Tetraflic acid (1,1,2,2-Tetrafluoroethanesulfonic acid, HC 2F4SO3H) and gallium tetraflate as effective catalysts in organic synthesis

Article abstract of DOI:10.1002/adsc.201200111

Tetraflic acid offers ample acidity for various organic reactions that require high acidity. Its gallium(III) salt is an efficient catalyst under mild condtions for synthetic transformations such as the ketonic Strecker reaction for the synthesis of fluorinated α-amino nitriles and condensation- cyclzation reactions using suitable fluoro ketones and 1,2-disubstituted benzenes for the direct preparation of 5-membered or 6-membered fluorinated heterocycles. Copyright

Full text of DOI:10.1002/adsc.201200111

FULL PAPERS
DOI: 10.1002/adsc.201200111
Tetraflic Acid (1,1,2,2-Tetrafluoroethanesulfonic Acid,
HC F SO H) and Gallium Tetraflate as Effective Catalysts
2
4
3
in Organic Synthesis
a,
a,
a
a
G. K. Surya Prakash, * Thomas Mathew, * Chiradeep Panja, Aditya Kulkarni,
a
b,
George A. Olah, and Mark A. Harmer *
Loker Hydrocarbon Research Institute, and Department of Chemistry, University of Southern California, Los Angeles,
a
California 90089-1661, USA
E-mail: gprakash@usc.edu or tmathew@usc.edu
Central Research and Development, The DuPont Company, Wilmington, Delaware 19880-0356, USA
b
E-mail: mark.a.harmer@usa.dupont.com
Received: February 9, 2012; Revised: May 7, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200111.
Abstract: Tetraflic acid offers ample acidity for vari- stituted benzenes for the direct preparation of 5-
ous organic reactions that require high acidity. Its membered or 6-membered fluorinated heterocycles.
gallium ACHTUNGTRENNUNG( III) salt is an efficient catalyst under mild
condtions for synthetic transformations such as the
ketonic Strecker reaction for the synthesis of fluori- Keywords: adamantylation; condensation–cycliza-
nated a-amino nitriles and condensation–cyclzation tion; gallium
ACHTUNGTRENNUNG( III) tetraflate; isomerization; tetraflic
reactions using suitable fluoro ketones and 1,2-disub- acid; 1,1,2,2-tetrafluoroethanesulfonic acid
Introduction
laborative effort with DuPont, we found that tetraflic
acid is a suitable catalyst for many strong acid-cata-
As part of sustainable growth, the development of lyzed organic reactions that can be carried out in
active catalysts that significantly reduce waste, high BF ·H O as well as triflic acid. The catalyst is less vol-
3
2
energy requirements, process expenditure making the atile than triflic acid, which makes it easier to handle.
overall process safer and economical is always Herein, we would like to report the results of some
a major focus of research in the area of synthetic or- organic synthetic transformations catalyzed by tetra-
ganic chemistry. Such catalysts can often replace
highly expensive acids in many organic synthetic
transformations in which high acidity and catalytic ac-
ACTHUNGTRENNUfGN lic acid and its gallium ACHTUNGTNERNUNG( III) salt.
tivity are often required. During our studies on such Results and Discussion
superacidic systems, a few of us previously found that
[
1–3]
boron trifluoride monohydrate (BF ·H O)
and Tetraflic acid is found to be an efficient superacid for
trifluoroethanol catalyzing a variety of organic reactions that require
BF ·2CF CH OH) complexes are very effective high acidity such as the isomerization of pivaladehyde
3
2
boron
trifluoride
[
4]
(
3
3
2
catalysts with high acidity. Efforts to find acid systems to isopropyl methyl ketone, isomerization of cyclohex-
of high acidity that are environmentally safer and can ane to methylcyclopentane, conversion of n-pentane to
be handled easily still remain highly desirable. Recent a mixture of alkanes and electrophilic bromination of
AHCTUNGTRENNUNG
[
5–7]
studies by Harmer et al.
from DuPont have re- deactivated aromatics using N-bromosuccinimide, etc.
vealed that 1,1,2,2-tetrafluoroethanesulfonic acid (tet-
raflic acid) offers high acidity, closer to that of tri-
fluoromethanesulfonic acid (triflic acid) and can be Isomerization of Pivalaldehyde to Methyl Isopropyl
employed in a wide range of organic reactions as Ketone
a suitable and less expensive superacid. As a continua-
tion to study organic transformations, which require During our studies of various methods for the isomer-
high acidity using less expensive acid systems, in a col- ization of pivalaldehyde (obtained by direct super-
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Scheme 1. Acid-atalyzed
pivalaldehyde–ethyl
isopropyl
ketone rearrangement.
