Alkylations of Arylboronic Acids including Difluoroethylation/Trifluoroethylation via Nickel-Catalyzed Suzuki Cross-Coupling Reaction

Article abstract of DOI:10.1002/adsc.201500346

An efficient alkylation method of functionalized alkyl halides under mild nickel-catalyzed C(sp³)-(sp²) Suzuki cross-coupling conditions is described. The features of this approach are excellent functional group compatibility, low cost nickel catalyst, and the use of a mild base. This is also the first successful example of the nickel-catalyzed direct 2,2-difluoroethylation or 2,2,2-trifluoroethylation of aryl-/heteroarylboronic acids.

Full text of DOI:10.1002/adsc.201500346

UPDATE
DOI: 10.1002/adsc.201500346
Alkylations of Arylboronic Acids including Difluoroethylation/
Trifluoroethylation via Nickel-Catalyzed Suzuki Cross-Coupling
Reaction
Xiaofei Zhanga and Chunhao Yanga,*
a
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu
Chong Zhi Road, Shanghai 201203, Peopleꢀs Republic of China
E-mail: chyang@simm.ac.cn
Received: April 7, 2015; Revised: June 10, 2015; Published online: August 13, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500346.
Abstract: An efficient alkylation method of func-
attention. With different cross-coupling partners (aryl
halides, Grignard and boronic reagents, etc.), the C
(sp3)-C (sp2) formation betweern alkyl halides and
aryl compounds can be achieved by Pd,[4] Ni,[5] Fe,[6]
and Co[7] catalysts.
Among these transition metal-catalyzed cross-cou-
plings such as Suzuki, Stille, Kumada, Negishi and
tionalized alkyl halides under mild nickel-catalyzed
3
2
À
C(sp ) C(sp ) Suzuki cross-coupling conditions is
described. The features of this approach are excel-
lent functional group compatibility, low cost nickel
catalyst, and the use of a mild base. This is also the
first successful example of the nickel-catalyzed
direct 2,2-difluoroethylation or 2,2,2-trifluoroethyla-
tion of aryl-/heteroarylboronic acids.
Hiyama reactions, Suzuki cross-couplings are arguably
3
À
the most powerful tool for the creation of C(sp )
C(sp2) bonds. This may be attributed in a large part to
the stable, commercially available boronic reagents
and relatively mild reaction conditions. Over 30 years
since the discovery of Suzuki cross-coupling in 1979,[8]
palladium catalysts were the most frequently investi-
gated until 2004, when the first nickel-catalyzed
Suzuki cross-coupling of alkyl halides with arylboron-
ic acids (Scheme 1, a) was described by Fu and co-
workers.[5c] About two years later, the same group re-
ported a more practical and robust method[5d] in
3
2
À
Keywords: C(sp ) C(sp ) bond formation; 2,2-di-
fluoroethylation; nickel catalyst; Suzuki cross-cou-
pling; 2,2,2-trifluoroethylation
Introduction
À
Transition metal-catalyzed C C coupling reactions which the replacement of Ni(cod)2 by stable NiI2 sig-
are ubiquitous in both academia and industry, and nificantly simplified the process of the operation
these protocols make it possible to build up complex (Scheme 1, b). Furthermore they successfully enabled
structures quickly from readily available compounds. the cross-couplings of secondary and primary alkyl
Among the C C coupling reactions, aryl and vinyl chlorides (Scheme 1, c)[5d] and unactivated tertiary
À
electrophiles play an important role because of their alkyl halides (Scheme 1, d)[5k] with arylboronic re-
high selectivity, high tolerance of functional groups agents by employing an NiCl2·glyme or NiBr2·glyme
and the wide availability of substrates.[1] Compared to complex.
