Transformation of Trifluorotoluenes Triggered by Titanium(IV) Chloride-Catalyzed Hydrodefluorination using Hydrosilanes

Article abstract of DOI:10.1002/adsc.201500920

The titanium tetrachloride-catalyzed hydrodefluorination reaction of trifluorotoluene derivatives was developed using triethylsilane as the reducing agent. The reaction produced various toluene derivatives with high chemoselectivities by means of readily accessible reagents. This hydrodefluorination process was extended to the intramolecular Friedel-Crafts benzylation reaction, furnishing polycyclic aromatics of various ring sizes in good yields.

Full text of DOI:10.1002/adsc.201500920

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DOI: 10.1002/adsc.201500920
Transformation of Trifluorotoluenes Triggered by Titanium(IV)
Chloride-Catalyzed Hydrodefluorination using Hydrosilanes
Takayuki Yamada,a Kodai Saito,a and Takahiko Akiyamaa,*
a
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Fax: (+81)-3-5992-1029; e-mail: takahiko.akiyama@gakushuin.ac.jp
Received: September 30, 2015; Revised: October 18, 2015; Published online: December 9, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500920.
Abstract: The titanium tetrachloride-catalyzed hy-
drodefluorination reaction of trifluorotoluene deriv-
atives was developed using triethylsilane as the re-
ducing agent. The reaction produced various tolu-
ene derivatives with high chemoselectivities by
means of readily accessible reagents. This hydrode-
fluorination process was extended to the intramo-
lecular Friedel–Crafts benzylation reaction, furnish-
ing polycyclic aromatics of various ring sizes in
good yields.
of harsh reducing agents and results in low chemose-
lectivities of carbon-fluorine bonds. Hydrodefluorina-
tion, based on single electron reduction using low-
valent metals or electrochemistry, is also limited to
C(sp ) F bond cleavage. On the other hand, Ozerov
et al. developed the Lewis acidic silylium cation-cata-
2
[6]
À
3
À
lyzed hydrodefluorination of the C(sp ) F bond using
triethylsilane as a reducing agent.[7] This method al-
lowed an access to various hydrocarbons under mild
reaction conditions with an easily handled reducing
agent. However, the precatalyst for the generation of
the silylium cation is less accessible and the reaction
must be performed under strictly anhydrous condi-
tions. Furthermore, although carbon-carbon bond for-
mation reactions initiated by activation of carbon-flu-
orine bonds have been reported,[7] there are few ex-
Keywords: carbon-fluorine bond; hydrocarbons; hy-
drodefluorination; hydrosilanes; transition metal
catalysts
À
amples of multi-functionalization of the C F bond to
C C bonds involving hydrodefluorination based on
À
Activation and transformation of inert chemical Lewis acidic activation of the trifluoromethyl group.[8]
bonds remains a major task of modern organic Therefore, the development of a new system for acti-
chemistry.[1] A carbon-fluorine bond is the most stable vation and transformation of trifluorotoluene deriva-
chemical bond among single bonds attached to tives based on Lewis acidic cleavage of carbon-fluo-
carbon atom and its activation is extremely challeng- ride bonds by means of an easily accessible catalyst is
ing.[2] On the other hand, a large number of organo- in high demand. We wish to report herein the titani-
fluorine compounds are commercially available, and um chloride(IV)-catalyzed hydrodefluorination[9] of
the robustness of the carbon-fluorine bond is an ad- trifluorotoluene derivatives using triethylsilane as the
vantage as a reagent among fairly reactive and labile terminal reductant. This method was successfully ex-
organohalogen compounds.[3] Therefore, the develop- tended to the intramolecular Friedel–Crafts alkylation
ment of transformation reactions of carbon-fluorine reaction for the construction of polycyclic compounds
bonds would not only be challenging, but would, (Scheme 1).
more importantly, give a direct access to valuable
compounds.
