Opened-ring adducts of 5-methylcytosine and 1,5-dimethylcytosine with amines and water and evidence for an opened-ring hydrate of 2'-deoxycytidine

A variety of nucleic acid components and related compounds undergo photoreaction with water to form so-called "photohydrates" (e.g. uracil forms 6-hydroxy-5,6-dihydrouracil). However, the corresponding hydrates of 5-methylcytosine (a minor nucleobase in eukaryotic DNA) and related compounds have not been characterized. We report the preparation of opened-ring forms of such products for 5-methylcytosine (m5C) and 1,5-dimethylcytosine (DMC). This was accomplished via thermal reaction of ring-opened amine adducts (e.g. N-carbamoyl-3-amino-2-methylacrylamidine (IVa) or N-(N'-methylcarbamoyl)-3-amino-2-methylacrylamidine (IVb)) produced by photo-induced reactions of m5C with ammonia or methylamine. When these adducts were treated with dilute trifluoroacetic acid, the amino group at the 3-position was replaced with a hydroxyl group; with IVa, N-carbamoyl-3-hydroxy-2-methylacrylamidine (Va) was formed, while reaction of IVb led to N-(N'-methylcarbamoyl)-3-hydroxy-2-methylacrylamidine (Vb). These compounds are ring-opened isomers of 5,6-dihydro-6-hydroxy-5-methylcytosine (Ia and IIa) and 5,6-dihydro-6-hydroxy-1,5-dimethylcytosine (Ib and IIb). Compounds Va and Vb each undergo thermal ring closure reactions to form two unstable compounds with chemical and UV spectral properties expected for Ia and IIa (or Ib and IIb). The latter compounds have been identified as minor products in UV-irradiated aqueous solutions of m5C and DMC. Evidence is also presented that the 2'-deoxycytidine photohydrates coexist with an opened-ring form, possibly similar in nature to Vb.     

Photoexchange products of cytosine and 5-methylcytosine with N alpha-acetyl-L-lysine and L-lysine

The photoinduced exchange reactions of cytosine (Ia) and 5-methylcytosine (IIa) with N alpha-acetyl-L-lysine, a derivative of the common amino acid L-lysine, were studied. These reactions of Ia and IIa at pH 7.5 produce, respectively, 2-N-acetylamino-6-(1-cytosinyl)hexanoic acid (Ib) and 2-N-acetylamino-6-(1-(5-methylcytosinyl]hexanoic acid (IIb) as major final products. In addition, small amounts of the corresponding deamination products were formed in the 5-methylcytosine-N alpha-acetyl-L-lysine and cytosine-N alpha-acetyl-L-lysine systems, namely 2-N-acetylamino-6-(1-thyminyl)-hexanoic acid and 2-N-acetylamino-6-(1-uracilyl)hexanoic acid. The compounds Ib and IIb were deacetylated by acid hydrolysis to yield the corresponding lysine products: 2-amino-6-(1-cytosinyl)hexanoic acid (Ic) and 2-amino-6-(1-(5-methylcytosinyl]hexanoic acid (IIc). The compound Ic was identified as a product in the photoreaction of cytosine with L-lysine at near neutral pH, while IIc is found as a product in the corresponding reaction of 5-methylcytosine. The occurrence of the above photoexchange reactions at pH values near those found in physiological systems could have biological implications; in particular, our observations suggest that cytosine and 5-methylcytosine residues, contained in DNA, might react with the epsilon-amino groups of lysine residues in proteins upon UV irradiation of nucleosomes and other DNA-protein complexes under physiological conditions.     

