From bench-stable carbenes to Blatter-type radicals Matthew S. Smith, Jacob Murray and AnnMarie O’Donoghue Durham University, UK
Nitron is a commercially available 1,2,4-triazolium accessible for over a century and normally employed as a gravimetric reagent for nitrate detection. Recently, a range of new potential applications of Nitron have been highlighted, including as a N -heterocyclic carbene catalyst or as a metal-ligand, however, there has been minimal synthetic exploration of this scaffold. 1 We recently reported the unusual rearrangement of Nitron and four derivatives to new Blatter-type persistent radicals including 1 and 2 , with C(3)-N substitution difficult to obtain using previously reported methodologies. 2 These radicals have applications in functional organic materials, quantum computing and medicinal chemistry. 3 In particular, substantially enhanced characteristics of radicals 1 and 2 as switchable signal enhancement agents for NMR spectroscopy were highlighted very recently. 4 Prior to our work, only the synthesis of the parent Nitron was reported, 5 which we used to access derivatives with simple alkyl substituents. 2 We now report an improved synthesis to Nitron, which permits access to a broader range of derivatives encompassing both strong electron donating and withdrawing substituents. The range of Nitron derivatives were evaluated in the Nitron to Blatter radical rearrangement, with synthetic evidence for radical formation being highly dependent on the zwitterion to carbene equilibrium position. Exploration of proton transfer dynamics and DFT computational analysis have provided insight into the key role of tautomerism in Blatter radical formation. Determination of NH and C(3)-H p K a s enabled access to p K T values, which could be correlated with extent of radical formation. This work sheds new light on a historically unexplored triazolium class.
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