Isolable fluorinated triphenylmethyl cation salts of [HCB11Cl11]−: demonstration of remarkable hydride affinity

Significantly fluorinated triarylmethyl cations have long attracted attention as potentially accessible highly reactive carbocations, but their isolation in a convenient form has proved elusive. We show that abstraction of chloride with a cationic silylium reagent leads to the facile formation of di-, tetra-, and hexafluorinated trityl cations, which could be isolated as analytically pure salts with the [HCB11Cl11]− counterion and are compatible with (halo)arene solvents. The F6Tr+ cation carrying six meta-F substituents was computationally predicted to possess up to 20% higher hydride affinity than the parent triphenylmethyl cation Tr+. We report that indeed F6Tr+ displays reactivity unmatched by Tr+. F6Tr+ at ambient temperature abstracts hydrides from the C–H bonds in tetraethylsilane, mesitylene, methylcyclohexane, and catalyzes Friedel–Crafts alkylation of arenes with ethylene, while Tr+ does none of these.


Introduction
The triphenylmethyl or trityl cation (Ph 3 C + or Tr + ) is a textbook example of a carbocation that is isolable owing to the high degree of benzylic conjugation and the steric protection afforded to the central carbon by the three phenyl substituents. 1 In organometallic chemistry and catalysis, salts of Tr + are frequently used to study the thermodynamics and kinetics of hydride transfer, 2,3,4 or to generate reactive main-group and transition-metal cations through hydride or alkyl anion abstraction. 5,6,7 Tr + can also serve as a convenient one-electron oxidant. 8 Trityl cation derivatives bearing stabilizing electrondonating groups can even exist in aqueous solutions, with a rich history of use as indicators and dyes. 9 The trityl cation versions bearing electron-withdrawing substituents have proven more challenging to obtain. Fluorinated trityl cations, up to (C 6 F 5 ) 3 C + (A, Fig. 1), have been of particular fundamental interest, [10][11][12] including as isoelectronic analogs of the widely used borane (C 6 F 5 ) 3 B, 13,14 and more recently have been studied by Horn and Mayr 15 and Dutton et al. 16 The more reactive A or other ortho-and/or meta-uorinated trityl cations were not isolated in those studies, but were generated in situ, or their intermediacy was indicated by kinetic studies. While generation of uorinated trityl cations in oleum and other superacidic media, [10][11][12]16 or by in situ abstraction of halides with element halide Lewis acids 15 is possible, these media and counteranions are not fully compatible with either the more electron-decient trityl cations themselves or with their potential use in the synthesis of other reactive main-group or transition metal cations. Thus, the full extent of the reactivity of the uorinated trityl cations can only be accessed when paired with more robust weakly coordinating anions in weakly coordinating solvents. 17 The only example of an isolated trityl-type cation uorinated in the ortho-/meta-positions is B (Fig. 1), obtained by Douvris and Reed in an undened yield, and not studied further. 18 The perchlorotrityl cation has also been isolated. 19 Our group has been attracted to the highly reactive carboand main-group cations in the context of our work on silylium and alumenium cation-catalyzed activation of aliphatic C-F bonds, 20-23 which permitted exhaustive deuorination of per-uoroalkyl groups under mild conditions. The chemistry of abstraction of uoride from certain uoroarenes with silylium cations has led to innovative reactivity, as well. 24,25 Trialkylsilylium cations are typically generated by hydride abstraction from trialkylsilanes (R 3 SiH) using Tr + , 26 but our theoretical analysis suggested that the parent Tr + only barely has the thermodynamic hydride affinity (HA) to abstract hydrides from even the relatively electron-rich SiH bonds in trialkylsilanes. Given the perceived challenge 10,16 in the isolation of the fully uorinated (C 6 F 5 ) 3 C + , we decided to rst explore the partially uorinated derivatives. Here, we report the isolation of analytically pure di-, tetra-, and hexauorosubstituted trityl cation salts, and the remarkable contrast in the hydride abstraction reactivity with the parent Tr + .

