Formates isotope effects

M. Pankova, M. Svoboda, and J. Zavada, Tetrahedron Lett. 1972 2465. The analysis of the data also requires that account be taken of (a) isotope effects and (b) formation of 4-decene. The method of analysis is described in detail by J. Sicher, J. Zavada, and M. Pankova, Collect. Czech. Chem. Commun. 36 314 (1971). no. J. Sicher, Angew. Chem. Int. Ed. Engl 11 200 (1972). 

Friedel-Crafts acylation sometimes shows a modest kinetic isotope effect. This observation suggests that the proton removal is not much faster than the formation of the (j-complex and that the formation of the n-complex may be reversible under some conditions. 

Indicate mechanisms that would account for the formation of each product. Show how the isotopic substitution could cause a change in product composition. Does your mechanism predict that the isotopic substitution would give rise to a primary or secondary deuterium kinetic isotope effect Calculate the magnitude of the kinetic isotope effect from the data given. 

Despite its apparent simplicity, the PK pyrrole synthesis has retained its mystique since being discovered. Several investigations into the PK mechanism have been reported, including a gas phase study. Current evidence (intermediate isolation, kinetics, isotope effects) suggests the following (abbreviated) mechanism for the formation of pyrrole 13. However, the specific PK mechanism is often dependent on pH, solvent, and amine and dicarbonyl structure, especially with regard to the ringclosing step. 

The deuterium isotope effect for each hydrogen atom ortho to the diazonio group ( H/ D = 1.22, Swain et al., 1973b) is the largest secondary aromatic hydrogen isotope effect yet observed. It is comparable to those observed for a-deuterium in reactions involving carbocation formation from secondary aliphatic esters. Ob-... 

Penton and Zollinger (1979, 1981 b) reported that this could indeed be the case. The coupling reactions of 3-methylaniline and A,7V-dimethylaniline with 4-methoxy-benzenediazonium tetrafluoroborate in dry acetonitrile showed a number of unusual characteristics, in particular an increase in the kinetic deuterium isotope effect with temperature. C-coupling occurs predominantly (>86% for 3-methylaniline), but on addition of tert-butylammonium chloride the rate became much faster, and triazenes were predominantly formed (with loss of a methyl group in the case of A V-di-methylaniline). Therefore, the initial attack of the diazonium ion is probably at the amine N-atom, and aminoazo formation occurs via rearrangement. 

It is apparent from equation (16) that if k x becomes much larger than k 2, the rate will depend upon k 2 and so a kinetic isotope effect will be observed. Now kL j will become large if there is steric hindrance to formation of the intermediate, and a number of examples are now known where an electrophile which normally gives no isotope effect, does so if formation of the intermediate is hindered. 

Jouzel, J. and Merlivat, L. (1984). Deuterium and oxygen 18 in precipitation modelling of the isotopic effects during snow formation. /. Geophys. Res. 89(D7), 11749-11757. 

The first step, as we have already seen (12-3), actually consists of two steps. The second step is very similar to the first step in electrophilic addition to double bonds (p. 970). There is a great deal of evidence for this mechanism (1) the rate is first order in substrate (2) bromine does not appear in the rate expression at all, ° a fact consistent with a rate-determining first step (3) the reaction rate is the same for bromination, chlorination, and iodination under the same conditions (4) the reaction shows an isotope effect and (5) the rate of the step 2-step 3 sequence has been independently measured (by starting with the enol) and found to be very fast. With basic catalysts the mechanism may be the same as that given above (since bases also catalyze formation of the enol), or the reaction may go directly through the enolate ion without formation of the enol ... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Henrich and WolP have studied the formation of Mo(CO)5 by catching Mo and Mo recoils in Cr(CO)g. The Molybdenum isotopes were produced in several different reactions, so that the recoil energy varied over a wide range. It was found that the yields of Mo(CO)g with the two isotopes differed from each other, but varied only slightly as a function of initial recoil energy. These authors were also able to show that the isotope effect of about 8% is nearly insensitive to radiation received by the sample (and catcher) during the bombardment. They argued that there remains only one possible cause of this isotope effect, that is, differences in the de-excitation schemes of the product nuclei. 

The experiments of Bott (17) and Noyce (19-21) show that a vinyl cation best represents the intermediate in the hydration of phenylacetylenes. In particular, the large solvent Isotope effects observed indicate a rate-limiting protonation and formation of a vinyl cation, for these values are not in agreement with solvent isotope effects observed for compounds which react by other possible mechanisms, such as one involving equilibrium formation of the vinyl cation followed by the slow attack by water. 

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