A convenient route to tetraalkylammonium perfluoroalkoxides from hydrofluoroethers

Article abstract of DOI:10.1002/anie.201410639

Hydrofluoroethers are shown to alkylate tertiary amines readily under solvent-free conditions, affording valuable tetraalkylammonium perfluoroalkoxides bearing α-fluorines. The reaction of R_FCF₂-OCH₃ (R_F = CF₂CF₃, CF₂CF₂CF₃, and CF(CF₃)₂) with NR¹R²R³ produces twenty new α-perfluoroalkoxides, [(CH₃)NR¹R²R³][R_FCF₂O] under mild conditions. These α-perfluoroalkoxides are easy to handle, thermally stable, and can be used for the perfluoroalkoxylation of benzyl bromides.

Full text of DOI:10.1002/anie.201410639

Angewandte
Chemie
DOI: 10.1002/anie.201410639
Perfluorinated Alkoxides
A Convenient Route to Tetraalkylammonium Perfluoroalkoxides from
Hydrofluoroethers**
Benson J. Jelier, Jon L. Howell, Craig D. Montgomery, Daniel B. Leznoff, and
Chadron M. Friesen*
Abstract: Hydrofluoroethers are shown to alkylate tertiary
amines readily under solvent-free conditions, affording val-
uable tetraalkylammonium perfluoroalkoxides bearing a-flu-
Unlike their hydrocarbon analogues, perfluoroalkoxides
with a-fluorines cannot be derived from the corresponding
perfluoroalcohols and therefore have been difficult to pre-
pare and isolate.[9] Perfluoroalcohols are thermodynamically
unstable, because the elimination of HF affords the corre-
sponding acyl fluoride even at very low temperatures.[10]
Rather, perfluoroalkoxides have been accessed through the
formation of adducts between alkali metal fluorides and
perfluorinated carbonyl derivatives as first reported by Willis
and Redwood more than 50 years ago.[11] However, these
products can readily revert back to their starting point with
elimination of an alkali metal fluoride (Scheme 1) and hence
À
orines. The reaction of RFCF2 OCH3 (RF = CF2CF3,
CF2CF2CF3, and CF(CF3)2) with NR1R2R3 produces twenty
new a-perfluoroalkoxides, [(CH3)NR1R2R3][RFCF2O] under
mild conditions. These a-perfluoroalkoxides are easy to
handle, thermally stable, and can be used for the perfluor-
oalkoxylation of benzyl bromides.
T
he fully fluorinated (perfluoro-) alkoxy moiety can impart
a range of desirable properties including high metabolic
stability, increased lipophilicity, and a degree of thermal and
chemical stability that is unprecedented with other functional
groups.[1] Furthermore, perfluoroalkoxides have demon-
strated ability as inductively electron-withdrawing ancillary
ligands in the design of olefin metathesis catalysts,[2] have
been proposed as a precursor to solvents for metal/air
batteries,[3] and are the active, but not well-studied, species
in the anionic ring-opening polymerization of commercially
available perfluoropolyalkylethers.[4] Whereas the number of
strategies for the late stage incorporation of fluorine,[1c,5]
perfluoroalkyl,[6] and perfluorothiolate[7] moieties continues
to increase, the perfluoroalkoxylation of aliphatic and aryl
substrates has been hindered in part by the lack of a con-
venient, fluoride-free preparation of thermally stable per-
fluoroalkoxides. Recent reports have shown the need for
a convenient and practical preparation of this underrepre-
sented functional group for pharmaceutical, agrochemical,
and industrial applications.[1c,f,8]
Scheme 1. Elimination of fluoride from a-perfluoroalkoxides.
have limited synthetic utility beyond ring opening of per-
fluoro-epoxides.[4]
However, the presence of a nonmetal cation tends to shift
the equilibrium to the perfluoroalkoxide and improve the
synthetic utility significantly.[12] The first structurally charac-
terized nonmetal perfluoroalkoxide, tris(dimethylamino)sul-
fonium (TAS+) trifluoromethoxide, was prepared by Farnham
and Middleton in 1985[13] and has been utilized in the
trifluoromethoxylation of aryl boronic acids and stannanes
by Ritter et al.[14] More recently, four additional examples
were reported between the reaction of an anhydrous fluoride
ion and the corresponding perfluorocarbonyl or trifluoro-
methyl trifluoromethanesulfonates (Scheme 2).[12,15] Particu-
larly noteworthy is the preparation of the only two isolated
transition metal complexes bearing a trifluoromethoxide
moiety by Vicic et al.[8f,16] However, the high cost, toxicity of
low-molecular-weight carbonyl fluorides, air sensitivity of the
required reagents, the need for an anhydrous fluoride source,
the difficulty of having a fluoride-free perfluoroalkoxide as
well as the often in situ preparation of the perfluoroalkoxides,
had severely limited synthetic applications to date.
