A Ferrocenyl Kaleidoscope: Slow Interconversion of Six Diastereo-atropisomers of 2,6-Di-tert-butyl-9,10-diferrocenyltriptycene
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The title triptycene, 6, has been isolated as the product of 9,10-cycloaddition of benzyne to 9,10-diferrocenyl-2,6-di-tert-butylanthracene, 5, whose X-ray crystal structure is reported. Each ferrocenyl unit in 6 has access to the same three non-equivalent molecular environments, and their rotations relative to the molecular paddlewheel give rise to six slowly interconverting atropisomers. Their dynamic behaviour in solution is a challenging NMR puzzle that can be successfully solved by taking advantage of the recently described very large diamagnetic anisotropy of the ferrocenyl moiety, together with the C2 symmetry of particular atropisomers. Application of one- and two-dimensional NMR techniques over a range of temperatures together, with a detailed analysis of the homo- and heteronuclear correlations in 6, resulted in unequivocal mapping of the 99 (1)H and 162 (13)C positions in the six interconverting systems. Variable-temperature 2D-EXSY measurements revealed that, while the stability of the atropisomers is almost identical, they are separated by energy barriers which the ferrocenyls must overcome in the course of their interconversions. The heights of two different rotational barriers have been identified and these experimental findings are in good agreement with DFT calculations.
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