Developing organoboranes as phase transfer catalysts for nucleophilic fluorination using CsF

Despite the general high fluorophilicity of boron, organoboranes such as BEt3 and 3,5-(CF3)2C6H3–BPin are shown herein for the first time, to our knowledge, to be effective (solid to solution) phase-transfer catalysts for the fluorination of certain organohalides with CsF. Significant (up to 30% e.e.) chiral induction during nucleophilic fluorination to form β-fluoroamines using oxazaborolidine (pre)catalysts and CsF also can be achieved. Screening different boranes revealed a correlation between calculated fluoride affinity of the borane and nucleophilic fluorination reactivity, with sufficient fluoride affinity required for boranes to react with CsF and form Cs[fluoroborate] salts, but too high a fluoride affinity leading to fluoroborates that are poor at transferring fluoride to an electrophile. Fluoride affinity is only one component controlling reactivity in this context; effective fluorination also is dependent on the ligation of Cs+ which effects both the phase transfer of CsF and the magnitude of the [Cs⋯F-BR3] interaction and thus the B–F bond strength. Effective ligation of Cs+ (e.g. by [2.2.2]-cryptand) facilitates phase transfer of CsF by the borane but also weakens the Cs⋯F–B interaction which in turn strengthens the B–F bond – thus disfavouring fluoride transfer to an electrophile. Combined, these findings indicate that optimal borane mediated fluorination occurs using robust (to the fluorination conditions) boranes with fluoride affinity of ca. 105 kJ mol−1 (relative to Me3Si+) under conditions where a signficant Cs⋯F–B interaction persists.


Introduction
Boranes are ubiquitous in chemistry and most commonly utilised for their Lewis acidic character. The established dogma is that boranes (BY 3 ) are strong Lewis acids towards uoride, with the derived uoroborates, [F-BY 3 ] À , being highly stable towards loss of uoride. 1 Many of the most widely used boranes, such as BX 3 (X ¼ halide) and B(C 6 F 5 ) 3 , are indeed strong Lewis acids towards uoride and form robust uoroborates, 2 with [BF 4 ] À being an archetypal weakly coordinating anion. 1 Furthermore, boranes such as B(C 6 F 5 ) 3 , and even HBR 2 , 3 are increasingly applied in deuorinative functionalisation of uorocarbons, with uoride abstraction by the borane to form a uoroborate anion a key step (Fig. 1). 4 However, by controlling the relative Lewis acidity of the carbon and boron electrophiles it is possible to effect uoride transfer from uoroborates to carbon electrophiles. One classic example is [BF 4 ] À reacting as a stoichiometric uoride source in the Balz-Schiemann reaction, but this requires a highly reactive aryl + electrophile. 5 To expand the utility of uoroborates in nucleophilic uorinations it is highly desirable to: (i) use sub-stoichiometric uoroborate and stoichiometric MF, i.e. use boranes as MF solid to solution phase transfer catalysts; (ii) uorinate carbon electrophiles less reactive than e.g. aryl + .
To expand the electrophile scope amenable to uorination with uoroborates requires an understanding of the factors controlling the uoride ion affinity (FIA) of boranes, thereby enabling its rational modulation. Analysis of calculated FIA values reveals that borane uorophilicity can be attenuated by: (i) the presence of signicant B]Y multiple bond character; (ii) reducing the partial positive charge localised at boron using less electron withdrawing substituents, and (iii) increasing the pyramidalisation energy at boron. 6 The rst two points combined explains the trend in the uoride affinity of the simple (herein simple refers to facile to make or commercially available and inexpensive) boranes: BF 3 (most Lewis acidic, FIA ¼ 258 kJ mol À1 ) [ trialkylboranes (FIA of BMe 3 ¼ 132 kJ mol À1 ) > B(OH) 3 (FIA ¼ 106 kJ mol À1 , FIA values relative to Me 3 Si + ). 6 Despite the facile ability to tune uoride affinity at boron there are no reports, to the best of our knowledge, that utilise low FIA boranes as catalysts for MF phase transfer uorination. Due to the importance of uorinated molecules in pharmaceuticals and agrochemicals 7 and the attractive nature of using metal uoride (MF) salts and simple boranes to effect nucleophilic uorination, we sought to: (i) demonstrate that low uoride affinity boranes can be used as MF phase transfer catalysts and (ii) develop the structure activity relationships key to enabling this reactivity.
