Reactivity of N-Heterocyclic Carbene Half-Sandwich Ru-, Os-, Rh-, and Ir-Based Complexes with Cysteine and Selenocysteine: A Computational Study

The structure and the reactivity of four half-sandwich metal complexes of Ru^II, Os^II, Rh^III, and Ir^III were investigated by means of density functional theory approaches. These piano-stool complexes, grouped in cym-bound complexes, Ru^II(cym)(dmb)Cl₂, 1, and Os^II(cym)(dmb)Cl₂, 2, and Cp*-bound complexes, Rh^III(Cp*)(dmb)Cl₂, 3, and Ir^III(Cp*)(dmb)Cl₂, 4, with cym = η⁶-p-cymene, Cp* = η⁵-pentamethylcyclopentadienyl, and dmb = 1,3-dimethylbenzimidazol-2-ylidene, were recently proposed as anticancer metallodrugs that preferably target Cys- or Sec-containing proteins. Thus, density functional theory calculations were performed here to characterize in detail the thermodynamics and the kinetics underlining the targeting of these metallodrugs at either neutral or anionic Cys and Sec side chains. Calculations evidenced that all these complexes preferably target at Cys or Sec via chloro exchange, although cym-bound and Cp*-bound complexes resulted to be more prone to bind at neutral or anionic forms, respectively, of these soft protein sites. Further decomposition analyses of the activation free energies for the reaction between 1-4 complexes and either Cys or Sec, paralleled with the comparison among the optimized transition-state structures, allowed us to spotlight the significant role played by solvation in determining the overall reactivity and selectivity expected for these prototypical metallodrugs.

Dual utility of a single diphosphine-ruthenium complex: a precursor for new complexes and, a pre-catalyst for transfer-hydrogenation and Oppenauer oxidation

The diphosphine-ruthenium complex, [Ru(dppbz)(CO)₂Cl₂] (dppbz = 1,2-bis(diphenylphosphino)benzene), where the two carbonyls are mutually cis and the two chlorides are trans, has been found to serve as an efficient precursor for the synthesis of new complexes. In [Ru(dppbz)(CO)₂Cl₂] one of the two carbonyls undergoes facile displacement by neutral monodentate ligands (L) to afford complexes of the type [Ru(dppbz)(CO)(L)Cl₂] (L = acetonitrile, 4-picoline and dimethyl sulfoxide). Both the carbonyls in [Ru(dppbz)(CO)₂Cl₂] are displaced on reaction with another equivalent of dppbz to afford [Ru(dppbz)₂Cl₂]. The two carbonyls and the two chlorides in [Ru(dppbz)(CO)₂Cl₂] could be displaced together by chelating mono-anionic bidentate ligands, viz. anions derived from 8-hydroxyquinoline (Hq) and 2-picolinic acid (Hpic) via loss of a proton, to afford the mixed-tris complexes [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], respectively. The molecular structures of four selected complexes, viz. [Ru(dppbz)(CO)(dmso)Cl₂], [Ru(dppbz)₂Cl₂], [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], have been determined by X-ray crystallography. In dichloromethane solution, all the complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows redox responses within 0.71 to -1.24 V vs. SCE. [Ru(dppbz)(CO)₂Cl₂] has been found to serve as an excellent pre-catalyst for catalytic transfer-hydrogenation and Oppenauer oxidation.

Anticancer activity of benzoxazole derivative (2015 onwards): a review

Background

According to the report published recently by the World Health Organization, the number of cancer cases in the world will increase to 22 million by 2030. So the anticancer drug research and development is taking place in the direction where the new entities are developed which are low in toxicity and are with improved activity. Benzoxazole and its derivative represent a very important class of heterocyclic compounds, which have a diverse therapeutic area. Recently, many active compounds synthesized are very effective; natural products isolated with benzoxazole moiety have also shown to be potent towards cancer.

Main text

In the last few years, many research groups have designed and developed many novel compounds with benzoxazole as their backbone and checked their anticancer activity. In the review article, the recent developments (mostly after 2015) made in the direction of design and synthesis of new scaffolds with very potent anticancer activity are briefly described. The effect of various heterocycles attached to the benzoxazole and their effect on the anticancer activity are thoroughly studied and recorded in the review.

