Synthesis and Characterization of CoII Macrocycles and its Use for Promoting Hydrolysis of Esters

The design and synthesis of hydrolytically active macrocycles mimic the substrate selectivity and rate enhancements for the hydrolysis of various organic substrates in high/low temperatures and extreme pH conditions, which is extremely challenging. In the present study, we synthesized two CoIIHMTAA-14 and CoIIHMTAA-16 macrocycles (HMTAA=hexamethyl-dibenzo-tetraaza-azulene) and used them to promote the hydrolysis of 4-nitrophenyl-2-benzamide carbonate and 4-nitrophenyl-4-benzamide carbonate esters. The effect of pH on hydrolysis of the carbonate esters was also studied at different pH 4.0, 6.5, and 8.5. The results of these studies showed that the reaction follows the first order with respect to ester concentration and is independent of the medium pH and water concentration. For the account of mechanism, hydrolysis of 4-nitrophenyl-2-benzamide carbonate and 4-nitrophenyl-4-benzamide carbonate proceeds either through oxygen or nitrogen intermolecular attack and normal H2O or OH- attacks, respectively. The plots of logkobs vs. pKa of the conjugate acid nucleophile showed leveling beyond pKa of about β = 0.3. The present macrocyclic complexes were found to provide enhanced hydrolysis of the esters.

One-step synthesis of imidazoles from Asmic (anisylsulfanylmethyl isocyanide)

Substituted imidazoles are readily prepared by condensing the versatile isocyanide Asmic, anisylsulfanylmethylisocyanide, with nitrogenous π-electrophiles. Deprotonating Asmic with lithium hexamethyldisilazide effectively generates a potent nucleophile that efficiently intercepts nitrile and imine electrophiles to afford imidazoles. In situ cyclization to the imidazole is promoted by the conjugate acid, hexamethyldisilazane, which facilitates the requisite series of proton transfers. The rapid formation of imidazoles and the interchange of the anisylsulfanyl for hydrogen with Raney nickel make the method a valuable route to mono- and disubstituted imidazoles.

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O)⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O)⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Multiphase buffer theory explains contrasts in atmospheric aerosol acidity

&lt;p&gt;Aerosol acidity largely regulates the chemistry of atmospheric particles, and resolving the drivers of aerosol pH is key to understanding their environmental effects. We find that an individual buffering agent can adopt different buffer pH values in aerosols and that aerosol pH levels in populated continental regions are widely buffered by the conjugate acid-base pair NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;/NH&lt;sub&gt;3&lt;/sub&gt; (ammonium/ammonia). We propose a multiphase buffer theory (Zheng et al., 2020, &lt;em&gt;Science&lt;/em&gt;) to explain these large shifts of buffer pH, and we show that aerosol water content and mass concentration play a more important role in determining aerosol pH in ammonia-buffered regions than variations in particle chemical composition. Our results imply that aerosol pH and atmospheric multiphase chemistry are strongly affected by the pervasive human influence on ammonia emissions and the nitrogen cycle in the Anthropocene.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Zheng, G., Su, H.*, Wang, S., Andreae, M. O., P&amp;#246;schl, U., and Cheng, Y.*: Multiphase buffer theory explains contrasts in atmospheric aerosol acidity, &lt;em&gt;Science&lt;/em&gt;, 369, 1374-1377, 2020.&lt;/p&gt;