Aminodifluorosulfinium salts: Selective fluorination reagents with enhanced thermal stability and ease of handling

Article abstract of DOI:10.1021/jo100504x

Diethylaminodifluorosulfinium tetrafluoroborate (XtalFluor-E) and morpholinodifluorosulfinium tetrafluoroborate (XtalFluor-M) are crystalline fluorinating agents that are more easily handled and significantly more stable than Deoxo-Fluor, DAST, and their analogues. These reagents can be prepared in a safer and more cost-efficient manner by avoiding the laborious and hazardous distillation of dialkylaminosulfur trifluorides. Unlike DAST, Deoxo-Fluor, and Fluolead, XtalFluor reagents do not generate highly corrosive free-HF and therefore can be used in standard borosilicate vessels. When used in conjunction with promoters such as Et₃N3HF, Et₃N2HF, or DBU, XtalFluor reagents effectively convert alcohols to alkyl fluorides and carbonyls to gem-difluorides. These reagents are typically more selective than DAST and Deoxo-Fluor and exhibit superior performance by providing significantly less elimination side products.

Full text of DOI:10.1021/jo100504x

pubs.acs.org/joc
Aminodifluorosulfinium Salts: Selective Fluorination Reagents with
Enhanced Thermal Stability and Ease of Handling†,‡
Alexandre L’Heureux,§ Francis Beaulieu,§ Christopher Bennett, David R. Bill,^
Simon Clayton, Franc-ois LaFlamme,§ Mahmoud Mirmehrabi,# Sam Tadayon,#
David Tovell, and Michel Couturier*,§
§OmegaChem Inc., 480 Rue Perreault, St-Romuald (Qc) G6W 7V6, Canada, Manchester Organics Limited,
The Heath Business and Technical Park, Runcorn, Cheshire WA7 4QX, U.K., ^Process Safety Laboratory,
Chemical Research and Development, Pfizer Inc., Eastern Point Road, Groton, Connecticut 06340, and
#Crystallization Engineering Technologies, Chemical Development, Pfizer Canada,
1025 Boulevard Marcel Laurin, St-Laurent (Qc) H4R 1J6, Canada
michelcouturier@omegachem.com
Received March 17, 2010
Diethylaminodifluorosulfinium tetrafluoroborate (XtalFluor-E) and morpholinodifluorosulfinium
tetrafluoroborate (XtalFluor-M) are crystalline fluorinating agents that are more easily handled and
significantly more stable than Deoxo-Fluor, DAST, and their analogues. These reagents can be
prepared in a safer and more cost-efficient manner by avoiding the laborious and hazardous
distillation of dialkylaminosulfur trifluorides. Unlike DAST, Deoxo-Fluor, and Fluolead, XtalFluor
reagents do not generate highly corrosive free-HF and therefore can be used in standard borosilicate
vessels. When used in conjunction with promoters such as Et3N 3HF, Et3N 2HF, or DBU,
3
3
XtalFluor reagents effectively convert alcohols to alkyl fluorides and carbonyls to gem-difluorides.
These reagents are typically more selective than DAST and Deoxo-Fluor and exhibit superior
performance by providing significantly less elimination side products.
Introduction
atoms in a safe, efficient and controlled fashion. Among the
myriad of fluorination methodologies currently available,
the direct replacement of oxygen by fluoride, commonly
referred to as deoxofluorination, is one of the most effective
and routinely used approaches that is particularly applicable
to a wide variety of functional groups.2
Deoxofluorination using nucleophilic-type fluorinating
reagents has a long development history, notably with
the early recognition that SF4 could directly transform such
groups as hydroxyl to fluoride, carbonyl to difluoromethyl-
ene, and carboxylic acid to trifluoromethyl.3 Since this
It is well recognized that the introduction of a C-F
structural feature within a bioactive molecule often imparts
desirable physiological properties.1 As a consequence, fluori-
nated biologically active compounds are increasingly com-
mon in the realm of pharmaceuticals and agrochemicals.
This widespread interest and increasing demand for site
selective fluorination of organic compounds has prompted
the extensive development of novel specialized fluorination
technologies and reagents capable of incorporating fluorine
† Dedicated to Professor Paul Brassard on the occasion of his 80th birthday.
‡ Patent pending.
(1) (a) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc.
Rev. 2008, 37, 320. (b) Hagmann, W. K. J. Med. Chem. 2008, 51, 4359.
€
(c) Muller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881.
(2) For a recent review on fluorination methods and strategies employed
in medicinal chemistry, see: Kirk, K. L. Org. Process Res. Dev. 2008, 12, 305.
(3) Hasek, W. R.; Smith, W. C.; Engelhardt, V. A. J. Am. Chem. Soc.
1960, 82, 543.
DOI: 10.1021/jo100504x Published on Web 04/21/2010
r
J. Org. Chem. 2010, 75, 3401–3411 3401
2010 American Chemical Society

L’Heureux et al.
JOCArticle
initial discovery, several reagents were developed, such as
DAST (diethylaminosulfur trifluoride),4 Deoxo-Fluor (bis(2-
methoxyethyl)aminosulfur trifluoride),5 Yarovenko’s reagent
(N,N-diethyl-2-chloro-1,1,2-trifluoroethylamine),6 Ishikawa’s
reagent (N,N-diethyl-1,1,2,3,3,3-hexafluoropropylamine),7
1,1,2,2-tetrafluoroethyl-N,N-dimethylamine (TFEDMA),8
2,2-difluoro-1,3-dimethylimidazolidine (DFI),9 and perfluoro-
1-butanesulfonyl fluoride (PBSF).10 DAST and Deoxo-
Fluor are the most widely used deoxofluorinating agents
and arguably the best in this class in terms of broad spectrum
of applicability.11
in many aspects of their life cycle. First, purification of crude
reagents by vacuum distillation is a hazardous process that
requires extensive safety measures due to their explosiveness.
Later, after manufacturing, dialkylaminosulfur trifluorides
are subject to stringent shipping regulations. Then, in hand,
great care must be exercised in handling these fuming liquids
as they react extremely violently with water. Over time, these
liquids are known to discolor, and a redistillation can be
required to be used satisfactorily.15 Upon use, DAST and
Deoxo-Fluor generate free HF which is very volatile (bp 20 ꢀC),
highly toxic, extremely corrosive to skin and other tissues
including bone, and readily etches glass.16 Finally, deoxo-
fluorinations using dialkylaminosulfur trifluorides are pro-
blematic in certain cases in that dehydration to an olefin
often occurs. In light of these considerations, the develop-
ment of safer deoxofluorinating reagents is warranted. In
this context, enhanced thermal stability and crystallin-
ity would be desirable attributes that would address most
shortcomings of DAST and Deoxo-Fluor. A crystalline
reagent would not only facilitate its isolation and purifica-
tion but would also offer the convenience of handling a solid
reagent.
From a historical perspective, dialkylaminosulfur trifluor-
ides were developed as liquid alternatives to the parent
reagent, i.e., SF4. The latter is a gaseous substance that is
extremely toxic and corrosive. Its handling therefore neces-
sitates extensive safety measures and specialized equipment.
