Eyring plot analyses

A remarkable solvent-controlled enantiodivergence has been reported for the (DHQD)2PHAL-catalysed chlorocyclization of unsaturated carbamates (6) with l,3-dichloro-5,5-dimethyl hydantoin (7). Thus, in Pr"OH, the reaction produces (8), whereas ent- %) was obtained in a CHCI3-hexane mixture. The latter system exhibited an unusual increase in selectivity with increasing temperature. Eyring plot analyses showed a strongly solvent-dependent entropy-enthalpy balance between the two pathways. ... 

One analysis of experimental data involves carrying out rate constant measurements at a series of temperatures, and then plotting iQ(k/T) against l/T (a so-called Eyring plot). We may rearrange Eq. (15.28) to... 

These parameters were calculated from Eyring plots that were based on the temperature dependence of kJK. Although the plots were linear for reactions of FKBP, and thus readily provided values of A//t and ASt, they were curved for reactions of CyP (Harrison and Stein, 1992). A detailed analysis of the Eyring plots for CyP catalysis suggests that the nonlinearity can be accounted for by a mechanism in which the CyP exists in two interconvertible forms, only one of which is enzymatically active. This analysis allowed values of AGt, A//t, and ASt to be extracted from the experimental data (Table IV). 

Band-shape analysis can be a powerful mechanistic tool, affording exchange rates, from which activation parameters can be derived via Arrhenius and/or Eyring plots. Thus, possible exchange/reaction mechanisms can be distinguished. Care, however, is needed since ... 

Despite considerable efforts, it has proven difficult to distinguish between these two mechanisms, partly because the (3-1-2) and (2-1-2) pathways are kineti-cally indistinguishable. Experimental support for a multi-step mechanism primarily stems from two experimental areas non-linear Eyring plots and analysis of the electronic effects in the amine acceleration, whereas theoretical studies of... 

Analysis of the distribution of 4a-7a shows a definite preference for erythro arrangements in addition to the stereocontrol provided by the auxiliary compare n = 1 selectivity (Table 3) with selectivity in n = 2 telomers (Fig. 4). The temperature dependence of selectivity was studied and A(A//) and A(A5) were obtained. As expected, selectivity was greater at lower temperatures. Eyring plots for the three selectivities (addition of the second monomer to the n = 1 radical, allyl transfer... 

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. 

In a report describing acid-promoted Ritter reactions involving a-methylene-)5-hydroxyesters, an I -type process was found to be the preferred mechanism. For compound (104), both Ritter reaction products (105 and 106) are obtained. In order to rule out 5 2 or Sf 2 mechanisms, kinetic analysis was performed, kinetic isotope effects were evaluated, and both Hammett and Eyring plots were done. The mechanistic studies were consistent with an I -type process being preferred with initial formation of the oxonium cation, loss of water, and formation of the allylic acarbocation (107). DFT calculations indicated nucleophilic attack at the terminal carbon (107b, 5 10 was favored by about 2.6kcalmol over attack at the benzylic position (107a, 5 1) (Scheme 23). 

The strain rate dependence of the yield stress is shown at various temperatures in Fig. 20. To go further in the analysis, it is interesting to use the Eyring approach presented in Sect. 2.2.1.1. For this purpose, the ratio oy/T, K is plotted versus log( , s-1) at various temperatures in Fig. 21. A linear dependence is observed at each temperature, in agreement with the Eyring expression. However, the slopes show two different temperature regimes at low and high temperatures. Of course, the activation volume, Vo, directly related to the slope, reflects the change in behaviour, as shown in Fig. 22. At low temperature, the activation volume is small (around 0.1 nm3) and independent of temperature, whereas it increases rapidly above room temperature... 

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