Conclusion

These compiled data in the article will surely update the scientific community with the recent development in this area and will provide direction for further research in this area.

Ligand photodissociation in Ru(ii)–1,4,7-triazacyclononane complexes enhances water oxidation and enables electrochemical generation of surface active species

Ligand transformations involved in metal complexes during water oxidation (WO), such as ligand decomposition, partial oxidation, or complete dissociation have been reported, however, ligand photodissociation has not been reported yet.

Ru(ii)-Peptide bioconjugates with the cppH linker (cppH = 2-(2'-pyridyl)pyrimidine-4-carboxylic acid): synthesis, structural characterization, and different stereochemical features between organic and aqueous solvents

Three new Ru(ii) bioconjugates with the C-terminal hexapeptide sequence of neurotensin, RRPYIL, namely trans,cis-RuCl2(CO)2(cppH-RRPYIL-κNp) (7), [Ru([9]aneS3)(cppH-RRPYIL-κNp)(PTA)](Cl)2 (8), and [Ru([9]aneS3)Cl(cppH-RRPYIL-κNp)]Cl (11), where cppH is the asymmetric linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid, were prepared in pure form and structurally characterized in solution. The cppH linker is capable of forming stereoisomers (i.e. linkage isomers), depending on whether the nitrogen atom ortho (No) or para (Np) to the carboxylate on C4 in the pyrimidine ring binds the metal ion. Thus, one of the aims of this work was to obtain pairs of stereoisomeric conjugates and investigate their biological (anticancer, antibacterial) activity. A thorough NMR characterization clearly indicated that in all cases exclusively Np conjugates were obtained in pure form. In addition, the NMR studies showed that, whereas in DMSO-d6 each conjugate exists as a single species, in D2O two (7) or even three if not four (8 and 11) very similar stable species form (each one corresponding to an individual compound). Similar results were observed for the cppH-RRPYIL ligand alone. Overall, the NMR findings are consistent with the occurrence of a strong intramolecular stacking interaction between the phenol ring of tyrosine and the pyridyl ring of cppH. Such stacking interactions between aromatic rings are expected to be stronger in water. This interaction leads to two stereoisomeric species in the free cppH-RRPYIL ligand and in the bioconjugate 7, and is somehow modulated by the less symmetrical Ru coordination environments in 8 and 11, affording three to four very similar species.

Synthesis, characterisation, molecular docking, biomolecular interaction and cytotoxicity studies of novel ruthenium(ii)–arene-2-heteroarylbenzoxazole complexes

Ru(ii)–arene-2-heteroarylbenzoxazole complexes were synthesized and implemented for their biological evaluation.

Organoruthenium Prodrugs as a New Class of Cholinesterase and Glutathione-S-Transferase Inhibitors

A small library of 17 organoruthenium compounds with the general formula [Ru^II (fcl)(chel)(L)]ⁿ⁺ (in which fcl=face capping ligand, chel=chelating bidentate ligand, and L=monodentate ligand) were screened for inhibitory activity against cholinesterases and glutathione-S-transferases of human and animal origins. Compounds were selected to include different chelating ligands (i.e., N,N-, N,O-, O,O-, S,O-) and monodentate ligands that can modulate the aquation rate of the metal species. Compounds with a labile ruthenium chloride bond that provided rapid aquation were found to inhibit both sets of enzymes in reversible competitive modes and at pharmaceutically relevant concentrations. When applied at concentrations that completely abolish the activity of human acetylcholinesterase, the lead compound [(η⁶ -p-cymene)Ru(pyrithionato)Cl] (C1 a) showed no undesirable physiological responses on the neuromuscular system. Finally, C1 a was not cytotoxic against non-transformed cells at pharmaceutically relevant concentrations.

