Trifluoromethylselenolation of Aryldiazonium Salts: A Mild and Convenient Copper-Catalyzed Procedure for the Introduction of the SeCF3 Group

Article abstract of DOI:10.1002/chem.201504601

The straightforward introduction of the trifluoromethylseleno group into aromatic and heteroaromatic compounds is accomplished by the utilization of readily available aryldiazonium tetrafluoroborates, potassium selenocyanate, and Ruppert-Prakash reagent. The reaction tolerates a wide range of aromatic and heteroaromatic diazonium salts and is also amenable to the synthesis of pentafluoroethyl selenoethers. Furthermore, the methodology can be applied directly starting from anilines.

Full text of DOI:10.1002/chem.201504601

DOI: 10.1002/chem.201504601
Communication
&
Synthetic Methods |Hot Paper|
Trifluoromethylselenolation of Aryldiazonium Salts: A Mild and
Convenient Copper-Catalyzed Procedure for the Introduction of
the SeCF3 Group
Pavlo Nikolaienko[a] and Magnus Rueping*[a, b]
Pioneering work from Yagupolskii and Haas, followed by Bil-
liard, Langlois, and Dolbier[7] disclosed the introduction of the
Abstract: The straightforward introduction of the trifluo-
romethylseleno group into aromatic and heteroaromatic
compounds is accomplished by the utilization of readily
available aryldiazonium tetrafluoroborates, potassium sele-
nocyanate, and Ruppert–Prakash reagent. The reaction tol-
erates a wide range of aromatic and heteroaromatic diazo-
nium salts and is also amenable to the synthesis of penta-
fluoroethyl selenoethers. Furthermore, the methodology
can be applied directly starting from anilines.
SeCF3 group into molecules. Recently, Weng and co-workers re-
ported elegant protocols for the synthesis of [{Cu(bpy)-
(SeCF3)}2] and [{Cu(phen)(SeCF3)}2] reagents, and showed their
usefulness in reactions with aryl and heteroaryl halides.[8] How-
ever, the above-mentioned methods still engage the usage of
equimolar quantities of metals or pre-functionalized starting
materials.[9] Therefore, a simple one-pot procedure for the in-
troduction of a SeCF3 group by employing a cheap and readily
available metal salt as catalyst would be desirable. Hence, we
decided to develop such a reaction starting from readily avail-
able diazonium salts.
Small perfluorinated groups can be found in numerous phar-
maceuticals, agrochemicals, and materials. Compounds which
possess fluorine-rich residues have usually different, improved
biological and physical properties compared to their non-fluo-
rinated analogs.[1–3] Therefore, the introduction of such groups
became an important topic in medicinal and pharmaceutical
chemistry[1] as well as agrochemistry.[2] Recently, the SCF3
group turned out to be the subject of numerous investigations
due to its unique properties, including high lipophilicity
(Hansch constant p=1.44), metabolic stability and electron
withdrawing effect.[4] To date, we have a general collection of
The Sandmeyer reaction is a powerful tool for the prepara-
tion of differently functionalized compounds starting from di-
azonium salts, which can be easily obtained from readily avail-
able anilines. The reactions are performed with or without cat-
alytic amounts of Cu salts and typically do not show byproduct
formations. Therefore, we wondered whether a copper-cata-
lyzed Sandmeyer reaction for the introduction of selenium[10]
followed by a reaction with the Ruppert–Prakash reagent
would give the trifluoromethylseleno-functionalized com-
pounds.[11,12] This would in principle allow the preparation of
different trifluoromethyl aryl selenides from anilines.
