analysis of conventional and hydrothermal liquefaction–esterification processes of microalgae for biodiesel production
  159. Advancing biodiesel production from microalgae Spirulina sp. by a simultaneous extraction–transesterification process using palm oil as a co-solvent of methanol
  160. Topical Issue on Applications of Mathematics in Chemistry
  161. Omega and the related counting polynomials of some chemical structures
  162. M-polynomial and topological indices of zigzag edge coronoid fused by starphene
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