[
8]
2
.6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
acid-catalyzed carbonylation of isobutane)
to
Scheme 3. Isomerization of endo-tricyclo[5.2.1.0 ]decane to
3
,7
methyl isopropyl ketone as a tool for the synthesis of adamantane (tricyclo[3.3.1.1 ]decane).
methyl isopropyl ketone 1 (Scheme 1), we found that
the complete rearrangement of pivalaldehyde 2 to
2
.6
methyl isopropyl ketone 1 requires high acidity (ÀH
Isomerization of Tricyclo[5.2.1.0 ]decane to
o
[4,9]
ca. 10.9).
The pivalaldehyde–methyl isopropyl Adamantane
ketone rearrangement proceeded quantitatively at
room temperature with neat HCF CF SO H showing The synthesis of adamantane by Lewis acid-catalyzed
2
2
3
that the acidity of the acid is high, comparable to isomerization of tetrahydrodicyclopentadiene
5
2
.6
those of other related Brønsted superacids.
(endo-tricyclo[5.2.1.0 ]decane) was first carried out
[11]
by Schleyer. Many reports of this transformation in-
volving both Lewis and Brønsted acids of high acidity
are known in the literature. Olah reported the isomer-
ization of tetrahydrodicyclopentadiene 5 in fluoroanti-
Isomerization of Cyclohexane to Methylcyclopentane
[
12]
In superacid media, an equilibrium exists between cy- monic acid (HF·SbF5)
while McKervey et al. re-
[
10]
clohexane and methylcyclopentane. Depending on ported the process in the gas phase over a chlorinated
[
13]
the strength of the acid and the conditions, the equi- platinum-alumina catalyst.
Adamantane was ob-
librium can be shifted in the desired direction. This is tained in quantitative yield by the reductive isomeri-
2
.6
a well-established catalytic reaction occurring through zation of dicyclopentadiene (tricyclo[5.2.1.0 ]deca-
the formation of a carbocation intermediate 3, leading 3,8-diene) using sodium borohydride/triflic acid in
[
14]
to a ring contraction and the formation of the thermo- Freon 113. Tetraflic acid also catalyzes the isomeri-
dynamically more stable tertiary methyl cyclopentyl zation of 5 to adamantane 6 under optimized condi-
carbocation intermediate 4, which subsequently un- tions. When 5 was heated in tetraflic at 1208C for 6 h,
dergoes hydride transfer to form methylcyclopentane in a closed pressure tube, it isomerized to adamantane
(
Scheme 2). The reaction is catalyzed by strong Lewis (6, 53%) which sublimed to the top of the tube
as well as Brønsted acids. (Scheme 3). GC-MS analysis shows the formation of
Tetraflic acid is found to be strong enough to drive some amount of dimers and isomerized products.
the equilibrium towards methycyclopentane. Though
the conversion is very slow at room temperature,
Conversion to 55% methylcyclopentane was observed n-Pentane Isomerization and Disproportionation
at 808C in 16 h (GC-MS analysis).
n-Pentane under superacidic conditions undergoes
isomerization and disproportionation to yield a mix-
[
10,15]
ture of hydrocarbons.
Similar to the results ob-
tained using other superacidic catalysts, n-pentane in
the presence of tetraflic acid gives a mixture of but-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
perature (Scheme 4). Isopentane is obtained by iso-
merization whereas butanes and hexanes are obtained
by disproportionation.
Scheme 2. Isomerization of cyclohexane to methylcyclopen-
tane.
Scheme 4. Isomerization and disproportionation of n-pent-
ACHTUNGERTNNUNGa ne to a mixture of alkanes.
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Tetraflic Acid (1,1,2,2-Tetrafluoroethanesulfonic Acid, HC F SO H) and Gallium Tetraflate
2
4
3
Table 1. Bromination of deactivated aromatics with NBS in
HCF CF SO H.
