Kinetic isotope effects halide reactions

The secondary -deuterium kinetic isotope effects in reactions of -perdeuterated ethyl-, isopropyl- and rm-butylmagnesium halides with four different ketones were described [24] and were found to be small (within 5%). There, too, hyperconjugative stabilization was supposed to play a role, but this effect was opposed by the steric effects. The role of hyperconjugation in reactions of Grignard reagents, therefore, seems complicated and requires further studies. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

Schowen (1972) used the Swain-Bader theory to explain solvent kinetic isotope effects on SN2 reactions involving halides, in terms of the numer of water molecules solvating a halide ion in the transition state (assumed to be three) vs. the number solvating it in the bulk (four). The contribution of a single halide-water hydrogen bond was also taken to depend on the partial charge on the halide, which could be consistent with either theory. 

It is possible that in other halogenations with measurable isotope effects the same mechanism is effective as in the bromination of 2-naphthol-6,8-disulphonic acid (fc i[Br-] > fci[Br2] fc2[ ])- The observation of Grovenstein (1958) that the kinetic isotope effects in the iodination of phenol-2,4,6- 3 (reaction no. 22) are independent of the concentration of halide ions might be explained by such a mechanism. 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

While these four schemes by no means exhaust the possibilities, they represent a broad spectrum of mechanisms in terms of molecularity and polarity of the transition state. Some organolithium reactions, e.g., thermal decomposition, very probably proceed via radical formation. Furthermore, there is evidence that the coupling reaction with an alkyl halide may involve radicals (72), although these systems must be more extensively investigated. Reaction scheme iv) can probably be ruled out immediately it would require an inordinately high activation energy, would probably lead to a negligible lithium kinetic isotope effect, and would not account for stereospecificity in olefin polymerization 68, 73). It possibly becomes an important pathway, however, in the presence of more polar molecules such as ethers. 

It is generally agreed that the mechanism of the solvolysis of benzyl halides lies near the region which marks the transition from Sjfl to Sjf2 solvolysis. Considerations of the kinetic chlorine isotope effect have recently led to the conclusion that even 4-nitrobenzyl chloride undergoes Sul solvolysis (Hhl and Pry, 1962) but an examination of the data suggests that this effect does not represent a sensitive test of solvolytic mechanism (Kohnstam, 1967). On the other hand, the values oi AC, AC I AS, and AS show that the solvolysis of the parent compoimd has the characteristic featvues of an 8 2 reaction (Tables 5, 6), and other evidence also supports this conclusion (see Bensley and Kohnstam, 1957). 

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