On the other hand, DAST is a more easily handled fluorinat-
ing agent and generally superior to SF4 in that the latter
requires much higher temperatures (typically 100 ꢀC) and
leads to undesired side products. As such, DAST became
widely used in deoxofluorinations and has found applica-
tions in a myriad of transformations. However, it was soon
recognized that liquid DAST was thermally unstable and
highly explosive.12 Investigation of this phenomenon by
thermal analyses has shown that the decomposition occurs
in two distinct stages. The first event occurring at approxi-
mately 90 ꢀC is a nonexothermic disproportionation of
DAST leading to SF4 and bis(diethylamino)sulfur difluo-
ride. Upon further heating, the latter undergoes detonation/
explosion with generation of unidentified gases and black
char.13 The safety concerns over DAST prompted the deve-
lopment of a safer dialkylaminosulfur trifluoride reagent,
namely bis(2-methoxyethyl)aminosulfur trifluoride (Deoxo-
Fluor).3 In this context, it has been shown by differential
scanning calorimetry (DSC) that DAST and Deoxo-Fluor
have the same decomposition temperature, but the latter
degrades more slowly with somewhat lower heat evolution.
In spite of these significant advances, the hazards associated
with this class of reagent are considered unsafe for industrial
use in a batch-type process. As a matter of fact, considerable
efforts were deployed to develop continuous-flow processes
to prevent reagent accumulation and circumvent the inherent
problems associated with both DAST and Deoxo-Fluor.14
From their initial preparation to their final use, handling
dialkylaminosulfur trifluoride reagents remains problematic
In the quest to identify a safe deoxofluorinating reagent
capable of incorporating fluorines in a controlled fashion, we
surmised that dialkylaminodifluorosulfinium salts could act
as surrogates for aminosulfur trifluorides and provide a safer
alternative reagent. Preliminary investigations in our labora-
tories have shown that dialkylaminodifluorosulfinium salts
were promising leads in that regard and warranted further
investigation.17 Herein, we report the results of our recent
findings.
Results and Discussion
Synthesis of Dialkylaminodifluorosulfinium Salts. The first
examples of dialkylaminodifluorosulfinium salts were re-
ported over three decades ago by Markovskii et al. In this
account, the authors describe that DAST and the dimethy-
lamino, piperidino and morpholino analogues all react with
BF3 Et2O to form the corresponding dialkylaminodifluor-
3
osulfinium tetrafluoroborates.18 In this context, although
DAST possesses electron lone pairs, BF3 does not act as a
Lewis acid but rather as an irreversible fluoride ion acceptor,
and the resulting dialkylaminosulfinium ion is stabilized as
the tetrafluoroborate.19 Shortly thereafter, Cowley et al.
found that dimethylaminosulfur trifluoride reacted with
BF3, PF5, and AsF5 to form the corresponding dimethyla-
minodifluorosulfinium salts.20 A year later, Mews and Henle
also reported that dimethylaminosulfur trifluoride readily
loses a fluoride to BF3.21 Over a decade later, Pauer et al.
(4) Middleton, W. J. J. Org. Chem. 1975, 40, 574.
(5) (a) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.; Prozonic, F. M.; Cheng, H.
J. Org. Chem. 1999, 71, 7048. (b) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.;
Prozonic, F. M. J. Chem. Soc., Chem. Commun. 1999, 215.
(6) Yarovenko, N. N.; Raksha, M. A. Zh. Obshch. Khim. 1959, 29, 2159.
(7) Takaoka, A.; Iwakiri, H.; Ishikawa, N. Bull. Chem. Soc. Jpn. 1979,
52, 3377.
(8) Petrov, V. A.; Swearingen, S.; Hong, W.; Petersen, W. C. J. Fluorine
Chem. 2001, 109, 25.
(9) Hayashi, H.; Sonoda, H.; Fukumura, K.; Nagata, T. J. Chem. Soc.,
Chem. Commun. 2002, 1618.
(15) Fauq, A. H.; Singh, R. P.; Meshri, D. T. In Handbook of Reagents for
Organic Synthesis - Fluorine Containing Reagents; Paquette, L. A., Ed.; John
Wiley & Sons Ltd.: England, 2007; pp 180-194.
(16) Yin, J.; Zarkowsky, D. S.; Thomas, D. W.; Zhao, M. M.; Huffman,
M. A. Org. Lett. 2004, 6, 1465.
€
(10) (a) Bennua-Skalmowski, B.; Vorbruggen, H. Tetrahedron Lett. 1995,
36, 2611. (b) Savu, P. M.; Snustad, D. U.S. Patent 6 248 889, 2001.
(11) Singh, R. P.; Shreeve, J. M. Synthesis 2002, 17, 2561.
(12) (a) Cochran, J. Chem. Eng. News 1979, 57 (12), 74. (b) Middleton,
W. J. Chem. Eng. News 1979, 57 (21), 43.
(17) Beaulieu, F.; Beauregard, L.-P.; Courchesne, G.; Couturier, M.;
LaFlamme, F.; L’Heureux, A. Org. Lett. 2009, 11, 5050.
(18) Markovskii, L. N.; Pashinnik, V. E.; Saenko, E. P. Zh. Org. Khim.
1977, 13, 1116.
(13) Messina, P. A.; Mange, K. C.; Middleton, W. J. J. Fluorine Chem.
1989, 42, 137.
(19) Minkwitz, R.; Molsbeck, W.; Oberhammer, H.; Weiss, I. Inorg.
Chem. 1992, 31, 2104.
€
(14) (a) Negi, D. S.; Koppling, L.; Lovis, K.; Abdallah, R.; Geisler, J.;
(20) Cowley, A. H.; Pagel, D. J.; Walker, M. L. J. Am. Chem. Soc. 1978,
100, 7065.
(21) Mews, R.; Henle, H. J. Fluorine Chem. 1979, 14, 495.
Budde, U. Org. Process Res. Dev. 2008, 12, 345. (b) Baumann, M.;
Baxendale, I. R.; Martin, L. J.; Ley, S. V. Tetrahedron 2009, 65, 6611.
3402 J. Org. Chem. Vol. 75, No. 10, 2010

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SCHEME 1. One-Pot Preparation of XtalFluor-E from N,N-
Diethyltrimethylsilylamine
Powder X-ray diffraction (XPRD) of the latter confirmed
the generation of a distinctly different polymorph, herein
referred to as Type II polymorph. Whereas Markovskii
reportedly obtained needles (herein referred to as Type I
polymorph) melting at 74-76 ꢀC, the new morphology
consists of flakes with a melting point of 89.8 ꢀC measured
by DSC and 83-85 ꢀC measured by capillary. The fusion
enthalpy of the higher-melting type II polymorph was mea-
sured at 101.3 J/g, whereas the lower melting type I poly-
morph gave 86.4 J/g, confirming that the new polymorph is
more stable. Moreover, the Type II polymorph is less
moisture sensitive, exhibits superior handling properties
and is storage stable. Similarly, the morpholinodifluorosul-
finium tetrafluoroborate that we have obtained using a
different procedure (vide infra) exhibited a higher mp of
133.2 ꢀC measured by DSC and 129-132 ꢀC measured by
capillary as compared with the 104-106 ꢀC mp previously
disclosed by Markovskii. Although we were not able to
reproduce the original prisms, the significantly higher melt-
ing crystals we have now obtained is indicative of a new and
more stable polymorphic form.
