The Missing Corners of the Square: ¹³C NMR Challenges in Monosquarate-Amides

Squaric acid, 3,4-dihydroxycyclobut-3-ene-1,2-dione, is a highly functionalised and electron-deficient four-membered carbocycle that provides a versatile platform for the preparation of structurally diverse derivatives. Conversion of squaric acid to squarate esters provides readily functionalised intermediates that undergo facile substitution with amines, enabling access to monosquarate-amides and bis-squaramides. These compound classes have attracted interest across synthetic, medicinal and supramolecular chemistry. Despite their relatively simple structural motifs, however, complete characterisation by routine ¹³C NMR spectroscopy can sometimes prove challenging, particularly for the quaternary carbon atoms of the core squaric acid ring.

In this study, the ¹³C NMR characterisation of representative monosquarate-amides is investigated using modified acquisition parameters designed to improve the detection and resolution of weak or poorly observed resonances. Particular attention is given to 3-((4-bromobenzyl)(methyl)amino)-4-ethoxycyclobut-3-ene-1,2-dione (1), a tertiary benzylamine-derived monosquarate-amide, for which conventional ¹³C NMR analysis provides limited resolution of the squarate carbon environments. Extended acquisition experiments are compared with routine spectra to assess the extent to which altered acquisition conditions can improve structural characterisation. A related aniline-derived monosquarate-amide is additionally examined as a comparative example.

To provide complementary structural confirmation, the previously undisclosed single-crystal X-ray diffraction structure of 1 is also presented. Together, these results highlight practical considerations in the ¹³C NMR analysis of monosquarate-amides and demonstrate the value of combining optimised spectroscopic methods with crystallographic characterisation where routine NMR data alone may be incomplete.