Results and discussion
Theoretical HA analysis Wilson and Dutton calculated gas-phase and CH 2 Cl 2 solvent continuum HA values for a series of symmetric polychloro-and polyuorosubstituted trityl cations. 27 They discussed the t to the known experimental values provided by the various computational methods and settled on the use of B3LYP/aug-cc-pVTZ//B3LYP/def2-TZVPP. 28, 29 The Wilson-Dutton calculations showed that replacement of H with F in the para-position has an essentially zero effect on HA, whereas introduction of each ortho-or a meta-uorine increases HA by ca. 2.4-2.7 kcal mol À1 (CH 2 Cl 2 continuum) or ca. 3.5 kcal mol À1 (gas phase). This is in line with the more negative pK R+ values for the various ortho-and meta-uorinated trityls compared to Tr + or the para-F substituted trityls, determined by Filler et al. 10 The ortho-and para-CF positions are conjugated to the central carbon by resonance and the para-CF has been identied as a site of alternative nucleophilic attack on (C 6 F 5 ) 3 C + related to its decomposition pathways. 10, 16 We decided to avoid uorination in the ortho-or para-positions and focus on meta-uorination. The Wilson-Dutton HA values for F 6 Tr + (213.0 and 108.3 kcal mol À1 ) were 11% and 17% higher than for Tr + (191.4 and 92.5 kcal mol À1 ) in the gas phase and CH 2 Cl 2 continuum, respectively.
In 2011, 30 we analyzed the HA and FA values for a series of cations relevant to the silylium-catalyzed HDF using the M05-2X functional with the basis sets 6-311+G(d) for F, and 6-31++G(d,p) for C and H. 31 Utilizing the DFT approach from our 2011 paper, we calculated the gas-phase and the chlorobenzene solvent continuum HA values for F 6 Tr + to be 229.4 and 135.0 kcal mol À1 , representing a 13% and a 20% increase vs. Tr + . These relative increases are similar to those in the Wilson-Dutton work. 27 The substantial increase suggests that the HA of F 6 Tr + is thermodynamically sufficient to abstract a hydride from a range of Si-H containing molecules, and rivals the HA values calculated (also in PhCl) for Me 3 C + (126.6 kcal mol À1 ), PhCH 2 + (137.8 kcal mol À1 ), and Me 2 CH + (138.9 kcal mol À1 ). 30 Without assessing quantitative accuracy, we nonetheless surmised that F 6 Tr + might be able to abstract hydrides from tertiary and possibly secondary and benzylic C(sp 3 )-H bonds.

Synthesis and characterization of F x Tr + salts
We envisioned the synthesis of uorinated trityl cations partnered with the exceptionally robust and weakly coordinating [HCB 11 Cl 11 ] À anion ([Cl11], Fig. 1 43 as well as the 1 H and 19 F NMR spectral data did not suggest any signicant interaction of the cations with the [Cl11] À anion, the arene or CD 2 Cl 2 solvents, or the Me 3 SiCl/Me 3 SiOTf by-products. Single-crystal X-ray diffractometry (Fig. 3) 3 SiH in a C 6 D 6 /o-C 6 H 4 Cl 2 solvent mixture led to the quantitative formation of F 6 TrH or TrH, respectively. The fate of the "Et 3 Si + " species in arene solvents is not straightforward, as has been studied in detail 45 by Heinekey and coworkers: the presence of varying amounts of Et 4 Si betrays complexity arising from the H/Et redistribution in the Si species and/or reactions with the arenes.
The reaction of Tr[Cl11] with a substoichiometric (0.9 equiv.) amount of Et 3 SiH did not lead to the complete disappearance of the Si-H moiety (16% of the original Si-H intensity remained) and only 82% of the possible TrH was observed (Fig. 4). In contrast, the reaction of F 6 Tr[Cl11] with substoichiometric (0.75 equiv.) amount of Et 3 SiH led to the production of the expected quantity of F 6 TrH, the complete disappearance of the Si-H signals, and without the concomitant observation of Et 4 Si.

H-D exchange
In the reactions of F 6 Tr[Cl11] with Et 3 SiH, signicant H/D scrambling was observed among the neutral aromatic compounds present in solution: C 6 D 6 , o-C 6 H 4 Cl 2 , and F 6 TrH (but the C(sp 3 )-H bond in F 6 TrH was never deuterated). The extent of H-D exchange was analyzed via 1 H, 13 C, or 19 F 46 NMR spectroscopy (see ESI † for details). The mechanism of the H/D exchange likely involves the generation of superacidic protonated arenes in situ, 47 which should enable rapid H/D exchange via H + /D + shuttling (Fig. S4 †). 47,48 The product of addition of either Et 3 Si + or F 6 Tr + to a neutral arene can be alternatively viewed as a protonated arene. 47 It is also possible that analogous cations are accessed via reactions involving the minor components of the mixture. The Oestreich group recently examined this type of H/D exchange catalysis in greater detail. 49