As mentioned above, the preparation of DAST has the
disadvantage of requiring a distillation. This purification
step is hazardous, requires extensive safety measures and
specialized equipment, and is a major cost contributor to
this relatively expensive reagent. In that regard, a crystalline
reagent derived from DAST would also be desirable by
allowing isolation via simple filtration. In this context, we
envisaged using crude undistilled DAST in the prepara-
tion of XtalFluor-E, thereby eliminating the need for a
time-consuming and hazardous distillation. Gratifyingly,
FIGURE 1. Structures of fluorinating reagents.
determined the crystal structure of dimethylaminodifluoro-
sulfinium hexafluoroarsenate.22 More recently, Pashinnik et al.
reported that morpholinosulfur trifluoride reacts with
SeF4 to form morpholinodifluorosulfinium pentafluorosele-
nate.23 Throughout these accounts, the chemical reactivity of
isolated dialkylaminodifluorosulfinium salts has been scar-
cely studied. They were only employed in the reaction with
trimethylsilylmorpholine to give tris(morpholino)sulfinium
tetrafluoroborate, and to the best of our knowledge, the
fluorination of an allylic alcohol in a prostaglandin repre-
sents the sole example of deoxofluorination using a dialkyl-
aminodifluorosulfinium salt.24 It is noteworthy that the
presence of a dialkylaminosulfinium species in solution has
been inferred in the fluorination of thiocarbonyls using
Deoxo-Fluor in the presence of catalytic amounts of SbCl3.25
In this account, the identification of an in situ generated
dialkylaminosulfinium species by 19F NMR is described but
the counterion was not characterized. A similar intermediate
had been previously proposed for the reaction of DAST with
ZnI2 or SbCl3 in the specific context of transforming sulf-
oxides to R-fluorothioethers.26
In order to further evaluate the thermal stability of
dialkylaminodifluorosulfinium salts and their potential use
as fluorinating reagents, we initially secured some diethylamino-
difluorosulfinium tetrafluoroborate (XtalFluor-E, 1) and
morpholinodifluorosulfinium tetrafluoroborate (XtalFluor-
M, 2) by reproducing the original Markovskii procedure
(Figure 1).27 As we previously communicated, XtalFluor-E
crystallizes directly out of solution upon the reaction of
addition of BF3 THF to crude DAST (prepared from N,
3
N-diethyltrimethylsilylamine and SF4 in dichloromethane)
directly led to crystalline XtalFluor-E in the desired poly-
mophic form (Type II) with an isolated yield of 90% (Scheme 1).
All the previously reported dialkylaminodifluorosulfi-
nium salts were prepared via fluoride transfer to Lewis acids
such as BF3, PF5, AsF5, and SbF4, and the types of salts
were limited to the corresponding counteranions. However,
during the course of mechanistic investigations (vide infra),
DAST and BF3 Et2O in diethyl ether, but the isolated solid
3
was somewhat amorphous and highly hygroscopic. In the
hopes of obtaining a less moisture-sensitive material, the
forgoing salt was initially recrystallized in warm 1,2-dichloro-
ethane (DCE), which upon rapid cooling led to needles
melting at 72-76 ꢀC, consistent with Markovskii’s published
results (vide supra). A second crystallization trial at higher
temperature did not lead to the same morphology, even when
seeded with aforementioned needles. Instead, a denser and
cleaner product with higher melting point was obtained.
SCHEME 2. Novel Synthesis of Diethylaminodifluorosulfinium
Salts Using Strong Brønsted Acids
(22) Pauer, F.; Erhart, M.; Mews, R.; Stalke, D. Z. Naturforsch., B:
Chem. Sci. 1990, 45, 271.
(23) Pashinnik, V. E.; Martynyuk, E. G.; Shermolovich, Y. G. Ukr. Khim.
Zh. (Russ. Ed.) 2002, 68, 83.
(24) Bezuglov, V. V.; Pashinnik, V. E.; Tovstenko, V. I.; Markovskii,
L. N.; Freimanis, Y. A.; Serkov, I. V. Russ. J. Bioorg. Chem. 1996, 22, 688.
(25) Lal, G. S.; Lobach, E.; Evans, A. J. Org. Chem. 2000, 65, 4830.
(26) (a) McCarthy, J. R.; Peet, N. P.; LeTourneau, M. E.; Inbasekaran,
M. J. Am. Chem. Soc. 1985, 107, 735. (b) Wnuk, S. F.; Robins, M. J. J. Org.
Chem. 1990, 55, 4757.
(27) XtalFluor-E (1) and XtalFluor-M (2) can be obtained from the
Aldrich Chemical Co. (catalogue nos. 719439 and 719447, respectively) and
Manchester Organics Ltd. (catalogue nos. J11026 and K11027, respectively).
J. Org. Chem. Vol. 75, No. 10, 2010 3403

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SCHEME 3. Reaction of Hydrocinnamyl Alcohol with Diethylaminodifluorosulfinium Tetrafluoroborate
DAST was found to react exothermically with tetrafluoro-
boric acid and provides diethylaminodifluorosulfinium tetra-
fluoroborate with concomitant elimination of HF. In fact,
the exothermic reaction proceeded nearly quantitatively with
an isolated yield of 96% (Scheme 2). This finding constitutes
a novel method for the preparation of dialkylaminodifluoro-
sulfinium salts. Moreover, this unprecedented Brønsted acid
exchange method complements the traditional fluoride
transfer procedure, and now provides access to aminodi-
fluorosulfinium species bearing other types of countera-
nions. For example, diethylaminodifluorosulfinium trifluoro-
methanesulfonate salt (3) can be readily prepared by reacting
DAST with triflic acid and constitutes the first example of a
triflate salt of a dialkylaminodifluorosulfinium. An addi-
tional benefit is that the foregoing triflate salt has a higher
melting point (97-101 ꢀC) as compared to the corresponding
tetrafluoroborate.
Fluorination of Alcohols with XtalFluor-E and XtalFluor-
M Reagents. As mentioned above, Pashinnik et al. reported
the deoxofluorination of an allylic alcohol using morpholi-
nodifluorosulfinium tetrafluoroborate (2) in acetonitrile and
claim an 85% yield of the corresponding fluoride as a
mixture of epimers. This represents the sole record of a
deoxofluorination reported in the literature using an amino-
difluorosulfinium salt. We therefore set out to assess the
potential scope of such salts from a broader perspective.
Fluorinations of various alcohols using XtalFluor-M were
initially attempted under conditions similar to those re-
ported in the Pashinnik procedure, i.e., using acetonitrile
as solvent. Unexpectedly, geraniol led to an intractable
mixture, whereas hydrocinnamyl alcohol (4) provided acet-
amide 5 as the major product, presumably via a Ritter-type
reaction with the solvent. These indications point to an
incompatibility with acetonitrile. However by using dichloro-
methane as solvent, we found that XtalFluor-E (1) did
convert hydrocinnamyl alcohol into the desired fluoride,
albeit sluggishly. More specifically, all the starting material
was consumed within 5 min and led to a complex reaction
mixture from which fluoride 6, ether 7, and sulfinate 8 were
isolated in 32%, 27%, and 9% yield, respectively (Scheme 3).
As we previously communicated, the mechanistic path-
ways of deoxofluorinations with DAST and XtalFluor-E are
similar in that they both involve a common dialkylaminodi-
fluorosulfane intermediate.4,28 However, since XtalFluor-E
releases tetrafluoroboric acid instead of the DAST-gener-
ated HF, the reactions with XtalFluor-E are fluoride-
starved, and side reactions occur.17
In this context, formation of ether 7 would be ascribed to a
nucleophilic displacement of the activated sulfane species by
the alcohol instead of fluoride. To a lesser extent, ejection of
diethylamine from the transient sulfane, followed by alcohol
(28) (a) Tewson, T. J.; Welch, M. J. J. Org. Chem. 1978, 43, 1090.
(b) Sutherland, A.; Vederas, J. C. Chem. Commun. 1999, 1739.