Phase transfer catalysts are well established in the eld of nucleophilic uorination as the very low solubility of MF in nonprotic solvents (required for sufficient uoride nucleophilicity) necessitates their use. 8,9a Established phase transfer agents include metal chelators (e.g. cryptands), organic cations (e.g. [R 4 N] + ), 9 Lewis acids that weakly bind uoride (e.g. in hypercoordinated silicates) and compounds that function as multiple hydrogen bond donors to uoride, e.g. bis-ureas. 8,9 Highly notable recent work using the latter class also achieved excellent (>85% e.e.) enantioselectivity during phase transfer nucleophilic uorination of certain alkylhalides (e.g. b-haloamines) with MF. 8 Boranes with low FIA (relative to BF 3 ) have been largely overlooked in this area. Even the stoichiometric use of uoroborates derived from lower uoride affinity boranes in nucleophilic uorination is rare, with the very limited exceptions including: the use of PinBF in the ring opening uorination of epoxides; 10 the use of uoroborate A (Fig. 2, top) to uorinate a range of organic electrophiles; 11 the use of Mes 2 -B(aryl) compounds to bind, and on addition of [CN] À , to release uoride. 12 Note, when using compound A (or when adding an exogenous nucleophile to [Mes 2 B(aryl)F] À ), the formation of a B)SR 2 dative bond (or a B-CN bond) contributes to making uoride transfer from boron to carbon thermodynamically favourable. This factor will be absent using Lewis base free conditions/boranes in MF phase transfer/nucleophilic uorination cycles (Fig. 2, bottom).
Herein we demonstrate that simple (and Lewis base free) boranes are useful CsF phase transfer uorination catalysts. Furthermore, we have elucidated important factors controlling the effectiveness of low FIA boranes as CsF phase transfer uorination catalysts. Demonstrating that simple boranes can act as CsF phase transfer uorination catalysts opens the door to using the plethora of readily synthesised enantioenriched boranes 13 in enantioselective nucleophilic uorination.

Results and discussion
Initially we sought to determine if the uoroborates derived from low uoride affinity triorganoboranes will transfer uoride to weaker (than aryl + ) carbon electrophiles, as suggested by previous computational studies. 14 3 ], resulted in an analogous outcome (BEt 3 and Ph 3 CF formation). Therefore in contrast to [BF 4 ] À (which does not transfer uoride to Ph 3 C + ), these [R 3 BF] À anions do transfer uoride to Ph 3 C + (note Ph 3 C + is a signicantly weaker carbon electrophile than the aryl + species uorinated in the Balz-Schiemann reaction by [BF 4 ] À ).
To guide subsequent studies and identify other boranes with potential as phase transfer uorination catalysts we calculated uoride ion affinity values using a closely related method to that reported by Greb et al. 6 These values are a useful initial indicator of utility in this context, as sufficient uoride affinity is required for the borane to react with MF and form the uoroborate salt, but if the FIA is too great then subsequent transfer of uoride from the uoroborate to an electrophile will be disfavoured. Therefore the borane with the lowest uoride affinity value that enables phase transfer of a MF salt was our initial target as this should have the maximum uorination scope as it will form the most nucleophilic uoroborate (i.e. the uoroborate with the weakest B-F bond).
These calculations (Fig. 3) enabled us to identify commercially available boranes (including two enantioenriched examples) spanning a range of uoride affinity values for study, with the value for BF 3 at this level provided for comparison. The calculations were consistent with the expected outcomes e.g. electron withdrawing groups (in 1-3) increase uoride affinity (relative to PhBPin). While increased multiple bond character, e.g. B]NR 2 double bond character being greater than B]OR double bond character, leads to CBS (Corey-Bakshi-Shibata) oxazaborolidine catalyst 4 being a weaker Lewis acid towards uoride than PhBPin. Several boranes with very similar calculated uoride affinity values also were identied to probe the effect different functional groups (e.g. NO 2 vs. CF 3 in 1 and 3x), ortho vs. meta vs. para substitution (in 3x) and substituent size (e.g. BEt 3 vs. 5) have on borane reactivity towards MF and the subsequent reactivity of the uoroborate. This is important as in contrast to [R 4 N] + , solvation of M + and F À needs to be considered along with the effect of strong interactions between M + and the uoride of the uoroborate persisting in solution.

