Sterically Hindered Aromatic Compounds. VI. A Remote ε-Deuterium Kinetic Isotope Effect in the Solvolysis of Perdeutero-2,4,6-tri-tert-butylbenzyl Chloride
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2,4,6-Tri-tert-butylbenzyl chloride deuterated at the three tert-butyl groups was synthesized. Conductimetric solvolysis studies of the normal and perdeutero-2,4,6-tri-tert-butylbenzyl chloride at 30.06 °C in 80% ethanol–water provides evidence for an inverse remote ε-deuterium isotope effect, k H /k D = 0.873−0.874. Under the same conditions the α-deuterium isotope effect was k H /k αD = 1.166 per deuterium, indicative of limiting solvolytic behavior. The remote ε-deuterium isotope effect for the perdeutero compound is discussed in terms of the inductive effect of deuterium and steric effects on the transition state conformation.Keywords:
Solvolysis
Kinetic isotope effect
Kinetic isotope effect
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The secondary alpha-deuterium kinetic isotope effect (alpha-kie) for the solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate in 97% w/w aqueous 2,2,2-trifluoroethanol (97T) at 25 degrees C has been measured (k(H)/k(D) = 1.33). The alpha-kie is abnormally high compared to the value of 1.23 for the corresponding limiting S(N)1 solvolysis of 2-adamantyl p-bromobenzenesulfonate, which proceeds via an extended ion-pair mechanism. A novel mechanism for the solvolysis of the tin compound is proposed that accommodates not only the high alpha-kie but also the absence of internal return.
Solvolysis
Kinetic isotope effect
Limiting
Heavy water
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The rates of reaction of 2-(4-methoxyphenyl)oxirane (4-methoxystyrene oxide), trans-3deutereo-2-(4-methoxyphenyl)oxirane and 3,3-dideutereo-2-(4-methoxyphenyl)oxirane in water solutions were measured as functions of pH. Kinetic deuterium isotope effects for the reactions of the mono- and di-deuterated (4-methoxyphenyl)oxiranes were determined for both the acidcatalyzed hydrolysis to diols and the pH-independent reactions leading mostly to rearranged aldehyde and involving a 1,2-hydrogen migration. The inverse kinetic deuterium isotopes for acid-catalyzed hydrolyses of the deuterated (4-methoxyphenyl)oxiranes to diols are consistent with rate-limiting epoxide ring opening. The magnitudes of the normal kinetic deuterium isotope effects on the pH-independent reactions of deuterated 4-methoxyphenyloxiranes are significantly smaller than the deuterium isotope effect on the aldehyde-forming step, and are rationalized by a reversible epoxide ring opening step that is partially rate-limiting. The magnitude of the partitioning isotope effect on the hydrogen migration step is consistent with isotope effects determined by Professor Coxon’s laboratory on the Lewis acid-catalyzed rearrangements of deuterated phenyloxiranes in organic solvents.
Kinetic isotope effect
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Analysis of the products of solvolysis of aryl esters of anilidophosphoric acids in water-2-propanol mixture excludes that the reaction proceeds via E1cB mechanism. Analysis of the reaction products of solvolysis of methyl-substituted compounds made it possible to evaluate the importance of the reported steric hindrance by CH 3 group. The dependence of rate constants of the solvolysis on solvent system has been established.
Solvolysis
Steric factor
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Isotope effects have been measured for the abstraction of hydrogen from a series of organic substrates by the perfluoro radical, Na+ -O3SCF2CF2OCF2CF2*, in water. Both primary and secondary deuterium isotope effects were measured, with the primary isotope effects ranging in value from 4.5 for isopropanol to 19.6 for acetic acid. The values for the alpha- and beta-secondary deuterium isotope effects were 1.06 and 1.035, respectively. It was concluded that tunneling contributes significantly to the production of the observed, large primary kinetic isotope effects in these C-H abstraction reactions.