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TABLE 1. Effects of Additives on the Reaction Profiles of Hydrocin-
namyl Alcohol with Diethylaminodifluorosulfinium Tetrafluoroboratea
addition order was critical, and increased conversion was
observed when the reagent was added last. Although the
reaction rates with DBU were slower than those using
yieldc (%)
Et3N 3HF, none of the ether 7 and sulfinate 8 side products
3
entry
additive
none
TEA 3HF
addition orderb time (h)
6
7
8
4
were observed, thereby suggesting that the deprotona-
ted alkoxy-N,N-dialkylaminodifluorosulfane adduct is more
stable.
With these promising leads in hand, substrate scope was
next assessed on a wide variety of alcohol substrates, includ-
ing primary, secondary, tertiary, allylic, and anomeric alco-
hols (Table 2). In the case of hydrocinnamyl alcohol, greater
conversion to the corresponding fluoride was achieved when
the reactions were initiated at lower temperatures. Conver-
1
2
3
4
5
6
n/a
A
B
0.1
1.0
0.5
0.2
32 27
78
84
39 26
92
76
9
0
0
6
0
0
0
5
3
0
6
23
4
2
3
TEA 3HF
TEA 3HF
3
C
3
DBU
DBU
A
B
17
19
0
0
aAll reactions were performed at room temperature using 1.5 equiv of
reagent and additive. bMethod A: reagent was added to a mixture of
alcohol and additive. Method B: alcohol was added to a mixture of re-
agent and additive. Method C: additive was added to a mixture of
alcohol and reagent;. cNonisolated HPLC yields using m-xylene as inter-
nal standard.
sions using Et3N 3HF were initially more problematic in
3
sec-alcohols. However, this problem was easily resolved
through the use of Et3N 2HF which is considered a more
nucleophilic and less basic fluoride ion source than
3
addition on the resulting sulfoxonium would account for the
formation of the sulfinate 8. In light of the fast reaction rate
and product distribution observed, the dialkylaminodifluor-
osulfinium salt is a potent electrophile that leads to a reactive
alkoxy-N,N-dialkylaminodifluorosulfane species, but the re-
action mixture is fluoride-starved. In this context, we sur-
mised that an exogenous source of fluoride would be required
to mitigate side reactions. Indeed, a cleaner conversion of
hydrocinnamyl alcohol (4) to 1-fluoro-3-phenylpropane (6)
could be achieved when the reaction was performed in the
Et3N 3HF.30 Although not commercially available, this
reagent is easily generated in situ from the appropriate
3
amounts of Et3N 3HF and Et3N. Thus, in the fluorination
3
of 1-phenyl-3-butanol (9) using Et3N 2HF in conjunction
3
with XtalFluor-M, 4% of the starting material remained
unreacted as compared with 17% and 31% using Et3N 3HF
3
and Et3N HF, respectively (Table 2, entries 5-7).
3
Another important consideration is the amount of elimi-
nation side products which is typically observed in deoxo-
fluorinations reactions using DAST and Deoxo-Fluor. Not
only does the formation of these undesired products con-
sume valuable reagent and negatively impact the yields, but
the purification of the desired fluoride is oftentimes challen-
ging due to similar physical properties between the two. The
presence of Et3N 3HF as a promoter (Table 1, entry 3). The
3
addition order was a key parameter in this reaction. In fact,
adding the exogenous fluoride immediately after the sub-
strate was contacted with XtalFluor-E led to essentially the
same product distribution as a reaction without Et3N 3HF
3
(Table 1, entries 1 and 4). In other words, coupling of
XtalFluor-E with the alcohol and subsequent reactions of
the adduct are quasi instantaneous, thereby exemplifying the
highly reactive nature of the salt.
Unlike anhydrous hydrogen fluoride and Olah’s reagent, it is
important to emphasize that Et3N 3HF is much less corrosive
use of Et3N 2HF is also advantageous in that regard since
3
less elimination is observed as compared with Et3N 3HF. As
3
shown in entries 13 and 14, the XtalFluor-E-mediated
deoxofluorination of cyclooctanol (11) using the latter as
promoter provided a mixture of cyclooctyl fluoride (12) and
cyclooctene in a 3.7:1 ratio. However, combination of both
3
and can be handled in borosilicate glassware up to 150 ꢀC
without etching.29 Moreover, Et3N 3HF is soluble in organic
colder temperature and the use of Et3N 2HF markedly
3
3
increased the selectivity to 12.1:1. These results compare
favorably with those reported using DAST and Deoxo-
Fluor (2.3:1 and 5.7:1, respectively).31
solvents, anhydrous, and inexpensive. The role of the latter is
to serve as a source of HF, and it is unlikely that HF causes
the dialkylaminodifluorosulfinium tetrafluoroborate to re-
vert back to dialkylaminosulfur trifluoride and generate
tetrafluoroboric acid. On the contrary, DAST reacts exother-
mically with tetrafluoroboric acid to provide dialkylamino-
difluorosulfinium tetrafluoroborate and HF in 96% isolated
yield. Moreover, 19F NMR analysis of a 1:1 mixture of
The foregoing method was next challenged on enantio-
pure (R)-benzyl 3-hydroxypyrrolidine-1-carboxylate (13) to
determine the stereochemical course of the substitution.
Thus, treatment of this reagent with XtalFluor-E and
Et3N 2HF provided the inverted fluoride 14 in 98.0% ee
3
admixed with benzyl 2,5-dihydro-1H-pyrrole-1-carboxylate
in a 5.9:1 ratio, respectively. Even better results in terms of
selectivity and stereochemical integrity were observed using
the same fluorinating reagent and DBU (Table 2, entry 19).
XtalFluor-E and Et3N 3HF in deuterated dichlorometh-
3
ane did not show the characteristic DAST peak expected at
40.9 ppm.
While considering the mechanism at play, we surmised
that the presence of a non-nucleophilic strong base could
serve to deprotonate the putative alkoxy-N,N-dialkylamino-
difluorosulfane species and prevent release of diethylamine.
This would not only suppress the pathway leading to the
sulfinate 8 byproduct but also now allow ejection of requisite
fluoride anions. Validation of the concept was realized when
DBU allowed formation of the desired fluoride, i.e., in the
absence of exogenous fluoride (Table 1, entries 5, 6). Again,
On the basis of these results, both the Et3N 2HF- and DBU-
3
promoted fluorinations mainly proceed in an SN2 fashion
with minimal loss of enantiopurity.
Further along the substrate-scope evaluation, 2,3,4,6-
tetra-O-benzyl-D-glucose (15) provided glycosyl fluoride 16
in essentially quantitative yield as a mixture of anomers.32
(30) (a) Giudicelli, M. B.; Picq, D.; Veyron, B. Tetrahedron Lett. 1990, 31,
6527. (b) Yin, J. Y.; Zarkowsky, D. S.; Thomas, D. W.; Zhao, M. M.;
Huffman, M. A. Org. Lett. 2004, 6, 1465.
(29) (a) Haufe, G. J. Prakt. Chem. 1996, 338, 99. (b) McClinton, M. A.
Aldrichim. Acta 1995, 28, 31. (c) Gatner, K. Pol. J. Chem. 1993, 67, 1155.
(d) Franz, R. J. Fluorine Chem. 1980, 15, 423.
(31) Lal, G. S.; Pez, G. P. US patent 6,222,064 B1, 2001.
(32) Kovac, P.; Yeh, H. J. C.; Jung, G. L. J. Carbohydr. Chem. 1987,
6, 423.
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TABLE 2. Deoxofluorinations of Alcohols with XtalFluor-E and XtalFluor-M Reagents
aAll reactions were performed using 1.5 equiv of XtalFluor reagent. bNonisolated HPLC yields using m-xylene as internal standard. cFluoro/alkene
e
f
ratio calculated by 1H NMR on crude product. dRemaining alcohol estimated by HPLC. ee of starting material: 99.9%. R/β ratio calculated by
1H NMR.
