On the Structure of [K(crypt‐222)]⁺

Abstract

Due to disorder, [K(crypt‐222)]+ CF3− (1) is crystallographically indistinguishable from its isomer, a 1:1 fluoroform clathrate of deprotonated [K(crypt‐222)]+, [K(crypt‐222{‐H+})]·CHF3 (2). In our preceding publications, 1 was characterized on the basis of combined X‐ray diffraction, NMR, reactivity, labeling, acid‐base, and DFT studies. Herein we report that neither incorporation of deuterium nor any other transformation of [K(crypt‐222)]+ is observed in the presence of CD3OD/CD3OK at 70 °C or CD3S(O)CD2K/(D6)DMSO at 23 °C over hours and even days. Since fluoroform is easily and quickly deprotonated under such conditions, the demonstrated greater acidity of CHF3 relative to [K(crypt‐222)]+ provides additional evidence in favor of 1 and against 2. Likewise, crystal packing analysis and DFT calculations support 1, which has now been refined in the ultimately correct space group R32. Our previously drawn conclusions regarding [K(crypt‐222)]+ CF3− and the existence of a ‘naked’ trifluoromethyl anion in a condensed phase remain valid. The recent alternative refinement (Becker and Müller, Chem. Eur. J. 2017, 23, 7081 – 7086) of our raw diffraction data has been improperly performed in the wrong space group to yield a non‐charge‐balanced model [K(crypt‐222)]+·CHF3, which is hereby repudiated. Becker and Müller's attempts to resolve the charge balance issue have produced models that are inadequate from the perspective of both crystallography and chemistry.

Anion-Initiated Trifluoromethylation by TMSCF3: Deconvolution of the Siliconate–Carbanion Dichotomy by Stopped-Flow NMR/IR

“…In 2014, Prakash showed that the elusive trifluoromethyl anion­(oid) can be detected in situ ( 13 C, 19 F NMR) at low temperatures after addition of KO t Bu/18-crown-6 to 1a . With the much bulkier reagent TIPSCF 3 ( 1c ), the generation of ion-paired [K­(18-c-6)] + [CF 3 ] − ( E ) proceeds quantitatively at −78 °C over a period of 30 min.…”

“…Subsequent addition of PhCOMe (11 equiv) or PhCHO (4 equiv) afforded CF 3 -addition products (22–68%) after quenching with H 2 O, mechanism IV . In 2015, Grushin demonstrated that use of crypt-222 (L, Scheme ) facilitates generation of the free CF 3 – carbanion, a tetrahydrofuran (THF) solution-phase “noncovalently bound ionic species” . The structure of the highly air- and temperature-sensitive salt, [K­(crypt-222)] + [CF 3 ] − ( E ), was confirmed by single-crystal X-ray diffraction. , …”

“…The pioneering studies summarized above have been highly enlightening regarding the structure and stability of pentacoordinate (trifluoromethyl)­siliconates ( C , D ) , and their ability to release the trifluoromethane anion­(oid) ( E ) under specific conditions. , However, they do not yield direct detail on the kinetics and mode of transfer of CF 3 from TMSCF 3 ( 1a ) to a carbonyl electrophile, using a catalytic fluoride-based initiator (M + X – ), at ambient temperature. , …”

Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

“…The authors evidenced that the anion with [K(18‐crown‐6)] + as counter ion is persistent at low temperatures (−78°C); that is, the CF 3 − could be a long‐lived intermediate under certain experimental conditions. Moreover, Lishchynskyi et al showed that CF 3 − may be a free carbanion in the presence of [K(crypt‐222)] + . Both findings have opened new possibilities for fluoroalkylation reactions and mechanistic studies in organofluorine chemistry.…”

Synthesis of trifluoromethyl ketones by nucleophilic trifluoromethylation of esters under a fluoroform/KHMDS/triglyme system

“…In earlier studies, the solvent N,N -dimethylformamide (DMF), was essential for nucleophilic trifluoromethylation by HCF 3 since DMF acts as a CF 3 anion reservoir that is used as a hemiaminaloate adduct [Me 2 NCH(O)CF 3 ]- ( Scheme 1b ) [ 19 – 28 ]. Although the taming CF 3 anion had been an elusive problem for decades, it has been dramatically progressed in recent years by the substantial works by Grushin [ 51 – 53 ] and Prakash [ 54 ]. Our group reported novel DMF-free systems for the nucleophilic trifluoromethylation reaction using HCF 3 , including the phosphazene base P4- t -Bu (P4- t -Bu), in 2013 ( Scheme 1c ) [ 45 ] and a potassium tert -butoxide ( t -BuOK) or potassium hexamethyldisilazide (KHMDS)/glyme combination in 2018 ( Scheme 1d ) [ 46 ].…”