OH Nucleophile Research Articles

Reactions of dipivaloylketene dimer with nucleophiles: new access to the 2,6,9-trioxabicyclo[3.3.1]nona-3,7-diene ring system (bridged bis-dioxines)

Novel and convenient methods to prepare several functionalized 2,6,9-trioxabicyclo[3.3.1]nonadienes (''bridged bis-dioxines'') 6, 10, 11, and 12 are described. In general, these are obtained by addition of nucleophiles to alpha-oxoketene 5 in a multistep reaction sequence, where the specific reaction pathway depends strongly on the type of nucleophile employed: Aromatic amines with electron-donating substituents afford the trioxabicyclononadienes 6, while with aliphatic amines or thiolates, 2 equiv of the corresponding dipivaloylacetamides 13 or thioesters 14, respectively, are formed. In the case of OH nucleophiles, the primary 1:1 adducts 8 are isolable, characterized by an X-ray analysis of 8a. Under acidic conditions, 8 can be cyclized to the bifunctionalized bis-dioxines 10. The bis-carboxylic acid 10c serves as a valuable intermediate to synthesize other derivatives, e.g. the bis-acid chloride 11 and the bis-ester 12. Mechanistic pathways leading to the various reaction products are discussed in detail and supported by semiempirical molecular orbital calculations using the AM1 method. The spontaneous formation of the bis-dioxine skeleton in the reaction 5 --> 6 is likely to proceed via an unusual [4 + 4] type tandem cyclization reaction.

Electrostatic catalysis of the hydrolysis of a phosphate diester in water

The efficiency of general acid catalysis by protonated amines of the hydrolysis of a phosphate diester with a neighbouring nucleophilic OH group depends less-on the pKa of the acid than on the proximity of a second positive charge.

Intramolecular attack by the hydroxy group on phosphate diesters is both general acid and general base catalysed

Buffers catalysing the hydrolysis of a phosphate diester with a neighbouring nucleophilic OH group act as general acids, general bases, or both, depending on their pKa, though the rate determining step remains the same.

Fluorimetric studies of solutions of pyronin dyes: equilibrium constants in water and partition coefficients in organic-solvent—water systems

The cytotoxic pyronin dyes provide simplified models for the xanthene squeleton of popular rhodamines. Kinetic analysis of fluorescence spectra of pyronin Y (PY) and pyronin B (PB) aqueous solutions allows us to study the reversible equilibria between R + zwitterions (λ Fluo, max=566 nm (PY), 570 nm (PB)) and colorless ROH xanthydrols (λ Fluo, max=463 nm (PY), 467 nm (PB)) formed through ion-pair reactions with nucleophile OH −. The rate and equilibrium constants of R + hydrolysis by OH − in water gives pK R+(S 0)= -2.6 for the two pyronins while fluorescent species are too short lived for any shift in the equilibrium to be detected in the S 1 state. Covalent ROH species are stable in basic solutions and poor ion solvating solvents; the reaction of R + with OH − is slow in neutral solutions. For further understanding and applications of pyronins on biomembranes, biphasic mixtures of an organic solvent (OS) with water (W) provide partition coefficients P and extraction percentages E of the different species in the OS phase. In the n-octanol-water (ratio 1:1) system, R + and ROH co-exist in both phases, resulting in P ion=1.8 (PY) and 2.6 (PB) at physiological pH with E ROH=0.09 (PY) and 0.16 (PB); most pyronin exists as R + in water owing to its pK R+ but the presence of the octanol phase shifts the equilibrium toward ROH. The ROH structure presents other features of interest because of its oxidization by air into RO xanthone in aprotic non-polar solvents. In the OS-W system (1:1) where OS is cyclohexane, the partitioning of pyronins between the OS and W phases is strongly one sided with R + remaining in water RO(λ Fluo, max=365–366 nm) completely trapped in the cyclohexane. The E percentages differ for PY and PB since E RO(PY)=0.8 but only 0.3 for PB. The implication of hydrophobic interactions and hydrogen-bonded (solvated) water in OS is also discussed.