2
2
3
Scheme 5. Disproportionation of p-xylene catalyzed by
TFESA.
Disproportionation of p-Xylene
Xylene disproportionation to a mixture of isomers,
and trimethylbenzenes including mesitylene is well
[16]
known.
tetraflic acid was stirred at room temperature for
4 h, no appreciable change was observed. However,
When a solution of p-xylene in excess of
1
when heated to 1208C for 6 h, a mixture of products
containing toluene, mixture of xylenes, mesitylene, its
ring isomers, and higher derivatives was obtained due
to disproportionation and transalkylation (Scheme 5).
Halogenation of Deactivated Aromatics
A combination of NXS (N-halosucccinimides) with
strong acids such as CF SO H and BF ·H O, etc., has
3
3
3
2
been used as effective halogenating agents for the hal-
ogenations (chlorination, bromination and iodination)
of various deactivated aromatics under mild condi-
[17,3e]
[a]
tions.
As already demonstrated, tetraflic acid
From GC-MS, not optimized.
Rest is mainly a mixture of dibrominated products.
[
[
[
[
b]
c]
d]
e]
(
TFESA) catalyzes various organic synthetic transfor-
6
8% conversion.
mations that require high acidity with catalytic activi-
ty comparable to that of triflic acid. Intrigued by
these results, we decided to study the N-halosuccin-
At 808C, 83% conversion.
Rest is a mixture of dibromonitrobenzenes.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
imide-tetraflic acid system as a potential halogenating
agent for deactivated aromatics. We found that, as ex- Gallium Tetrafluoroethanesulfonate [Gallium
pected, all N-halosuccinimides in tetrafluoroethane- Tetraflate, (HCF CF SO ) Ga]-Catalyzed Synthetic
AHCTUNGTRENNUNG
2
2
3 3
sulfonic acid are effective halogenating agents for de- Transformations
activated aromatics (Scheme 6). The halogenation re-
actions of all the substrates mentioned in Table 1 and Strong and efficient Lewis acids such as AlCl , AlBr ,
3
3
Table 2 (except the bromination of nitrobenzene) SbF , etc. are prone to fast hydrolysis and consequent
5
were carried out at room temperature. However, deactivation leaving them non-recyclable. Most of the
during the reactions, sucessive halogenation always reactions with these catalysts require water-free con-
remained a major competing reaction resulting in ditions and large amounts of the catalysts. However,
lower selectivity for monohalogenation prompting us gallium
ACHTUNGTRENNUNG( III) trifluoromethanesulfonate [Ga ACHTUNGTRENNUNG( OTf) ,
3
to stop the reaction before complete conversion in gallium triflate] acts as an effective but mild and
some cases.
aqueous-stable Lewis acid catalyst for many organic
synthetic transformations such as Friedel–Crafts alky-
lations, dehydration of oximes, Beckman rearrange-
ment, Strecker reaction, synthesis of fluorinated and
[
18,19]
non-fluorinated heterocycles etc.
Based on its
structural similarity, high acidity and low volatitily, in
comparison with trifluoromethanesulfonic acid, we
anticipated a similar trend in the physical and chemi-
cal properties including the catalytic behavior of the
gallium
Scheme 6. Halogenation of deacivated aromatics using the flate was prepared by treating Ga metal with excess
NBS/HCF CF SO H system. of tetraflic acid at 1208C (Scheme 7). After work-up,
ACTHUNGERTNNUNG( III) salt of tetraflic acid. Gallium ACHTUNGRTENNUNG( III) tetra-
2
2
3
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Table 2. Chlorination/iodination of deactivated aromatics
with NCS/NIS in HCF CF SO H.
2
2
3
Figure 1. TGA data of gallium tetraflate.