PHOTOCHEMICAL EXCHANGE REACTIONS OF THYMINE, URACIL AND THEIR NUCLEOSIDES WITH SELECTED AMINO ACIDS

The photoinduced exchange reactions of thymine with lysine at basic pH, using 254 nm light, have been studied. Three products have been isolated, namely, 6-amino-2-(1-thyminyl)hexanoic acid ( Ia ), 2-amino-6-(1-thyminyl)hexanoic acid ( IIa ) and 1-amino-5-(1-thyminyl)pentane ( IIIa ). Compound IIIa was shown to be a secondary product, produced by photochemical decarboxylation of Ia . Photochemical reaction of thymine with glycine and alanine at basic pH led, respectively, to formation of 2-(1-thyminyl)acetic acid ( Ic ) and 2-(1-thyminyl)propionic acid ( Id ). Compounds Ic and Id underwent photolysis to produce the decarboxylated secondary products 1-methylthymine and 1-ethylthymine, respectively. Thymidine reacts photochemically with glycine and alanine to produce the same products.
Irradiation of DNA in the presence of lysine at basic pH led to the formation of the same products formed in the thymine-lysine system, namely Ia , IIa and IIIa .
Uracil was found to undergo analogous photochemical exchange reactions with lysine to form 6-amino-2-(1-uracilyl) hexanoic acid ( Ib ), and 2-amino-6-(1-uracilyl)hexanoic acid ( IIb ). Compound Ib was found to undergo photodecarboxylation to form 1-amino-5-(1-uracilyl)pentane ( IIIb ), analogous to the secondary photoreaction of Ia . Photoreaction of uracil with 1,5-diaminopentane (cadaverine) likewise led to formation of IIIb .     

THE PHOTOCHEMISTRY OF 5-METHYLCYTOSINE AND 5-METHYL-2''-DEOXYCYTIDINE IN AQUEOUS SOLUTION

The nucleobase 5-methylcytosine (I) is a minor component of eukaryotic DNA thought to be important in regulation of gene expression. The photochemical reactions of this nucleobase and its 2'-deoxyribonucleoside, 5-methyl-2'-deoxycytidine (II), in water have been studied. These reactions lead, respectively, to 3-amino-2-methylacrylamidine (Ib) and 3-(2-erythro-D-pentopyranos-1-yl)amino-2-methylacrylamidine (IIb) as the main photoproducts. The structure of the photoproducts was established by spectroscopic methods (1H and 13C NMR, UV spectroscopy, electron impact and liquid secondary ion mass spectrometry); in the case of Ib, confirmatory evidence was obtained by chemical methods (photolysis of 5-methyl[2-13C]cytosine, hydrolysis of N-carbomethoxy-3-amino-2-methylacrylamidine and reaction of Ib with 1,1'-carbonyldiimidazole to give I). The quantum yield for formation of Ib was determined to be 1.8 x 10(-3) at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 x 10(-3) at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 x 10(-3) at pH 7.0 (Ib) and 0.6 x 10(-3) at pH 5.0 (IIb). The mechanism of formation of Ib (or IIb) is proposed to involve nucleophilic attack of water on the C-2 position of photoexcited I (or II), followed by ring opening and decarboxylation of an intermediate carbamic acid.     

Ring opening photoreactions of cytosine and uracil with ethylamine

The photochemical reactions of cytosine (Cyt) and uracil (Ura) with ethylamine, an analog of the side chain of the amino acid lysine, have been studied. After irradiation of Cyt in aqueous ethylamine at lambda = 254 nm, N-(N'-ethylcarbamoyl)-3-aminoacrylamidine (Ia) and N-(N'-ethylcarbamoyl)-3-ethylaminoacrylamidine (Ib) were isolated as products, while irradiation of Ura gave N-(N'-ethylcarbamoyl)-3-aminoacrylamide (IIa) and N-(N'-ethylcarbamoyl)-3-ethylaminoacrylamide (IIb) as products. Studies in which Ia and IIa were incubated with ethylamine at various pH values indicate that Ib and IIb are secondary products produced via thermal reactions of Ia and IIa with ethylamine. Heating of Ia and Ib leads to ring closure with the resultant formation of 1-ethylcytosine; small amounts of 1-ethyluracil are also produced. Heating of IIa and IIb produces 1-ethyluracil as the sole product. Spectroscopic properties were determined for each of these opened ring products, as well as for N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamidine (III) and N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamide (IV). Quantum yield measurements showed that Ia was formed with a phi of 1.6 x 10(-4) at pH 9.8, while phi for formation of IIa was 7.2 x 10(-4) at pH 11.5. A profile of the relative quantum yield for formation of Ia, determined as a function of pH, showed that the maximum quantum yield occurs at around pH 9.5; the analogous profile for IIa shows a maximum quantum yield at pH 11.3 and above. Acetone sensitization does not produce Ia in the Cyt-ethylamine system, which indicates that the known triplet state of Cyt is not involved in reactions leading to this opened ring product.     

The preparation and properties of metallofluorene complexes of transition metals.

