Prebiotic-Like Condensations of Cyanamide and Glyoxal: Revisiting
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DOI:10.1002/chem.201601999 Full Paper & Prebiotic Chemistry Prebiotic-LikeCondensationsofCyanamideand Glyoxal: Revisiting Intractable Biotars Nieves Lavado,* Juan CarlosEscamilla, Martín valos, Reyes Babiano,* Pedro Cintas, JosØ Luis JimØnez, and Juan Carlos Palacios[a] Abstract: We report adetailed investigation into the nature in particularNMR and mass spectroscopy (ESI mode), which of products that are generated by the reactions of cyana- are all consistent with the generation of afew functional mide and glyoxal, two small molecules of astrochemicaland groupsthat are embedded into regularchains of five- and prebiotic significance, under differentexperimental condi- six-memberedrings, therebypointing to asupramolecular tions. The experimental data suggest that the formation of organization.Three different modelsofcross-condensation oligomeric structures is relatedinpart to the formation of and chain growth are suggested. These synthetic explora- insoluble tholins in the presence of oxygen-containing mole- tions provide further insightsinto the formation of complex cules. Althougholigomerization proceeds well in water, organic matter in interstellarscenarios and extraterrestrial productisolation turned out to be impractical. Instead, solid bodiesthat might have played apivotal role in chemical precipitates wereobtained easily in acetone. Crude mixtures evolution. have been thoroughly scrutinized by spectroscopic methods, Introduction withinthe contextofplanetary simulation experimentsthat lead to aerosol or solid deposits that contain organic constitu- Astrochemical observations of protoplanetary disks that sur- ents of variable chemical composition, usually with low C/N round distant stars along with space missions that targeted icy ratios.[8] However,these simulationsmay not reproduce the moonsand interstellar bodies provide enoughevidenceon actualconditions present on Titan or Jovian moons, for in- these huge chemical factories to suggestthat complex organic stance. Analysesconducted so far on such complexmixtures chemistry is ubiquitous beyondour tiny planet.[1,2] In such sce- show that the formation of prebioticmolecules, including tria- narios, the chemistry appears to be influenced or largelydomi- zine bases or amino acids, is not uncommon alongside linear nated by surface and solid-state reactions (e.g.,ondust and cyclic macromolecular structures(PAHs among others), in [3] grains), which are clearly different from processes that occur whichCH2,NH, or CN functionalities serve as growth units.If in gas-rich atmospheres or wet environments.[4] Aplethora of one leaves aside these astrochemical reactions, combinations neutralspecies and ions have been detected in the interstellar of prebiotic precursors do represent plausible routes to medium, which range from simple diatomic molecules to poly- modelsofearly evolution, as exemplified by Sutherland’s ami- atomic carbon-bearingspecies, such as glycolaldehyde, cyana- noxazole chemistry on the currently held view of the prebiotic mide, glycine, or low-mass alcohols.[5] Theterm tholin has formation of nucleosides and nucleotides.[9] That chemistry, become synonymouswith complex organic matter,and they linkedtoglycolaldehyde, cyanamide, and inorganic phospho- are often intractable mixtures.[6] In astrict sense,the term was rus, may subsequently triggerthe formation of nucleoside ana- first coined by Sagan and Khare in the late 1970s to describe loguesand other biomoleculesinthe presence of other small sticky brownish residuesthat were formedbyphotochemical precursors. irradiation or electrical discharges of cosmically relevant gas Prominent among prebiotic-like polymersare the so-called mixtures.[7] Accordingly,tholinsshould usually be defined HCN-oligomers, which have been the subject of numerous studies and structural proposals.[10] Heterooligomeric sequen- ces from two or more prebiotically credible speciesshould also [a] Dr.N.Lavado, Dr.J.C.Escamilla, Dr.M.valos, Dr.R.Babiano, yield other valuable, yet underestimated, polymers. The forma- Dr.P.Cintas, Dr.J.L.JimØnez, Dr.J.C.Palacios Departamento de Química Orgµnica eInorgµnica tion of such condensed phases would have served as asource FacultaddeCiencias-UEX and methodofaccumulationofprebiotic monomers. The pur- IACYS-Unidad de Química Verde yDesarrollo Sostenible pose of this article is to scrutinize in detail the chemical nature 06006 Badajoz (Spain) of the oligomeric species that are generated by reaction of E-mail:[email protected] [email protected] glyoxal and cyanamide in acetone, from which asolid material Supporting information and ORCID fromthe author for this article are can easily be isolated.Certainly,the direct condensations of cy- available on the WWW under http://dx.doi.org/10.1002/chem.201601999. anamide with electrophiles have long been known,although Chem. Eur.J.2016, 22,13632 –13642 13632 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper the resulting reactionmixturesare often too complex for con- and trimeric hydrates form above 1m concentrations.