Deoxofluorination of geraniol using either XtalFluor-E or
but a greater selectivity of 4.1:1 was observed using DBU as
promoter. While the DBU/XtalFluor-E reagent combina-
tion was optimal in this case, the use of Et3N 2HF/
XtalFluor-M and Et3N 2HF proceeded smoothly and ex-
3
clusively in a SN20 fashion. Initial attempts to fluorinate a
homoallylic alcohol, namely androstenolone (17) using the
same reagent system were low yielding and more proble-
matic. However, with further optimization with the former
3
XtalFluor-M on 2-hydroxy-2-methylpropanoate (21) gave
a superior selectivity of 21:1. Thus, on the basis of these
results, there are no general trends for the ideal combination
of XtalFluor-E and XtalFluor-M fluorinating reagents with
Et3N 2HF, Et3N 3HF, and DBU promoters. However,
fluorinating reagent, Et3N 3HF gave superior results and
3
provided 77% isolated yield of the 3β-isomer consistent with
previous results obtained with DAST.33 Finally, deoxofluor-
inations were tested on two tertiary alcohol substrates, i.e.,
2-hydroxy-2-methyl-1-phenylpropan-1-one (19) and ethyl
2-hydroxy-2-methylpropanoate (21). On the former alcohol,
3
3
various reagent/promoter combinations are complementary
to each other and provide opportunities for optimization.
Geminal Difluorination of Carbonyls with XtalFluor-E and
XtalFluor-M Reagents. Prior to our original communication,
the use of aminodifluorosulfinium salts for the geminal
difluorination of carbonyls was unprecedented. In prelimin-
ary trials, we found that XtalFluor-E alone was incapable of
performing such transformations. For example, when 4-tert-
butylcyclohexanone (27) was treated with XtalFluor-E, no
XtalFluor-E and Et3N 2HF led to a mixture of desired
3
fluoride 20 and elimination product in a modest 1.9:1 ratio,
(33) Rozen, S.; Faust, Y.; Ben-Yakov, H. Tetrahedron Lett. 1979,
20, 1823.
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TABLE 3. Deoxofluorinations of Aldehydes and Ketones with XtalFluor-E and XtalFluor-M Reagents
aIsolated yields of products. bUnless otherwise noted, 1.5 equiv of reagent was employed. cUnseparated mixture of fluoro and alkene. dFluoro/alkene
ratio calculated by 1H NMR on crude product.
detectable conversion to 4-tert-butyl-1,1-difluorocyclohex-
ane (28) was observed even after 4 days at room tempera-
ture. Interestingly, Merck chemists have also reported that
Although not supported by conclusive experimental
proof, the first step toward geminal difluorination is gener-
ally considered to be the addition of HF across the carbonyl
group to provide a fluorohydrin, which then partakes in
dehydroxyfluorination with the reagent. In the case of DAST
and Deoxo-Fluor, the initial source of free HF arises by
hydrolysis of the reagents with trace amounts of water or
by the deliberate addition of ethanol. Likewise, exogenous
HF would be required to initiate deoxofluorinations of
carbonyls using dialkylaminodifluorosulfinium salts. Accord-
ingly, we found that the deoxofluorination of hydrocinna-
maldehyde (25) using either XtalFluor-E or XtalFluor-M
10 mol % of BF3 Et2O substantially retards the rate of
3
reaction in the deoxofluorination of a ketone by Deoxo-
Fluor.34 These results are somewhat contradictory to pre-
vious claims to the effect that BF3 Et2O catalyzes the
3
dialkylaminosulfur trifluoride mediated deoxofluorination
of ketones.31 All indications suggest that BF3 Et2O does not
3
act as a Lewis acid but rather as an irreversible fluoride ion
acceptor to form dialkylaminodifluorosulfinium tetrafluor-
oborate salts with attenuated reactivity.
was indeed promoted with Et3N 3HF and provided the
3
geminal difluoride 26 with no detectable amounts of vinyl-
fluoride side product (Table 3, entry 1, 2). Although no
reaction was observed between XtalFluor-E and 4-tert-
(34) Mase, T.; Houpis, I. N.; Akao, A.; Dorzoitis, I.; Emerson, K.;
Hoang, T.; Iida, T.; Itoh, T.; Kamei, K.; Kato, S.; Kato, Y.; Kawasaki,
M.; Lang, F.; Lee, J.; Lunch, J.; Maligres, P.; Molina, A.; Nemoto, T.;
Okada, S.; Reamer, R.; Song, J. Z.; Tschaen, D.; Wada, T.; Zewge, D.;
Volante, R. P.; Reider, P. J.; Tomimoto, K. J. Org. Chem. 2001, 66, 6775.
butylcyclohexanone, the addition of Et3N 2HF efficiently
3
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TABLE 4. Miscellaneous Reactions Using XtalFluor-E and XtalFluor-M Reagents
aIsolated yields of products. bUnless otherwise noted, 1.5 equiv of reagent was employed.
allowed the desired transformation to occur and provided a
91% yield of gem-difluoride 28 admixed with vinyl fluoride
in a 62:1 ratio. This result favorably compares with the 5:1
and 2:1 ratios reportedly observed with Deoxo-Fluor and
DAST, respectively. As was the case for alcohols, conver-
higher selectivities than those reported using DAST and
Deoxo-Fluor.
Miscellaneous Reactions using XtalFluor-E and XtalFluor-
M Reagents. A brief survey of other types of oxygen-based
functional groups was undertaken to further expand on the
scope of fluorinating capabilities of XtalFluor salts and to
compare with known reactions with DAST and Deoxo-
Fluor (Table 4). Both aliphatic and aromatic carboxylic
acids 43 and 45 were easily fluorinated to the corresponding
acyl fluorides by either reagent. However, it is noteworthy
that, even under forcing conditions, exhaustive fluorination
to the corresponding trifluoromethyls was unsuccessful. This
limitation could be ascribed to the fact that free-HF is not
generated in the process.36 Further along the substrate-
scope studies, the well-precedented fluorination of sulfoxides
to R-fluorothioethers was also successfully performed on
methyl phenyl sulfoxide (47). Lastly, using XtalFluor-E, a
one-pot process was developed where an O-trimethylsilyl
cyanohydrin generated in situ from hydrocinnimalhehyde
and TMSCN afforded the R-fluoronitrile 50 in excellent
yields.37
Thermal Safety Assessments of XtalFluor-E and XtalFluor-
M. A preliminary thermal screening was performed using
differential scanning calorimetry (DSC) to establish if they
are safe to handle from a thermal hazard safety perspective.