Inhibition of the Ras/Raf interaction and repression of renal cancer xenografts in vivo by an enantiomeric iridium(iii) metal-based compound

Targeting protein-protein interactions (PPIs) offers tantalizing opportunities for therapeutic intervention for the treatment of human diseases. Modulating PPI interfaces with organic small molecules has been found to be exceptionally challenging, and few candidates have been successfully developed into clinical drugs. Meanwhile, the striking array of distinctive properties exhibited by metal compounds renders them attractive scaffolds for the development of bioactive leads. Here, we report the identification of iridium(iii) compounds as inhibitors of the H-Ras/Raf-1 PPI. The lead iridium(iii) compound 1 exhibited potent inhibitory activity against the H-Ras/Raf-1 interaction and its signaling pathway in vitro and in vivo, and also directly engaged both H-Ras and Raf-1-RBD in cell lysates. Moreover, 1 repressed tumor growth in a mouse renal xenograft tumor model. Intriguingly, the Δ-enantiomer of 1 showed superior potency in the biological assays compared to Λ-1 or racemic 1. These compounds could potentially be used as starting scaffolds for the development of more potent Ras/Raf PPI inhibitors for the treatment of kidney cancer or other proliferative diseases.

Platinum, palladium, gold and ruthenium complexes as anticancer agents: Current clinical uses, cytotoxicity studies and future perspectives

Metallodrugs offer potential for unique mechanism of drug action based on the choice of the metal, its oxidation state, the types and number of coordinated ligands and the coordination geometry. This review illustrates notable recent progress in the field of medicinal bioinorganic chemistry as many new approaches to the design of innovative metal-based anticancer drugs are emerging. Current research addressing the problems associated with platinum drugs has focused on other metal-based therapeutics that have different modes of action and on prodrug and targeting strategies in an effort to diminish the side-effects of cisplatin chemotherapy. Examples of metal compounds and chelating agents currently in clinical use, clinical trials or preclinical development are highlighted.

Pyrithione-based ruthenium complexes as inhibitors of aldo-keto reductase 1C enzymes and anticancer agents

Four ruthenium complexes of clinically used zinc ionophore pyrithione and its oxygen analog 2-hydroxypyridine N-oxide were prepared and evaluated as inhibitors of enzymes of the aldo-keto reductase subfamily 1C (AKR1C). A kinetic study assisted with docking simulations showed a mixed type of inhibition consisting of a fast reversible and a slow irreversible step in the case of both organometallic compounds 1A and 1B. Both compounds also showed a remarkable selectivity towards AKR1C1 and AKR1C3 which are targets for breast cancer drug design. The organoruthenium complex of ligand pyrithione as well as pyrithione itself also displayed toxicity on the hormone-dependent MCF-7 breast cancer cell line with EC50 values in the low micromolar range.

DNA binding properties, histidine interaction and cytotoxicity studies of water soluble ruthenium(ii) terpyridine complexes

In this study, two representatives of previously synthesized ruthenium(ii) terpyridine complexes, i.e., [Ru(Cl-tpy)(en)Cl][Cl] (1) and [Ru(Cl-tpy)(dach)Cl][Cl] (2), were chosen and a detailed study of the kinetic parameters of their reactivity toward l-histidine (l-His), using the UV-Vis and (1)H NMR techniques, was developed. The inner molecular rearrangement from N3-coordinated l-His to the N1 bound isomer, observable in the NMR data, was corroborated by DFT calculations favoring N1 coordination by nearly 4 kcal mol(-1). These two ruthenium(ii) terpyridine complexes were investigated for their interactions with DNA employing UV-Vis spectroscopy, DNA viscosity measurements and fluorescence quenching measurements. The high binding constants obtained in the DNA binding studies (Kb = 10(4)-10(5) M(-1)) suggest a strong binding of the complexes to calf thymus (CT) DNA. Competitive studies with ethidium bromide (EB) showed that the complexes can displace DNA-bound EB, suggesting strong competition with EB (Ksv = 1.5-2.5 × 10(4) M(-1)). In fact, the results indicate that these complexes can bind to DNA covalently and non-covalently. In order to gain insight of the behavior of a neutral compound, besides the four previously synthesized cationic complexes [Ru(Cl-tpy)(en)Cl][Cl] (1), [Ru(Cl-tpy)(dach)Cl][Cl] (2), [Ru(Cl-tpy)(bpy)Cl][Cl] (3) and [Ru(tpy)Cl3] (P2), a new complex, [Ru(Cl-tpy)(pic)Cl] (4), was used in the biological studies. Their cytotoxicity was investigated against three different tumor cell lines, i.e., A549 (human lung carcinoma cell line), HCT116 (human colon carcinoma cell line), and CT26 (mouse colon carcinoma cell line), by the MTT assay. Complexes 1 and 2 showed higher activity than complexes 3, 4 and P2 against all the selected cell lines. The results on in vitro anticancer activity confirmed that only compounds that hydrolyze the monodentate ligand at a reasonable rate show moderate activity, provided that the chelate ligand is a hydrogen bond donor.