3
2
À
aryl and alkenyl (pseudo) halides, alkyl halides and
Although the Ni(II)-catalyzed C(sp ) C(sp )
their equivalents also represent a prominent class of Suzuki cross-coupling reactions were greatly promot-
coupling reactions. However, alkyl electrophiles in ed by Fu and other scientists, the inevitable applica-
these reactions are considered to be more challenging tion of strong bases like Li/KO-t-Bu, Na/KHMDS
due to the difficulties in the oxidative addition step strictly limited the scope of substrates and the rele-
and side reactions, such as b-H elimination and hydro- vant functional groups. Recently, Zhang[9] and co-
dehalogenation.[2] Since 1992, when the first palladi- workers successfully achieved the nickel-catalyzed
um-catalyzed cross-coupling of alkyl iodides with aryl- Suzuki cross-coupling reaction of arylboronic acids
borane reagents was reported by Suzuki and co-work- with difluoromethyl bromides and chlorides in the
3
2
ers,[3] the development of C(sp ) C(sp ) cross-cou- presence of K2CO3. This was one of the few reported
À
3
2
À
pling reactions of alkyl (pseudo) halides with pre- examples of Ni(II)-catalyzed C(sp ) C(sp ) Suzuki
functionalized aryl compounds has gained much more cross-couplings under relatively mild conditions. As
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Xiaofei Zhang and Chunhao Yang
Table 1. Survey of reaction conditions[a]
Entry Ni catalyst
Ligand Base
Yield [%][b]
1[c]
2[d]
3[d]
4
5
6
7
8
9
10
11
12
13
14
15[e]
Ni(NO3)2·6H2O
NiCl2·DME
NiCl2·6H2O
NiCl2·6H2O
NiCl2
Ni(PPh3)2Cl2
NiCl2·dppp
Ni(OAc)2·6H2O L1
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
L1
L1
L1
L1
L1
L1
L1
K2CO3
16
21
35
45
13
41
27
trace
55
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
3
2
À
Scheme 1. Previous Ni-catalyzed C(sp ) C(sp ) Suzuki
cross-couplings reported by Fu and Zhang.
L1
L2
L3
L4
L3
L3
L3
far as we know, very few functional groups were men-
tioned in previous works and the yields were relative-
ly low. Many important functional groups in organic/
medicinal chemistry were rarely reported in those lit-
erature reports, including aldehyde, lactone, vinyl and
cyclopropyl groups. Meanwhile, maybe because of the
alkaline conditions, the direct 2,2-difluoroethylation
of arylboronic reagents with ICH2CF2H still remains
unexplored due to the high tendency for b-H elimina-
tion of ICH2CF2H. Moreover, 2,2,2-trifluoroethylation
of ICH2CF3 was performed only by expensive palladi-
trace
81
50
Na2CO3 22
Cs2CO3
K2CO3
58
<30
[a]
Reaction conditions (unless stated otherwise): 1a
(0.5 mmol, 1.0 equiv.), 2a (1.0 mmol, 2.0 equiv.), base
(1.5 mmol, 3.0 equiv.), anhydrous 1,4-dioxane (3 mL),
12 h.
[b]
[c]
Yield of isolated product.
um catalyst so far[10]
.
2a (0.75 mmol, 1.5 equiv.), K2CO3 (1.0 mmol, 2.0 equiv.),
commercially available 1,4-dioxane (3 mL), 608C, 24 h.
Commercially available 1,4-dioxane (3 mL).
Anhydrous toluene, tetrahydrofuran, N,N-dimethylfor-
mamide and dimethyl sulfoxide; DME=dimethyl ether,
dppp=1,3-bis(diphenylphosphino)propane.
[d]
[e]
Results and Discussion
As one part of our continuing efforts on the transition
[11]
À
metal-catalyzed C C bond construction,
and also
because of the novel interesting features of target
molecules in medicinal chemistry, we are keen about monly used in nickel-catalyzed Suzuki cross-coupling
developing a cheap, mild and robust method for reactions were examined (Table 1, entries 2–12).
Suzuki cross-couplings of generally unactivated alkyl Ni(NO3)2·6H2O and 4,4’-di-t-Bubpy (L3) were found
halides including functionalized alkyl halides. We first to be the most effective catalytic systems. Interesting-
tested Zhangꢀs optimized conditions for arylboronic ly, water in the solvent decreased the yield of the re-
acid and unactivated alkyl halides. 4-(Methoxycarbo- action (Table 1, entry 3 vs. 4) while the crystal water
nyl)benzeneboronic acid (1a) and iodocyclohexane in nickel catalyst was beneficial (Table 1, entry 4 vs.
(2a) were used as model substrates. Not surprisingly, 5). In addition, compared to other bases and solvents,
the desired cross-coupling product was obtained with K2CO3 and anhydrous 1,4-dioxane were the best
only 16% yield under the optimized conditions choice
for
this
reaction.
Finally,
5 mol%
(Table 1, entry 1). To suppress the side reactions and Ni(NO3)2·6H2O, 5 mol% 4,4’-di-t-Bubpy (L3) and
improve the reaction efficiency, different nickel cata- K2CO3 (3.0 equiv.) in anhydrous 1,4-dioxane at 808C
lysts and bipyridine (bpy) ligands which were com- for 12 h under an argon atmosphere were identified
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Alkylations of Arylboronic Acids
Table 2. Nickel-catalyzed cross-couplings of 1a with various alkyl halides[a]
[a]
Reaction conditions (unless stated otherwise): 1a (0.5 mmol, 1.0 equiv.), 2 (1.0 mmol, 2.0 equiv.),
K2CO3 (1.5 mmol, 3.0 equiv.), anhydrous 1,4-dioxane (3 mL), 12 h, isolated yield.
