Superelectrophilic Activation of Phenylglyoxamides: Efficient Synthesis of Triarylacetamides and Fluorenecarboxamides by Superacid Catalysis

Superelectrophilic Activation of Phosphacoumarins towards Weak Nucleophiles via Brønsted Acid Assisted Brønsted Acid Catalysis

The electrophilic activation of various substrates via double or even triple protonation in superacidic media enables reactions with extremely weak nucleophiles. Despite the significant progress in this area, the utility of organophosphorus compounds as superelectrophiles still remains limited. Additionally, the most common superacids require a special care due to their high toxicity, exceptional corrosiveness and moisture sensitivity. Herein, we report the first successful application of the "Brønsted acid assisted Brønsted acid" concept for the superelectrophilic activation of 2-hydroxybenzo[e][1,2]oxaphosphinine 2-oxides (phosphacoumarins). The pivotal role is attributed to the tendency of the phosphoryl moiety to form hydrogen-bonded complexes, which enables the formation of dicationic species and increases the electrophilicity of the phosphacoumarin. This unmasks the reactivity of phosphacoumarins towards non-activated aromatics, while requiring only relatively non-benign trifluoroacetic acid as the reaction medium.

Superelectrophiles: Recent Advances

Superelectrophiles are reactive species that often carry multiple positive charges. They have been useful in numerous synthetic methods and they often exhibit highly unusual reactivities. Recent advances in superelectrophile chemistry are discussed in this review.

Thermal decomposition of syn- and anti-dihydropyrenes; functional group-dependent decomposition pathway

Syn and anti dihydropyrene (DHP) are excellent thermochromes, and therefore extensively studied for their thermochromic and photochromic properties, respectively. However, they suffer from thermal decomposition due to thermal instability. In this study, we thoroughly investigated pathways for the thermal decomposition of anti- and syn- dihydropyrenes through computational methods. The decomposition pathways include sigmatropic shift and hemolytic and heterolytic (cationic and anionic) cleavages. The decomposition pathway is influenced not only by the dihydropyrene (syn- or anti-) but also by the functional groups present. For anti-dihydropyrenes, sigmatropic shift is the most plausible pathways for CN and CHO internal groups. The cascade of sigmatropic shifts is followed by elimination to deliver substituted pyrenes. For CH₃- and H- dihydropyrenes, hemolytic cleavage of the internal groups is the most plausible pathway for decomposition to pyrenes. The pathway is changed to heterolytic cleavage when the internal groups on the dihydropyrenes are Cl^-, Br^-, and SMe^-. Comparison of the activation barriers for syn (30.18 kcal mol^-1) and anti (32.10 kcal mol^-1) dimethyldihydropyrenes for radical pathway reveal that decomposition of syn- DHP is more facile over anti-, which is consistent with the experimental observation. The decomposition pathway for syn-dihydropyrene is also hemolytic in cleavage when the internal groups are methyl and hydrogen. Syn-dihydropyrenes (symmetrical or unsymmetrical) bearing CN group do not follow sigmatropic shift, quite contrary to the anti-dihydropyrene. The lack of tendency of the syn-dihydropyrene for sigmatropic shift is rationalized on the planarity of the scaffold. The results of the theoretical study are consistent with the experimental observations. The results here help in understanding the behavior of substituents on the dihydropyrene scaffold, which will be useful in designing new molecules with improved thermal stabilities. Graphical abstract Functional group dependent decomposition pathways of dihydropyrenes.