Thermodynamics, of redistribution reactions

These ring-opening polymerizations are referred to as equilibration reactions. Since a variety of Interchange reactions can take place, a quantitative conversion of the tetramer to high polymer Is not achieved and there Is, at thermodynamic equilibrium, a mixture of linear and cyclic species present. Scheme III shows examples of the types of redistribution reactions thought to be occurring In these systems. It Is generally convenient to use D" to refer to a dlfunctlonal slloxane unit and M to refer to a monofunctional slloxane unit. Thus, D4 represents the cyclic slloxane tetramer and MM represents the linear hexamethyldlslloxane. 

The kinds of catalysts that have been used for these reactions are similar to those for Z i5-(3-indolyl)methanes [194-201]. In one study, I2 was found generally superior to Lewis acids [202]. This same study found evidence of redistribution in the case of unsymmetrical fr T-indolylmethanes. Specifically, reaction of indole-3-carboxaldehyde with 1-methylindole gave only the frw-(l-methyl-3-indolyl) methane and gave indole as a by-product, suggesting thermodynamic control and enhanced stability of the 1-substituted trimer. 

However, some of the recent experiments cast doubt on the applicability of this assumption. First, experiments done in the gas phase are few-body problems where taking the thermodynamic limit is not always appropriate. In other words, we have to take into account the fact that the size of the environment is finite. Second, initial states prepared by laser are so highly excited that the timescale for the energy redistribution would be comparable to that of the reaction. Third, the timescale for observing reactions can be much shorter than that for relaxation. Therefore, dynamical behavior of reactions should be studied without assuming local equilibrium. 

Given their extraordinary reactivity, one might assume that o-QMs offer plentiful applications as electrophiles in synthetic chemistry. However, unlike their more stable /tora-quinone methide (p-QM) cousin, the potential of o-QMs remains largely untapped. The reason resides with the propensity of these species to participate in undesired addition of the closest available nucleophile, which can be solvent or the o-QM itself. Methods for o-QM generation have therefore required a combination of low concentrations and high temperatures to mitigate and reverse undesired pathways and enable the redistribution into thermodynamically preferred and desired products. Hence, the principal uses for o-QMs have been as electrophilic heterodienes either in intramolecular cycloaddition reactions with nucleophilic alkenes under thermodynamic control or in intermolecular reactions under thermodynamic control where a large excess of a reactive nucleophile thwarts unwanted side reactions by its sheer vast presence. 

Fig. 6 Dynamic combinatorial peptide library that expioits enzyme reactions to control self-assembly processes under thermodynamic controi. (a) Emergence of the potentiai peptide derivatives of varying length in a library of interconverting molecules formed from the staring materials of Fmoc L/L2 system. Fmoc-Ls is preferentially formed. Corresponding AFM images of the fibrillar structures formed at 5 min after the addition of enzyme, and the sheet-like structures observed after 2000 h show that redistribution of the derivatives is accompanied by the remodelling from fibres (Fmoc L3) to sheet-like structures (Fmoc L5). (b) HPLC analysis of the composition of the system reveals the formation and the stabilisation of Fmoc-Ls over time. Modified from [21]...

Second, formation of weak bonds between substrate and enzyme also results in desolvation of the substrate. Enzyme-substrate interactions replace most or all of the hydrogen bonds between the substrate and water. Third, binding energy involving weak interactions formed only in the reaction transition state helps to compensate thermodynamically for any distortion, primarily electron redistribution, that the substrate must undergo to react. 

These recent experimental and theoretical results make it clear that control of selectivity depends on the kinetics rather than thermodynamics. It is interesting to speculate how one might improve the [4 + 2] addition selectivity of a Si(100)-(2 x 1) surface toward the diene systems. By replacing hydrogens with other appropriate groups, one may be able to alter the barrier for either the [2 + 2] reaction or the isomerization reaction. The former may control the initial distribution of surface products, while the latter may change the selectivity of the surface by thermal redistribution. 

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