In previously disclosed DSC values, DAST reportedly de-
composed at 140 ꢀC releasing 1700 J/g, whereas Deoxo-
Fluor decomposed at 140 ꢀC with 1100 J/g of energy.5
However, recently published DSC data on DAST has shown
strong exothermic activity from 131 to 163 ꢀC with a Tmax at
162 ꢀC and a release of 1252 J/g.35b Since the latter value is
much lower than the one previously reported, we elected to
retest DAST and Deoxo-Fluor using the same calibrated
sions were generally greater when Et3N 2HF was employed
3
in lieu of Et3N 3HF. However, greater selectivities were
3
generally obtained using the latter, but in this context, higher
reaction temperatures, i.e., refluxing DCE, were required to
achieve full conversion. For example, the selectivity of
deoxofluorination on 4-(Boc-amino)cyclohexanone (29)
could be upgraded from 1.8:1 to 5.1:1 by using Et3N 3HF
3
instead of Et3N 2HF and by performing the reaction under
3
more concentrated conditions in refluxing DCE. Likewise,
high-yielding selectivities were obtained in the synthesis of
ethyl 4,4-difluorocyclohexanecarboxylate (32), a key inter-
mediate in the commercial drug Maraviroc.35 In this case,
XtalFluor-M gave a higher 15:1 selectivity than XtalFluor-E
(13:1), results which again compare favorably to the 1:1
selectivity reported using DAST. Application of the fore-
going procedure on cyclohexane-1,4-dione mono(ethylene
glycol) acetal (33) provided the desired gem-difluoride (34) in
a 9:1 selectivity, whereas Deoxofluor was reported to yield a
0.8:1 ratio.16 Even more impressive results were obtained
when 2 equiv of XtalFluor-M was employed which led to no
detectable amounts of vinyl side product (Table 3, entries 8
and 9). Likewise, no elimination was observed in the fluor-
ination of N-Cbz-protected 3-pyrrolidinone (35), whereas
N-Cbz-protected 4-piperidinone (37) led to a 13:1 selectivity
in favor of the desired gem-difluoride 38. Finally, both ethyl
2-oxo-2-phenylacetate (39) and 1-phenoxypropan-2-one
(41) were readily converted to their corresponding difluo-
rides using XtalFluor-E and Et3N 2HF. Throughout these
3
examples, all reactions were essentially completed within
1 day, were generally high yielding, and consistently exhibited
(36) Although there are a few examples of perfluoration of carboxylic
acids using DAST and Deoxo-Fluor, these potentially hazardous reactions
are conducted neat at high temperatures and are of very limited scope; see
ref 5.
(37) (a) Effenberger, F.; Osswald, S. Tetrahedron: Asymmetry 2001, 12,
279. (b) LeTourneau, M. E.; McCarthy, J. R. Tetrahedron Lett. 1984,
25, 5227.
(35) (a) Price, D. A.; Gayton, S.; Selby, M. D.; Ahman, J.; Haycock-
Lewandowski, S.; Stammen, B. L.; Warren, A. Tetrahedron Lett. 2005, 46,
5005. (b) Haycock-Lewandowski, S. J.; Wilder, A.; Ahman, J. Org. Process
Res. Dev. 2008, 12, 1094. (c) Ahman, J.; Birch, M.; Haycock-Lewandowski,
S. J.; Long, J.; Wilder, A. Org. Process Res. Dev. 2008, 12, 1104.
3408 J. Org. Chem. Vol. 75, No. 10, 2010

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FIGURE 2. Overlapped DSC thermograms of DAST, Deoxo-
Fluor, XtalFluor-E, and XtalFluor-M.
FIGURE 3. ARC thermograms of DAST, Deoxo-Fluor, XtalFluor-
E, and XtalFluor-M: temperature over time.
DSC instrument as the one to be used for assessing
XtalFluor-E and XtalFluor-M (Figure 2). Thus, DAST
exhibited a very sharp Tmax peak at 155 ꢀC and a release of
1641 J/g, whereas Deoxo-Fluor was much broader with a
Tmax at 158 ꢀC and a release of 1031 J/g. For comparison,
DSC analysis on XtalFluor-E showed a decomposition
temperature (Tmax) at 205 ꢀC with an exothermic heat
(-ΔH) of 1260 J/g. In general, a higher decomposition
temperature and a lower exothermic heat generated during
decomposition are indicative of a more stable compound
and provide greater safety. On the basis of these results,
XtalFluor-E is significantly more stable as it decomposes at
approximately at 50 ꢀC above DAST and Deoxo-Fluor.
Even superior results were observed with XtalFluor-M,
which decomposed at an even higher temperature of 243 ꢀC
(Tmax) while releasing only 773 J/g. Moreover, isothermal
DSC of both XtalFluor-E and XtalFluor-M set at 90 ꢀC
showed no observable degradation in the time frame tested,
i.e., 5000 min. At the same temperature, DAST and Deoxo-
Fluor were reported to degrade within 300 and 1800 min,
respectively.5
FIGURE 4. ARC thermograms of DAST, Deoxo-Fluor, XtalFluor-
E and XtalFluor-M: self-heat rate versus temperature.
In order to accurately assess the thermal hazards and set
the safety limits for process, storage, and transport tempera-
tures, the onset temperature self-accelerated thermal decom-
position must be identified by more reliable methods.
Accelerated rate calorimetry (ARC), isothermal calorimetry
(IC), and thermal screening unit (TSU) measurements pro-
vide more realistic estimates of the run-away temperature.38
For example, previous hazard safety studies reported by
Rossi et al. on Deoxo-Fluor showed that the onset of
decomposition is around 70 ꢀC using a Calvet calori-
meter (C80), a value much lower than that detected by
DSC (120 ꢀC).39 Likewise, DAST reportedly undergoes a
very energetic decomposition between 50 and 60 ꢀC.40 It is
noteworthy that these figures are somewhat lower than those
previously obtained by ARC which showed onsets of decom-
position at 85 ꢀC for DAST and 100 ꢀC Deoxo-Fluor.5
Therefore, commercial samples of DAST and Deoxo-Fluor
were reevaluated by ARC using the same unit model and
similar sample loading as previously reported (Figure 3).41
Thus, the onset of decomposition started at 60 ꢀC for both
DAST and Deoxo-Fluor, a value which is more aligned with
recent reports. Furthermore, the maximum heat generation
rate of Deoxo-Fluor occurred at 170 ꢀC with a recorded
505 ꢀC/min, a marked difference with the previously reported
10 ꢀC/min (Figure 4).5 These discrepancies could be attrib-
uted to the presence of impurities in the commercial samples,
as it is well recognized that dialkylaminosulfur trifuorides
discolor with aging, and several impurities were previously
detected in commercial sources of Deoxo-Fluor.42 These
(38) Dale, D. J. Org. Process Res. Dev. 2002, 6, 933.
(39) Rossi, F.; Corcella, F.; Caldarelli, F. S.; Heidempergher, F.;
Marchionni, C.; Auguadro, M.; Cattaneo, M.; Ceriani, L.; Visentin, G.;
Ventrella, G.; Pinciroli, V.; Ramella, G.; Candiani, I.; Bedeschi, A.; Tomasi,
A.; Kline, B. J.; Martinez, C. A.; Yazbeck, D.; Kucera, D. J. Org. Process
Res. Dev. 2008, 12, 322.
(40) DAST decomposition data: Bretherick’s Handbook of Reactive
Chemical Hazards, 6th ed.; Urben, P. G., Ed.; Butterworth-Heinemann:
Oxford, Boston; 1999; Vol. 1, p 560.
(41) The ARC tests were conducted under a “heat-wait-search” mode;
i.e., the sample is heated to a predetermined temperature, and the instrument
waits and searches for exothermic activity generated by self-heating of
the sample. The tests were done between the range of 30 to 300 ꢀC with
heating at 5 ꢀC/min, 10 ꢀC steps, a 20 min wait time, and a detection limit of
0.02 ꢀC/min.
(42) Laali, K. K.; Borodkin, G. I. J. Fluorine Chem. 2002, 115, 169.
