Sterically Hindered Aromatic Compounds. V. Solvolysis of p-Methyl-, 2,4,6-Trimethyl-, and 2,4,6-Tri-t-butylbenzyl Chloride. Effect of Solvent and α-Deuterium Substitution

1972 ◽  
Vol 50 (24) ◽  
pp. 3965-3972 ◽  
Author(s):  
L. R. C. Barclay ◽  
J. R. Mercer ◽  
J. C. Hudson
Keyword(s):  
Chemical Shifts ◽  
Isotope Effects ◽  
Activation Parameters ◽  
Butyl Alcohol ◽  
Deuterium Isotope Effect ◽  
Limiting Behavior ◽  
Kinetic Isotope ◽  
Deuterium Substitution ◽  
Alcohol Water ◽  
Chloride Effect

Rates, activation parameters, and kinetic α-deuterium isotope effects are reported for solvolysis of p-methyl-benzyl chloride in 80% trifluoroethanol–water, and in ethanol–water and t-butyl alcohol–water of varying solvent compositions. The α-deuterium isotope effect in trifluoroethanol–water (kH/kD = 1.14 per D) indicates approximately limiting behavior. Results in the other solvents (kH/kD = 1.04–1.08) indicate considerable nucleophilic participation by solvent in the transition state. 2,4,6-Trimethylbenzyl chloride gave kH/kD = 1.09 in 80% ethanol–water but kH/kD = 1.16 in 80% trifluoroethanol–water. 2,4,6-Tri-t-butylbenzyl chloride showed kH/kD effects of 1.13–1.15 in t-butyl alcohol–water and 1.15–1.16 in ethanol–water. The higher kinetic isotope effects for solvolysis of these trialkylbenzyl chlorides are discussed in terms of steric hindrance to nucleophilic participation by solvent. N.m.r. chemical shifts give evidence for steric compression of the benzylic protons in 2,4,6-tri-t-butylbenzyl chlorides.

The reaction between 2,2-di(4-nitrophenyl)1,1,1-trifluoroethane and tert-butoxide ion in tert-butyl alcohol solvent

1980 ◽  
Vol 58 (18) ◽  
pp. 1979-1982 ◽  
Author(s):  
Arnold Jarczewski ◽  
Kenneth T. Leffek
Keyword(s):  
Isotope Effects ◽  
Activation Parameters ◽  
Butyl Alcohol ◽  
Blue Colour ◽  
Butyl Ether ◽  
Tert Butyl ◽  
Kinetic Isotope ◽  
Tert Butyl Alcohol ◽  
Reaction Proceeds ◽  
Alcohol Solvent

The reaction between 2,2-di(4-nitrophenyl) 1,1,1-trifluoroethane and excess sodium tert-butoxide in tert-butyl alcohol takes place rapidly to yield first the olefin, by elimination of HF. This is followed by a slower addition of tert-butoxide to the olefin, which upon reaction with the solvent gives the tert-butyl ether as the final product. The blue anions involved in each of these reactions are observed.Rate constants, the primary deuterium kinetic isotope effects, and the activation parameters have been measured for the appearance of the initial blue colour and also for the formation of the olefin. It is concluded that the reaction proceeds by a reversible ElcB mechanism.

DEUTERIUM KINETIC ISOTOPE EFFECTS: V TEMPERATURE DEPENDENCE OF β-DEUTERIUM EFFECTS IN WATER SOLVOLYSIS OF ISOPROPYL COMPOUNDS

1961 ◽  
Vol 39 (10) ◽  
pp. 1989-1994 ◽  
Author(s):  
K. T. Leffek ◽  
R. E. Robertson ◽  
S. E. Sugamori
Keyword(s):  
Temperature Dependence ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Zero Point ◽  
Kinetic Isotope Effects ◽  
Methyl Groups ◽  
Deuterium Isotope Effect ◽  
Point Energy ◽  
Kinetic Isotope ◽  
Enthalpy Of Activation

The secondary β-deuterium isotope effect (kH/kD) has been measured over a range of temperature for the water solvolysis reactions of isopropyl methanesulphonate, p-toluenesulphonate, and bromide. In these cases the isotope effect is due to a difference in entropies of activation of the isotopic analogues rather than a difference in the enthalpies of activation. It is suggested that the observed isotope effect is due to internal rotational effects of the methyl groups in the isopropyl radical, and the lack of an isotope effect on the enthalpy of activation is accounted for by a cancellation of an effect from this source and one from zero-point energy.

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].