Nucleophilic uorination with CsF
Fluorination of 6 to form b-uoroamine, 7, using MF (M ¼ K or Cs) catalysed by boranes was explored as a test reaction to determine if there is any correlation between borane uoride affinity and phase transfer/nucleophilic uorination reactivity (Table 1). Attempts to perform the uorination of 6 with KF (with 1 or BEt 3 as catalyst) led to no uorination in CHCl 3 , thus all further uorination studies were performed using CsF. The disparity between KF and CsF is attributed to the greater lattice energy of KF relative to CsF effecting the energetics of the reaction with borane (vide infra). It is noteworthy that the use of ground CsF led to substantial rate enhancements versus reactions using as received CsF. This is consistent with an increase in surface area facilitating the phase transfer reaction between solid CsF and the dissolved borane. Ground and dried CsF is used throughout this study. With both BEt 3 and ArBPin based boranes haloalkane solvents gave better outcomes than other solvents, e.g. MeCN, thus only results in DCM or chloroform are discussed in depth. Anhydrous conditions are essential, as the presence of water (either using non-puried chloroform, or a 99.5 : 0.5 chloroform/H 2 O volume ratio) led to a signicant retardation in the rate of uorination of 6 using 1. The use of protic additives was not explored with BR 3 species due to their propensity to undergo protodeboronation with ROH. Finally, a control in the absence of borane led to no uorination of 6 with CsF in chloroform.
From this borane scoping, phase transfer uorination of 6 using CsF was most effective with 10 mol% BEt 3 and 1. This demonstrates that borane phase transfer catalysts can be used to access important uorinated molecules. 8 As expected the identity of the borane is all important, with weaker Lewis acids e.g. PhBPin, and stronger Lewis acids (e.g. BPh 3 ) both giving poorer outcomes. The former is consistent with a minimum uoride affinity being required to form the Cs[uoroborate] salt, while the latter indicates that if the uoride affinity is too high then this disfavours transfer of uoride from boron in the uoroborate to the electrophile (uoroborate formation is observed with the higher FIA boranes). However, there are additional factors beyond uoride affinity controlling uorination using boranes, as 3p was a relatively poor catalyst despite having an identical calculated uoride affinity to 1. Furthermore, the meta and ortho derivatives, 3m and 3o were more active than 3p, despite similar FIA values. Finally, a Hammett analysis (see Fig. S5 †) using a range of 4-Y-C 6 H 4 -BPin (Y ¼ MeO, H, F, Cl, Br, CF 3 , NO 2 ) boranes led to effectively no correlation, indicating other effects are impacting the uorination outcome (vide infra).
A brief electrophile scoping study was performed using BEt 3 and 1 as catalysts and this revealed the uoroborates derived from these boranes to be poorer sources of uoride relative to the Lewis base incorporated borate A. For example, no  uorination of octyl bromide or benzyl halides was observed even aer prolonged periods reuxing with excess borane/CsF (Scheme 1). In contrast, using two eq. of A generated high yields of PhCH 2 F, 11 demonstrating the positive effect the B) SR 2 dative bond has in enhancing uoride transfer ability. Stronger electrophiles (than PhCH 2 Br) did undergo uorination with CsF using 1 or BEt 3 as catalysts. Reaction of bbromo sulphide 8 with CsF with either BEt 3 or 1 as catalyst in CHCl 3 led to signicant formation of stilbene (mixture of cistrans isomers) with only traces of 9 formed. Serendipitously, we found that the outcome of this reaction is effected dramatically by solvent. Using DCM/n-hexane (6 : 1) as the reaction medium, stilbene formation was negligible (ca. 3%) and 9 could be formed in moderate yield using BEt 3 (Fig. 4). We attribute this disparity to the solvent effecting the equilibrium position between 8 and the thiiranium cation essential for uorination. 8 Notably, the use of the more soluble (than CsF) uoride source [NMe 4 ]F (in the absence of any borane) under identical conditions led to signicant stilbene formation (2 : 1 ratio of stilbene : 9) in contrast to the outcome using CsF/BEt 3 . The reaction of Ph 3 CCl with CsF in CHCl 3 catalysed by either BEt 3 or 1 proceeded in moderate to good yield. Benzoyl chloride proved to be more challenging, with 1 as the catalyst uorination proceeded to only ca. 5% conversion. However, using 10 mol% BEt 3 benzoyl uoride was formed in good yield.