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and XtalFluor-M were poor fluorinating agents of alcohols
and found totally inert toward carbonyls, they excelled when
used in conjunction with promoters, such as Et3N 3HF,
3
Et3N 2HF, or DBU. A set of general reaction conditions has
3
been developed and the fluorinations were generally high
yielding and significantly more selective in providing less
elimination side products as compared to Deoxo-Fluor and
DAST. Contrary to the latter reagents, XtalFluor-E and
XtalFluor-M are easily handled free-flowing solid which do
not generate highly toxic and corrosive free HF, and are
therefore compatible with borosilicate glassware and com-
mon multipurpose industrial equipment. The aforementio-
ned desirable attributes address most of the shortcomings
associated with the use of Deoxo-Fluor and DAST. Above
and beyond handling, safety, and selectivity considerations,
XtalFluor-E and XtalFluor-M are easily prepared and cost-
effective reagents.
FIGURE 5. ARC thermograms of DAST, Deoxo-Fluor, XtalFluor-
E, and XtalFluor-M: pressure generation rate versus temperature.
Experimental Section
Preparation of XtalFluor-E (1) Using DAST and BF3 THF.
3
ARC results were next compared with those of XtalFluor-E
and XtalFluor-M by using the same set of parameters. Thus,
ARC testings on approximately 1 g samples showed decom-
position onsets at 119 ꢀC for XtalFluor-E and 141 ꢀC for
XtalFluor-M, a significant increase in margin safety over
DAST and Deoxo-Fluor.
To a solution of diethylaminosulfur trifluoride (8.2 mL, 62 mmol)
in anhydrous 1,2-dichloroethane (150 mL) at room tempera-
ture was added, dropwise and under nitrogen, neat BF3 THF
3
(6.8 mL, 62 mmol) over a period of 45 min while keeping the
reaction temperature below 30 ꢀC. The suspension was stirred
for an additional 30 min and then filtered under a blanket of
nitrogen. The solid material was rinsed twice with diethyl ether
(2ꢀ50 mL) and then dried under vacuum to provide 1 (12.1 g,
85%) as colorless crystals: mp 83-85 ꢀC (lit.18 mp 74-76 ꢀC);
1H NMR (CD3CN, 300 MHz) δ 3.87 (m, 4H), 1.35 (t, J = 7.2
Hz, 6H); 19F NMR (CD3CN, 282 MHz) δ 12.9 (m, 2F), -151.1
(s, 4F); 13C NMR (CD3CN, 75 MHz) δ 45.5, 12.6.
Since ARC is performed under near adiabatic conditions,
the information provided is generally considered as highly
reliable and provides valuable heat and pressure generation
rates.43 In this context, a maximum pressure generation rate
of 855 psi/min was observed at 149 ꢀC for DAST, whereas
Deoxo-Fluor decomposed at a marginally lower rate of
788 psi/min at 170 ꢀC (Figure 5). In contrast, XtalFluor-E
showed a maximum pressure generation rate of 357 psi/
min at 191 ꢀC while 85 psi/min at 246 ꢀC was recorded for
XtalFluor-M. Thus, the maximum rates of both XtalFluor
salts occur at significantly higher temperatures than those of
DAST and Deoxo-Fluor. Moreover, the maximum rate of
pressure rise for XtalFluor-E is approximately half the rates
of DAST and Deoxo-Fluor whereas XtalFluor-M is about 1
order of magnitude lower. A similar trend is also observed
when comparing heat generation rates. Maximum rates
of temperature rise of 310 ꢀC/min and 50 ꢀC/min were
recorded for XtalFluor-E and M respectively, as compared
to 711 ꢀC/min and 505 ꢀC/min for DAST and Deoxo-Fluor.
Concluding Remarks. Our investigations on the physico-
chemical properties of dialkylaminodifluorosulfinium salts
have led to important findings, including a novel and com-
plementary preparative method using strong acids and the
discovery of a novel and more stable polymorphic form of
diethylaminodifluorosulfinium tetrafluoroborate. Detailed
safety hazard assessments have demonstrated the enhanced
thermal stability of XtalFluor-E and XtalFluor-M over
Deoxo-Fluor and DAST. Moreover, a new preparative
method of XtalFluor-E has eliminated the need to perform
the hazardous distillations of DAST, a major cost contribu-
tor in the preparation of the latter. Although XtalFluor-E
One-Pot Preparation of XtalFluor-E Using N,N-Diethyltri-
methylsilylamine, SF4, and BF3 THF. To a 5 L flange necked
3
flask fitted with magnetic stirrer, temperature probe, bubbler,
and nitrogen inlet was added dichloromethane (150 mL) and
then the flask cooled to -78 ꢀC. Sulfur tetrafluoride (70 g,
0.65 mmol) was subsurfaced while keeping the temperat-
ure below -65 ꢀC. To the resulting pale purple solution was
added dropwise a solution of diethylaminotrimethylsilane (90 g,
0.62 mol) in dichloromethane (42 mL) while keeping the tem-
perature below -60 ꢀC. The resulting solution was allowed to
slowly warm to room temperature and stirred overnight. To
the resulting dark amber solution was added dichloromethane
(558 mL) followed by boron trifluoride tetrahydrofuran com-
plex (68 mL, 0.61 mol) dropwise over 30 min keeping the
temperature between 15 and 25 ꢀC. The suspension was stirred
for an additional 60 min and then filtered under a blanket of
nitrogen. The solid material was rinsed with diethyl ether (3ꢀ
150 mL)and then dried under vacuum to provide 1 (126 g, 89%)
as off-white crystals: mp 83-85 ꢀC; the NMR spectra were
identical in all respects to those reported above.
Preparation of XtalFluor-M (2) Using Morpho-DAST and
BF3 THF. To a solution of morpholinosulfur trifluoride (13.8 mL,
3
114 mmol) in anhydrous 1,2-dichloroethane (200 mL) at room tem-
perature was added, dropwise and under nitrogen, a solution of
BF3 THF (12.6 mL, 114 mmol) in anhydrous 1,2-dichloroethane
3
(100 mL) over a period of 45 min, while keeping the reaction
temperature below 25 ꢀC. The suspension was stirred for an addi-
tional 30 min and then filtered under a blanket of nitrogen. The solid
material was rinsed twice with anhydrous 1,2-dichloroethane (2 ꢀ
25 mL) and then dried under vacuum to provide morpholinodi-
fluorosulfinium tetrafluoroborate (25.8 g, 94%) as colorless crystals:
mp 129-132 ꢀC (lit.18 mp 104-106 ꢀC); 1H NMR (CD3CN,
300 MHz) δ 3.90-3.85 (m, 8H); 19F NMR (CD3CN, 282 MHz) δ
(43) Information on heat and pressure generation rates are particularly
important in a manufacturing plant setting as it allows the determination of
required cooling capacities and the size of emergency pressure-relief valves
on process reactors.
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10.2 (s, 2F), -151.3 (s, 4F); 13C NMR (CD3CN, 75 MHz) δ 65.7,
48.3 (br).
Representative Procedure Using an XtalFluor Reagent and
dried over magnesium sulfate, and filtered through a pad of
silica gel. Solvents were evaporated, and the resulting crude
material was purified by silica gel flash chromatography using
hexanes/EtOAc (9/1) to provide the title compound (220 mg,
TEA 3HF in DCM: Preparation of 1,1-Difluoro-3-phenylpro-
3
pane (26)17. To a solution of triethylamine trihydrofluoride
(326 μL, 2.0 mmol) in dichloromethane (3.0 mL) at room
temperature were successively added XtalFluor-E (344 mg,
1.5 mmol) and 3-phenylpropionaldehyde (132 μL, 1.0 mmol).