Gallium Tetraflate-Catalyzed Strecker Reactions of
Aldehydes, Ketones and Fluorinated Ketones
[
19e]
Our previous studies
have shown shown that
gallium(III) triflate [(CF SO ) Ga] is capable of cata-
AHCTUNGTRENNUNG
3
3 3
lyzing the three-component Strecker reaction of alde-
hydes/ketones with amines and trimethylsilyl cyanide
for the synthesis of a-amino nitriles (8), the precur-
sors of a-amino acids. Such Strecker reactions were
again carried out using a mixture of aldehydes/ke-
tones, amines and trimethylsilyl cyanide in the pres-
ence of (HC F SO ) Ga as a catalyst (Scheme 8,
2
4
3 3
Table 3). The new gallium salt shows high catalytic ac-
[
a]
Rest of the product contains a mixture of dihalo deriva- tivity similar to that observed with gallium (III) tri-
tives (trihalo derivatives also in some cases).
flate for this reaction.
Friedel–Crafts Alkylation (Adamantylation) of
Toluene Using Gallium Tetraflate
Scheme 7. Preparation of (HCF CF SO ) Ga.
2
2
3 3
Gallium triflate was found to be a very effective and
strong Lewis acid catalyst with its activity exceeding
gallium tetraflate was obtained as a white powder. Al-
though it is slightly less stable than gallium triflate, it
is less hygroscopic with similar chemical properties.
The initial weight loss in the TGA diagram is proba-
bly due to the loss of coordinated water molecules.
Decomposition of (HC F SO ) Ga occurs above
2
4
3 3
2208C (Figure 1). Gallium AHCTUNGRETNNUN(G III) tetraflate was found to
be an effective catalyst for many acid-catalyzed or-
ganic transformations previously studied using
gallium ACHTUNGTRENNUNG( III) triflate in our group.
Scheme 8. Strecker reaction of aldehydes and ketones using
gallium tetraflate.
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Tetraflic Acid (1,1,2,2-Tetrafluoroethanesulfonic Acid, HC F SO H) and Gallium Tetraflate
2
4
3
Table 3. Gallium tetraflate-catalyzed Strecker reactions of aldehydes, ketones and fluorinated
ketones.
that of other well-known lanthanide triflates and it re- ed the p-isomer as the major product, lessening the
[
18d]
mains active at ambient temperature.
least reactive triflates produced p-tolyladamantane se- tion showing that the Lewis acidity of gallium
lectively, a significant shift in selectivity was observed raflate is slightly lower than that of gallium (III) tri-
with Ga(OTf) , due to an extensive isomerization re- flate. When the reaction was carried out at 358C for
While the amount of m-isomer due to a lower rate of isomeriza-
AHCTUNGTR(ENNNUG III) tet-
ACHTUNGTRENNUNG
3
action to the m-product. However, the adamantyla- 1 h, 25% conversion of toluene to p-adamantylto-
tion reaction in the presence of (HC F SO ) Ga yield- luene was observed. Increasing the temperature to
2
4
3 3
5
08C resulted in quantitative conversion with the for-
mation of 63% p-isomer as the major product in 1 h
Scheme 9).
(
Synthesis of 5-Membered and 6-Membered
Fluorinated Heterocycles using Gallium Tetraflate
Benzimidazolines, benzothiazolines, benzoxazolines and
dihydrobenzoxazinones are important biogically active
heteroaromatics which have wide applications in the ag-
ricultural and pharmaceutical arenas and are useful
[20–23]
building blocks in organic synthesis.
As it is well-
known that the presence of fluorine can induce signifi-
cant changes in many biological and physicochemical
properties such as liophilicity, bioavailability, pharmaco-
Scheme 9. Adamantylation of toluene using gallium tetra- dynamics, etc., we were interested in synthesizing the
[24]
flate.
fluorinated analogues of these classes of compounds.
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Scheme 10. Reactions of benzaldehyde/ketones with diamines and synthesis of fluorinated heterocycles from fluorinated bu-
tynoic acids.
Benzimidazolines 10 are generally synthesized from
the reaction of 1,2-phenylenediamines and benzalde-
[
25]
hyde (Scheme 10).
However, when ketones (when
1
2
R , R =alkyl, phenyl) were used under similar condi-
tions, 1,5-benzodiazepine derivatives 11 were formed
as the major products (Scheme 10). Reports on the
direct synthesis of fluorinated benzimidazolines are
[
26]
[27]
[28]
rather rare.
Funabiki et al.
have reported the
preparation of fluorinated benzimidazolines 13 from
fluorinated butynoic acids 12 (Scheme 10). The syn-
thesis of more diverse and functionalized starting ma-
terials for this reaction is difficult. Furthermore, the
reaction conditions were harsh, and chemical yields
were not so significant.
