Formation of Cyanamide–Glyoxal Oligomers in Aqueous Environments Relevant to Primeval and Astrochemical Scenarios: A Spectroscopic and Theoretical Study

Abstract

The condensation of cyanamide and glyoxal, two well-known prebiotic monomers, in an aqueous phase has been investigated in great detail, demonstrating the formation of oligomeric species of varied structure, though consistent with generalizable patterns. This chemistry involving structurally simple substances also illustrates the possibility of building molecular complexity under prebiotically plausible conditions, not only on Earth, but also in extraterrestrial scenarios. We show that cyanamide-glyoxal reactions in water lead to mixtures comprising both acyclic and cyclic fragments, largely based on fused five- and six-membered rings, which can be predicted by computation. Remarkably, such a mixture could be identified using high-resolution electrospray ionization (ESI) mass spectrometry and spectroscopic methods. A few mechanistic pathways can be postulated, most involving the intermediacy of glyoxal cyanoimine and further chain growth, thus increasing the diversity of the observed products. This rationale is supported by theoretical analyses with clear-cut identification of all of the stationary points and transition-state structures. The properties and structural differences of oligomers obtained under thermodynamic conditions in water as opposed to those isolated by precipitation from organic media are also discussed.

Synthesis of CxNy-rich polycyclic oligomers from primeval monomers in aqueous media

“…For other oligomeric species, however, their formation can be rationalized by the iterative condensation of cyanamide and glyoxal units with removal of water (Table 2, formulae marked in red), in agreement with previous synthetic patterns. 25,26 Examples are shown in Fig. 4 with prevalence of cyanamide units, which are otherwise consistent with the starting stoichiometries employed.…”

“…In principle, the direct and repetitive addition of cyanamide to glyoxal gives rise to open-chain structures susceptible to undergo cyclization to either 2-amino-2-oxazolines or imidazolines. Theoretical calculations predict lower barriers for ring closure to oxazolidines, 26 thereby accounting for the incorporation of oxygen into mass spectra (Table 2). The reaction of neighboring hydroxy and amino groups with another molecule of glyoxal would render six-membered rings, both morpholine and piperazine structures, often fused to the above-mentioned pentagonal heterocycles.…”

“…For lower cyanamide-molar stoichiometries (1 : 1 and 1 : 2), oxygen-rich oligomers are obtained with an obvious increase in the incorporation of cyanamide starting from 2 : 1 mixtures. 25,26 It is pertinent to mention that intense m/z peaks containing glyoxal result from exchange reactions with ammonia (Table 2, highlighted in red). As pointed out earlier, both hydrolysis and deamination of cyanamide are performed under thermal conditions, 23,24 and older cyanamide addition procedures were reported in aqueous ammonia solutions as well.…”

“…On the other hand, tertiary peaks in the 110-75 ppm range are the most abundant signals in reactions between cyanamide and glyoxal. 25,26,39 For high cyanamide ratios, most peaks concentrate in the 74-64 ppm range (Fig. 6) and can be assigned to N-CH-N bonds.…”

“…24 Our contributions to cyanamide oligomerization under milder aqueous conditions have focused on cyanamide-glyoxal heterooligomers, obtained at 1 : 1, 1 : 2 and 2 : 1 molar ratios. 25,26 Using acetone as the antisolvent, precipitates containing oligomeric mixtures could be isolated. In water, however, supramolecular networks generated through self-assembly remain soluble, although using 2 : 1 cyanamide-glyoxal stoichiometries, brownish to black powders were obtained in low yields, in line with previous observations of unidentified resinous solids in similar reactions.…”