[19] Hy- clusive analysis. In general, the base-catalyzed reactionof droxylated dioxolane and dioxanestructures coexistinsolution glyoxal with amides affords 1,2-bisacylamino-1,2-ethanediols.[11] in an equilibrium with their acyclic derivatives, andtheir rela- Similar reactions, conducted in the late 1960s, between glyoxal tive importance can be assessed by IR spectroscopy.[20] Glyoxal, and formamide or sulfonamide derivatives gave rise to tetrahy- along with other aldehydes and sugar precursors, hasbeen de- droxypiperazines together with the corresponding N,N’-disub- tected recently in experiments that reproduce the photo/ther- stituted1,2-diamino-1,2-ethanediols (Scheme 1).[12] However, mochemical conditions of cometary ice analogues.[21] More- this protocol was not generalenoughand someamides failed over,cyanamide can undergo hydrolysis, although this appears to give the expectedadducts. As noted by the authors:“trifor- to be arelatively slow process, which would only be feasible mamidomethane,cyanamide, cyanacetamide, glycolamide, for- through its carbodiimide tautomer.[22] Also, cyanamide can un- mamidoxime, sulphamide, and ethanesulphonamidedid not dergo dimerization and cyclotrimerization reactions, often give solid precipitates on treatment with basic aqueous glyoxal, under hydrothermalconditions.[23,24] Cyanamide dimerizes to and evaporation of the solutions gave dark resinous residues give 2-cyanoguanidine (dicyandiamide) and stabilizes itself as which were not further examined”. However,fused piperazines acyclic trimer (melamine). The latter can hydrolyze to cyanuric as well as tetra-aza-5,10-dioxaperhydroanthracenes could be acid. Dicyandiamide can eliminate ammonia to give dicyana- identified later in the reactions of glyoxal and ethylenedia- mide. Cyanuric acid can aminolyzetoafford triuret;dicyandia- mines (Scheme 1).[13] Structurally related compounds arise from mide can hydrolyze to give biuret. Melamine, cyanuricacid, the condensation of a-diketones with ethanolamines, which biuret, and triuret are likely to be components of “biotars”, lead to trans-bis(morpholino) and trans-morpholino-dioxane which mightbecaged in the polymers. All in all, the existence rings.[14] of water molecules renders acomplex scenarioinwhich multi- ple equilibria may be involved. The coexistence of acetone molecules in Earth-like scenarios mighthave taken place after cometary impacts or further chemicalevolution. Acetone belongs to one of the four organ- ic molecules (the others being acetamide, methyl isocyanate, and propanal) that were recently detected on comet 67P/Chur- yumov–Gerasimenko by the Rosetta mission,which have never beforebeen observed on acomet.[25] Results and Discussion Syntheses and structural clues Scheme1.Ring patterninginreactionsofglyoxal with amines and amides. Different isomers (either cis-ortrans-fused) can be generated in piperazine– With the aim of simulating conditions akin to aprimeval terres- oxazineand bis(piperazine) heterocycles. trial scenario, we conducted preliminaryscreenings by mixing equimolar amountsofcyanamide and glyoxal (as 40 %aque- Glyoxal(as hydrate) reacts with methylamine in aerosol ous solution)inneat water.That mixture did not afford any phases (clouddroplet evaporation), which leads to high-molec- precipitate after prolongedtime, although monitoring by 13 ular-mass oligomers that contain 1,3-dimethylimidazol-5-ones CNMR spectroscopy (D2Osolutions) showed carbon peaks, that are substituted with formyl and N-methylglycineside which were absent in acontrol experiment with glyoxal alone chains. This oligomerization, which appears to be relevantin and suggest the formation of condensation products.Inpartic- the formationoforganic aerosols, occurs by repeated addition ular,quaternary peaks between 160 and 167 ppm presumably of imine units.[15] In arelatedcontext,the prebiotic-like con- point to carbonyl and imine derivatives, one carbon peak at densation of glycolaldehyde and cyanamide, reported by Su- around121 ppm would be consistent with the nitrile function- therland et al.,gave rise to the formation of complex mixtures, ality,and upfield shifts below 100 ppm could be attributed to althoughcontemporary NMR experiments did shed light into hemiacetalic or N C Ocarbons (in the range 93–80 ppm) and À À their putative composition with the identification of some key CH O peaks ( 65 ppm), which arise from the glyoxal core. 2À À products (e.g.,nucleoside analogues, as noted above).[16] The reactionmixture evolvesslowly