Abstraction of hydride from C-H bonds
Given the computational prediction of the enhanced hydride affinity of F 6 Tr + vs. Tr + , we wished to examine their reactivity towards benzylic and aliphatic C-H bonds. As expected, no reaction was observed between Tr[Cl11] and (1) 1 equiv. of mesitylene or (2) 1 equiv. of methylcyclohexane in o-C 6 H 4 Cl 2 aer 1 week at ambient temperature. In contrast, the reaction of F 6 Tr[Cl11] with mesitylene (as solvent) resulted in 66% yield (NMR evidence) or F 6 TrH aer 48 h. We propose that hydride abstraction from mesitylene by F 6 Tr[Cl11] generates a 3,5dimethylbenzyl cation, which rapidly undergoes Friedel-Cras 20,21 addition to mesitylene. GC-MS analysis of the mixture aer quenching with water showed the presence of a m/z signal at 238, consistent with compound 4 (Fig. 5). Treatment of F 6 Tr [Cl11] in o-C 6 H 4 Cl 2 with 1 equiv. of methylcyclohexane resulted in the >95% yield (NMR evidence) of F 6 TrH aer 96 h. The aliphatic region of the 1 H NMR spectrum presented a large  number of overlapping aliphatic signals, indicating a complex mixture (Fig. 5b).
The methylcyclohexyl cation presumed to be formed initially may undergo isomerization 50 and Friedel-Cras addition to o-C 6 H 4 Cl 2 , with many potential products. Abstraction of a hydride from alkanes, with generation of rearranged tertiary carbocations, was previously reported by the Reed group using Me [HCB 11 Me 5 Br 6 ]. 51,52 The key difference between Reed's "Me + " reagents and the F 6 Tr + reported here is that the latter can be prepared in bulk analytical purity and is stable in haloarene solutions.
Abstraction of a hydride from the b-position in trialkylaluminums with Tr + has been used to generate reactive alumenium (R 2 Al + ) cations. 6,22,53 The analogous abstraction of bhydride from alkylsilanes by Tr + is not known, and we have conrmed that no reaction takes place between Tr[Cl11] and Et 4 Si in C 6 D 6 /o-C 6 H 4 Cl 2 . However, an analogous reaction of Et 4 Si with F 6 Tr[Cl11] resulted in the formation of 82% F 6 TrH aer 96 h (and complete disappearance of Et 4 Si aer 10 d). The major Si product appeared to be "Et 3 Si", but instead of the stoichiometric complement of free ethylene, we observed ethane and other aliphatic resonances. Ethane may result from the protonolysis of Et 4 Si by the highly Brønsted acidic cations generated in the reaction (extensive H/D exchange was concomitantly observed), a process reported on by Oestrich and co-workers. 54 As a control experiment, we examined the reaction of F 6 Tr[Cl11] with 6.3 equiv. of ethylene in C 6 D 6 /o-C 6 H 4 Cl 2 . Within 18 h at ambient temperature, all ethylene had been consumed, with the concomitant generation of ethylbenzene (1.8 equiv.) and other alkylarenes, and quantitative production of F 6 TrH. It is reasonable to propose that F 6 Tr[Cl11] abstracts a hydride from the benzylic positions of ethylbenzene or other alkylarenes generated through Friedel-Cras alkylation. In complete contrast, no reaction occurred between Tr[Cl11] and ethylene under analogous conditions.

Conclusion
Introduction of six meta-F substituents in F 6 Tr + brought about remarkable contrast with the reactivity of the parent triphenylmethyl (Tr + ) cation, understood primarily through the greatly enhanced hydride affinity of especially the hexa-uorinated F 6 Tr + . Interestingly, while F 6 Tr + catalyzes the Friedel-Cras alkylation of arenes with ethylene, and generates alkyl cations via hydride abstraction which then readily engage in Friedel-Cras addition, F 6 Tr + itself is stable in combination with (halo)arene solvents and dichloromethane. This shows that uorinated trityl cations represent a promising class of reagents for achieving the extremes of hydride affinity while minimizing reactivity with other potential substrates.

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