Deuterium kinetic isotope effects structure

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

Deuterium kinetic isotope effects, secondary, and transition state structure, 31,143 Diazo compounds, aliphatic, reactions with acids, 5, 331... 

Secondary Deuterium Kinetic Isotope Effects and Transition State Structure... 

Secondary a-deuterium kinetic isotope effects (KIEs) have been widely used to determine the mechanism of SN reactions and to elucidate the structure of their transition states (Shiner, 1970a Westaway, 1987a). Some of the significant studies illustrating these principles are presented in this section. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

The rate of catalytic dehydrohalogenation is influenced by the structure of the reactants, but the extent of this effect varies from one catalyst to another with change of mechanism, i.e. with the timing of the fission of the Ca—X and Cp—H bonds. This is best seen from the published data on the deuterium kinetic isotope effect in Table 8. Their significance for the elucidation of the mechanism will be dealt with in Sect. 2.4.4 and here we can simply state that the value of the isotope effect depends on the nature of the catalyst. However, with a different reactant and within a series of related catalysts, kH/kD values independent of the catalyst were obtained (Table 9) [183],... 

Typical acidic catalysts are silica—alumina, transition metal sulphates or chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8,11 and 12). These results correspond to the E2cA or El mechanisms, between which a transition may be observed due to the influence of the structure of the reactants, i.e. according to the polarity of the Ca—X and Cp—H bonds. Again, the reactions of 1,2-dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica—alumina was 1.0 for the dibromo-derivative, which indicates a pure El mechanism, whereas for the tetrachloro-derivative, the value of 1.5 was found. 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

These recent calculations for cyclopropanes and trimethylene transition structures, and for isotopically labeled analogs, have provided vibrational frequencies from which secondary deuterium isotope effects have been calculated. Getty, Davidson and Borden found that reactions dependent upon C,(ts) at 422.5 °C should be associated with secondary deuterium kinetic isotope effects favoring access to 1,2-d2-Cj(ts) over l,3-d2-C (ts) structures by a factor of 1.13269. 

It would have been possible to rescue the concerted hypothesis at this point by claiming that the transition state was structurally similar to the diolate, that is, that the alkene extrusions involved little C O bond cleavage prior to the transition state. However, the measurement of a significant secondary deuterium kinetic isotope effect (kHJkw = 1.25 0.05 at 100°C) gave evidence of significant C—O bond cleavage, whatever the mechanism. 

This solvation rule for 5n2 reactions can be useful in predicting the influence of a change in solvent on the structure of activated complexes. It is in agreement with studies involving leaving group heavy atom and secondary a-deuterium kinetic isotope effects, as well as theoretical calculations of solvent effects on transition-state structures. Possible limitations of this solvation rule have been discussed see [498] and relevant references cited therein. 

Gajewski, J. J. Conrad, N. D. Variable transition-state structure in the Cope rearrangement as deduced from secondary deuterium kinetic isotope effects, J. Am. Chem. Soc. 1978,100, 6269-6270. 

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