Enthalpy Michael addition

Table 6.3 Reaction enthalpies of the Michael additions studied.

If we compare the estimated enthalpies of formation (Table 1) for (Z,Z)-mucon-aldehyde (14a) and rroRj-cyclobutene-l,2-dicarboxaldehyde (see Eq. 4), we find that the isomerization to 22b is endothermic by 16.9 kcal/mol, about 11 kcal/mol more endothermic than isomerization to the 2f/-pyran 19. This plausibly explains the rapid Z,Z to Z,E isomerization coupled with very slow Z, to , isomerization in the absence of nucleophiles. Thus, the first step could be unimolecular thermal rearrangement, while the second step under metabolic conditions could be the reversible Michael addition-isomerization. Of course, both steps could result from attack by nucleophiles either in the medium or on biological macromolecules. 

In addition to the yield investigations, the reachon enthalpies were determined, as many Michael addihons exhibit considerable heat release when carried out rapidly [38]. Indeed, four of the reachons were found to be highly exothermic, while two are mildly endothermic (Table 6.3). 

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