We have found commercially available hydrofluoroethers
(HFEs) such as methoxy heptafluoropropane (HFE-7000) (1)
and methoxy nonafluorobutane (HFE-7100) (2) to be partic-
ularly good and inexpensive, laboratory cosolvents for the
manipulation of organofluorine compounds as both are
fluorophilic and relatively polar solvents (Figure 1).[17] Bear-
ing both fluorinated and hydrocarbon segments, HFEs display
a combination of physical properties such as low surface
[*] B. J. Jelier, Prof. D. B. Leznoff
Dept of Chemistry, Simon Fraser University
8888 University Dr., Burnaby, BC, V5A 1S6 (Canada)
Dr. J. L. Howell
Experimental Station
E. I. du Pont de Nemours, Wilmington, DE 19803 (USA)
Prof. C. D. Montgomery, Prof. C. M. Friesen
Department of Chemistry, Trinity Western University
7600 Glover Rd, Langley, B.C., V2Y 1Y1 (Canada)
E-mail: chad.friesen@twu.ca
[**] The authors acknowledge funding from the Natural Sciences and
Engineering Research Council of Canada (NSERC), Simon Fraser
University (SFU), and Trinity Western University (TWU). In addition,
we thank J. Ovens (SFU) for assistance with the structural
determination of 6a and A. Simonson (U of Utah) for assistance
with TGA-MS.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410639.
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Table 1: Substrate scope for the methylation of tertiary amines by
hydrofluoroethers.[a]
Scheme 2. Representative methods for the preparation of perfluoroalk-
oxides bearing a-fluorines.
Figure 1. Commercially available hydrofluoroethers HFE-7000 (1) and
HFE-7100 (2), a nearly equimolar mixture of n- and iso-isomers.
tension, low flammability, and low toxicity, which allow this
class of compounds to find application in a wide range of
commercial uses as refrigerants, solvents in precision clean-
ing, fingerprinting agents, blowing agents, and lithium battery
cosolvents.[18] Overall, this utility is underpinned by the
chemical inertness of HFEs.
[a] Yields refer to the isolated material of >98% purity unless otherwise
noted. n.r.=no reaction.
Despite their chemical inertness, we observed a rather
unexpected but noteworthy degradation pathway for
HFEs.[19] Thus, rather than being inert, methoxy hydrofluoro-
ethers (HFE-7000 and HFE-7100) alkylate tertiary amines
under mild conditions in moderate to good yields, affording
tetraalkylammonium perfluoroalkoxides as highly moisture-
sensitive solids. To probe the scope of this reaction, we
investigated the reactivity of 12 tertiary amines, including
trialkylamines, diamines, and aromatic amines with both
HFE-7000 and HFE-7100 (Table 1). In most cases, the
perfluoroalkoxide, which is the only detectable product,
precipitates out of the reagent mixture, enabling a clean
workup by filtration or, if the amine is volatile, both reagents
can be simply removed under reduced pressure to afford
multigram quantities of the desired perfluoroalkoxide adduct.
Each of the twenty new perfluoroalkoxides reported
1
herein was characterized by H and 19F NMR spectroscopy,
IR spectroscopy, and elemental analysis (see the Supporting
Information, SI). In addition, the X-ray crystal structure of 6a
was determined (Figure 2) and clearly depicts a methylated
N,N-dimethylbenzylamine with a perfluoropropoxide anion.
The bond lengths are consistent with the other four known
perfluoroalkoxide structures, including a significantly short
Figure 2. X-ray structure of 6a showing the thermal ellipsoids at the
30% probability level (P21/c, 150 K, R=7.11%).
ambient temperature, the a-fluorines of the perfluoroalk-
oxide tend to appear as a broad signal in the 19F NMR
spectrum, a not well-understood phenomenon which has been
attributed to slow fluoride exchange.[13,15d] However, at no
point do we observe free fluoride or the corresponding acyl
fluoride (see SI). Upon cooling in CH2Cl2 in 108 intervals
À
À
C O bond (1.28 ꢀ) and elongated C F bonds, ostensibly due
to negative hyperconjugation. This is further corroborated by
the increased nCÀ frequency (1540 cmÀ1) in each of the
O
reported perfluoroalkoxides due to an increase in bond order
À
of the C O bond, irrespective of the associated cation. At
2
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from room temperature to À808C, the a-difluoromethylene
signal of 6a sharpens and then splits into a multiplet below
À308C.
However, at this temperature, the tetramethylammonium
perfluoropropoxide underwent immediate decomposition of
the tetramethylammonium fluoride to fluoroform and trime-
thylamine based on gas-phase IR and GC/MS analysis of the
trapped off-gas and 19F NMR analysis of the residue (see SI).