At the outset, we selected 1a as the model substrate
and treated it with an excess amount of metal salts
The trifluoromethyl group is one of the most MXn in the presence of triethylsilane in dichloro-
common representative functional groups having ethane at 808C (Table 1). No reaction took place with
3
À
a C(sp ) F bond and can be transformed into other AlCl3 and BF3·OEt2 (entries 1–4). When we used zir-
3
À
C(sp ) R bonds. Hydrodefluorination, which is the conium(IV) chloride or hafnium(IV) chloride, the
À
À
conversion of a C F bond to a C H bond, is the sim- target product 2a was obtained in low yields (entries 5
plest transformation of carbon-fluorine bonds.[4] Al- and 6). Among the metal halides examined, titanium
though transition metal-catalyzed hydrodefluorination tetrachloride (1 equiv.) gave the highest yield of 2a
has been extensively investigated,[5] this reductive (90% yield, entry 7). Fortunately, the loading of TiCl4
À
cleavage of the C F bond generally requires the use
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Transformation of Trifluorotoluenes
Scheme 2. Substrate scope of the hydrodefluorination.
are summarized in Scheme 2. Upon subjection of sub-
strates bearing aryl groups to the optimized condi-
tions, hydrodefluorination proceeded smoothly to
give 2b–2e in good to high yields. Interestingly, com-
plete chemoselectivities were observed in the reaction
3
À
of substrates 1f and 1g containing C(sp ) F and
2
À
C(sp ) halogen bonds, and the corresponding aryl
halides were obtained in 87% and 83% yields, respec-
tively. A substrate 2h bearing a methoxy group, which
potentially possesses coordinating ability to Lewis
acids, also participated in the reaction.[10] Additional-
ly, substituents such as benzyl (2i), fluorine (2j),
phenyl (2k) and long alkyl (2l) groups at the ortho-
position were tolerated under the conditions.
Scheme 1. Representative approach for the hydrodefluorina-
tion of trifluorotoluene derivatives.
Table 1. Examination of the reaction conditions.
In order to extend the synthetic utility of the pres-
ent hydrodefluorination, we subjected o-benzyltri-
fluorotoluene (3a) to the hydrodefluorination under
the optimized conditions to explore the intramolecu-
lar Friedel–Crafts alkylation (Scheme 3).[8] While hy-
drodefluorination preferentially occurred under the
optimized conditions, sequential hydrodefluorination–
intramolecular Friedel–Crafts alkylation successfully
proceeded to afford 4a in 75% isolated yield under di-
luted conditions.
Entry
MXn
Y [mol%]
Time [h]
Yield [%][a]
1
2
3
4
5
6
AlCl3
300
300
300
300
300
300
100
30
13
13
20
20
4
11
3
0
0
0
0
17
29
90
90
Et2AlCl
BF3·OEt2
BBr3
ZrCl4
HfCl4
TiCl4
The scope of the reaction is summarized in
Scheme 4. The present carbodefluorination reaction
7
8[b]
TiCl4
20 min
[a]
Isolated yields.
Reaction was carried out at room temperature.
[b]
could be reduced to 30 mol% without sacrificing the
yield (90%, entry 8).
Next, we explored the scope of the TiCl4-catalyzed
hydrodefluorination of trifluorotoluene derivatives
Scheme 3. Initial attempt for sequential hydrodefluorination
under the optimized reaction conditions. The results with carbodefluorination.
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Takayuki Yamada et al.
Scheme 4. Substrate scope for sequential hydrodefluorina-
tion with carbodefluorination.
Scheme 5. Deuterium labelling experiments and proposed
reaction mechanism.
was effective for the formation of medium-sized rings, alyzed hydrodefluorination of trifluorotoluenes which
in particular 6- to 8-membered rings.[11] The Friedel– involves a niobium carbenoid intermediate.[5e,8c]
Crafts alkylation for the construction of 9-membered
To understand the reaction mechanism of the se-
ring 4h also proceeded, albeit in low yield. This quential hydrodefluorination–intramolecular Friedel–
method was also applicable to the synthesis of fluo- Crafts alkylation, we compared the reactivities of var-
renes 4i in moderate yields. It should be noted that ious benzyl fluorides (Scheme 6). Interestingly, al-
this sequential hydrodefluorination–carbodefluorina- though difluorotoluene 6b and fluorotoluene 7b were
tion based on the Lewis acidic activation of a trifluoro-
methyl group had not been reported to date.
To gain further insight into the reaction mechanism
of this hydrodefluorination, deuterium labelling ex-
periments were performed on the reaction of 1a and
3b using the deuterated silane, Et3SiD (Scheme 5).
