Nickel-catalyzed amination of aryl fluorides with primary amines

Article abstract of DOI:10.1039/c7cc08181b

The Ni-catalyzed cross-coupling reaction between aryl fluorides and primary amines was enabled by the 1,2-bis(dicyclohexylphosphino)benzene (DCYPBz) or 1,2-bis(dicyclohexylphosphino)ethane (DCYPE) ligands. Both N-alkyl- and N-aryl-substituted primary amin

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Nickel-Catalyzed Amination of Aryl Fluorides with Primary Amines
Tomoya Harada, Yusuke Ueda, Tomohiro Iwai* and Masaya Sawamura*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The Ni-catalyzed cross-coupling reaction between aryl fluorides functionalizations leading to various derivatives such as tertiary
and
primary
amines
was
enabled
by
the
or
1,2- amines and amides.
1,2- Here, we report the Ni-catalyzed cross-coupling between aryl
and primary amines enabled by 1,2-
bis(dicyclohexylphosphino)benzene
(DCYPBz)
bis(dicyclohexylphosphino)ethane (DCYPE) ligands. Both N-alkyl- fluorides
and N-aryl-substituted primary amines participated in the bis(dicyclohexylphosphino)benzene (DCYPBz),11 which features a
selective reaction to form secondary amines. This protocol would rigid backbone connecting two dicyclohexylphosphino moieties, as a
potentially be useful for late-stage diversification of fluorinated ligand (Scheme 1, bottom). Both N-alkyl- and N-aryl-substituted
compounds with complex structures for the synthesis of primary amines successfully participated in this transformation,
functionally interesting aniline derivatives.
giving secondary arylamines in high yields.
Specifically, the reaction between an electron-rich aryl fluoride,
4-fluoroanisole (1a, 0.25 mmol), and N-octylamine (2a, 1.5 equiv) in
the presence of [Ni(cod)2] (5 mol%), DCYPBz (7.5 mol%) and
NaOtBu (1.5 equiv) in toluene at 120 °C over 20 h occurred cleanly
and gave the secondary amine product 3a in 94% isolated yield
(Scheme 2). Importantly, the corresponding tertiary amine was not
detected in the crude product. Even with an excess amount of 1a
(1a/2a 1.5:1), 3a was the sole product (94% NMR yield). IPr and
PCy3, which have often been employed as effective ligands in Ni-
catalyzed C–F bond transformations,7,8,10 induced no reaction.
Compounds with F-substituted aryl groups are widely available from
reagent suppliers, and their structural diversity in pharmaceuticals
and functional materials is significant.1 Thus, the transformation of
C–F bonds in these compounds is expected to be a useful strategy to
create large-scale biased compound libraries.2 However, the intrinsic
inertness of C–F bonds limits their utility as a functional group for
chemical transformations.3 Therefore, development of new ways for
transforming C–F bonds of aryl fluorides is highly desirable.
Transition metal catalyzed cross-coupling reactions of aryl
halides are powerful tools for C–C and C–heteroatom bond
formation.4 Since the seminal report by Tamao, Kumada and a co-
worker,5 Ni catalysis has been recognized as a favorable strategy for
derivatization of aryl fluorides through C–F bond activation. Recent
progress in this field has been spectacular.6 However, compared to
C–C bond formation reactions using nucleophilic organometallic
reagents,7 C–heteroatom bond formation reactions have been less
developed.8 For C–N bond forming reactions to prepare arylamines,9
Wang and a co-worker disclosed Ni catalysis for amination of aryl
fluorides using a bulky N-heterocyclic carbene ligand (NHC), IPr
(Scheme 1, top).10 While various N-dialkyl- or N-alkylaryl secondary
amines participated in this transformation, its applicability toward
primary amines has not been well-demonstrated despite the
importance of secondary arylamines as handles for further N-
Scheme 1 Ni-catalyzed amination of aryl fluorides.
Scheme 2 Ni-catalyzed amination of 1a with 2a.
Screening of chelating bisphosphines identified DCYPBz as the
most effective ligand for the reaction between 1a and 2a (Scheme
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3).12 Among bisphosphines having a rigid o-phenylene linker reaction between 4-chloro-4’-fluorobiphenyl and 1 equiv of 2a at
between the two atoms, less-hindered P-alkyl-substituted 120 °C, the C–Cl amination product 3h was obtained as the sole
P
bisphosphines such as DIPPBz and DETPBz were less effective. A product, indicating that the C–Cl bond underwent the amination
perarylated bisphosphine DPPBz gave no catalytic activity. Thus, preferentially over the C–F bond. The use of 3 equiv of 2a gave a
steric bulk and strong electron-donation of the bisphosphines were mixture of 3h and diamination product 3h’, which formed through
crucial. The polystyrene-cross-linking bisphosphine PS-DPPBz,13 the consecutive C–Cl and C–F amination.
which allowed controlled bisphosphine monochelation to metals,
was also ineffective.