Having established a convenient route to thermally stable
perfluoroalkoxides, we then explored the reactivity of 1a with
benzyl bromide substrates in solution. We had a particular
interest in the ratio of perfluoroalkoxylation to fluoro
derivatives. After optimizing the solvent and temperature,
we found that acetonitrile at 458C gave the highest con-
version (see SI). Surprisingly, even at 458C, with 2.0 equiv-
alents of tetramethylammonium perfluoropropoxide, only
1% of 4-bromobenzyl fluoride was obtained. However, at
758C the same conversion of ether was obtained but an
increase in 4-bromobenzyl fluoride. We hypothesized that the
newly formed ether was in a reversible reaction. With the
addition of 1.0 equivalent of AgBF4, the conversion reached
94% with less than 1% benzyl fluoride after 8 h. The
volatility of the ethers resulted in a lower isolated yield
(Figure 3).
To further survey the methylating ability of HFEs, we
targeted analogous Group 15 substrates and observed that the
yield unsurprisingly decreases (Table 2). The spectroscopic
Table 2: Methylation of homologous Group 15 substrates by
CH3OCF2CF2CF3.[a]
[a] 258C, 48 h, yields are of isolated products.
details of 13a are entirely consistent with a tetramethylphos-
phonium perfluoropropoxide rather than a pentacoordinate
phosphorane with perfluoroalkoxy or fluoride moiety.[20]
Since perfluoroalkoxides tend to eliminate fluoride with
concomitant formation of perfluoroacyl fluoride at elevated
temperatures, we hypothesized that thermal decomposition of
tetraalkylammonium perfluoroalkoxides could enable a facile
route to truly anhydrous tetraalkylammonium fluorides. The
prepared tetramethylammonium perfluoropropoxide was
subjected to thermal analysis in a glovebox equipped with
a thermogravimetric analyzer/mass spectrometer (TGA-MS).
As shown in Scheme 3, adventitious amounts of moisture
Figure 3. Perfluoroalkoxylation of alkyl bromides with
NMe4OCF2CF2CF3 at 458C in CH3CN for 8 h. Conversions were
determined by GC/MS and yields of the pure isolated compound are
reported in parentheses.
The solution state stability of perfluoroalkoxides may be
related to methods of preparation as was recently observed by
Vicic et al. in the preparation of the analogous [NMe4]-
[SCF3].[7b] Even though the substrates reported herein are
simple (Figure 3) with no competing elimination pathway, the
perfluoroalkoxides are able to tolerate higher temperatures
than previously reported with high conversion of the ether
and only minor amounts of fluoride substitution (Fig-
ure 3).[15a,c,21] However, even if the perfluoroalkoxide is not
in equilibrium with free fluoride, the newly formed ethers can
still be susceptible to b-fluoride elimination. For instance, no
product was observed with 2-bromoacetophenone derivative
18 but substantial amounts of benzoyl fluoride derivatives.
In summary, we have observed that two commercially
available methyl hydrofluoroethers are not inert but act as
electrophilic methylating agents of tertiary amines and PMe3,
affording quaternary ammonium or phosphonium perfluoro-
alkoxides, respectively. Furthermore, these thermally stable
salts are isolable, stable in the absence of moisture, and react
with benzyl bromides to form perfluoropropoxy benzyl
ethers. Hydrofluoroethers provide access to new tetraalkyl-
ammonium perfluoroalkoxides and future work will seek to
Scheme 3. Thermal and hydrolysis decomposition routes for tetrame-
thylammonium perfluoropropoxide (1a) determined by TGA-MS, IR
spectroscopy, GC/MS, and 19F NMR spectroscopy.
were sufficient to catalytically decompose the tetraalkylam-
monium perfluoroalkoxides to the carboxylic acid. The
carboxylic acid then underwent decarboxylation of the
anion and concomitant Hofmann elimination of the cation
between 185–2008C, affording a mixture of volatile products,
including tetrafluoroethylene. However, with careful han-
dling in the appropriate Schlenkware on a stainless steel
vacuum line equipped with a gas cell IR spectrometer, we
were able to determine that the tetramethylammonium
perfluoroalkoxide were surprisingly stable at 1508C at 1 torr
for a period of 1 h and partial decomposition did not occur
until 1808C.
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General procedure for the preparation of perfluorinated alkoxides: In
a sealed 20 mL vial with a resin-lined screw cap under nitrogen
atmosphere, the respective tertiary amine (0.6 mol) and hydrofluoro-
ether (5 equiv) were combined and the vial was sealed. The vials were
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Received: October 31, 2014
Revised: December 10, 2014
Published online: && &&, &&&&
Keywords: alkylation · fluorine · hydrofluoroether ·
.
perfluoroalkoxide · reactive intermediates
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Communications
Perfluorinated Alkoxides
B. J. Jelier, J. L. Howell,
C. D. Montgomery, D. B. Leznoff,
Fluoride free! The preparation of a series
of tetraalkylammonium perfluoroalkox-
ides under mild, fluoride-free conditions
is achieved by the methylation of tertiary
amines with commercially available, par-
tially fluorinated ethers. This practical
synthesis paves a new way to an under-
represented class of thermally stable
compounds bearing a-fluorines.
C. M. Friesen*
&&& — &&&
A Convenient Route to
Tetraalkylammonium Perfluoroalkoxides
from Hydrofluoroethers
6
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