Isotope effects intramolecular

KINETIC ISOTOPE EFFECT EQUILIBRIUM ISOTOPE EFFECT SOLVENT ISOTOPE EFFECT HEAVY ATOM ISOTOPE EFFECT INTRAMOLECULAR KINETIC ISOTOPE EFFECT... 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

A true intramolecular proton transfer in the second step of an azo coupling reaction was found by Snyckers and Zollinger (1970a, 1970b) in the reaction of the 8-(2 -pyridyl)-2-naphthoxide ion (with the transition state 12.151). This compound shows neither a kinetic deuterium isotope effect nor general base catalysis, in contrast to the sterically similar 8-phenyl-2-naphthoxide ion. Obviously the heterocyclic nitrogen atom is the proton acceptor. 

As has been indicated, since there is a ring isotope effect there must be a degree of C-H bond breaking in the transition state of the rate-determining stage. Clearly further work is required in this system before a definitive mechanism can be established for the intramolecular rearrangement. 

Intramolecular Isotope Effects. The data in Figure 2 clearly illustrate the failure of the experimental results in following the predicted velocity dependence of the Langevin cross-section. The remark has been frequently made that in the reactions of complex ions with molecules, hydrocarbon systems etc., experimental cross-sections correlate better with an E l than E 112 dependence on reactant ion kinetic energy (14, 24). This energy dependence of reaction presents a fundamental problem with respect to the nature of the ion-molecule interaction potential. So far no theory has been proposed which quantitatively predicts the E l dependence, and under these circumstances interpreting the experiment in these terms is questionable. 

Intramolecular isotope effect studies on the systems HD+ + He, HD+ + Ne, Ar+ + HD, and Kr + + HD (12) suggest that the E l dependence of reaction cross-section at higher reactant ion kinetic energy may be fortuitous. In these experiments the velocity dependence of the ratio of XH f /XD + cross-sections was determined. The experimental results are presented in summary in Figures 5 and 6. The G-S model makes no predictions concerning these competitive processes. The masses of the respective ions and reduced masses of the respective complex reacting systems are identical for both H and D product ions. Consequently, the intramolecular isotope effect study illuminates those... 

Intramolecular isotope effects were studied in the systems N2-HD, CO-HD, 02-HD and C02-HD (20). Product decomposition directly associated with rupture of OH or OD bonds was not observed in these reactions. Isotope effects in decomposition processes which gave OH + or OD+ from reactions of 02+ with HD and COH+ or COD + from... 

Intermediate ions, unreactivity of 260 Intramolecular isotope effects... 95... 

Measurements of the deuterium isotope effect for unsymmetrical di-Schiff bases fully confirmed the interrelation between proton transfer equilibria in both intramolecular hydrogen bonds.46... 

Monosulfoxide 13 undergoes the Pummerer rearrangement when treated with acetic anhydride in the presence of sodium acetate.85 The experiments with tetradeuterated and 180-labeled sulfoxide confirm intermediate formation of a dication.86 The ratio of 2,8,8-trideuteriated to 4,4,6,6-tetradeuteriated product 37 is equal to the intramolecular isotope effect ku k0 =1.7 (Scheme 21).85... 

The intramolecular and intermolecular deuterium isotope effects in the cycloaddition of acrylonitrile to allene (equation 98) have been studied by Dolbier and Dai231,232. The intramolecular KIEs in the allene-acrylonitrile system were found to be 1.21 0.02 at 206°C and 1.14 0.02 at 225°C. A negligible intermolecular SKIE was found in the reaction of the mixture of tetradeuteriated and undeuteriated allene using a limited amount of acrylonitrile (ku/ku) = 1.04 0.05 at 190-210 °C for D0/D4 allene. An equilibrium deuterium IE of 0.92 0.01 was found at 280-287 5°C (15-45 h reaction time). 

Song and Beak161 have used intramolecular and intermolecular hydrogen-deuterium kinetic isotope effects to investigate the mechanism of the tin tetrachloride catalysed ene-carbonyl enophile addition reaction between diethyloxomalonate and methylenecy-clohexane (equation 105). These ene reactions with carbonyl enophiles can occur by a concerted (equation 106) or a stepwise mechanism (equation 107), where the formation of the intermediate is either fast and reversible and the second step is slow k- > k-i), or where the formation of the intermediate (the k step) is rate-determining. 

Song and Beak found intramolecular and intermolecular hydrogen-deuterium kinetic isotope effects of 1.1 0.2 and 1.2 0.1, respectively, for the tin tetrachloride catalysed ene reaction. Since significant intramolecular and intermolecular primary deuterium kinetic isotope effects of between two and three have been found for other concerted ene addition reactions161, the tin-catalysed reaction must proceed by the stepwise pathway with the k rate determining step (equation 107). 

The isotope effects have been interpreted in terms of a mechanism involving two equilibrating zwitterionic intermediates (equation 110). In this instance, the k step is partially reversible and both the intermolecular and intramolecular isotope effects are a composite of the isotope effects in several steps (Schemes 26 and 27). 

A considerable amount of the strain in l,8-bis(dimethylamino)naphtha-lene is relieved by protonation and the N—H N bond length (260 pm) in the protonated amine shows that the molecule is able to adopt a conformation [55] with an intramolecular hydrogen bond (Truter and Vickery, 1972). The infra-red spectrum of protonated l,8-bis(dimethylamino)naphthalene and the chemical shift (5 19.5) of the acidic proton in the nmr spectrum confirm the presence of an intramolecular hydrogen bond (Alder et al., 1968). The magnitude of the isotope effect on the chemical shift (Altman et al., 1978) and the appearance of two Nls peaks in the photoelectron spectrum... 

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