basic procedures for the formation of C(sp,sp2
,
ÀSCF3 bonds
Various reactions for the introduction of selenium atoms are
highly dependent on the selenium source. Polymeric “grey” se-
lenium shows very poor reactivity and often side products are
formed. In contrast, amorphous “red” selenium is more reac-
tive. However, it is less stable with regard to storage and thus
needs to be prepared freshly prior to use. To overcome such
obstacles we decided to use potassium selenocyanate, consid-
ering its availability, low price, and long-term shelf stability.[13a]
Initially, p-methoxyphenyl diazonium tetrafluoroborate (1)
was chosen as model substrate to react with potassium seleno-
cyanate. Considering that the oxidation potential of NCSeÀ
(Eo =À0.59 V)[13b] is similar to that of IÀ (Eo =À0.54 V),[13c] it is
reasonable to assume that the reaction might occur without
any catalyst. Indeed, upon treatment of 1 with KSeCN in dry
CH3CN at r.t. for 15 min, evolution of nitrogen occurred and
two products were isolated in 55% total yield (Table 1,
entry 1). The main product was obtained in 35% yield after
column chromatography and it was identified by 1H and
13C NMR spectroscopy and MS measurement to be p-methoxy-
phenyl selenocyanate 1a’. As a second product we isolated
bis(p-methoxyphenyl)selenide in 20% yield. These results most
probably relate to the ambident nucleophilicity of the seleno-
sp3)
that includes the straightforward introduction of a SCF3 group
in a nucleophilic, electrophilic, or radical manner.[5]
In contrast to sulfur derivatives, selenium compounds are
less applied[6] although the SeCF3 group for instance provides
a stable, more bulky, and more lipophilic functional group.[4]
Therefore, it is surprising that only few methods are known
which allow the direct introduction of the SeCF3 group in
a straightforward, convenient, and mild way, especially in aro-
matic and heteroaromatic substrates.
[a] P. Nikolaienko, Prof. Dr. M. Rueping
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: Magnus.Rueping@rwth-aachen.de
[b] Prof. Dr. M. Rueping
King Abdullah University of Science and Technology (KAUST)
Kaust Catalysis Center (KCC)
Thuwal 239556900 (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504601.
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Communication
In order to accomplish the trifluoromethylselenylation, we
tested the trifluoromethylation of the corresponding seleno-
cyanate 1a’ with TMSCF3 in the presence of different activating
agents, such as organic and inorganic bases or fluorides
(Table 2). It was found that Cs2CO3 is the best initiator and
product 2a was obtained in 84% yield (entry 2). Finally, to fur-
ther improve the protocol, we carried out the reaction in
a one-pot manner starting from 1a, and obtained the desired
product 2a in 74% overall yield (entry 6).
Table 1. Optimization of the reaction conditions for the selenocyanation
of p-methoxyphenyldiazonium tetrafluoroborate.[a]
Entry
Catalyst [10 mol%]
Additive [10 mol%]
x
Yield [%][b]
1
2
3
4
5[c]
6[d]
7
8
9
none
CuOAc
CuTC
CuCl
CuCl
CuCl
(CuOTf)2·C6H6
Cu(OTf)2
CuCl/CuCl2
CuCl/CuCl2
CuBr/CuBr2
(CH3CN)4CuBF4/
Cu(BF4)2
CuCl/CuCl2
–
–
–
–
–
–
–
–
2
2
2
2
2
2
2
2
1.5
1.5
1.5
1.5
35
44
55
56
58
51
38
46
78
82
80
81
Table 2. Optimization of the trifluoromethylation reaction conditions of
p-methoxyphenylselenocyanate[a]
–
10
11
12
1,10-phen
1,10-phen
1,10-phen
Yield [%][b]
13
1,10-phen
1.5
88[e]
Entry
Additive [equiv]
x
1
2
3
4
5
6[c]
CsF (1)
1.5
1.5
2.0
1.5
1.5
2.0
16
84
24
10
5
[a] All reactions were carried out under an argon atmosphere on
a 0.2 mmol scale. [b] Yields of isolated products after column chromatog-
raphy. [c] 50 mol% of CuCl was used. [d] 1.0 equiv of CuCl was used.
[e] reaction was performed at À258C. CuTC=copper 2-thiophenecarbox-
ylate; 1,10-phen=1,10-phenanthroline.
Cs2CO3 (2)
TBAF (0.1)
TBAF (1)
tBuOK (3)
Cs2CO3 (1.5)
74[d]
[a] All reactions were carried out under an argon atmosphere on
a 0.2 mmol scale. [b] Yields of isolated products after column chromatog-
raphy. [c] The reaction was performed starting from 1a and applying the
conditions from Table 1, entry 13 for the first step. [d] Yield for the overall
transformation 1a!2a. TBAF=tetrabutylammonium fluoride in THF
(1m).
cyanate anion and the different stability of the corresponding
arylseleno- and arylisoselenocyanates. Hence, further investiga-
tions were targeted to suppress the side product formation by
catalyst involvement. Cu salts are typical catalysts for the Sand-
meyer reaction; hence, different Cu sources were tested. Inter-
estingly, the desired p-methoxyphenylselenocyanate 1a’ was
almost exclusively formed (Table 1). CuI and CuII salts reacted
equally well (Entries 2–8) and comparable results were ob-
tained with catalytic as well as stoichiometric amounts of CuCl
(Entries 4–6). We also tested heterovalent CuI/CuII systems[14]
(Entries 9–12). Pleasingly, with 10 mol% of a CuCl/CuCl2 system
the desired product 1a’ was obtained in 78% yield. Further
optimization disclosed 1,10-phenanthroline as appropriate
ligand and the product was obtained in 82% yield with little
side product formation.