After 2 h, the reaction mixture was quenched at room tempera-
ture with a 5% aqueous sodium bicarbonate solution and stirred
for 15 min, and the resulting mixture was extracted twice using
dichloromethane. The organic phases were combined, dried
over magnesium sulfate, and filtered through a pad of silica
gel. Solvents were evaporated, and the resulting crude material
was purified by silica gel flash chromatography using pentane to
91%) as a clear oil: H NMR (CDCl3, 300 MHz) δ 7.31-7.26
1
(m, 5H), 5.08 (s, 2H), 3.73-3.53 (m, 4H), 2.29-2.21 (m, 2H); 19
F
NMR (CDCl3, 282 MHz) δ -102.7 (m, 2F); 13C NMR (CDCl3,
1
75 MHz) δ 154.6, 136.5, 127.6 (t, JC-F = 249.2 Hz), 127.0
=
(t, 1JC-F=248.7 Hz), 128.7, 128.4, 128.2, 67.4, 53.0 (t, 2JC-F
32.4 Hz), 52.9 (t, 2JC-F=32.9 Hz), 43.9, 34.3 (t, 2JC-F=24.4 Hz),
33.7 (t, 2JC-F=23.9 Hz).
Representative Procedure Using an XtalFluor Reagent and
DBU: Preparation of (S)-N-Cbz-3-fluoropyrrolidine (14)44. To
a solution of (R)-N-Cbz-3-hydroxypyrrolidine (221 mg, 1.0 mmol,
ee >99.9%) in dichloromethane (3.0 mL) cooled at -78 ꢀC were
successively added DBU (224 μL, 1.5 mmol) and XtalFluor-E
(344 mg, 1.5 mmol). After being stirred under nitrogen for
30 min, the reaction mixture was allowed to warm to room
temperature and stirred for 24 h. The reaction mixture was
quenched with a 5% aqueous sodium bicarbonate solution and
stirred for 15 min, and the resulting mixture was extracted twice
with dichloromethane. The organic phases were combined,
dried over magnesium sulfate, and filtered through a pad of
silica gel. Solvents were evaporated, and the resulting crude
material was purified by silica gel flash chromatography using
hexanes/EtOAc (3/1) to afford the title compound (192 mg,
86%) admixed with N-Cbz-2,5-dihydropyrrole (6.9:1 ratio
1
provide the title compound (119 mg, 76%) as a clear oil: H
=
NMR (CDCl3, 300 MHz) δ 7.38-7.22 (m, 5H), 5.65 (tt, 2JH-F
56.7 Hz, JH-H =4.4 Hz, 1H), 2.82 (t, J = 7.7 Hz, 2H), 2.20
=
3
(m, 2H). 19F NMR (CDCl3, 282 MHz) δ -117.5 (dt, JH-F
2
56.7 Hz, 3JH-F=16.9 Hz, 1F); 13C NMR (CDCl3, 75 MHz) δ
140.2, 128.9, 128.6, 126.7, 117.0 (t, JC-F = 238.9 Hz), 35.9
1
(t, 2JC-F=20.5 Hz), 28.7 (t, 3JC-F=6.1 Hz).
Representative Procedure Using an XtalFluor Reagent and
TEA 3HF in DCE: Preparation of 4-Carbethoxy-1,1-difluoro-
3
cyclohexane (32).35 To a solution of triethylamine trihydro-
fluoride (163 μL, 1.0 mmol) in 1,2-dichloroethane (2.0 mL)
was added at room temperature XtalFluor-M (362 mg, 1.5
mmol) followed by 4-carbethoxycyclohexanone (159 μL, 1.0
mmol), and the reaction mixture was heated to reflux. After 2 h,
the reaction mixture was cooled to room temperature, quenched
with a 5% aqueous sodium bicarbonate solution, and stirred for
15 min, and the resulting mixture was extracted twice using
dichloromethane. The organic phases were combined, dried
over magnesium sulfate, and filtered through a pad of silica
gel. Solvents were evaporated, and the resulting crude material
was purified by silica gel flash chromatography using pentane
to provide the title compound (166 mg, 86%) admixed with
4-carbethoxy-1-fluorocyclohex-1-ene (15:1 ratio respectively) as
1
respectively) as a clear oil. Major product: H NMR (CDCl3,
300 MHz) δ 7.37-7.26 (m, 5H), 5.15 (d, 2JH-F=52.5 Hz, 1H),
5.08 (s, 2H), 3.79-3.46 (m, 4H), 2.24-1.91 (m, 2H); 19F NMR
(CDCl3, 282 MHz) δ -177.8 (m, 1F); 13C NMR (CDCl3,
1
75 MHz) δ 154.9, 136.9, 128.7, 128.2, 128.1, 93.0 (d, JC-F
176.8 Hz), 92.2 (d, JC-F =176.2 Hz), 67.1, 53.0 (d, JC-F
=
=
=
1
2
27.1 Hz), 52.7 (d, 2JC-F=27.1 Hz), 44.2, 43.8, 32.4 (d, 2JC-F
2
57.6 Hz), 32.1 (d, JC-F =57.6 Hz). Enantiomeric excess was
determined to be 98.2% ee (HPLC on a Chiralcel OJ 250 ꢀ
4.6 mm column, 215 nm, hexanes/ethanol/2-propanol/TFA
1000:20:5:1, 25 ꢀC, 1.2 mL/min, tR =22.00 min (major), tR =
27.06 min (minor)).
1
a clear oil. Major compound: H NMR (CDCl3, 300 MHz) δ
4.11 (q, J = 7.0 Hz, 2H), 2.53-1.61 (m, 9H), 1.23 (t, J = 7.0 Hz,
3H); 19F NMR (CDCl3, 282 MHz) δ -94.3 (d, 2JF-F=237.5 Hz,
Acknowledgment. We are grateful to Dr. Richard Barnhart
(Pfizer) for acting as a consultant for the thermal experi-
mental screenings. We also thank Professor Pierre Deslong-
2
1F), -99.8 (d, JF-F = 237.4 Hz, 1F); 13C NMR (CDCl3,
1
75 MHz) δ 174.2, 127.2 (t, JC-F=241.6 Hz), 60.5, 40.5, 32.5
(t, 2JC-F=24.3 Hz), 25.0, 14.1.
Representative Procedure Using an XtalFluor Reagent and
ꢀ
champs (Universite de Sherbrooke), Professor Paul Brassard
(Universite Laval, retired), and Ms. Marie-Marthe Leroux for
ꢀ
TEA 2HF: Preparation of N-Cbz-3,3-difluoropyrrolidine (36)44.
3
proofreading the manuscript.
To a solution of triethylamine trihydrofluoride (326 μL, 2.0 mmol)
and triethylamine (139 μL, 1.0 mmol) in dichloromethane (3.0 mL)
at room temperature were successively added XtalFluor-E
(344mg,1.5mmol) andN-Cbz-pyrrolidin-3-one (219 mg, 1.0 mmol).
After 24 h, the reaction mixture was quenched at room tem-
perature with a 5% aqueous sodium bicarbonate solution and
stirred for 15 min, and the resulting mixture was extracted twice
using dichloromethane. The organic phases were combined,
Supporting Information Available: Alternative experimen-
tal methods for the preparation of 1; experimental procedures,
compound characterization data for all compounds, including
1
references to the known compounds; copies of H NMR, 13C
NMR, and 19F NMR spectra for all compounds; DSC and ARC
thermograms of 1, 2, DAST, and DeoxoFluor; XPRD spectra
of 1. This material is available free of charge via the Internet at
http://pubs.acs.org.
(44) Giardina, G.; Dondino, G.; Grugni, M. Synlett 1995, 55.
J. Org. Chem. Vol. 75, No. 10, 2010 3411