Enantioselective uorination studies
One attractive feature of using boranes as CsF phase transfer uorination catalysts is the ready accessibility of many enantioenriched boranes. 13 Herein in proof of principle studies commercially available 4 and 5 were assessed in the enantioselective uorination of 6 and 8 (which proceed via ring opening of the meso aziridinium and thiiranium cations, respectively). 8 While 5 was ineffective as a catalyst in halocarbon solvents, it did function in the presence of MeCN. However, the use of stoichiometric Cs[5-F] in DCM/MeCN mixtures while leading to formation of 7 and 9, resulted in no e.e. being observed by chiral HPLC analysis. Furthermore, signicant amounts of hydrodehalogenation also was observed using Cs[5-F] alongside formation of 7/9, possibly via a mechanism related to the Midland reduction (Scheme 2). 13c The use of commercially available CBS catalyst 4 (0.5 M in toluene) also was explored as it is not prone to loss of hydride. Surprisingly (given its low calculated uoride affinity), as received 4 effectively catalysed uorination of 6 with CsF and led to appreciable e.e. in 7 (maximum e.e. observed using commercial 4 was in CHCl 3 at 20 C ¼ 30% e.e.). 15 In addition to 7, ca. 5% of the b-amino-alcohol, 10 (inset Fig. 5), was formed at early stages of the reaction, attributed to the presence of low quantities of water that leads to hydroxide transfer to 6. 16 A range of CBS catalysts were bought or made (see ESI †) and used as crude mixtures (as per CBS-catalysed hydroboration procedures). However, none gave better e.e. than commercial 4 in the catalytic uorination of 6 with CsF. Notably, commercial CBS catalyst 11, supplied as a solid, only enabled uorination aer an induction period. Due to this disparity detailed analysis of the commercial batches of 4 and 11 was performed. This revealed a number of impurities present at signicant levels (up to 30% by 11 B NMR spectroscopy), including resonances consistent with products derived from reaction of 4/11 with water as previously reported (e.g. 12/13/14; Fig. 5). 17 Attempts were made to isolate high purity CBS catalysts for further studies. This proved challenging, but the formation of several in signicantly higher purity (ca. 90-99% purity) than the commercial material was achieved. 18 These higher purity CBS catalysts gave worse outcomes than using commercial batches of 4 in the uorination of 6 with CsF. In addition, all >90% purity CBS catalysts (including independently synthesised 4, termed "higher purity 4") displayed an induction period before signicant uorination occurred (Fig. 6). This indicated that CBS catalysts are actually pre-catalysts for phase transfer uorination. It should be noted that 1 and BEt 3 did not display induction periods during the uorination of 6 under identical conditions. Attempts were made to elucidate the structure of the catalytically active species derived from CBS precatalysts under uorination conditions, however this study was inconclusive, and these results can be found in the ESI. † While this work with CBS (pre)catalysts provides proof of principle that enantioselective borane phase transfer uorination catalysis is feasible, the ill-dened and complex mixtures produced using CBS (pre)catalysts under these conditions is a complicating factor presumably contributing to the maximum  e.e. being 30%, despite using multiple CBS (pre)catalyst structures. This highlights the importance of using borane catalysts that are robust under these conditions to allow for rational control of reactivity (note under these uorination conditions both 1 and BEt 3 show no observable decomposition, e.g. by protodeboronation or BPin hydrolysis).

MF binding studies
To understand why only certain borane/MF combinations are effective uorination catalysts, their ability to form M[uoroborate] salts was explored. With BEt 3 and with 1/2 no change to the NMR spectra (including the amount of borane observed in solution vs. an internal standard) was observed on addition to KF suspended in CHCl 3 , consistent with the higher lattice enthalpy of KF relative to CsF (KF ¼ 194.4 kcal mol À1 and CsF ¼ 178.7 kcal mol À1 ). 19 The absence of any uoroborate formation is presumably why there is no uorination of 6 using these boranes and KF. In contrast, combining BEt 3 with CsF formed the uoroborate in a range of solvents (  by cryptand). The different chemical shis and coupling constants observed suggests signicantly different B-F bond strengths in these systems, presumably due to different Cs/F-B interactions. Therefore Cs + ligation will effect not just the energetics of solid to solution phase transfer of CsF using boranes, but also the ability of the formed Cs[FBR 3 ] to act as a nucleophilic source of uoride. The NMR data indicate that CsF/BR 3 in halocarbon solvents (e.g. entries 1/2) should be the most nucleophilic source of uoride using BEt 3 as catalyst, due to the downeld shied 11 B resonance (which is generally associated with less electron density located at boron which would correlate with a weaker B-F bond in this context). This is consistent with the catalytic uorination results where halocarbon solvents gave better outcomes than using MeCN.