Amberlite IR-120(H)-mediated “on water” synthesis of novel anticancer ruthenium(ii)–p-cymene 2-pyridinylbenzothiazole (BTZ), 2-pyridinylbenzoxazole (BOZ) & 2-pyridinylbenzimidazole (BIZ) scaffolds

A class of novel Ru(ii)–p-cymene BTZ, BOZ & BIZ complexes have been synthesized “on water” using Amberlite IR-120(H) resin and their anticancer activities were evaluated.

The contrasting catalytic efficiency and cancer cell antiproliferative activity of stereoselective organoruthenium transfer hydrogenation catalysts

The rapidly growing area of catalytic ruthenium chemistry has provided new complexes with potential as organometallic anticancer agents with novel mechanisms of action.

Development of ruthenium-based complexes as anticancer agents: toward a rational design of alternative receptor targets

In the search for novel anticancer agents, the development of metal-based complexes that could serve as alternatives to cisplatin and its derivatives has received considerable attention in recent years. This becomes necessary because, at present, cisplatin and its derivatives are the only coordination complexes being used as anticancer agents in spite of inherent serious side effects and their limitation against metastasized platinum-resistant cancer cells. Although many metal ions have been considered as possible alternatives to cisplatin, the most promising are ruthenium (Ru) complexes and two Ru compounds, KP1019 and NAMI-A, which are currently in phase II clinical trials. The major obstacle against the rational design of these compounds is the fact that their mode of action in relation to their therapeutic activities and selectivity is not fully understood. There is an urgent need to develop novel metal-based anticancer agents, especially Ru-based compounds, with known mechanism of actions, probable targets, and pharmacodynamic activity. In this paper, we review the current efforts in developing metal-based anticancer agents based on promising Ru complexes and the development of compounds targeting receptors and then examine the future prospects.

Metal coordination study at Ag and Cd sites in crown thioether complexes through DFT calculations and hyperfine parameters

Structural and electronic properties of [C12H24S6X], [C13H26S6OX], and [C14H28S6OX] (X: Ag(+), Cd(2+)) crown thioether complexes were investigated within the framework of the density functional theory (DFT) using the projector augmented wave (PAW) method. The theoretical results were compared with time-differential perturbed γ-γ angular correlations (TDPAC) experiments reported in the literature using the (111)Ag→(111)Cd probe. In the case of X=Ag(+), a refinement of the structure was performed and the predicted equilibrium structures compared with available X-ray diffraction experimental data. Structural distortions induced by replacing Ag(+) with Cd(2+) were investigated as well as the electric-field gradient (EFG) tensor at the Cd(2+) sites. Our results suggest that the EFG at Cd(2+) sites corresponds to the Ag(+) coordination sphere structure, i.e., before the structural relaxations of the molecule with X=Cd(2+) are completed. The results are discussed in terms of the characteristics of the TDPAC (111)Ag→(111)Cd probe and the time window of the measurement, and provide an interesting tool with which to probe molecular relaxations.

Syntheses, crystal structures and antimicrobial activities of Cu(ii), Ru(ii), and Pt(ii) compounds with an anthracene-containing tripodal ligand

Three new Cu(ii), Ru(ii), and Pt(ii) compounds of an anthracene-containing tripodal ligand were synthesized. Their crystal structures was determined by a single-crystal X-ray diffraction method and their antimicrobial activities were investigated.

