N-Heterocyclic carbene stabilised fluoroboranes for 18 F PET imaging: promoter-free labelling and water stability Anna Booth, B. Cornelissen, S. Faulkner, S. Aldridge University of Oxford, UK Positron Emission Tomography (PET) is an important imaging modality in modern medicine and requires new tracers in order to increase its scope and specificity. Traditional routes to create 18 F tracers often require extensive optimisation to make their multistep syntheses viable. Hence new approaches have instead utilised boron-fluoride coordination chemistry as a route to fast, single step labelling at a late-stage. 1,2 These new B–F based tracers are, however, susceptible to hydrolysis. 1,2 A range of systems have consequently been explored to limit this hydrolytic instability with the recently reported N-heterocyclic carbene boron trifluoride adducts (NHC-BF 3 , Figure) showing unprecedented stability in aqueous solutions. 3,4 These NHC-BF 3 adducts do however require the presence of a Lewis Acid promoter in order to undergo 19 F– 18 F isotopic exchange radiolabelling. Moreover, the radiolabelled 18 F-products are inseparable from their 19 F-precursors, thus limiting their efficacy. In view of this, we have investigated carbene stabilised boranes (with a selection of ligand frameworks around the boron) as potential precursors for 18 F-PET imaging. Herein we report the synthesis of a library of new NHC stabilised catecholborane systems. Promoter-free fluoride binding is achieved by 1 and its analogues resulting in fluoroboranes with remarkable water stability. Systematic variation of the carbene donor and ligand framework is used to tune the rate of fluoride binding by these complexes, in addition to their water stability. Thus, these optimisations allow rapid, promoter-free fluoride binding to be achieved with a bench-stable complex.
References 1. D. M. Perrin, Acc. Chem. Res., 2016, 49, 1333–1343. 2. K. Chansaenpak, B. Vabre and F. P. Gabbaï, Chem. Soc. Rev., 2016, 45, 954–971.
3. K. Chansaenpak, M. Wang, Z. Wu, R. Zaman, Z. Li and F. P. Gabbaï, Chem. Commun., 2015, 51, 12439– 12442. 4. K. Chansaenpak, M. Wang, H. Wang, B. C. Giglio, F. P. Gabbaï, Z. Wu and Z. Li, RSC Adv., 2017, 7, 17748–17751.
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