Borane 5 also was studied as it is a triorganoborane with the same calculated uoride affinity as BEt 3 but a different environment around the boron centre, which signicantly impacts its performance in catalysing nucleophilic uorination (vide supra). Compound 5 showed no propensity to bind CsF in halocarbon solvents (by NMR spectroscopy) in contrast to BEt 3 , consistent with the disparate catalytic nucleophilic uorination performance observed in DCM. This further conrms that calculated uoride affinity values must be used with caution for predicting reactivity when there is a coordinating cation present. Using DCM/MeCN mixtures or neat MeCN did enable formation of the uoroborate, Cs[5-F] ( Table 2 entry 6), consistent with the observation of uorination using this borane in these solvents. This again indicates that interaction of Cs + with MeCN provides a signicant contribution to the solubilisation of CsF.   Single crystals of Cs[5-F] were obtained from a saturated MeCN solution at À25 C with its solid state structure consisting of {Cs 2 (FBR 3 ) 2 } units propagated into a 1D-coordination polymer by three acetonitrile molecules bridging two adjacent caesium centres (Fig. 7, inset right). In Cs[5-F] each Cs + cation is interacting with only ve Lewis base donor atoms. Note the only other close contacts involving Cs + in the extended structure of Cs[5-F] are C-H/Cs + interactions with the shortest being 3.133 A, these are presumably signicantly weaker interactions than those involving N/Cs + /F/Cs + /O/Cs + . Solid state structures of Cs[FBR 3 ] salts are rare, but Aldridge and co-workers have reported a monomeric example, (18-crown-6)Cs-F-Baryl 3 (B; Fig. 7), in which Cs + is interacting with seven Lewis base donor atoms. 20 A comparison of the two structures is informative with different degrees of aggregation/Cs + ligation signicantly effecting key bond distances, in B: B-F ¼ 1.496(5)Å and Cs/F ¼ 3.034Å, whereas in Cs[5-F]: B-F ¼ 1.524(5)Å and Cs/F ¼ 2.945(3)Å. This is consistent with: (i) the presence of a more Lewis acidic caesium centre more strongly interacting with the B-F unit, thereby reducing the B-F bond strength; (ii) the observed impact of caesium ligation (e.g. with cryptandsvide infra) on the ability of uoroborates to transfer uoride from boron to carbon electrophiles. The low formal coordination number of Cs + in Cs[5-F] may explain the disparity in reactivity between 5 and BEt 3 towards CsF, particularly in halocarbon solvents. The larger hydrocarbyl groups in 5 (relative to Et in BEt 3 ) may prevent additional interactions to Cs + (e.g. formation of higher Cs n F n aggregates containing additional Cs/FB interactions) thus leading to unfavourable solvation energetics (and thus no reaction) when 5 is combined with CsF in halocarbon solvents. This again emphasises that appropriate ligation of caesium in Cs[F-BR 3 ] is vital alongside the appropriate borane uoride affinity in enabling borane catalysed phase transfer uorinations.
Moving to dioxaborolanes, with ArBPin/CsF combinations only the free ArBPin was visible by NMR spectroscopy in halocarbon solvents, although solid is present in these reactions. Assessing these mixtures by NMR spectroscopy using an internal standard revealed a signicant decrease in the intensity of ArBPin resonances on addition of CsF for 1 (and 2). This indicates the formation of poorly soluble (in halocarbons) uoroborate salts derived from 1 (and 2). Thus 1 does react with CsF consistent with its ability to catalyse uorination. In contrast, no evidence for formation of the uoroborate was observed on combining CsF/PhBPin (by NMR spectroscopy versus an internal standard which showed no decrease in the amount of PhBPin present in halocarbon solutions). The disparity can be attributed to the lower uoride affinity of PhBPin which will disfavour reaction with CsF and is presumably why PhBPin is a poor catalyst for nucleophilic uorination of 6.
