Matrix isolation of a reactive o-quinoid compound - 6-methylene-2,4-cyclohexadien-1-imine

Aza-ortho-Quinone Methide and Its Conjugated Acid: Reactivity, Stability and Acidity

Aza-o-quinone methides and their conjugated acids are reactive drug metabolites that might react with nucleophilic sites of DNAs and proteins to cause cancer or immune responses, and their reactivity with water is the key information to judge if these metabolites are harmful in living systems. For the first time, aza-o-quinone methide (1) and its conjugated acid (2) are observed by laser flash photolysis, and their reactivity, stability and acidity in water are determined.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Synthetic exploitation of halogenated alkenes containing EWG: benzotriazole-mediated synthesis of benzoazetines and thermal transformations

The stable benzoazetines (5a–d) based on nitrovinyl moieties have been synthesizedviatransamination of bisbenzotriazolyl derivatives (2a–c).

Substituent effects in the interconversion of phenylcarbene, bicyclo[4.1.0]hepta-2,4,6-triene, and 1,2,4,6-cycloheptatetraene

The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functional theory. It is found that substituents have a large effect on both the thermochemistry and activation energy of these rearrangements. For instance, para-substitution yields a range of overall activation energies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH(2) and NO(2) substituents, respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTE species are more exothermic than that of the parent PC. The proximity of the substituent to the carbene center can also affect the overall chemistry as in the case of ortho-substituted species. Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures, can compete with the ring-expansion process. Also, as calculated herein, the ortho substituents can, by a combination of mesomeric and steric interactions with the carbene center, affect the overall barrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens (F and Cl) raise the activation barrier to ring expansion by approximately 5 kcal/mol. This is reminiscent of the effect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.

Computational study of the electronic structure of substituted phenylcarbene in the gas phase

Gas-phase calculations for ortho-, meta-, and para-substituted phenylcarbenes demonstrate that aromatic ring substituents can have a large effect on the singlet-triplet splitting in these molecules. For example, p-aminophenylcarbene is predicted to have a singlet-triplet splitting (DeltaE(ST)) of 0.7 kcal/mol, which is 4.7 kcal/mol smaller than that of the parent phenylcarbene. However, when the mesomeric effect of pi-donation is removed, as in the meta-substituted molecules, this affect is greatly attenuated. m-Amino substitution lowers the DeltaE(ST) to 5.2 kcal/mol, a change of only 0.2 kcal/mol from the parent phenylcarbene. The sensitivity of phenylcarbene to substitution is judged by the slope of the line correlating the singlet-triplet gaps to linear free energy (LFE) parameters. In this study, the best linear correlation for DeltaE(ST) in the para-substituted phenycarbenes is observed when plotted against sigma(p)(+). The resultant slope (rho) is 5.0. The origin of this effect is preferential interaction of aryl substituents with the singlet rather than the triplet species as demonstrated by electron density analyses.