Issue 12, 2024

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate (‘TBAT’)

Abstract

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.

Graphical abstract: Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate (‘TBAT’)

Supplementary files

Article information

Article type
Edge Article
Submitted
29 okt 2023
Accepted
06 dec 2023
First published
07 dec 2023
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2024,15, 4331-4340

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate (‘TBAT’)

M. M. Kucharski, A. J. B. Watson and G. C. Lloyd-Jones, Chem. Sci., 2024, 15, 4331 DOI: 10.1039/D3SC05776C

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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