Scheme 11. ACHTUGNTRENN(NUG HC F SO ) Ga-catalyzed synthesis of fluorinat-
2 4 3 3
ed heterocycles from 1,1,1-trifluoroacetone and disubstitut-
ed benzenes.
On the other hand, the direct syntheses of fluori-
nated benzimidazolines/benzoxazolines/benzothiazo-
lines 13, and dihydrobenzoxazinones/dihydrobenzo- 13a, benzoxazolines 13b, benzothiazoline 13c, dihy-
thiinones 14 from fluorinated ketones and o-phenyle- dro-3,1-benzoxazin-4-one 14a and 3,1-benzoxathiin-4-
nediamines/o-aminophenols/o-aminothiophenols
as one 14b) in high yield and purity.
well as anthranilic acid/thiosalicylic acid have been re-
[
19f,29]
ported only recently.
nated benzimidazolines directly from fluorinated ke- Conclusions
tones and diamines using gallium triflate as the Lewis
acid catalyst was found to be successful, we decided Tetraflic acid has proven to be an efficient non-oxidiz-
Since the synthesis of fluori-
ACHTUNGTRENNUNG
to explore the same methodology using gallium
tetraflate as a catalyst.
ACHTUNGTRENNUNG
We found that 1,1,1-trifluoroacetone undergoes hydrocarbon isomerization reactions and halogena-
a smooth condensation–cyclization reaction with 1,2- tion of deactivated aromatics. Gallium(III) tetraflate,
phenylenediamine or suitable 1,2-disubstituted ben- (HC F SO ) Ga shows good catalytic activity compa-
AHCTUNGTRENNUNG
2
4
3 3
zenes using 5 mol% of gallium
ACTHUNGTRNENUNG( III) tetraflate in rable to that of gallium ACHTGNUTRENNUNG( III) triflate for the ketonic
CH Cl at 50–1008C (Scheme 11, Table 4). Many of Strecker reaction and condensation–cyclization reac-
2
2
these reactions are feasible even at room temperature tions. Both tetraflic acid and its gallium salt are ex-
with longer reaction times. The reactions are clean, pected to be effective catalysts and can be potentially
and removal of solvent afforded the corresponding useful for many other acid-catalyzed organic transfor-
trifluoromethylated heterocycles (benzimidazoline mations.
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Tetraflic Acid (1,1,2,2-Tetrafluoroethanesulfonic Acid, HC F SO H) and Gallium Tetraflate
2
4
3
Table 4. ACTHUNGTRENNGU(N HC F SO ) Ga-catalyzed preparation of 5- and 6-membered fluoroheterocylcles.
2 4 3 3
Experimental Section
were identified by comparison of their spectral data (GC-
MS, NMR) with those of the authentic samples.
General Method for Isomerization and Dispropor-
tionation Reaction using Tetraflic Acid
Preparation of Gallium ACHTUNGERTNNUNG( III) Tetrafluoroethane-
sulfonate [(HCF CF SO ) Ga]
2
2
3 3
In a pressure tube, 4 mmol of the substrate (pivalaldehyde/
cyclohexane/tetracyclodicyclopentadiene/n-pentane/p-
Gallium tetraflate was prepared following a procedure simi-
lar to the one for the preparation of gallium triflate. In a 15-
mL pressure tube, gallium metal (0.7 g, 10 mmol) was
placed. Tetrafluoroethanesulfonic acid (10.4 g, 57 mmol) was
then added and the mixture stirred at 1208C for 24 h. After
cooling the mixture to 08C, it was poured into 200 g of ice
and then filtered to remove any unreacted gallium metal
and other insoluble materials. Water and excess tetraflic
acid were removed by evaporation and the residue was then
dried by heating at 1508C for 12 h using a P O trap under
xylene) were added to 3 mL of tetraflic acid with cooling
and the tube closed. The mixture was stirred at the required
temperature for the specified period of time. In the case of
pivaladehyde and tetracyclodicyclopentadiene, the reaction
mixture was quenched with ice, neutralized with NaHCO3
and extracted with CH Cl (10 mL), washed with water,
2
2
dried over anhydrous Na SO and analyzed by GC-MS. In
2
4
other cases, the organic layer was carefully separated and
neutralized with 10% NaHCO , washed with water and
3
2
5
dried over Na SO and the product was analyzed by GC-
2
4
vacuum. Gallium (III) tetrafluoroethanesulfonate (gallium
tetraflate) was obtained as a white powder; yield: 4.3 g
MS.