Molecular dynamics of serine proteases.

The motion of catalytic residues and water molecules in the active site of acyl-chymotrypsin was studied with stochastic boundary molecular dynamics. A water molecule expected to provide the nucleophilic OH- for the deacylation reaction was found trapped between His57 and the ester linkage of the acyl group. The trapped water oxygen and hydrogen occasionally approach the carbonyl carbon and the ester oxygen, respectively, while keeping the hydrogen bond to His57 Nε2. Such configurations lead us a stepwise mechanism with very small imidazole movement or one step mechanism in which the tetrahedral intermediate is at the saddle point of the reaction.

The role of hydration in an enzyme reaction

The aim of the work was to elucidate the role of water in the reaction between acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) and methanesulfonyl fluoride, accelerated by accelerators. The reaction between the enzyme and methanesulfonyl fluoride in the presence of individual monovalent cations of the Hofmeister series was investigated. The results obtained were analyzed in comparison with the effect of methanesulfonylation of the specific accelerators tetramethylammonium and tetraethylammonium under various experimental conditions. The monovalent cations of the Hofmeister series accelerate the reaction. Their effect—as well as that of specific accelerators—significantly correlates with the effect of these agents on the structure of water. These findings, together with others, led to the following model of the role of hydration water in acylation of acetylcholinesterase. The accelerator, which may also be the cationic head of the natural substrate, binds to the anionic site of the enzyme and reduces the hydration of the nucleophilic serine OH in the esteratic site, thus enhancing the nucleophilicity of OH. This results in an improvement of the binding between the acylating agent and the esteratic site of acetylcholinesterase.

Reactivity of 2-furylethylenes with nucleophilic groups and its biological significance

The paper describes addition-nucleophilic reactions of 1-(5-R1-2-furyl)-2-R2-R3-ethylenes with SH (thioglycolic acid), NH2 (glycine) and OH (phenol) nucleophiles, and with hydroxyl ions in aqueous media. The determined rate constants, of second order (dm3 mol-1 s-1), show that in the physiological conditions the preferential reaction will be that with thiols. The amino acid composition of proteins modified with 1-(5-nitro-2-furyl)-2-nitro-2-methoxycarbonylethylene has revealed that the only group affected was the SH group of cysteine. This finding points out the significance of addition-nucleophilic SH-interactions of 2-furylethylenes with the protein macromolecules and accounts for the high cytotoxicity of these compounds.

31P nuclear magnetic resonance of phosphoenzyme intermediates of alkaline phosphatase.

Covalent (E-P) and noncovalent (E X P) phosphoenzyme intermediates exist on the reaction path of alkaline phosphatase of Escherichia coli. Zn(II) and Cd(II) alkaline phosphatases both form E-P and E X P from inorganic phosphate. These intermediates show well separated 31P NMR resonances in slow chemical exchange with respect to each other and to unbound phosphate. The 31P signals of E X P of all forms of the 113Cd(II) enzyme are doublets (J = approximately 30 Hz) due to 113Cd-O-31P coupling. Heteronuclear decoupling shows the phosphate of E X P to be coordinated to the A site metal of the two metal ions, A and B, approximately 3.9 A apart at each catalytic center. The chemical shifts of E X P vary from approximately 4 ppm for the Zn(II) enzyme to 12.6-13.4 ppm for forms of the Cd(II) enzyme and indicate a major influence of the metal ions on the conformation around phosphorus. The phosphoryl group of E-P is not coordinated to either of the two metal ions at the active center as shown by the absence of 113Cd-O-31P coupling on the 31P signals of E-P formed by the 113Cd(II) enzymes. The chemical shift of E-P is not sensitive to metal ion species or stoichiometry and is 8-9 ppm for all forms of the Zn(II) and Cd(II) enzymes. The E-P in equilibrium E X P in equilibrium E + Pi equilibria are described by analogous pH functions for the Zn(II) and Cd(II) enzymes. At acid pH E-P predominates and is converted to E X P as the pH is raised, following a sigmoid pH profile. For the Zn(II) enzyme the midpoint of the E-P in equilibrium E X P equilibrium occurs at pH 5, while for the Cd(II)6 and Cd(II)2 enzymes the midpoints are pH 8.7 and 10, respectively. The ionization controlling the equilibrium between E-P and E X P may be that of a metal-bound H2O (-OH nucleophile) whose pKa will depend strongly on the hardness of the coordinating metal ion. For the Zn(II)4 enzyme one of 2 mol of E-P formed by the enzyme at acid pH dissociates readily at pH 7.5-8 where dissociation of E X P (Kd approximately equal to mM) is rate-limiting. Phosphate binds more tightly to the Cd(II) enzyme and 2 mol of phosphate remain bound until above pH 9 where E X P begins to dissociate at mM concentrations. The low Kd for E X P and the alkaline shift in the E-P in equilibrium E X P pH profile probably account for the slow turnover of the Cd(II) enzyme. Precise chemical shifts of the 113Cd and 31P NMR signals as well as the ratio of E-P/E X P at one active center of the dimer are altered by metal ion binding at the other active center indicating significant subunit-subunit interactions.