Kinetic isotope effect
Primary (astronomy)
Hydrogen atom abstraction
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Rates and secondary deuterium kinetic isotope effects for the solvolysis of endo-and exo-bicyclo[3.2.1]octan-3-yl toluene-p-sulphonates [(4a and c) and (5a and c)] have been measured. Rate constants, rate ratios, and apparent m values, when compared with results for model compounds, suggest that the solvolyses of (4a) and (5a) are close to the limiting (SN1) extreme in all solvents. The α-deuterium kinetic isotope effects are high (ca. 1.19—1.20 at 25 °C) for (5c) in formic and acetic acids, aqueous 50, 80, and 98% ethanol, and aqueous 97% 2,2,2-trifluoroethanol, and for (4c) in all these solvents except aqueous 98% ethanol (1.141 at 60.4 °C). The β-2H4 kinetic isotope effects are also high for both (4a) and (5a). The results are interpreted in terms of a principal mechanism involving rate-determining formation of intimate ion-pair intermediates. For the solvolysis of (4a) in aqueous 98% ethanol some direct SN2 reaction of solvent with covalent tosylate is also invoked. Mechanisms of solvolysis generally are discussed including the roles of intimate and solvent-separated ion-pairs, and solvent-induced SN2.
Solvolysis
Kinetic isotope effect
SN2 reaction
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Nitrogen and deuterium kinetic isotope effects were measured in the Menshutkin reaction between methyl iodide and a series of para-substituted N,N-dimethylanilines in ethanol. The nitrogen kinetic isotope effect increases for the more electron-donating substituents [0·9989, 1·0032, and 1·0036 for 4-C(O)Me, H and 4-Me, respectively], in agreement with the Hammond postulate. The secondary deuterium isotope effect, however, exhibits the reverse trend (1·045, 0·989, 0·975 per deuterium, for the respective substituents). This discrepancy is rationalized in terms of solvent molecule participation in the transition state.
Kinetic isotope effect
Methyl iodide
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Kinetic data are reported for the solvolysis of 1-methyl cyclopentyl chloride in a 50% by volume ethanol+ water mixture. These data are combined with kinetic data for solvolysis of three deuterated analogues; 1-methyl-d 3 cyclopentyl chloride, 1-methyl-2,2,5,5-d 4 cyclopentyl chloride, and 1-methyl-d 3 -2,2,5,5-d 4 cyclopentyl chloride, being the d 3 d 4 , and d 7 , derivatives respectively. The kinetic data are used to calculate the kinetic deuterium isotope effect, k H /k D . For these systems, the d 7 -isotope effect is close to the product of the d 3 and d 4 effects.
Solvolysis
Kinetic isotope effect
Hydrogen chloride
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The rates of the reactions of 1-nitro-1-(4-nitrophenyl)alkanes and their deuteriated analogues with two bicyclic guanidines of comparable basicity, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), in tetrahydrofuran have been measured. The results are discussed in terms of the effects of steric hindrance in the C-acid and the base on the rates and deuterium kinetic isotope effects (KIEs). The reactions of TBD are 118–287 times faster than reactions of MTBD with the same nitroalkanes. The stabilization of the transition state of the TBD reactions by the N–H· · ·O hydrogen bond is plausible. With the most sterically crowded C-acid, the steric hindrance in the base gives a reduced deuterium KIE. Deuterium KIEs for the reactions of MTBD with various C-acids decreases with the steric hindrance in the C-acid but the reverse is true for TBD reactions. Results of this work disagree with the notion that steric hindrance leads to enhanced kinetic isotope effects.
Kinetic isotope effect
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This chapter contains sections titled: Introduction Hyperconjugation and Secondary β-Deuterium Isotope Effects The Conformational Dependence of Secondary β-Deuterium Isotope Effects and the Structures of Cationic Transition States More Remote Isotope Effects. Models for the Origin of γ-Deuterium Isotope Effects
Kinetic isotope effect
Carbocation
Hyperconjugation
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