Conjugative stabilization

The other necessary reaction for a BN to VN isomerization is a well precedented 1,5 H shift to convert the linearly conjugated substituted cyclopentadiene (LCC) into the cross conjugated cyclopenta-diene (CCC). The relative lability of BN relative to VN is thus a reflection of the stabilizing conjugation of the substituent in the vinyl isomers and the fact that the formation of LCC from BN is more favorable than the formation of CCC from the retro Diels Alder of VN. The relative energetics for all of these processes is represented in a combined reaction profile diagram shown in Figure 1. 

Heinemann, C., Mueller, T., Apeloig, Y. et al.. On the question of stability, conjugation, and aromaticity in imidazol-2-ylidenes and their silicon analogs, /. Am. Chem. Soc., 118, 2023,1996. 

The contrasting behavior of the suhonamides (71), which exist in bicyclic form, and the monocyclic dihydroazocines (64) must be attributed to the stabilizing conjugation provided by the 7-carboxylate group in (64) which would be absent in the bicyclic isomer. 

The above KIE determinations have been taken as providing evidence against the rate-limiting TS involving diffusion-controlled separation of the buffer acid from the Cj-carbanion/enaminc 459 resonance-stabilized conjugated base515,518. 

Pyrazole is a much weaker base than imidazole. Both have resonance-stabilized conjugate acids (Figure 14) but in the case of the pyrazolium cation 64 the direct linking of the quaternary nitrogens has a base-weakening effect. This additional effect also contributes to the low basicities of isothiazoles and isoxazoles (65 and 66). 

The pKa of the methylene protons of 7 is close to that of cyanacetic ester. Thus, it was interesting to compare the reactions of these two species. We found that 7 can be acylated by various carboxylic acid halides, leading to compound 11. According to IR and NMR data 11 exists mostly as the enol-form, due to the influence of the carboranyl substituent, which strongly stabilizes conjugated double bonds.13 Compound 11 is a useful synthon for the preparation of heterocyclic compounds. It can be methylated, chlorinated and acylated twice ( compound 13). Compound 13 gives an adduct with phenylhydrazine (scheme 6). 

A sulfonic acid (RSO3H) is a strong acid because it forms a weaK resonance-stabilized conjugate base on deprotonation (19.13). 

Essentially Flat Structure Bond Dissociation Energies Radical Stabilities Conjugation Stabilizes Best, Substitution Stabilizes Slightly Interaction Diagrams for Radical Species (Supplementary)... 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

The product l,4-dihydro-4-(2-thienyl)pyridazine (57) undergoes thermal rearrangement to l,4-dihydro-3-(2-thienyl)pyridazine (58), which is induced by stronger stabilization conjugation between the two rings. 

The base peak (76) in the mass spectrum of chaetocin represents the favoured loss of sulphur and water and splitting in half to give the stabilized, conjugated ion. The structure of chaetocin, which was shown by labelling experiments to... 

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