Unequal primary kinetic isotope effects in the base-catalysed H–D exchange of diastereotopic protons

1980 ◽  
Vol 58 (1) ◽  
pp. 72-78 ◽  
Author(s):  
Robert R. Fraser ◽  
Philippe J. Champagne
Keyword(s):  
Transition State ◽  
Exchange Reaction ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Kinetic Isotope

Primary kinetic isotope effects have been measured for the base-catalyzed exchange reaction of 4′,1″-dimethyl-1,2,3,4-dibenzcyclohepta-1,3-diene-6-one, 1. It was found that the isotope effects kH/kT and kD/kT for the faster exchanging protons (13.6 and 3.8 respectively) are significantly larger than the corresponding values for the slower exchanging protons (4.6 and 1.6 respectively). These differences could result from truly unequal isotope effects due to transition state differences or intrusion of a second pathway for exchange of the less reactive proton in the dedeuteration reaction. The data appear to support the latter interpretation. The secondary deuterium isotope effect was found to be 1.18.

Secondary α-deuterium kinetic isotope effects for the E2 reaction of the 2-phenylethyl halides with tert-butoxide ion in tert-butyl alcohol

1985 ◽  
Vol 63 (1) ◽  
pp. 100-102 ◽  
Author(s):  
Peter James Smith ◽  
Kanchugarakoppal S. Rangappa ◽  
Kenneth Charles Westaway
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Order Rate Constant ◽  
Butyl Alcohol ◽  
Tert Butyl ◽  
Kinetic Isotope ◽  
Tert Butyl Alcohol ◽  
Elimination Reactions ◽  
Leaving Groups ◽  
High Base

Secondary α-deuterium kinetic isotope effects have been determined for the elimination reactions of 2-phenylethyl halides with tert-butoxide in tert-butyl alcohol at 40 °C in the presence and absence of the crown ether 18C6. The second-order rate constant k2 and the normal (kH/kD)α effect remained constant when the tert-butoxide concentration was varied for reaction of the iodo and bromo compounds. However, both the magnitude of k2 and the secondary α-deuterium isotope effect were significantly dependent on [t-BuO−] when chlorine and fluorine are the leaving groups. It is noteworthy that (kH/kD)α is inverse for the reaction of both the chloro and fluoro compounds at "low" base concentrations and normal at "high" base concentrations. These results are discussed in terms of both syn- and anti-elimination pathways promoted by various associated and dissociated base species. It is suggested that the (kH/kD)α effect may be useful as a criterion for determining the stereochemistry of E2 elimination reactions.

Model calculations of isotope effects. VIII. Deprotonation of nitroethane

1983 ◽  
Vol 36 (8) ◽  
pp. 1513
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Partial Negative Charge ◽  
Hydrogen Deuterium ◽  
State Models ◽  
Experimental Values ◽  
Comparison Of The Results

Transition-state models for the base-promoted deprotonation of nitroethane have been designed, and primary and secondary hydrogen-deuterium kinetic isotope effects have been calculated. Comparison of the results with experimental values of the primary isotope effects allows no firm conclusions to be reached concerning probable transition-state structures. However, the secondary α-deuterium isotope effect comparison disqualifies from consideration those transition states in which rehybridization of Cα and delocalization of the partial negative charge by the nitro group keep pace with the extent of deprotonation. Transition-state models wherein Cα is carbanionic and essentially pyramidal yield theoretical isotope effects lying within the experimental range.

The proton transfer reaction between bis(2,4-dinitrophenyl)methane and nitrogen bases in dimethyl sulfoxide and toluene solvents

1991 ◽  
Vol 69 (3) ◽  
pp. 468-473 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Dimethyl Sulfoxide ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Activation Parameters ◽  
Kinetic Isotope Effects ◽  
Proton Transfer Reaction ◽  
Kinetic Isotope ◽  
Deuteron Transfer ◽  
Classical Region

Rate constants have been measured for the proton and deuteron transfer reactions of bis(2,4-dinitrophenyl)methane (1) with 1,1,3,3-tetramethylguanidine (TMG) and 1,5-diazabicyclo[5.4.0]undec-7-ene (DBU) in dimethyl sulfoxide (DMSO) and toluene solvents. Equilibrium constants, primary deuterium kinetic isotope effects, and activation parameters are reported. The reaction of 1 with DBU is faster than that with TMG by factors of 5 and 50 in toluene and DMSO respectively. The primary deuterium kinetic isotope effects, kH/kD = 7–9, which are independent of the polarity of the solvent, indicate an uncoupled mechanism of proton transfer and are in the "classical" region with little or no indication of a tunnelling contribution to the enthalpy of activation for these reactions. Key words: proton transfer, bis(2,4-dinitrophenyl)methane, deuterium isotope effects.

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