Effects of linkers between two dicyclohexylphosphino moieties
were also investigated (Scheme 3). The thiophene-based ligand
DCYPT11b was less effective than DCYPBz, giving 3a in 68% yield.
DCYPE having a flexible ethylene (C2) linker showed similar
performance to DCYPT (48%), while product yields dropped for
ligands with C1, C3 and C4 linkers (DCYPM, DCYPP and DCYPB,
respectively; 0–6%). The P,P,P’,P’-tetracyclohexyl-substituted
Josiphos-type ligand L1 gave a moderate yield (73%), but did not
improve the yield obtained with DCYPBz.
The use of NaOtBu as a base was also important for the efficient
amination of 1a with 2a in the presence of the DCYPBz-Ni system
(5 mol% Ni, 120 °C for 20 h). LiOtBu (20%), KOtBu (17%), and
NaHMDS (22%) were much less effective. Aprotic solvents such as
CPME (82%) and octane (75%) could be used, but gave slightly
lower yields than toluene. Aprotic polar solvents such as DMA (0%)
or protic solvents such as t-AmOH (5%) were not suitable. A lower
reaction temperature (in toluene with NaOtBu) decreased the yields
(62% and 15%, at 110 °C and 100 °C, respectively). Removal of
either the Ni complex or ligand from the conditions shown in
Scheme 2 resulted in no reaction. Thus, the possibility of a
nucleophilic aromatic substitution reaction pathway was excluded.14
Scheme 4 Scope of aryl fluorides. Conditions: 1 (0.25 mmol), 2a (0.375
mmol), [Ni(cod)2] (5 mol%), DCYPBz (7.5 mol%), NaOtBu (0.375 mmol),
toluene (1 mL), 120 °C, 20 h. Isolated yields are shown. a 140 °C in m-xylene.
b
The substrate 4-chloro-4’-fluorobiphenyl (~96% purity) was contaminated
c
with inseparable 4,4’-dichlorobiphenyl. 2a (0.25 mmol). The isolated
Scheme 3 Ligand effects in the reaction between 1a and 2a. Conditions: 1a
product 3h contained N-octyl-[1,1'-biphenyl]-4-amine (4%). d 2a (0.75 mmol)
(0.25 mmol), 2a (0.375 mmol), [Ni(cod)2] (5 mol%), ligand (7.5 mol%),
and NaOtBu (0.75 mmol). e 1H NMR yields. f N,N-Diethylbenzamide (ca. 10%)
was detected in the 1H NMR spectrum of the crude product. g The tBu ester
cleavage product of 3k was observed in the crude mixture using DCYPBz
NaOtBu (0.375 mmol), toluene (1 mL), 120 °C, 20 h. 1H NMR yields are
shown. a DCYPP was generated in situ from DCYPP·2HBF4.
h
(22%) or DCYPE (6%). The tertiary amine product was observed in the
With the optimal conditions in hand, the scope of the aryl
fluorides was investigated using 2a as a representative primary
amine substrate (Scheme 4). Electronically neutral aryl fluorides
(fluorobenzene, 4-fluorotoluene and 4-fluorobiphenyl) underwent
efficient amination, giving the corresponding N-octylanilines 3b–3d
in high yields. Amination of electron-rich aryl fluorides having N,N-
dimethylamino (3e) or triisopropylsiloxy (3f) groups at the para
position occurred at 140 °C. 5-Fluoro-1-methylindole, an electron-
rich heteroaryl fluoride, served as a suitable substrate (3g). In the
crude mixture using DCYPBz (4%) or DCYPE (5%).