With the optimal conditions in hand, we decided to test the
scope of this novel one-pot copper-catalyzed trifluoromethyl-
selenolation of aryldiazonium tetrafluoroborates. Under the
standard conditions, we obtained the trifluoromethylselenolat-
ed products 2a–g bearing electron-donating and -withdrawing
groups at the para position in good yields (Table 3). We also
tested meta- and disubstituted substrates and obtained the
corresponding products 2h and 2k–n in good to high overall
yields. It is worth noting that different functional groups, such
as halogen (2 f, 2l), nitro (2b, 2i), and nitrile (2c), are well tol-
erated under the reaction conditions.
Furthermore, it was found that bromide and tetrafluorobo-
rate derivatives can also be employed and provide comparable
results (Entries 11, 12). The reaction is very fast and to fully
suppress the isoselenocyanate formation, the reaction was per-
formed at different temperatures. Performing the reaction at
À258C turned out to be optimal and p-methoxyphenyl seleno-
cyanate was obtained in 88% yield. (Entry 13). Evaluation of
various solvents (DMF, EtOAc, THF, DCM, AcOH) did not give
any improvement (see the Supporting Information for details).
The mechanism of the Sandmeyer reaction proposed by
Koshi[15a] and subsequent supportive studies by Galli[15b–d] in-
cludes two steps: a reductive formation of aryl radicals from
the diazonium cation by the CuI species in the first step, and
abstraction of the aryl radical with the introduction of nucleo-
phile by the CuII species in the second step. Hence, we suggest
that the CuI/CuII/ligand system simultaneously catalyzes both
steps involved in the Sandmeyer reaction mechanism.
To further extend the scope of this one-pot approach, differ-
ent heteroaromatic diazonium tetrafluoroborates 1p–t were
subjected to the standard reaction conditions (Table 3). To our
delight, nitrogen-containing heteroarenes did not affect the
catalytic system and the corresponding trifluoromethyl seleno-
lated quinolines and quinoxaline 2p–r were obtained in mod-
erate yields. The reaction with indolizine diazonium tetrafluoro-
borate 1s as well as thiophene derivative 1t also proceeded
well despite the presence of bulky ester groups at the ortho
position.
To broaden the applicability of this methodology further,
other TMS reagents were tested under the standard reaction
conditions. Among the applied pentafluoroethyl-, benzyl-, and
allyl-trimethylsilanes, the pentafluoroethyltrimethylsilane was
the most effective and the corresponding pentafluoroethyl se-
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Table 3. Scope of the trifluoromethylthiolation of arene diazonium salts[a]
Scheme 1. Scope of the pentafluoroethylselenolathion of arene diazonium
salts.
Scheme 2. One-pot approach for the trifluoromethylthiolation of anilines.
In summary, a mild procedure for the trifluoromethylseleno-
lation of aromatic compounds was developed. Various readily
available aryl diazonium tetrafluoroborates smoothly under-
went reactions to give products in good to high yields. Both
electron-rich and -poor substrates are suitable for the process,
and different functional groups, such as nitro, halogen, ether,
ester, amino, nitrile, and carbonyl, are tolerated under the reac-
tion conditions. The scope of the reaction was extended by
the utilization of pentafluoroethyltrimethylsilane and various
heteroaryldiazonium tetrafluoroborates. In addition, a one-pot
approach starting from the corresponding anilines provided
the trifluoroselenolated products with good overall yield on
a gram-scale.
Keywords: anilines · copper · diazonium salts · fluorine ·
organic chemistry
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[a] Reaction conditions: (Het)Ar-N2+BF4 (0.4 mmol), CuCl/CuCl2/1,10-phen
(10 mol%), KSeCN (0.6 mmol, 1.5 equiv), Cs2CO3 (0.6 mmol, 1.5 equiv),
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Received: November 16, 2015
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Published online on January 21, 2016
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