Metalluorene complexes (π-C₅H₅)M(CO)(C₁₂H₈) (IVa)-(IVc) and (π-C₅H₅) (CO)(C₁₂F₈) (IVd)-(IVf) (M = Co, Rh and Ir) have been prepared from reactions of the appropriate (π-cyclopentadienyl) carbonylmetal diiodides with 2,2′-dilithhiolbiphenyl (IIa) and 2,2′-dilithiooctafluorobiphenyl (IIb), respectively The triphenylphoshine substitution reactions of cobalt compounds (IVa) and (IVd) have also been studied. Reaction of (IIa) and (IIb) with norbornadieneplatinum dichloride result in the preparation of metallocyclic platinum compounds (π-C₇H₈) and (π-C₇H₈)Pt(C₁₂H₈). A reaction of (IIb) with zirconocene dichloride produces (π-C₅H₅)₂Zr(C₁₂F₈), the first example of a ziconium-containing metalofluorene.     

Chemistry of thienopyridines. XIX. Further studies on chlorination and S-oxidation in the thieno[2,3-b] pyridine system

Treatment of thieno [2,3-b] pyridine (1a) with chlorine gas in chloroform (plus water) gave a mixture of two 2,3-dichloro-2,3-dihydrothieno[2,3-b] pyridine 1-oxides [trans-syn (IIa), and cis-anti (IIb)) [and 2,3,3-trichloro-2,3-dihydrothieno[2,3-b]pyridine syn-1-oxide (IVa), as well as a non-isolated fourth product (prohably the anti isomer of IVa) and sometimes a small amount of thieno[2,3-b]pyridine 1,1-dioxide (III). Treatment of Ia in a solvent (water, chloroform-water, or THF-water) with sulfuric acid and sodium hypochlorite gave a mixture of IIb and III. Effects of variations of reaction conditions on the composition of the product mixture were ascertained through chemical isolation and/or pmr analysis. Products formed were rationalized in terms of the chlorine-water-hypochlorous acid equilibrium, plus attack of chlorine variously at positions 1 (S-atom), 2, and 3 of 1a, but of hypochlorous acid only at position 1. Thermal and chromatographic limitations on isolation procedures for some of the products were established. Stereochemistries of IIa, IIb, and IVa were assigned by means of pmr spectrometry with the aid of the shift reagent Eu(fod)₃. Spin-spin couplings between the proton at position 2 and those at positions 4 and 6 were observed at high resolution. In exploratory runs, 5-ethyl-la was converted into isolable 2,3-dichloro-5-ethyl-2,3-dihydrothieno[2,3-b]pyridine 1-oxide, and 5-acetyl-Ia yielded 3-chloro-5-acetylthieno] 2,3-b]pyridine. Mass spectral fragmentation patterns for the various products are presented.     

BIOLOGICAL CHEMILUMINESCENCE

Abstract
The nucleobase 5-methylcytosine ( I ) is a minor component of eukaryotic DNA thought to be important in regulation of gene expression. The photochemical reactions of this nucleobase and its 2'-deoxyribonucleoside, 5-methyl-2'-deoxycytidine ( II ), in water have been studied. These reactions lead, respectively, to 3-amino-2-methylacrylamidine ( Ib ) and 3-(2- erythro - d -pentopyranos-1-yl) amino-2-methylacrylamidine ( IIb ) as the main photoproducts. The structure of the photoproducts was established by spectroscopic methods (¹H and ¹³C NMR, UV spectroscopy, electron impact and liquid secondary ion mass spectrometry); in the case of Ib , confirmatory evidence was obtained by chemical methods (photolysis of 5-methyl[2–¹³C]cytosine, hydrolysis of N -carbomethoxy-3-amino-2-methylacryl-amidine and reaction of Ib with 1,1'-carbonyldiimidazole to give I ). The quantum yield for formation of Ib was determined to be 1.8 × 10^-3at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 × 10^-3 at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 × 10^-3 at pH 7.0 ( Ib ) and 0.6 × 10^-3 at pH 5.0 ( IIb ). The mechanism of formation of Ib (or IIb ) is proposed to involve nucleophilic attack of water on the C-2 position of photoexcited I (or II ), followed by ring opening and decarboxylation of an intermediate carbamic acid.     

