Isotope effects in eliminations

Saunders, W.H. (1985). Calculations of isotope effects in elimination reactions. New experimental criteria for tunneling in slow proton transfers. J. Am. Chem. Soc. 107, 164-169... 

Chiao, W.-B., Saunders, W. H., Jr. Mechanisms of elimination reactions. 29. Deuterium kinetic isotope effects in eliminations from amine oxides. The consequences of nonlinear proton transfer. J. Am. Chem. Soc. 1978,100, 2802-2805. 

The increased strength and electron-withdrawing power of the C -N bond relative to the C - bond should cause reactions of the former to proceed via a more carbanionic transition state. Under the same reaction conditions, the sulphur isotope effect for 2-phenylethyldimethylsulphonium ion (I I, Table 6) indicates more C -X bond breaking than the nitrogen isotope effect in elimination from the corresponding ammonium salt. 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

The existence of tr-complex intermediates in C-H activation chemistry has been suggested to explain inverse kinetic isotope effects in reductive elimination processes whereby alkanes are formed from alkyl metal hydrides (Scheme 3).9... 

Good selectivity for the oxidation of primary alcohols in the presence of secondary ones can be achieved. By appropriate choice of the reaction conditions, overoxidation of the aldehyde from a primary alcohol to carboxylic acid can be minimized. Kinetic isotope effects in the range of 2 to 3 testify about the relevance of the H+-elimination step upon the overall reactivity . In general, the efficiency of oxidation of alkanols is slightly lower... 

Another method of seeking evidence of the EIcBirr mechanism is to exam heavy-atom isotope effects in the leaving group. Of course, these should be much more significant in an E2 process because the bond is breaking in the transition state. For example, Thibblin and co-workersfound that in the base-induced elimination of an alkyl halide in which the p-carbon is unusually acidic (indene derivative, 12), moderately strong bases (triethylamine and methoxide) lead to a significant Cl/ Cl isotope effect = 1.010 1.009, where a maximum effect of... 

The secondary Hke/T H KIE in the eliminations of 373, 374 and 375 presented above which are higher than this maximum possible secondary IE value, are taken as strongly implicating tunnelling. This conclusion has been supported also by intercomparison of secondary H/T and D/T isotope effects in E2 reactions of RNM3 1 Br at 50 °C. The secondary IE is depressed markedly when deuterium rather than proton is transferred, which also implicates tunnelling ... 

Scheme 9.19 Kinetic hydrogen isotope effects in competing rearrangement and elimination from 1-(2-acetoxy-2 -propyl) i ndene.

Another procedure for sulfur isotope measurements has been developed where samples are converted to solid arsenic sulfide, AS0S3 (s), and measured by thermal ionization mass spectrometry (TIMS) (22). This technique offers several advantages over the gaseous methods in that both memory and isotope effects are eliminated, and the chemical procedure is simpler. A precision of 1 0/00, and the capability of making measurements on small samples, makes the TIMS technique competitive with gas phase MS techniques. 

In view of the above, a transition state with very little charge separation (A), or one with a reversed polarity (compared to that of the normal eliminations, e.g., esters) (B) was favored. A deuterium isotope effect in a-deuterodiphenylmethyl ether gave kg/kg 1.1. This very low value suggests either little Ca-H bond rupture in the transition state, or that the hydrogen atom is transferred with some... 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Bach, R. D., Braden, M. L. Primary and secondary kinetic isotope effects in the Cope and Hofmann elimination reactions. J. Org. Chem. 1991,56, 7194-7195. 

Lewis, D. E., Sims, L. B., Yamataka, H., McKenna, J. Calculations of kinetic isotope effects in the Hofmann eliminations of substituted (2-... 

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