Mechanism of nitrile-forming fragmentation reactions

Treatment of the 2,4-dinitrophenyl ethers of the anti-form of the α-keto-oximes PhCO·C(Ph)NOH (I) and 2,4,6-Me3C6H2CO·C(Ph)NOH (II) with the nucleophiles OH–, OMe–, OPri– leads to a Beckmann fragmentation with the formation of benzonitrile and the corresponding benzoic or 2,4,6-trimethylbenzoic acids. Similar reactions occur between the benziloxime ether and BuOt– and NH3, whereas, these nucleophiles substitute the 2,4-dinitrophenyl ring of the ether of (II). The reactions of the 2,4-dinitrophenyl ether of (II) with OH–, OMe– and OPri– follow second-order kinetics ruling out a mechanism involving the rate-determining formation of 2,4,6-trimethylbenzoylium cation. Examination of the oxygen-18 content of recovered 2,4-dinitrophenyl ether of (II) labelled at the carbonyl group after the ether had been subjected to solvolytic conditions, i.e. sodium hydroxide in dioxan–water, showed no loss of oxygen-18. The mechanism of the Beckmann fragmentation reaction is discussed in the light of these observations. When the 2,4-dinitrophenyl ethers of (I) and (II) are heated in acetonitrile or benzene, benzonitrile and the 2,4-dinitrophenyl ester benzoic or 2,4,6-trimethylbenzoic acid are formed. Kinetic measurements suggest that reaction involve an initial Beckmann rearrangement followed by collapse of the resulting nitrilium salt.

A kinetic study of some carbonyl substitutions at the CONH group, with special reference to the mechanism of ϵ‐caprolactam polymerisation

AbstractA kinetic study of some substitutions at the carbonyl group of simple N‐substituted amides has provided information on the mechanism of chemical interactions in polyamides generally, and in polymerizing caprolactam melts in particular.In polyamides the amide group may react in substitution either as the uncharged –CONH–, or as the amide conjugate acid –CONH+2–, giving uncatalyzed and acid‐catalyzed reactions respectively. Substituting groups may be the nucleophiles H2O, OH−, NH2, or COO−. Each of the resulting eight possible reactions has been discussed in turn, in the light of kinetic measurements in model systems. Some systematisation of these reactions has been made and the catalytic role of acid end‐groups clarified. There is agreement with other workers on the importance of acid‐catalyzed aminolysis in lactam systems, but it is possible that a different catalytic mechanism, not involving proton transfer to the amide, and not included in this investigation, may operate in certain circumstances.The need for an independent determination of the degree of ionisation of end‐groups in polyamides, as an aid to complete characterization of chemical interactions in such systems, is emphasized.