NaI (0.25 mmol, 0.5 equiv.) was added and isolated yields without NaI are given in parentheses.
[b]
Boc=t-butyloxycarbonyl.
as the optimal conditions and the product was ob- ry for unactivated alkyl bromides and iodides. For sp3
tained in 81% yield (Table 1, entry 11) (for details, tertiary halides, the coupling could be especially chal-
see the Supporting Information). No desired product lenging.[5k,12] Remarkably, 1-bromoadamantane afford-
was detected in the absence of the nickel catalyst or ed the coupling product 3an in 35% yield. Above all,
ligand.
many important functional groups, like ester, cyano,
The substrate scope of the alkyl halides was then ketone, lactone, cyclopropyl and vinyl groups, oxetane
examined, and various unactivated/functionalized and N-Boc-azetidine were quite well tolerated. In
alkyl halides 2 were treated with 1a under the optimal comparison, we synthesized compound 3ai, 3aj and
conditions (Table 2). Although the unactivated alkyl 3al by employing Fuꢀs method (Scheme 1, b), 3ai was
chlorides gave relatively poor yields (<30%) of unac- not detected in the reaction, and compounds 3aj and
tivated alkyl bromides/iodides(3aa, 3ab), the activated 3al were obtained with less than 30% yields.
alkyl chlorides, such as benzyl chloride, ethyl chloro-
To demonstrate the generality of boronic acids,
acetate, chloroacetonitrile and 2-chlorocyclohexa- a range of substituted aryl- and heteroarylboronic
none, reacted smoothly with 1a by addition of acids were employed with iodocyclohexane (2a) in
0.5 equiv. NaI, and the corresponding products were this protocol (Table 3). The reactions of arylboronic
obtained in good yields (3ac, 3ad, 3ae and 3af). It is acids that contain either electron-donating groups or
particularly worth noting that, with the addition of electron-withdrawing groups afforded the desired
NaI, direct 2,2,2-trifluoroethylation of 1a could be products in 65–83% yield, and the electronic proper-
achieved by employing 2,2,2-trifluoroethyl trifluoro- ties of the substituents on arylboronic acids had very
methanesulfonate in 42% yield (3ag). NaI was also limited influence over the yields of this reaction. Fur-
beneficial for activated alkyl bromides but unnecessa- thermore, the sterically hindered 2-tolylboronic acid
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Xiaofei Zhang and Chunhao Yang
Table 3. Nickel-catalyzed cross-couplings of iodocyclohexane (2a) with various aryl-
and heteroarylboronic acids[a]
[a]
Reaction conditions (unless stated otherwise): 1 (0.5 mmol, 1.0 equiv.), 2a (1.0 mmol,
2.0 equiv.), K2CO3 (1.5 mmol, 3.0 equiv.), anhydrous 1,4-dioxane (3 mL), 12 h, isolat-
ed yield.
[b]
Due to the difficult purification of the product, N-Boc-4-iodopiperidine was used in-
stead of 2a.
2a (1.5 mmol, 3.0 equiv.) was added.
[c]
reacted well with bulky iodocyclohexane leading to difluoromethylene motif can serve as a bioisostere for
an acceptable yield (3bb). This reaction was not re- a carbonyl group or an oxygen atom.[15] In contrast to
stricted to arylboronic acids, and the heteroarylboron- many practical methods for introducing trifluorometh-
ic acids were also investigated. This protocol was yl or difluoromethylene fragment into functionalized
noteworthy because heteroaryl groups are very prom- aromatic compounds, methods for the synthesis of
inent structural motifs in pharmaceuticals and materi- (2,2-difluoroethyl)arenes or (2,2,2-trifluoroethyl)ar-
al science, such as pyridine (3bi), benzofuran (3bk) enes are still limited.