(
70%). The initial weight loss in the TGA diagram is proba-
bly due to the loss of coordinated water molecules. Decom-
General Method for Halogenation of Deactivated
Aromatics using N-Halosuccinimide in Tetraflic Acid
1
9
position of (HC F SO ) Ga occurs above 2208C. F NMR
2
4
3 3
(
CD CN, 376.3 MHz): d=À123.3 (m, 2F), À136.9 (m, 2F);
3
7
1
Tetraflic acid (4 mL) was added to a mixture of the deacti-
vated aromatic compound (10 mmol) and N-halosuccinimide
Ga NMR (CD CN, 152.4 MHz using GaCl as external
3 3
standard with d=0 ppm): d=À327.14 (broad s). Elemental
(
10 mmol) in a pressure tube and the tube closed. The mix-
analysis of a sample gave the results close to the calculated
ture was stirred at the required temperature for the speci-
fied period of time (Table 1, Table 2). The progress of the
reactions was monitored by GC-MS analysis. The reaction
mixture was quenched with ice/water (15 mL) and extracted
with dichloromethane (2ꢁ15 mL). The organic layer was
washed with a saturated solution of sodium bisulfite or
sodium thiosulfate (15 mL) to remove any free halogen
present followed by saturated sodium bicarbonate solution
value of the dihydrate (HC F SO ) Ga·2H O: calcd. C
2 4 3 3 2
11.10, H 1.09, F 35.13, Ga 10.74; found C 10.96, H 1.13, F
33.5, Ga 10.6.
Friedel–Crafts Alkylation of Toluene using 1-
Bromoadamantane Catalyzed by (HC F SO ) Ga
2
4
3 3
To 1-bromoadamantane (0.215 g, 1 mmol) taken in a pres-
sure tube, toluene (2 mL) and (HC F SO ) Ga (0.122 g, 0.2
(
(
15 mL) and brine (20 mL). It was then washed with water
2ꢁ30 mL) and dried over anhydrous sodium sulfate. After
2
4
3 3
mol) were added and the tube was closed. When the reac-
removing the solvent under reduced pressure, the products
tion was carried out at 358C for 1 h, 25% conversion of tolu-
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G. K. Surya Prakash et al.
FULL PAPERS
ene to p-adamantyltoluene was observed. However, when
the reaction was conducted at 508C, conversion was found
to be 100% after 1 h. Progress of the reaction was moni-
tored at different time intervals by TLC and NMR. After
the reaction was complete, the reaction mixture was filtered
and the solvent was removed under reduced pressure. The
crude product was analyzed using NMR and GC-MS and
characterized by comparing its data with those of the au-
thentic sample.
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[6] C. P. Junk, M. A. Harmer, A. E. Feiring, F. L. Schadt,
Z. Schnepp, U.S. Patent 7,834,209, 2010.
General Method for (HC F SO ) Ga-Catalyzed
Strecker Reaction and Condensation–Cyclization
Reaction of Fluorinated Ketones
[7] a) M. A. Harmer, C. Junk, V. Rostovtsev, L. G. Carcani,
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Harmer, L. G. Carcani, J. Org. Chem. 2008, 73, 711–
2
4
3 3
For the Strecker reaction, a mixture of fluorinated ketone
7
14.
(
3 mmol) and amine (2.2 mmol) dissolved in 3 mL of
[
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2
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3 3
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FULL PAPERS
10 Tetraflic Acid (1,1,2,2-Tetrafluoroethanesulfonic Acid,
HC F SO H) and Gallium Tetraflate as Effective Catalysts
2
4
3
in Organic Synthesis
Adv. Synth. Catal. 2012, 354, 1 – 10
G. K. Surya Prakash,* Thomas Mathew,* Chiradeep Panja,
Aditya Kulkarni, George A. Olah, Mark A. Harmer*
10
asc.wiley-vch.de
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