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Kinetic isotope effects (KIEs) have been found to be the most powerful tool available to physical organic chemists for determining the mechanism of reactions and for estimating the structure of their transition states. Various types of KIEs including primary-leaving group-, nucleophile-, and α-carbon KIEs and secondary alpha- and beta-deuterium KIEs are introduced. The factors that affect the magnitude of each of these KIEs are covered in some detail. Finally, the use of these KIEs to determine the mechanism of a reaction and to estimate the structure of the transition state for a reaction is discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Isotopic labelling studies have shown that the E2 reaction of 1-chloro-2-phenylethane with potassium tert-butoxide in tert-butyl alcohol occurs via an anti-periplanar stereochemistry. This demonstrates that the different secondary alpha deuterium kinetic isotope effects found for the high and low base concentrations and in the presence of 18-crown-6 ether are because of changes in transition state structure that occur when the form of the reacting base changes rather than to a change in the stereochemistry of the reaction.Key words: E2 reaction, stereochemistry, secondary alpha deuterium kinetic isotope effects, transition state.

Theoretical calculations at the HF/6-31+G* level suggest the secondary α-deuterium kinetic isotope effects for hydride ion SN2 reactions are much larger than expected for the structure of the transition state. The secondary α-deuterium kinetic isotope effects for the SN2 reactions between sodium borohydride (hydride ion) and para-methyl- and para-chlorobenzyl chlorides are much larger than expected as the theoretical calculations suggest. It appears the secondary α-deuterium isotope effects are larger than expected for the structure of the SN2 transition state because the hydride ion is too small to affect the Cα—H(D) out-of-plane bending vibrations in the transition state.Key words: secondary α-deuterium kinetic isotope effects, SN2, nucleophilic substitution, transition state, substituent effects.

The secondary alpha deuterium and primary leaving group nitrogen KIEs and Hammett ρ values found for the free ion and ion-pair SN2 reactions between benzyldimethylphenylammonium ion and sodium para-substituted thiophenoxides in methanol at 20.000°C show how (i) ion-pairing of the nucleophile, (ii) a change in substituent in the nucleophile, and (iii) a change in solvent alters the structure of a Type II SN2 transition state. Ion-pairing shortens the weaker sulfur - alpha carbon (S—Cα) transition state bond significantly but does not alter the stronger alpha carbon - leaving group (Cα—N) transition state bond as the bond strength hypothesis predicts. However, the effect of ion pairing, i.e., the decrease in the S—Cα bond on ion-pairing, decreases as a more electron-withdrawing substituent is added to the nucleophile, and the S—Cα bond actually increases when the nucleophile is the p-chlorothiophenoxide ion. The identical Hammett ρ values of -0.85 and -0.84 for the free ion and ion-pair reactions, respectively, may be observed because, on average, the S—Cα bonds are identical in the free ion and ion-pair transition states. When a more electron-donating substituent is added to the nucleophile, an earlier transition state is found in both the ion-pair and free ion reactions. However, the substituent effect is smaller in the ion-pair reactions, presumably because the change in the negative charge on the sulfur atom with substituent is greater in the free ion than in the ion-pair. The substituent effect on transition state structure suggested by the KIEs is not predicted by any of the theories that are used to predict substituent effects on SN2 reactions. Both the secondary alpha deuterium and primary leaving group nitrogen KIEs and the Hammett ρ values indicate that the transition state is earlier when the solvent is changed from DMF to methanol as the "solvation rule for SN2 reactions" predicts. This probably occurs because an earlier, more ionic, transition state is more highly solvated (more stable) in methanol.Key words: nucleophilic substitution, SN2, isotope effect, transition state, substituent, ion-pair.

The secondary α deuterium and heavy atom kinetic isotope effects found for two different SN2 reactions suggest that the magnitude of secondary α deuterium kinetic isotope effects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical SN2 transition state rather than by the usual nucleophile−leaving group distance. Although this means the interpretation of these isotope effects is more complex than has been recognized, the results suggest that they can be used to determine whether an SN2 transition state is symmetrical or unsymmetrical.

1 Introduction 2 The Theory of Kinetic Isotope Effects 3 Using Kinetic Isotope Effects to Elucidate the Mechanism of Benzidine-Type Rearrangements of Amines 4 Using Kinetic Isotope Effects to Model the SN2 Transition States for Reactions Involving Quaternary Ammonium Salts Keywords: isotopically labelled amino and quaternary ammonium salts - synthesis and uses; kinetic isotope effects theory; primary hydrogen deuterium kinetic isotope effects; inverse secondary hydrogen deuterium kinetic isotope effect; nitrogen isotope effects and whole-molecule isotope ratio mass spectrometry; nitrogen, carbon-13 and carbon-14 kinetic isotope effects; BEBOVIB IV calculations modeling transition state; secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects

Publisher Summary This chapter is concerned with secondary deuterium kinetic isotope effects. The chapter illustrates some of the important recent advances in the interpretation and uses of these kinetic isotope effects to elucidate reaction mechanisms. Other excellent reviews of secondary deuterium kinetic isotope effects (KIEs) have also been discussed in the chapter. The chapter demonstrates the ways in which secondary deuterium and tritium KIEs can be used to elucidate the mechanisms of reactions and determine the structure of their transition states. Secondary a-deuterium KIEs have been widely used to determine the mechanism of S N reactions and to elucidate the structure of their transition states. Some of the significant studies illustrating these principles are presented in this chapter. In particular, the advantages of using both theoretical calculations and experimental data to solve these problems have been emphasized in the chapter.

A very large secondary α-deuterium kinetic isotope effect of 1.179 ± 0.007 (1.086 ± 0.003 per α-deuterium) has been observed for the SN2 reaction of thiophenoxide ion with benzyldimethylphenylammonium ion in DMF at 0°C. This large isotope effect which is far outside the range reported for SN2 reactions, is attributed to the fact that the extraordinarily large steric crowding around the Cα—H bonds in the substrate is reduced in the SN2 transition state. The structure of the transition state is shown to be consistent with this hypothesis.

The secondary α-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMSO–H2O at 60.0 °C. A large secondary α-deuterium isotope effect of 1.158 per α-D was found, which indicates significant [Formula: see text] bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/kD)α is discussed in terms of the use of secondary α-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylammonium salts with ethoxide are considered with respect to the relative transition state structures for syn- and anti-elimination reactions.

The unexpectedly small secondary alpha deuterium KIE in the 4-methoxybenzyl chloride-thiophenoxide ion reaction is attributed to the increased conjugation between the aryl group and the alpha carbon in the S N 2 transition state.

Because the secondary alpha-deuterium kinetic isotope effects in some SN2 and E2 reactions are strongly concentration dependent, isotope effects measured at a single concentration could lead to erroneous conclusions about the mechanisms and transition state structures.

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.