The (α-4) photoconjugates of 5-methylcytosine, 1,5-dimethylcytosine, 1-methylthymine and thymidine

The pyrimidine nucleobases contained in DNA undergo a variety of photoinduced reactions in which two moieties become joined to form a product (e.g. formation of cyclobutane dimers and [6-4] adducts). Herein, we describe a new type of photoconjugation reaction that has been shown to occur for 5-methylcytosine (5-MeC), 1,5-dimethylcytosine (1,5-diMeC), 1-methylthymine and thymidine; in this reaction the 5-methyl group of one nucleobase (or nucleoside) becomes attached to the 4-position of the second moiety. For example, 5-MeC forms α-4'-(5'-methylpyrimidin-2'-one)-5-methylcytosine. The various (α-4) conjugates are produced upon irradiation of the parent compound in frozen aqueous solution at -78.5°C. The UV spectra of these compounds display a characteristic "double humped" profile, similar to that expected from overlaying the spectrum of parent nucleobase with that of a 2'-pyrimidone moiety. Preliminary results suggest that thymine and 5-methyl-2'-deoxycytidine (5-MedCyd) form analogous photoproducts. A variety of other previously unreported photoproducts are described as well for the 5-MeC, 1,5-diMeC and 5-MedCyd systems.     

A highly active catalyst for Pd-catalyzed amination reactions: cross-coupling reactions using aryl mesylates and the highly selective monoarylation of primary amines using aryl chlorides

A catalyst system based on a new biarylmonophosphine ligand (BrettPhos) that shows excellent reactivity for C-N cross-coupling reactions is reported. This catalyst system enables the use of aryl mesylates as a coupling partner in C-N bond-forming reactions. Additionally, the use of BrettPhos permits the highly selective monoarylation of an array of primary aliphatic amines and anilines at low catalyst loadings and with fast reaction times, including the first monoarylation of methylamine. Lastly, oxidative addition complexes of BrettPhos are included, which provide insight into the origin of reactivity for this system.     

Synthese von in 2-Stellung substituierten 2-Amino- und 3-Amino-1-indanonen

An examination has been made of the reaction of secondary amines with 2-bromo-1-indanones II substituted at position C-2, and with 2-substituted indenones III. 2-Bromo-2-methyl-1-indanone (IIa) and dimethylamine yielded a mixture of the corresponding 2- and 3-amino-methylindanones IVa and Va. 2-Bromo-2-methyl-6-chloro-1-indanone (II b) and morpholine gave a mixture of the 2- and 3-aminocompounds IVe and Ve.     

Ammonolysis of Glyoxal at the Air-Water Nanodroplet Interface

The reactions of glyoxal (CHO)₂) with amines in cloud processes contribute to the formation of brown carbon and oligomer particles in the atmosphere. However, their molecular mechanisms remain unknown. Herein, we investigate the ammonolysis mechanisms of glyoxal with amines at the air-water nanodroplet interface. We identified three and two distinct pathways for the ammonolysis of glyoxal with dimethylamine and methylamine by using metadynamics simulations at the air-water nanodroplet interface, respectively. Notably, the stepwise pathways mediated by the water dimer for the reactions of glyoxal with dimethylamine and methylamine display the lowest free energy barriers of 3.6 and 4.9 kcal ⋅ mol⁻¹, respectively. These results showed that the air-water nanodroplet ammonolysis reactions of glyoxal with dimethylamine and methylamine were more feasible and occurred at faster rates than the corresponding gas phase ammonolysis, the OH+(CHO)₂ reaction, and the aqueous phase reaction of glyoxal, leading to the dominant removal of glyoxal. Our results provide new and important insight into the reactions between carbonyl compounds and amines, which are crucial in forming nitrogen-containing aerosol particles.     

Synthesis and Dynamic NMR Studies of Some New Symmetrical Podands of Dithiocarbamates Formed from Bis(N‐thiazol)chloroacetamides

Reactions of dithiocarbamates salts ( IVa , IVb , IVc ) with bis(N‐thiazol) chloroacetamides( IIa , IIb ) in DMF furnished corresponding podands as Va , Vb , Vc , Vd , Ve , Vf in high to excellent yields. Two reacting ligands ( IIa , IIb ) were obtained in the reaction of bis(aminothiazoles) ( Ia , Ib ) with chloroacetyl chloride. Dynamic NMR spectroscopic data of two series of podands ( Va , Vb , Vc and Vd , Ve , Vf ) are discussed and figured out their free energy of activation () at coalescence temperatures. The s of these podands were attributed to conformational isomerization in the range of 14.9–16.2 kcal/mol due to rotation and resonance effect about thioamide C―N bond.     

