Isomerization irreversible

The occurrence of the reaction can however be detected by N.M.R. spectroscopy (for a detailed discussion of the observed spectra, see Doering and Roth, 1963). It is estimated that 3,4-homotropilidene undergoes this rearrangement about 1000 times per second at 180° C and about once per second at — 60° C. These data require further verification since they lead to highly improbable values for the A factor and for the energy of activation of the reaction. At much higher temperatures the compound isomerizes irreversibly to tetrahydropentalene ... 

In reality, two compounds are formed,52 the five- and a six-membered rings shown to arise from the E and Z forms of the O-trimethylsilylated derivative of the starting acylhydrazine.53 Moreover, the six-membered chelate isomerized irreversibly upon heating into the five-membered chelate.54 The six-membered chelate has also been shown to exhibit a trigonal bipyramidal silicon atom.55... 

The initially obtained cw-oxaphosphetane can subsequently isomerize irreversibly to a /rarw-oxaphosphetane with the rate constant kAift x [ktmm (ktram + As)] (Figure 11.3). This isomerization is referred to as stereochemical drift. 

In its literal form, this reaction is only of academic interest because a molecule is unlikely to break up or isomerize irreversibly in two or more different ways. However, situations frequently encountered in practice are those of multistep parallel first-order decomposition reactions and of parallel reactions that involve coreactants but are pseudo-first order in the reactant A. An example of the first kind is dehydrogenation of paraffins, examples of the second kind include hydration, hydrochlorination, hydroformylation, and hydrocyanation of olefins and some hydrocarbon oxidation reactions. All these reactions are multistep, but the great majority are first order in the respective hydrocarbon, and pseudo-first order if any co-reactant concentration is kept constant. 

As a variation of the process according to Eq. (2), the process of N-protonation of N-silylmethylimines and subsequent desilylation also works well. N-Protonated iminium intermediates are first formed and their desilylation by the counter ion (X ) leads to N-unsubstituted azomethine ylides of nonstabilized types 32. Although the N-unsubstituted azomethine ylides 32 would be able to isomerize irreversibly into the corresponding N-methylimine tautomers, these unusual azomethine ylides show a remark-... 

As has been stated above, the trans-(CH)x is thermodynamically more stable than the cis isomer, and hence the latter isomerizes irreversibly to... 

All doubts as to the correctness of the structure of lysergic acid were removed by a synthesis first of all of rac-6,8-dimethylergoline and then rac-dihydrolysergic acid but the comparisons were made difficult because of the production of isomers (three asymmetric centres are involved). The early approaches to the synthesis of lysergic acid itself were frustrated by its tendency under acidic conditions to isomerize irreversibly into the isomeric naphthalene . A synthesis was finally accomplished in a simple elegant manner in which the key concept was the introduction of the indolic double bond as the last step (Chart 4.7). 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

The aza-Cope/Mannich reaction takes advantage of the facility with which a y,<5-unsaturated itninium ion, such as 6, participates in a [3,3] sigmatropic rearrangement to give an isomeric species which is suitably functionalized for an intramolecular and irreversible Mannich cyclization (see intermediate 7). The aza-Cope rearrangement substrate 6 is simply an unsaturated iminium ion which can be fashioned in a number of ways from a homoallylic... 

What quantity would Furchgott s irreversible antagonist method (Section 1.6.4) estimate if the occupied receptor, AR, must first isomerize to a second form, AR, which then attaches to another entity, such as a G-protein, in order to elicit a response (as in Eq. (1.38)) Assume that the G-protein is present in great excess in relation to the receptors. 

Secondly, in view of the generally accepted mechanism for the sulfur extrusion reaction, which involves valence isomerization of the thiepin ring to its corresponding thianorcaradiene (benzene episulfide) isomer followed by irreversible loss of sulfur,... 

This bi-exponential behavior confirms the presence of reversible isomerization steps coupled with irreversible degradation steps and accounts for the role of the di-cis isomers as reaction intermediates, according to the general reaction scheme presented in Figure 12.1. The dependence of the rate constant of each elementary step on temperature allowed the calculation of the respective activation... 

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