Ruthenium complexes as inhibitors of the aldo-keto reductases AKR1C1-1C3

The human aldo-keto reductases (AKRs) from the 1C subfamily are important targets for the development of new drugs. In this study, we have investigated the possible interactions between the recombinant AKR1C enzymes AKR1C1-AKR1C3 and ruthenium(II) complexes; in particular, we were interested in the potential inhibitory actions. Five novel ruthenium complexes (1a, 1b, 2a, 2b, 2c), two precursor ruthenium compounds (P1, P2), and three ligands (a, b, c) were prepared and included in this study. Two different types of novel ruthenium(II) complexes were synthesized. First, bearing the sulphur macrocycle [9]aneS3, S-bonded dimethylsulphoxide (dmso-S), and an N,N-donor ligand, with the general formula of [Ru([9]aneS3)(dmso)(N,N-ligand)](PF6)2 (1a, 1b), and second, with the general formula of [(η(6)-p-cymene)RuCl(N,N-ligand)]Cl (2a, 2b, 2c). All of these synthesized compounds were characterized by high-resolution NMR spectroscopy, X-ray crystallography (compounds a, b, c, 1a, 1b) and other standard physicochemical methods. To evaluate the potential inhibitory actions of these compounds on the AKR1C enzymes, we followed enzymatically catalyzed oxidation of the substrate 1-acenaphthenol by NAD(+) in the absence and presence of various micromolar concentrations of the individual compounds. Among 10 compounds, one ruthenium complex (2b) and two precursor ruthenium compounds (P1, P2) inhibited all three AKR1C enzymes, and one ruthenium complex (2a) inhibited only AKR1C3. Ligands a, b and c revealed no inhibition of the AKR1C enzymes. All four of the active compounds showed multiple binding with the AKR1C enzymes that was characterized by an initial instantaneous inhibition followed by a slow quasi-irreversible step. To the best of our knowledge, this is the first study that has examined interactions between these AKR1C enzymes and ruthenium(II) complexes.

New water-soluble ruthenium(II) terpyridine complexes for anticancer activity: synthesis, characterization, activation kinetics, and interaction with guanine derivatives

With the aim of assessing whether ruthenium(II) compounds with meridional geometry might be utilized as potential antitumor agents, a series of new, water-soluble, monofunctional ruthenium(II) complexes of the general formula mer-[Ru(L3)(N-N)X][Y]n (where L3 = 2,2':6',2″-terpyridine (tpy) or 4'-chloro-2,2':6',2″-terpyridine (Cl-tpy), N-N = 1,2-diaminoethane (en), 1,2-diaminocyclohexane (dach), or 2,2'-bipyridine (bpy); X = Cl or dmso-S; Y = Cl, PF6, or CF3SO3; n = 1 or 2, depending on the nature of X) were synthesized. All complexes were fully characterized by elemental analysis and spectroscopic techniques (IR, UV/visible, and 1D and 2D NMR), and for three of them, i.e., [Ru(Cl-tpy)(bpy)Cl][Cl] (3Cl), [Ru(Cl-tpy)(en)(dmso-S)][Y]2 [Y = PF6 (6PF6), CF3SO3 (6OTf)] and [Ru(Cl-tpy)(bpy)(dmso-S)][CF3SO3]2 (8OTf), the X-ray structure was also determined. The new terpyridine complexes, with the exception of 8, are well soluble in water (>25 mg/mL). (1)H and (31)P NMR spectroscopy studies performed on the three selected complexes [Ru(Cl-tpy)(N-N)Cl](+) [N-N = en (1), dach (2), and bpy (3)] demonstrated that, after hydrolysis of the Cl ligand, they are capable of interacting with guanine derivatives [i.e., 9-methylguanine (9MeG) or guanosine-5'-monophosphate (5'-GMP)] through N7, forming monofunctional adducts with rates and extents that depend strongly on the nature of N-N: 1 ≈ 2 ≫ 3. In addition, compound 1 shows high selectivity toward 5'-GMP compared to adenosine-5'-monophosphate (5'-AMP), in a competition experiment. Quantitative kinetic investigations on 1 and 2 were performed by means of UV/visible spectroscopy. Overall, the complexes with bidentate aliphatic diamines proved to be superior to those with bpy in terms of solubility and reactivity (i.e., release of Cl(-) and capability to bind guanine derivatives). Contrary to the chlorido compounds, the corresponding dmso derivatives proved to be inert (viz., they do not release the monodentate ligand) in aqueous media.

Mitigating UVA light induced reactivity of 6-thioguanine through formation of a Ru(ii) half-sandwich complex

Half-sandwich ruthenium complexes of 6-thioguanine.