1H NMR,IR and UV/VIS Spectroscopic Studies of Some Schiff Bases Derived from 2‐Aminobenzothiazole and 2‐Amino‐3‐Hydroxypyridine

By condensing 2‐aminobenzothiazole with 2‐hydroxy‐1‐naphthaldehyde, 2‐hydroxybenzaldehyde, 4‐methoxybenzaldehyde, 4‐hydroxybenzal‐dehyde, benzaldehyde and 4‐dimethylaminobenzaldehyde, and five Schiff bases Ia‐Ie are prepared. Also, two Schiff bases IIa and IIb are prepared by condensation of 2‐amino‐3‐hydroxypyridine with 2‐hydroxy‐1‐naphthaldehyde and 2‐hydroxybenzaldehyde. The ¹H NMR, IR and UV/Vis spectra of these seven Schiff bases are investigated. The signals of the ¹H NMR spectra as well as the important bands in the IR spectra are considered and discussed in relation to molecular structure. The UV/Vis absorption bands in ethanol are assigned to the corresponding electronic transitions and the electronic absorption spectra of Schiff bases Ib and IIb are studied in organic solvents of different polarities. The UV/Vis absorption spectra of 2‐amino‐3‐hydroxypyridine Schiff bases IIa and IIb are investigated in buffer solutions of different pH values containing 5% (v/v) methanol, and the results are utilized for the determination of pK_a and ΔG^* of the ionization of the phenolic OH‐groups. The fluorescence spectra of IIa and IIb are studied in organic solvents of different polarities. The obtained spectral results are confirmed by some molecular calculations using the atom super position and electron delocalization molecular orbital theory for the Schiff base IIb.     

Aminolysis of 4-nitrophenyl phenyl carbonate and thionocarbonate: effects of amine nature and modification of electrophilic center from C[double bond]O to C[double bond]S on reactivity and mechanism

A kinetic study is reported for the reactions of 4-nitrophenyl phenyl carbonate (5) and thionocarbonate (6) with a series of alicyclic secondary amines in 80 mol% H(2)O-20 mol% DMSO at 25.0 +/- 0.1 degrees C. The plots of k(obsd) vs. amine concentration are linear for the reactions of 5. On the contrary, the plots for the corresponding reactions of 6 curve upward as a function of increasing amine concentration, indicating that the reactions proceed through two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)). The Br?nsted-type plot for 5 the reactions of with secondary amines exhibits a downward curvature, i.e., the slope decreases from 0.98 to 0.26 as the pK(a) of the conjugate acid of amines increases, implying that the reactions proceed through T(+/-) with a change in the rate-determining step (RDS). The k(N) values are larger for the reactions of with secondary amines than for those with primary amines of similar basicity. Dissection of k(N) values for the reactions of 5 into the microscopic rate constants (i.e., k(1) and k(2)/k(-1) ratio) has revealed that k(1) is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k(2)/k(-1) ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k(1) values but smaller k(2)/k(-1) ratios than primary amines. The current study has shown that the reactivity and reaction mechanism are strongly influenced by the nature of amines (primary vs. secondary amines) and electrophilic centers (C[double bond]O vs. C[double bond]S).     

Benzo[b] thiophene derivatives XVII. Electrophilic substitution of 4-hydroxybenzo[b] thiophene derivatives

A variety of eleclrophilic substitution reactions have been carried out on 4-methoxybenzo[b]-thiophene (IIIa) and 4-benzoyloxybenzo[b ] thiophene (IVa). Substitution occurs in the 7-position of IIIa and, with the exception of bromination, in the 7-position of IVa. Bromination of IVa occurs in the 3-position. Bromination of 4-hydroxybenzo[b] thiophene (IIa) occurs in the 5-position. The nmr spectra of eleven disubstituted benzo[b] thiophenes have been tabulated.     

Enamine Rearrangement of Pyridinium Salts to Indole Ring: A Combined Experimental and Molecular Modeling Study

N‐Alkyl pyridinium ( II ) and N‐alkyl isoquinolinium salts V undergo cyclization reaction when heated with sodium bicarbonate to give the corresponding indolizine derivatives III and VIa , VIb , VIc , VId , VIe , respectively, which undergo ring opening and recyclization reactions when heated with aqueous sodium hydroxide to give the corresponding indole derivatives IV and IXa , IXb , IXc , IXd , IXe , respectively. Molecular modeling tools including Molecular Mechanics using Augmented MM3 parameters followed by geometry optimization calculations in MO‐G using PM3 parameters were performed to gain better understanding and more insights on the thermodynamic properties of the recyclization reactions of compounds IIa , IIb , IIc , IId , IIe , IIf , IIg to the corresponding IIIa , IIIb , IIIc , IIId , IIIe , IIIf , IIIg and IIIa , IIIb , IIIc , IIId , IIIe , IIIf , IIIg to the corresponding IVa , IVb , IVc , IVd , IVe , IVf , IVg . The results were in excellent agreement with the experimental data and hence were proven to be a good tool in explaining different yields % because of the steric and electronic effects of electron‐withdrawing groups on the reactivity of the pyridine ring for nucleophilic attack.     

