Effect of Copper Salts on Amide Hydrothermal Formation and Reactivity

Abstract

Unlike ambient conditions, water at elevated temperature and pressure can increasingly favor dissolution of hydrophobic organic molecules, act as an acid/base catalyst, and trigger unique hydrothermal pathways. In a recent study, we found that amides could be readily synthesized from simple amines and carboxylic acids in hydrothermal solutions, which implied a potential peptide and biomolecule synthetic pathway in natural hydrothermal environments. Formation of amide or peptide bonds is critical in producing building blocks of peptides and proteins, which are essential to living organisms, such as extremophiles, in hydrothermal systems. However, amide formation and degradation pathways under hydrothermal conditions are still not well understood, particularly how these pathways would be influenced by surrounding minerals or dissolved metals is unknown. Here, we describe the effects of copper(II) salts, such as copper chloride and copper sulfate, on the hydrothermal formation and reactivity of amides at 250 °C and 40 bar (P sat ). The copper salts were chosen to be studied because they are commonly distributed in seawater and hydrothermal fluids. We found that the copper salts greatly inhibited the amide formation, with a decreased amide yield by up to 90% within 2 h. We also observed that aldehydes were quickly formed through the oxidation of amines by copper and became the dominant products. Compared to copper chloride/sulfate, copper acetate also facilitated the amine oxidation but did not suppress the amide formation, suggesting different roles of copper salts in amide hydrothermal synthesis. In addition, we found that hydrothermal reactivity of amides was strongly dependent on the solution pH, with lower reactivity at neutral pH than at very acidic or alkaline conditions. Our findings thus suggest that both copper and pH are important factors for amide hydrothermal synthesis and reactivity, which provide new insights into the prediction of amide or peptide bond synthesis in natural hydrothermal environments.

Anaerobic oxidation of aldehydes to carboxylic acids under hydrothermal conditions

“…Our recent research on alcohols, carboxylic acids, and amides have shown that organic oxidations can readily occur in the presence of metal salts such as Cu(II) under O 2 -absent hydrothermal conditions. [17][18][19] In those reactions, water serves as a green solvent, while Cu(II) acts as an efficient oxidant. The metal-promoted hydrothermal reactions also mimic natural geochemical processes on Earth, which provides the new "geomimicry" concept of using Earth-abundant metals as the oxidant/reductant for green organic reactions.…”

“…The metal-promoted hydrothermal reactions also mimic natural geochemical processes on Earth, which provides the new "geomimicry" concept of using Earth-abundant metals as the oxidant/reductant for green organic reactions. 18,20,21 In this study, we investigated the oxidation of aldehydes to carboxylic acids in an anaerobic and mild hydrothermal environment, using simple Cu(II) and non-toxic Fe(III) salts as the oxidizing agent. The optimal reaction condition and the substrate scope were both studied.…”

Hydrothermal Transformations of Alcohols with Copper(II) and Iron(III) Salts

“…For example, iron- and copper-bearing minerals such as pyrrhotite and chalcopyrite are commonly observed in deep-ocean hydrothermal systems, − where they can undergo dissolution to form metal ions combined with a variety of anions in the surrounding seawater. In addition, our recent work suggests that copper­(II) salts can act as an oxidant to trigger unique pathways for organic acids (e.g., oxidative decarboxylation), , and we hypothesize that copper­(II) and iron­(III) may also provide an oxidizing power for alcohol hydrothermal reactions. Herein, we investigate the effects of a suite of copper­(II) and iron­(III) salts on three model alcohol compounds including 1-phenylethanol, 2-phenylethanol, and 2-phenyl-2-propanol.…”

“…These starting organic and inorganic concentrations were similar to those in our previous copper salt studies. 23,33 After the starting materials were loaded, the oxygen and air inside the tubes were removed by three freeze−pump−thaw cycles prior to sealing with an oxyhydrogen flame under vacuum. The tubes were sealed, while the solution was frozen by liquid nitrogen.…”

“…Hydrothermal experiments were conducted following protocols developed from previous work. ,, In short, liquid alcohols were injected into fused-silica glass tubes (6 mm OD and 2 mm ID) using a gas-tight microsyringe, and 0.3 mL of copper or iron salt solution (0.2 molal in deionized and deoxygenated water) was subsequently added. The final alcohol concentration in solution was made to be 0.1 molal.…”