Two modes of peri-interaction between an aldehyde group and a carboxylate anion in naphthalaldehydate salts

Gizem Saritemur,Simon J Coles,Mateusz B Pitak,Laura Nomen Miralles,John D Wallis,Deborah Husson

doi:10.1039/c5ce02282g

Gizem Saritemur, Simon J Coles + Show 4 more

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https://doi.org/10.1039/c5ce02282g

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Abstract

The naphthalaldehydate anion shows two modes of interaction between its functional groups: either a carboxylate oxygen atom makes an n–π* interaction with the aldehyde carbon atom, or the aldehyde hydrogen atom is directed to the face of the carboxylate group.

Highlights

Carbonyl/carbonyl interactions of type C O⋯C O, in which a sp[2] oxygen lone pair impinges on a carbonyl carbon, can be regarded as n–π* interactions and have been proposed to stabilise the conformations of proteins,[1] polymers[2] and small molecules such as substituted N-acetyl-proline methyl esters 1 (Scheme 1).[3,4] The interaction leads to a small pyramidalisation of the carbonyl carbon involved
The O⋯C distances from neutral oxygen observed in these interactions are typically >2.8 Å and calculations suggest stabilisations varying from 0.9 kcal mol−1 in aspirin to about 0.27 kcal mol−1 between two amide groups
Given the weak nature of interactions involving neutral oxygen, we decided to investigate the electrophile/nucleophile interactions involving the more nucleophilic negatively charged oxygen, and here we report the structures of a series of salts of 1,8-naphthalaldehydic acid (8-formyl-1-naphthoic acid) in whose anion a negatively charged carboxylate group could make a close approach to an aldehyde group

Summary

Introduction

Carbonyl/carbonyl interactions of type C O⋯C O, in which a sp[2] oxygen lone pair impinges on a carbonyl carbon, can be regarded as n–π* interactions and have been proposed to stabilise the conformations of proteins,[1] polymers[2] and small molecules such as substituted N-acetyl-proline methyl esters 1 (Scheme 1).[3,4] The interaction leads to a small pyramidalisation of the carbonyl carbon involved. The longer N⋯C separations observed for interactions with esters and amides can be considered as n–π* interactions corresponding to early stages in bond formation and are associated with small pyramidalisations of the carbonyl carbon towards the nucleophile Such an interaction may be the initiation for further chemical reactions. Given the weak nature of interactions involving neutral oxygen, we decided to investigate the electrophile/nucleophile interactions involving the more nucleophilic negatively charged oxygen, and here we report the structures of a series of salts of 1,8-naphthalaldehydic acid (8-formyl-1-naphthoic acid) in whose anion a negatively charged carboxylate group could make a close approach to an aldehyde group Such interactions would be the anionic equivalent of the n–π* interactions between carbonyl groups described above. The parent acid adopts a cyclic structure in the solid and solution state, so that in this case the groups have interacted fully and formed a bond (Scheme 3)

Discussion

Conclusion