Notably, the para-nitro derivative, 3p, also showed no reaction with CsF in CDCl 3 (by NMR spectroscopy versus an internal standard), despite 3p having an effectively identical calculated uoride affinity to that for 1. This is consistent with the relatively poor catalytic performance of 3p in the uorination of 6 (Table 1). Furthermore, in MeCN while 1 is converted signicantly to soluble uoroborates on reaction with CsF (e.g. Table  2, entry 8), combining 3p with excess CsF in MeCN led to only ca. 10% of Cs [3-F], with 3p being the dominant boron containing species observed. Thus despite a similar calculated uoride affinity to 1, borane 3p is much less disposed to react with CsF in a range of solvents. We propose that this is due to a sufficiently different (to effect reactivity) interaction with the Cs + cation in the uoroborates derived from 1 and 3p. This is attributed to intramolecular ArCF 3 /Cs + interactions using meta substituted 1 persisting in solution, in contrast intramolecular ArNO 2 /Cs + contacts are not feasible in para   effect the strength of the interaction between the borane and CsF for ortho and meta substituted, but not para substituted aryl boronate systems.
The importance of intramolecular ligation of Cs + was further indicated by the improved performance of 3o and 3m relative to the para derivative 3p in phase transfer uorination (Table 1). This was consistent with the NMR studies with 3m and 3o forming ca. 20% and 30% of the uoroborate in chloroform, respectively, and ca. 88% and 30% formation of the uoroborate in acetonitrile, respectively (Fig. 8, right). This is despite the slightly lower uoride affinity values for 3m and 3o relative to 3p (Table 1). Again this indicates that the FIA is only one of several factors that need to be considered for identifying effective borane based MF phase transfer uorination catalysts. The ability of borane substituents to interact with Cs + being another important factor enabling phase transfer, particularly for lower FIA boranes (e.g. compare the reactivity of 3o and PhBPin). A similar effect also was observed when comparing the ortho and para isomers of ((CF 3 )C 6 H 4 )BPin, 15o and 15p. Borane 15o was signicantly more active as a catalyst in the uorination of 6 with CsF (conditions as per ]-cryptand more effectively sequesters Cs + leading to a relatively strong B-F bond in the uoroborate that is a poorer nucleophilic source of uoride. This clearly highlights that careful control of caesium ligation is vital to enable binding of CsF (favoured by stronger binding of Cs + ) but also to maintain a signicant Cs/F-B interaction that labilises the B-F bond (favoured by weaker binding of Cs + ).

Conclusions
Despite the high uorophilicity of boron, certain organoboranes and boronate esters can be employed as CsF phase-transfer nucleophilic uorination catalysts. Chiral induction during uorination with borane catalysts also was demonstrated as proof of principle (up to 30% e.e.), however limited catalyst stability under these reaction conditions precluded realising high e.e. with CBS systems, highlighting the importance of using boranes robust to uorination conditions. Regarding the factors controlling effective catalysis, as expected, nucleophilic uorination reactivity is impacted by B-F bond strength, which is dependent on borane Lewis acidity towards uoride. Sufficient uoride affinity favours the borane reacting with CsF, however if uoride affinity is too high the resultant uoroborate does not effectively transfer uoride to electrophiles. Importantly, nucleophilic uorination is most effective under conditions that provide sufficient ligation of Cs + to enable solid to solution phase transfer. However, avoiding too effective a ligation of Cs + is also vital, as good ligation of Cs + weakens the Cs/F-B interaction, strengthening the B-F bond and thereby leading to less reactive uoroborates. In terms of predictability, boranes with calculated uoride affinity of 95-120 kJ mol À1 (vs. Me 3 Si + ) appear to be suitable candidates as nucleophilic uorination catalysts, with the caveat that other factors (e.g. borane stability under the reaction conditions/forming the correct uoroborate aggregation/Cs + ligation level in solution) are also important to consider. Finally, weak intramolecular ligation of Cs + by borane substituents appears an effective method to enable lower FIA boranes to achieve CsF phase transfer and nucleophilic uorination. When the various prerequisites are met, simple boranes are effective catalysts for nucleophilic uorination using CsF, including to access useful products (e.g. b-uoroamines).

Data availability
Full experimental procedures, NMR spectra, DFT and crystallographic details/data are provided in the ESI. †

Author contributions
MI and SK conceived the research concept and aims and analysed all data. SK performed the majority of the synthetic work and the majority of the analytical components of this project. MP performed preliminary investigations on commercial CBS, BEt 3 and 5 catalysed uorinations. KY performed all the computational investigations. MU collected and solved the crystal structure. SK and MI draed, reviewed and edited the manuscript.

Conflicts of interest
There are no conicts to declare.