Reversible Intramolecular Chemical Processes

substitute the value of the total rate constant k,. at coalescence [4.44 Avq, from Eq. (10.6)] intoEq. (10.2)  

One classic example of valence isomerization is bullva-lene  

This unique structure has a C3 axis (Section 4.1), and its hydrogens (as well as its carbons) constitute an AA A B-B B CC C D spin system. At low temperature ( -85°C) nothing extraordinary occurs, and the 100-MHz H spectrum shows four complex multiplets centered at 5 2.07 (H ), 2.13 (H(j), 5.62 (H, ), and 5.70 (H ). But as the temperature of the sample is raised, these multiplets broaden and move together. At 15°C the spectrum coalesces to one extremely broad 

The way in which all 10 hydrogens (or carbons) in bullva-lene become equivalent involves a series of degenerate (i.e., leading to an equivalent structure) isomerizations known as Cope rearrangements (or [3.3] sigmatropic shifts). One such rearrangement is shown below (the dot is there to keep track of Cj)  

Note how this particular bond reorganization of two n bonds and one cyclopropane bond interconverts three pairs of hydrogens (and the attached carbons) Ha with Hc, Ha with Hc , and Ha. with Hd. Repetition of this rearrangement with other sets of k and cyclopropane bonds ultimately renders all 10 hydrogens (and carbons) equivalent. 

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Electroswitching of structure takes place when a redox change induces a reversible structural or conformational process in a molecule, such as an electrochemically activated intramolecular rearrangement [8.259]. On the supramolecular level it consists of the electroinduced interconversion between two states of different superstructure. A case in point is the reversible interconversion of a double-helical dinuclear Cu(l) complex M2L22+ [8.260] and of a mononuclear Cu(ll) complex ML2+ in a sequential electrochemical-chemical process [8.261] ... 

At more cathodic potentials another quasi-reversible wave (-0.82V) that may be attributed to a one-electron reduction of the NJ -dimcthyl-2,2 -bipyridinium moiety is observed together with an irreversible second reduction rc ox process at -1.29V. The irreversible redox behaviour of simple NJ>T-dialkyl-2,2 -bipyridinium molecules has previously been attributed to the reduced species possessing a non-planar conformation which can undergo an intramolecular chemical reaction [10]. 

Development of such a catalytic cascade process requires a stable and electron-rich carbon species as a nucleophile, which should be compatible with electrophihc aldehyde functionality in one of the chemical entities 144 (Schane 1.53) [90]. Undesired reaction of 144 with the catalyst to produce an iminium or aiamine could significantly complicate the cascade process. Potentially, the iminium 144a could undergo reversible intramolecular cyclopropanation and thus slow down the desired cascade process. Moreover, the enamine 144b could participate in the Michael reaction with iminium 144c. 

The reversal of the thermal decomposition of 6 to ethylene and vinylacetylene cannot be utilized to generate 6, since, according to a quantum-chemical analysis, the reaction is slightly endergonic and requires a large free activation enthalpy (0.9 and 42 kcal mol-1, respectively) [59]. The intramolecular variant of this process as well as the addition of typical dienophiles of the normal Diels-Alder reaction to divinylace-tylenes will be discussed at the end of Section 6.3.3. 

In previous sections we have shown clearly that intramolecular dihydrogen bonds X-H- H-Y, with X and Y representing various chemical elements, can exist in both the solid state and in solution. In addition, the bonds can be a critical factor in the control of molecular conformational states or effects on rapid and reversible hydride-proton exchanges related to the process shown in Scheme 5.1, or the well-known H-D isotope exchanges in similar subsystems [23]. Such bonds could also play an important role in the stabilization of transition states, appearing as a reaction coordinate in many transformations. This is particularly... 

Unimolecular reactions with thermal, optical, or chemical activation are governed by a competition between intramolecular isomerization, dissociation, or the reverse association (or recombination) processes, and intermolecular energy transfer in collisions. In addition to these traditional unimolecular reactions, many other reaction systems may be considered from a unimolecular point of view when a particular intramolecular event can be separated from preceding or other subsequent processes. Following this more general use of the term, unimolecular reaction rate theory has found a quite general application, and has been harmonized with other theories of reaction dynamics. 

Isolated-molecule dynamics is expected to be a sufficiently elementary process to permit observation of microscopic reversibility in the dynamics and, hence, to display a dependence of the outcome of dynamics on initial conditions. This dependence is desirable since the ability to retain information about initial conditions is necessary in order to achieve the technologically desirable goal of externally influencing chemical reactions. However, a great many experiments, perhaps with insufficiently well-characterized preparation and measurement, have indicated that time-irreversible relaxation is a useful model for many intramolecular processes. Thus, isolated-molecule intramolecular dynamics serves as a laboratory for the study of the inter-relationship between irreversible relaxation behavior in systems that are fundamentally de-scribable by time-reversible equations of motion. It also presents an experimental challenge to prepare sufficiently well-characterized states to observe time reversibility and sensitivity to initial conditions. 

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