New uses for old drugs: attempts to convert quinolone antibacterials into potential anticancer agents containing ruthenium

Continuing the study of the physicochemical and biological properties of ruthenium-quinolone adducts, four novel complexes with the general formula [Ru([9]aneS3)(dmso-κS)(quinolonato-κ(2)O,O)](PF6), containing the quinolones levofloxacin (1), nalidixic acid (2), oxolinic acid (3), and cinoxacin (4), were prepared and characterized in solid state as well as in solution. Contrary to their organoruthenium analogues, these complexes are generally relatively stable in aqueous solution as substitution of the dimethylsulfoxide (dmso) ligand is slow and not quantitative, and a minor release of the quinolonato ligand is observed only in the case of 4. The complexes bind to serum proteins displaying relatively high binding constants. DNA binding was studied using UV-vis spectroscopy, cyclic voltammetry, and performing viscosity measurements of CT DNA solutions in the presence of complexes 1-4. These experiments show that the ruthenium complexes interact with DNA via intercalation. Possible electrostatic interactions occur in the case of compound 4, which also shows the most pronounced rate of hydrolysis. Compounds 2 and 4 also exhibit a weak inhibition of cathepsins B and S, which are involved in the progression of a number of diseases, including cancer. Furthermore, complex 2 displayed moderate cytotoxicity when tested on the HeLa cell line.

cis-1,3,5-Triaminocyclohexane as a facially capping ligand for ruthenium(II)

Reaction of cis-[RuCl2(DMSO-S)3(DMSO-O)] with cis-1,3,5-triaminocyclohexane (tach) results in the formation of [RuCl(tach)(DMSO-S)2]Cl, a valuable precursor for a wide range of other tach-containing Ru complexes. Reaction of [RuCl(tach)(DMSO-S)2]Cl with the chelating nitrogen-based ligands (N-N = bipyridine, phenanthroline, and ethylenediamine) affords [Ru(N-N)(DMSO-S)2(tach)][Cl]2. A similar reaction between [RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based ligands (P-P = dppm, dppe, dppp, dppb, dppv, and dppben) leads to the formation of [RuCl(P-P)(tach)]Cl. The structures of 10 examples of the tach-containing complexes have been determined by single crystal X-ray diffraction. An examination of the structural metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH2 groups in the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.

Factors that influence the antiproliferative activity of half sandwich Ru(II)-[9]aneS3 coordination compounds: activation kinetics and interaction with guanine derivatives

Half sandwich Ru(II)-[9]aneS3 complexes ([9]aneS3 = 1,4,7-trithiacyclononane) are being studied for their antiproliferative activity. We investigated here the activation kinetics of three such complexes, namely [Ru([9]aneS3)(en)Cl](PF(6)) (1), [Ru([9]aneS3)(bpy)Cl](PF(6)) (2) and [Ru([9]aneS3)(pic)Cl] (3) (en = 1,2-diaminoethane, pic = picolinate), and their interaction with DNA model bases. The aim of the study was to assess how they are affected by the nature and charge of the chelating ligand. The model reactions of 1-3 with the guanine derivatives 9-methylguanine (9MeG), guanosine (Guo), and guanosine 5'-monophosphate (5'-GMP) were studied by NMR spectroscopy. All reactions lead, although with different rates and to different extents, to the formation of monofunctional adducts with the guanine derivatives N7-bonded to the Ru center. Two products, the complexes [Ru([9]aneS3)(en)(9MeG-N7)](PF(6))(2) (4) and [Ru([9]aneS3)(pic)(9MeG-N7)](PF(6)) (10), were structurally characterized also by X-ray crystallography. The structure of 4 is stabilized by strong intramolecular H-bonding between an NH of en and the carbonyl O6 of 9MeG. The kinetics of aquation and anation of complexes 2 and 3, as well as the kinetics and the mechanism of the reaction of complexes 1-3 with the biologically more relevant 5'-GMP ligand were studied by UV-Vis spectroscopy. The rate of the reaction of 1-3 with 5'-GMP depends on the nature of the chelating ligand rather than on the charge of the complex, decreasing in the order 3≈2 > 1. The measured enthalpies and entropies of activation (ΔH(≠) > 0, ΔS(≠) < 0) support an associative mechanism for the substitution process.