HCN Pentamers

Two pentamers of HCN have been isolated N-(aminomethylidene)diami-nomaleonitrile (10) (79JOC4532) and adenine (11). Several studies on prebiotic chemistry suggest HCN as a source for adenine and other purines (62M12  

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The formation of purines in interstellar space has been considered feasible for some considerable time. A theoretical study (using ab initio methods) on the mechanism of adenine formation from monocyclic HCN pentamers has been reported and has afforded a deeper insight into the gas-phase chemistry of possible purine syntheses. The authors drew the following conclusions from their results ... 

A HCN-pentamer, formed by reaction of N-(methylimino)acetonitrile and AMN was also suggested as precursor of adenine 1 (Scheme 3). 

The application of HCN to fine-chemical synthesis has been considered, particularly in the steroid field, and very elaborate and valuable chemicals are also prepared from HCN derivatives. In this prospect, oligomers of HCN. such as triaxine (an HCN t rimer), diaminomaleonitrUe (an HCN tetramer) and adenine (an HCN pentamer), are interesting e.xamples of derivatives for pharmaceutical synthesis. 

Glaser, R., Hodgen, B., Farrelly, D., 8c Mckee, E. (2007). Adenine synthesis in interstellar space Mechanisms of prebiotic pyrimidine-ring formation of monocyclic HCN-pentamers. Astrobi-ology, 7, 455. 

It is more difficult to conceive how the nucleic acid bases such as adenosine may be achieved in prebiotic syntheses. However, adenine, C5H5N5, corresponds in composition to a pentamer of hydrogen cyanide and could result by way of a trimer of HCN, 20, and the adduct of ammonia with HCN, 21 ... 

There have been extensive experimental and theoretical studies devoted to the structural and bonding characterization of weakly bound van der Waals complexes of acetylene. Structures of these complexes can often be determinated experimentally by means of Fourier transform microwave and infrared spectroscopic techniques. On the theoretical side, advanced treatments are required to understand the complex nature of the weak bonding in terms of the relative contributions of polarization and dispersion interactions, interactions of multiple moments, and electrostatic interactions involved in these completes. To determine the interaction energy in a weak complex, it is necessary to use large basis sets with the inclusion of electron correlation interactions. Theoretical calculations have been reported for van der Waals complexes of acetylene with COj [160], CO [161, 162], AICI3 [163], NH3 [164], He [165], Ar [166], H2O [167], HCN [168], HF [169-172], HCl [173, 174], and acetylene itself in the forms of non-covalent dimer [175-180], trimer [175,181], tetramer [175, 182, 183], and pentamer [175]. These calculations are very useful for the determination of multiple isomeric forms of the complex. For example, calculations at the MP2/6-31G level along with IR spectra indicate that the HCN-acetylene complex exists in a linear form in addition to the T-shaped structure observed previously by microwave studies (see Fig. 1-5) [168]. 

Fig. 2.3 Adenine will self-assemble from HCN and may be considered its pentamer.

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