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Enhanced synthesis of oxo-verdazyl radicals bearing sterically-and electronically-diverse C3-substituents

Fuller, RO ORCID: 0000-0003-3926-8680, Taylor, MR, Duggin, M, Bissember, AC ORCID: 0000-0001-5515-2878, Canty, AJ, Judd, MM, Cox, N, Moggach, SA and Turner, GF 2021 , 'Enhanced synthesis of oxo-verdazyl radicals bearing sterically-and electronically-diverse C3-substituents' , Organic and Biomolecular Chemistry, vol. 19, no. 46 , pp. 10120-10138 , doi: 10.1039/d1ob01946e.

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Abstract

The synthetic viability of the hydrazine- and phosgene-free synthesis of 1,5-dimethyl oxo-verdazyl radicals has been improved via a detailed study investigating the influence of the aryl substituent on tetrazinanone ring formation. Although it is well established that functionalisation at the C3 position of the tetrazinanone ring does not influence the nature of the radical, it is crucial in applications development. The synthetic route involves a 4-step sequence: Schiff base condensation of a carbohydrazide with an arylaldehyde, alkylation, ring closure then subsequent oxidation to the radical. We found that the presence of strong electron-donating substituents and electron rich heterocycles, result in a significant reduction in yield during both the alkylation and ring closure steps. This can, in part, be alleviated by milder alkylation conditions and further substitution of the aryl group. In comparison, more facile formation of the tetrazine ring was observed with examples containing electron-withdrawing groups and with meta- or para-substitution. Density functional theory suggests that the ring closure proceeds via the formation of an ion pair. Electron paramagnetic resonance spectroscopy provides insight into the precise electronic structure of the radical with small variations in hyperfine coupling constants revealing subtle differences.

Item Type: Article
Authors/Creators:Fuller, RO and Taylor, MR and Duggin, M and Bissember, AC and Canty, AJ and Judd, MM and Cox, N and Moggach, SA and Turner, GF
Keywords: oxo-verdazyl radicals, heterocyclic chemistry
Journal or Publication Title: Organic and Biomolecular Chemistry
Publisher: Royal Soc Chemistry
ISSN: 1477-0520
DOI / ID Number: 10.1039/d1ob01946e
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© The Royal Society of Chemistry 2021

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