Tautomerism and Atropisomerism in Free‐Base ( meso )‐Strapped Porphyrins: Static and Dynamic Aspects

We report herein some outstanding examples of atropisomerism and tautomerism in five ( meso ‐)strapped porphyrins. Porphyrins S0 – S4 have been synthesised, characterised and studied in detail by spectroscopic and spectrometric techniques, and their isomeric purity verified by HPLC analysis. In particular, they exhibit perfectly well‐defined NMR spectra that display distinct patterns depending on their average symmetry at room temperature: C 2 v , D 2 d , C 2 h , C 2 v , and D 2 h for S0 – S4 , respectively. NH tautomerism was evidenced by variable‐low‐temperature 1 H NMR experiments in [D 2 ]dichloromethane performed on S0 (Δ ${G{{{\ne}\hfill \atop {\rm 298K}\hfill}}}$ =48±1 kJ mol −1 ) and S1 (Δ ${G{{{\ne}\hfill \atop {\rm 298K}\hfill}}}$ =55±3 kJ mol −1 ), which has led to an understanding of the average spectra observed for the five porphyrins at room temperature. On the other hand, S2 and S3 are stable atropisomers at room temperature, easily separated and characterised, as a result of restricted rotation of their strapped bridges due to their high rotational barrier energies. Upon heating to 82 °C, they slowly equilibrate to a thermodynamic ratio of 64:36 in favour of the more stable S2 isomer. This atropisomerisation process was evidenced by 1 H NMR spectroscopy and monitored by HPLC, from which high rotational energy barriers of 115.2 (Δ ${G{{{\ne}\hfill \atop {\rm S2}\rightarrow {\rm S3}\hfill}}}$ ) and 116.9 kJ mol −1 (Δ ${G{{{\ne}\hfill \atop {\rm S2}\rightarrow {\rm S3}\hfill}}}$ ) were deduced.