and benzothiophene (3bl). However, we failed in in-
troducing a cyclohexyl group to the 2/3-position of duce 2,2-difluoroethyl and 2,2,2-trifluoroethyl groups
indole and its derivatives by the same method. to the aromatic rings by reacting aryl- and heteroaryl-
Inspired by product 3ag, we tried to directly intro-
As we all know, fluorinated moieties in organic boronic acids with ICH2CF2H and ICH2CF3, respec-
molecules can powerfully modulate the biological ac- tively. To our delight, by applying this method, good
tivities of these molecules, and therefore, they have to excellent yields of (2,2-difluoroethyl)arenes and
gained much attention in the strategy of drug design (2,2,2-trifluoroethyl)arenes were obtained (Table 4
and life science research.[13] Among the fluorine-con- and Table 5). Importantly, the sterically hindered o-
taining groups, the difluoromethylene and trifluoro- substituted and Br-substituted arylboronic acids only
methyl groups have proved to be very important moi- slightly affected the yields of target compounds (3cb
eties, this is owing to their lipophilic electron-with- and 3cc in Table 4, 3db and 3dc in Table 5). The
drawing capacity and specific size.[14] For example, the heteroarylboronic acids also underwent the reaction
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Alkylations of Arylboronic Acids
Table 4. Nickel-catalyzed cross-couplings of ICH2CF2H (2b) with various aryl- and hetero-
arylboronic acids[a]
[a]
Reaction conditions (unless stated otherwise): 1 (0.5 mmol, 1.0 equiv.), 2a (1.0 mmol,
2.0 equiv.), K2CO3 (1.5 mmol, 3.0 equiv.), anhydrous 1,4-dioxane (3 mL), 12 h, isolated
yield.
[b]
Reaction carried out on a gram scale (5.56 mmol scale).
well to provide the corresponding products in accept- scale, whereby the (2,2-difluoroethyl)arene and (2,2,2-
able yields (3cm–cp, 4dm–dp). trifluoroethyl)arene 3ci and 3di were furnished with
To further demonstrate the synthetic utility of our yields of 82% and 74%, respectively. Then, some late-
protocol, we first scaled up the reaction to a gram stage direct 2,2-difluoroethylation and 2,2,2-trifluoro-
ethylation reactions of biologically active compounds
were explored. On treatment of the flavanone-derived
and the estrone-derived arylboronic acids[10,16] with
ICH2CF2H and ICH2CF3, the reactions gave the de-
sired products in quite good yields (Scheme 2), thus
significantly highlighting the importance of this
method.
Conclusions
In conclusion, a facile and mild method for nickel-cat-
3
2
À
alyzed C(sp ) C(sp ) Suzuki cross-couplings of alkyl
halides with aryl- and heteroarylboronic acids has
been developed. Due to the low cost of the nickel cat-
alyst and the mild conditions, this protocol provides
a general, efficient and practical synthesis for the con-
Scheme 2. 2,2-Difluoroethylation and 2,2,2-trifluoroethyla-
tion of the flavanone-derived and the estrone-derived aryl-
boronic acids.
3
2
À
struction of C(sp ) C(sp ) bonds. Moreover, this is
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Xiaofei Zhang and Chunhao Yang
Table 5. Nickel-catalyzed cross-couplings of ICH2CF3 (2c) with various aryl- and hetero-
arylboronic acids[a]
[a]
Reaction conditions (unless stated otherwise): 1 (0.5 mmol, 1.0 equiv.), 2a (1.5 mmol,
3.0 equiv.), K2CO3 (2.0 mmol, 4.0 equiv.), anhydrous 1,4-dioxane (3 mL), 12 h, isolated
yield.
[b]
Reaction carried out on a gram scale (5.56 mmol scale).
temperature. The reaction mixture was diluted with 30 mL
of water, and then extracted with EtOAc (10 mL”3). The
combined extracts were successively washed with brine,
dried over Na2SO4 and evaporated to dryness. The crude
product was purified by silica gel chromatography eluted
with PE: EtOAc=30: 1 to give the product as a colorless
oil: yield: 88 mg (81%). For further details of the synthesis
and characterization, see the Supporting Information.
the first nickel-catalyzed 2,2,2-trifluoroethylation of
aryl- and heteroarylboronic acids with ICH2CF3 and
the first example of direct 2,2-difluoroethylation by
ICH2CF2H. As well as the broad scope of substrates
and excellent compatibility of functional groups, this
synthetic method will have many promising applica-
tions in drug discovery and development.
Experimental Section
Acknowledgements
General Procedure
This work was financially supported by the National Natural
Science Foundation of China (81321092), and SKLDR/
SIMM (SIMM1403ZZ-01).
A sealed tube of 15 mL equipped with a magnetic stirrer
was charged with 4-(methoxycarbony)benzeneboronic acid
1a (90 mg, 0.5 mmol), iodoocyclohexane 2a (210 mg,
1.0 mmol), K2CO3 (207 mg, 1.5 mmol), Ni(NO3)2·6H2O
(7 mg, 0.025 mmol), 4,4’-di-t-Bubpy L3 (7 mg, 0.025 mmol)
and 3 mL anhydrous 1,4-dioxane (0.5 equiv. NaI was added
if necessary). This tube was purged with argon and the mix- References
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had been sealed under an argon atmosphere and the content
stirred at 808C for 12 h, the mixture was cooled to room
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