scholarly journals Kinetics and mechanism of the anilinolysis of aryl phenyl isothiocyanophosphates in acetonitrile

2013 ◽  
Vol 9 ◽  
pp. 615-620 ◽  
Author(s):  
Hasi Rani Barai ◽  
Hai Whang Lee
Keyword(s):  
Isotope Effects ◽  
Bond Formation ◽  
Kinetic Studies ◽  
Kinetic Isotope Effects ◽  
Bond Breaking ◽  
Stepwise Mechanism ◽  
Kinetic Isotope ◽  
Energy Relationships ◽  
Break Region ◽  
Rate Limiting

Kinetic studies on the reactions of Y-aryl phenyl isothiocyanophosphates with substituted X-anilines and deuterated X-anilines were carried out in acetonitrile at 55.0 °C. The free-energy relationships with X in the nucleophiles were biphasic concave upwards with a break region between X = H and 4-Cl, giving unusual positive ρX and negative βX values with less basic anilines (X = 4-Cl and 3-Cl). A stepwise mechanism with rate-limiting bond breaking for more basic anilines and with rate-limiting bond formation for less basic anilines is proposed based on the positive and negative ρXY values, respectively. The deuterium kinetic isotope effects involving deuterated anilines (XC6H4ND2) showed primary normal and secondary inverse DKIEs for more basic and less basic anilines, rationalized by frontside attack involving hydrogen-bonded four-center-type TSf and backside attack TSb, respectively. The positive ρX values with less basic anilines are substantiated by the tight TS, in which the extent of the bond formation is great and the degree of the bond breaking is considerably small.

scholarly journals Mechanistic Insights into RNA Transphosphorylation from Kinetic Isotope Effects and Linear Free Energy Relationships of Model Reactions

2014 ◽  
Vol 20 (44) ◽  
pp. 14336-14343 ◽  
Author(s):  
Haoyuan Chen ◽  
Timothy J. Giese ◽  
Ming Huang ◽  
Kin-Yiu Wong ◽  
Michael E. Harris ◽  
...  
Keyword(s):  
Free Energy ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Linear Free Energy Relationships ◽  
Linear Free Energy ◽  
Kinetic Isotope ◽  
Energy Relationships ◽  
Model Reactions

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

scholarly journals Kinetic isotope effects of peptidylglycine α-hydroxylating mono-oxygenase reaction

1998 ◽  
Vol 336 (1) ◽  
pp. 131-137 ◽  
Author(s):  
Kenichi TAKAHASHI ◽  
Tetsuo ONAMI ◽  
Masato NOGUCHI
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Tracer Method ◽  
Deuterium Isotope Effect ◽  
Critical Determinant ◽  
Product Release ◽  
Kinetic Isotope ◽  
Hydroxylation Reaction ◽  
Double Label ◽  
Rate Limiting

Many bioactive polypeptides or neuropeptides possess a C-terminal α-amide group as a critical determinant for their optimal bioactivities. The amide functions are introduced by the sequential actions of peptidylglycine α-hydroxylating mono-oxygenase (PHM; EC 1.14.17.3) and peptidylamidoglycollate lyase (PAL; EC 4.3.2.5) from their glycine-extended precursors. In the present study we examined the kinetic isotope effects of the frog PHM reaction by competitive and non-competitive approaches. In the competitive approach we employed the double-label tracer method with d-Tyr-[U-14C]Val-Gly, d-Tyr-[3,4-3H]Val-[2,2-2H2]-Gly, and d-Tyr-Val-(R,S)[2-3H]Gly as substrates, and we determined the deuterium and tritium effects on Vmax/Km as 1.625±0.041 (mean±S.D.) and 2.71±0.16 (mean±S.D.), respectively. The intrinsic deuterium isotope effect (Dk) on the glycine hydroxylation reaction was estimated to be 6.5–10.0 (mean 8.1) by the method of Northrop [Northrop (1975) Biochemistry 14, 2644–2651]. In the non-competitive approach with N,N-dimethyl-1,4-phenylenediamine as a reductant, however, the deuterium effect on Vmax (DV) was approximately unity, although the deuterium effect on Vmax/Km (DV/K) was comparable to that obtained by the competitive approach. These results indicated that DV was completely masked by the presence of one or more steps much slower than the glycine hydroxylation step and that DV/K was diminished from Dk by a large forward commitment to catalysis. The addition of PAL, however, increased the apparent DV from 1.0 to 1.2, implying that the product release step was greatly accelerated by PAL. These results suggest that the product release is rate-limiting in the overall PHM reaction. The large Dk indicated that the glycine hydroxylation catalysed by PHM might proceed in a stepwise mechanism similar to that proposed for the dopamine β-hydroxylase reaction [Miller and Klinman (1983) Biochemistry 22, 3091–3096].

Experimental and Theoretical Kinetic Isotope Effects for Asymmetric Dihydroxylation. Evidence Supporting a Rate-Limiting “(3 + 2)” Cycloaddition

1997 ◽  
Vol 119 (41) ◽  
pp. 9907-9908 ◽  
Author(s):  
Albert J. DelMonte ◽  
Jan Haller ◽  
K. N. Houk ◽  
K. Barry Sharpless ◽  
Daniel A. Singleton ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Asymmetric Dihydroxylation ◽  
Kinetic Isotope ◽  
Rate Limiting

scholarly journals Comment on “Topography of the free energy landscape of Claisen–Schmidt condensation: solvent and temperature effects on the rate-controlling step” by N. D. Coutinho, H. G. Machado, V. H. Carvalho-Silva and W. A. da Silva, Phys. Chem. Chem. Phys., 2021, 23, 6738

2021 ◽  
Vol 23 (38) ◽  
pp. 22199-22201
Author(s):  
Charles L. Perrin
Keyword(s):  
Temperature Effects ◽  
Isotope Effects ◽  
Energy Landscape ◽  
Kinetic Isotope Effects ◽  
Chem Phys ◽  
Kinetic Isotope ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting ◽  
Rate Controlling Step

The referenced article in PCCP presents calculations of solvent kinetic isotope effects that indicate that the rate-limiting step in base-catalyzed chalcone formation in aqueous solution becomes the second enolization.

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