A programmed polymer folding: click and clip construction of doubly fused tricyclic and triply fused tetracyclic polymer topologies

A tandem alkyne-azide addition, i.e., click, and an olefin metathesis condensation, i.e., clip, reactions in conjunction with an electrostatic self-assembly and covalent fixation (ESA-CF) process, have been demonstrated as effective means to produce constructions of programmed folding of polymers having doubly fused tricyclic and triply fused tetracyclic topologies. Thus, a series of cyclic poly(tetrahydrofuran), poly(THF), precursors having an allyloxy group and an alkyne group (Ia), an allyloxy group and an azide group (Ib), and two alkyne groups (Ic) at the opposite positions was prepared by means of the ESA-CF method. The subsequent click reactions of Ia with a linear telechelic poly(THF) precursor having azide end groups (Id) and of Ib with Ic afforded a bridged dicyclic polymer (IIa) and a tandem spiro tricyclic precursor (IIb), respectively, both having two allyloxy groups at the opposite positions of the ring units. Finally, the intramolecular metathesis condensation reaction of IIa and of IIb in the presence of a Grubbs catalyst was performed to construct effectively a doubly fused tricyclic and a triply fused tetracyclic polymer topologies (III and IV), respectively.     

Rapid, selective and direct spectrophotometeric determination of aliphatic amines with m-dinitrobenzene

Color reaction has been studied for identification and spectrophotometric determination of aliphatic amines at room temperature by m-dinitrobenzene (m-DNB) as reagent. The λ_max value ranges from 458 to 570 nm. This is a simple and rapid method for determination of aliphatic amines in the acidic, water and acetone medium. Beer's law is verified for methylamine, dimethylamine, trimethylamine and n-butylamine in the range of 0.5–8 mg l⁻¹. The effect of pH on the molar absorptivity is investigated for a representative primary amine i.e. methylamine and it was observed that molar absorptivity increases from acidic to basic pH, with a sharp increase at pH 12. The kinetic of reaction was also studied and found that reaction time has marked effect on the molar absorptivity of electron donor–acceptor (EDA) complex. The detection of methyl amine has been reported in three real samples of water.     

Kinetics and mechanism of the aminolysis of O-aryl S-methyl thiocarbonates

[reaction: see text] The reactions of secondary alicyclic (SA) amines and quinuclidines (QUI) with 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) and those of SA amines with 2,3,4,5,6-pentafluorophenyl S-methyl thiocarbonate (3) are subjected to a kinetic study in aqueous solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions of thiocarbonates 1, 2, and 3 were followed spectrophotometrically at 400, 360, and 220 nm, respectively. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Br?nsted-type plots (log kN vs pKa of aminium ions) are linear for all the reactions, with slopes beta = 0.9 for those of 1 with SA amines and QUI, beta = 0.36 and 0.57 for the reactions of 2 with SA amines and QUI, respectively, and beta = 0.39 for the reactions of SA amines with 3. The magnitude of the slopes indicates that both aminolyses of 1 are governed by stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T+/-), where expulsion of the nucleofuge from T+/- is the rate-determining step. The values of the Br?nsted slopes found for the aminolyses of thiocarbonates 2 and 3 suggest that these reactions are concerted. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions arise: (i) Thiocarbonate 2 is more reactive than 1 toward the two amine series. (ii) The change of the nonleaving group from MeO in 4-nitrophenyl methyl carbonate to MeS in thiocarbonate 1 results in lower kN values. (iii) The greater reactivity of this carbonate than thiocarbonate 1 is attributed to steric hindrance of the MeS group, compared to MeO toward amine attack. (iv) The change of a pyridine to an isobasic SA amine or QUI destabilizes the T+/- intermediate formed in the aminolyses of 2. (v) The change of 4-nitrophenoxy to 2,3,4,5,6-pentafluorphenoxy or 2,4-dinitrophenoxy as the leaving group destabilizes the tetrahedral intermediate formed in the reactions with SA amines, changing the mechanism from a stepwise process to a concerted reaction.