À
À
All C F bonds of 1a were fully converted to C D
bonds with 97% D. When 3b was employed for the
deuterium labelling experiment of sequential hydro-
defluorination and Friedel–Crafts alkylation, two
geminal deuterium atoms were incorporated at the
benzylic position (97% D). These results clearly indi-
cate that Et3SiH played a key role in the reduction of
À
C F bonds. This reaction was initiated by the abstrac-
tion of a fluorine atom by titanium tetrachloride, af-
fording a difluorobenzyl cation species. This inter-
mediate is rapidly reduced by Et3SiH, resulting in the
formation of difluorotoluene, Et3SiF, and regeneration
of TiCl4. The sequential reduction of fluorotoluenes
in a similar manner finally produces the toluene de-
rivatives. This reaction mechanism is totally different
Scheme 6. Hydrodefluorination–carbodefluorination using
from our previously reported low-valent niobium-cat- fluorotoluene derivatives and the effect of hydrosilane.
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Transformation of Trifluorotoluenes
1N aqueous HCl (5 mL) was added to quench the reaction.
After the mixture was extracted with AcOEt three times,
the organic phase was dried over Na2SO4. The filtrate was
concentrated under reduced pressure, and the residue was
purified by preparative thin layer chromatography on silica
gel (hexanes) to give 4a as colorless solid; yield: 31.7 mg
(0.176 mmol, 75%).
smoothly reduced to 4b at room temperature within
10 min, while 3b was not fully consumed at room tem-
perature for 30 min. These results suggest that reduc-
tion of trifluorotoluene 3b to difluorotoluene 6b is the
rate-determining step. Furthermore, when the reac-
tion of 3b was carried out without Et3SiH at 808C for
12 h, 3b was recovered in 85% and cyclized product
8b was obtained in low yield (3%). This result also in-
dicates that hydrodefluorination of the trifluorometh-
yl group of 3b is the first step of this transformation
and, 6b or 7b are precursors for the Friedel–Crafts al-
kylation step.
References
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In conclusion, we have developed a TiCl4-catalyzed
hydrodefluorination of trifluorotoluenes using trie-
thylsilane as reducing agent. This hydrodefluorination
was successfully extended to the sequential intramo-
lecular Friedel–Crafts alkylation, affording various
fused carbocycles in good yields. This method featur-
ing the combined use of an easily accessible catalyst
with a reducing agent is quite effective for the reduc-
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Experimental Section
Typical Experimental Procedure for Hydrode-
fluorination
The reaction of 1a is described. A magnetic stir bar was
placed in a test tube that was then dried with a heat gun
under reduced pressure and filled with nitrogen. 1a
(50.1 mg, 0.200 mmol), anhydrous 1,2-dichloroethane
(0.20 mL) and triethylsilane (320 mL, 2.00 mmol) were
added to the text tube successively under a nitrogen atmos-
phere. Titanium tetrachloride (0.20 mL,0.067 mmol, 0.3M
solution in 1,2-dichloroethane) was then added. After stir-
ring at room temperature for 30 min, the mixture was
cooled to 08C, then 1N aqueous HCl (5 mL) was added to
quench the reaction. After the mixture was extracted with
AcOEt three times, the organic phase was dried over
Na2SO4. The filtrate was concentrated under reduced pres-
sure, and the residue was purified by preparative thin layer
chromatography on silica gel (hexanes) to give 2a as color-
less solid; yield: 35.3 mg (0.180 mmol, 90%).
Typical Experimental Procedure for Sequential
Hydrodefluorination with Carbodefluorination
The reaction of 3a is described. A magnetic stir bar was
placed in a test tube that was then dried with a heat gun
under reduced pressure and filled with nitrogen. 3a
(55.4 mg, 0.235 mmol), anhydrous 1,2-dichloroethane
(7.8 mL) and triethylsilane (112 mL, 0.704 mmol) were
added to the test tube successively under a nitrogen atmos-
phere. Titanium tetrachloride (0.23 mL, 0.0700 mmol, 0.3M
solution in 1,2-dichloroethane) was then added. After stir-
ring at 808C for 12 h, the mixture was cooled to 08C, then
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À
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À
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À
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À
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