In contrast to electron-neutral and electron-rich aryl fluorides,
electron-deficient aryl fluorides underwent less efficient amination
with the DCYPBz-Ni system. Most typically, the reaction between
1-fluoro-4-(trifluoromethyl)benzene and 2a at 120 °C in toluene
over 20 h produced only a trace amount of 3i (Scheme 4). The
reaction of N,N-diethyl-4-fluorobenzamide produced 3j in only 48%
yield (NMR). However, the use of DCYPE instead of DCYPBz
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improved the yield to 66%. 1H NMR and GC-MS analysis of the atoms is favorable for a rigid metal chelation, which may suppress
crude product showed formation of N,N-diethylbenzamide (ca. undesired ligand exchange by primary amine substrates. The latter
10%), probably through protonation of a putative Ar–Ni(II)–F two points are supported by Hartwig’s mechanistic studies on a
oxidative addition intermediate,15-17 indicating lower efficiencies of bisphosphine-Ni system for amination of aryl chlorides with N-
transmetalation and/or subsequent reductive elimination. Although alkyl-substituted primary amines19 and by our own study with
the reaction of t-butyl 4-fluorobenzoate competed with the cleavage polystyrene-cross-linking bisphosphine PS-DPPBz.13
of the tBu ester moiety, the use of DCYPE provided a better yield of
Finally, utility of the DCYPBz-Ni system was demonstrated in
3k compared to DCYPBz. Double C–F amination of 1,3- the derivatization of fluorinated compounds, including a bioactive
difluorobenzene with 2a (3 equiv) proceeded smoothly with the molecule and a functional material (Scheme 6). The C–F amination
DCYPE-Ni system, giving the corresponding diaminobenzene 3l in of an N-p-methoxybenzyl-protected form of paroxetine,20 an
79% yield.18 The reasons for the higher performance of DCYPE than antidepressant of the selective serotonin reuptake inhibitor class,
DCYPBz in these cases are unclear at present. Similar ligand effects occurred with 3 equiv of octylamine or aniline (5 mol% Ni loading
were observed in the reactions of ortho-substituted substrates such as in m-xylene at 140 °C for 20 h), affording 3ac or 3ad, respectively,
o-tolyl fluoride (3m) and 1-naphthyl fluoride (3n), and in
fluoropyridine such as 2- and 3-fluoropyridines (3o and 3p).
high
yields.
Trans,trans-4-(4-fluorophenyl)-4’-
propylbicyclohexyl, which is a fluorinated liquid crystal material,21
The DCYPBz-Ni system was applicable to the C–F amination was converted to the corresponding C–F amination products 3ae and
with various primary amines. The results are summarized in Scheme 3af (in toluene at 120 °C). As demonstrated by these
5. Isoamyl- and 2-phenylethyl amines reacted with 1a at 120 °C, transformations, the present Ni-catalyzed C–F amination reaction
giving the secondary amines 3q and 3r, respectively, in high yields. would serve as a means for late-stage diversification of complex
In the reaction of benzylamine, 3 equiv each of the amine and molecules, taking advantage of the masked reactivity of the C–F
NaOtBu were used for obtaining the coupling product 3s in high bond.
yield. Ethylamine hydrochloride served as an amine source (in
CPME), giving 3t in a useful yield. Diethyl acetal (3u) and
morpholine (3v) moieties were tolerated in the primary alkylamines.
N-Secondary-alkyl-substituted amines such as cyclohexylamine and
benzhydrylamine were also suitable substrates (3w and 3x). An N-
tert-alkyl-substituted primary amine, 1-adamantylamine, induced
less efficient amination even at a higher temperature (3y, 140 °C in
o-xylene, 18%).
Primary arylamines also participated in the amination of aryl
fluorides (Scheme 5). The reaction of 1a with aniline (3 equiv)
occurred using 3 equiv of NaOtBu at 120 °C, giving diaryl amine 3z.
The use of IPr in place of DCYPBz induced no catalytic activity as
in the case of the reaction with N-octylamine (Scheme 2). p-
Methoxy-substituted aniline also served as a suitable substrate (3aa),
while p-CF3-substituted aniline gave no amination product (data not
shown). Sterically demanding 2,6-dimethylaniline coupled with 1a,
giving 3ab in an acceptable yield.
The effectiveness of DCYPBz in the Ni-catalyzed amination of
aryl fluorides with primary amines can be summarized as follows.
First, the electron-donating P-alkyl substituents increase electron
Scheme 5 Scope of N-primary amines. Conditions: 1a (0.25 mmol), 2 (0.375
density of the Ni(0) species, assisting oxidative addition of the C–F
bond.15-17 Second, the steric demand of the ligand is beneficial to
mmol), [Ni(cod)2] (5 mol%), DCYPBz (7.5 mol%), NaOtBu (0.375 mmol),
a
toluene (1 mL), 120 °C, 20 h. Isolated yields are shown. 3 equiv. each of
form a bisphosphine-monochelated monomeric Ni(0) species, which
would be highly active due to the coordinative unsaturation of the
b
amine and NaOtBu were used. 3 equiv. of EtNH2·HCl and 6 equiv. of
NaOtBu were used in CPME (1 mL). c 140 °C in o-xylene.
metal center. Third, the o-phenylene linker connecting the two P
Scheme 6 C–F Amination of N-PMB-protected paloxetine (top) and a fluorinated liquid crystal material (bottom).
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This work was supported by JSPS KAKENHI Grant
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