Stable nickel(0) phosphites as catalysts for C-N cross-coupling reactions

Article abstract of DOI:10.1002/adsc.201400201

Herein we describe the design and preparation of inexpensive, air-stable nickel phosphite-based catalysts for use in the C-N cross-coupling reaction. The combination of nickel tetrakis(triphenyl phosphite) {Ni[P(OPh) ₃]₄} and 1,1'-bis(diphenylphosphino)ferrocene (dppf), and in particular a newly developed catalyst (dppf)Ni[P(OPh)₃] ₂, were found to be extremely effective in catalyzing a range of amination reactions of anilines and amines with aryl chlorides. This new catalyst system offers an alternative to the bis(cyclooctadienyl)nickel [Ni(COD)₂] and palladium(0) catalysts commonly used for C-N bond formation.

Full text of DOI:10.1002/adsc.201400201

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DOI: 10.1002/adsc.201400201
ꢀ
Stable Nickel(0) Phosphites as Catalysts for C N Cross-Coupling
Reactions
Sven S. Kampmann,a Alexandre N. Sobolev,a George A. Koutsantonis,a
and Scott G. Stewarta,*
a
The School of Chemistry and Biochemistry, Bayliss Building, The University of Western Australia, Crawley, WA 6009,
Australia
Fax : (+61)-8-6488-1005; phone: (+61)-8-6488-3180; e-mail: scott.stewart@uwa.edu.au
Received: February 22, 2014; Revised: April 8, 2014; Published online: May 28, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400201.
Abstract: Herein we describe the design and prepa-
workers fine-tuned the reactivity of this system
through extensive modifications to the dppf
ligand.[16,17] Additionally, the combination of
ration of inexpensive, air-stable nickel phosphite-
ꢀ
based catalysts for use in the C N cross-coupling
reaction. The combination of nickel tetrakis(tri-
Ni
examined by several groups.[18–20] Although the use of
Ni(COD)2 has numerous benefits, this pre-catalyst
ACHTUNGRTENUN(NG COD)2 with other bidentate ligands has also been
phenyl phosphite) {Ni[P
phenylphosphino)ferrocene (dppf), and in particu-
lar a newly developed catalyst (dppf)Ni[P(OPh)3]2,
ACHTUGNRTEN(NUGN OPh)3]4} and 1,1’-bis(di-
AHCTUNGTRENNUNG
N
has the drawbacks of being highly air-sensitive, diffi-
cult to transport, costly and difficult to prepare.
were found to be extremely effective in catalyzing
a range of amination reactions of anilines and
amines with aryl chlorides. This new catalyst system
offers an alternative to the bis(cyclooctadienyl)nick-
Alternatives to the Ni
(COD)2 and phosphine cata-
ꢀ
lytic system for C N cross-coupling reactions have
also been investigated using either Ni(0) or Ni(II) as
precatalysts.[21,22] Some of these reactions require
strong bases and other additives such as Zn.[23] Nickel
NHC complexes, heterogeneous nickel catalysts and
nickel in combination with other metals (Cu and Ni
oxide/C) have also been examined.[24–29] Previously,
nickel phosphite complexes have been used in Heck
cross-coupling reactions; however, the substrate scope
was not fully established and catalyst exploration was
el [NiACHTUNGTRENNUNG(COD)2] and palladium(0) catalysts common-
ꢀ
ly used for C N bond formation.
ꢀ
Keywords: catalyst design; C N bond formation;
cross-coupling; nickel phosphites
limited.[30] Ni(II) in the form of Ni
ACHTUNTRGENUNG(OAc)2·4H2O has
Nickel catalysis is increasingly being used in cross- been used previously in combination with phosphites
coupling chemistry as a reactive and cost effective al- in aminocarbonylation reactions with DMF.[31] Ni(0)
ternative to palladium catalysis. Nickel catalysts have phosphite complexes are cheaper and more air-stable
[32,33]
many advantages for cross-coupling reactions includ- than Ni
G
and nickel phosphine complexes.
ing lower energy barriers for oxidative addition, al- Furthermore, simple nickel phosphites have been
lowing a wider range of leaving groups.[1] Although used in other unrelated processes such as promoting
Ni(II) catalysts are generally air-stable, they often re- the reaction of allylic acetates[34] with thiols and in hy-
quire an additional reducing agent. Unfortunately, in drocyanation reactions.[35,36]
many instances, zero-valent nickel catalysts suffer
from instability or low reactivity. In many cross-cou- alysis would be in the nickel(0) oxidation state, readi-
pling reactions, Ni(COD)2 has been used as pre-cata- ly dissociate the phosphite ligands, thus providing
lyst in combination with other ligands such as phos- a future stable alternative for Ni(COD)2. Such Ni(0)
phines[2,3] or N-heterocyclic carbenes (NHC).[4–7] In species could also potentially be used in combination
particular, several groups have used Ni(COD)2 in with other ligands to initiate cross-coupling reactions.
Ideally, the nickel phosphite complex used for cat-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
combination with dppf in reactions such as the Our exploration of phosphites as supporting ligands
Heck,[8,9] Suzuki[10] and Buchwald–Hartwig[5,11–13] reac- in catalysts or precatalysts began with an examination
tions. Furthermore, this combination of pre-catalysts of Ni[PACHTUGNTRENNNUG
and ligands has been recently used in reactions in- from inexpensive nickelocene.[37] When Ni[P
ACHTUNGTRENNUNG
[14,15]
ꢀ
volving C H functionalization.
Adv. Synth. Catal. 2014, 356, 1967 – 1973
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1967

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Sven S. Kampmann et al.
Table 1. Ligand-based screening of Ni[P
C N coupling between para-chloroanisole 1 and para-tolui-
dine 2.
G
sulting species catalyzed the reaction between para-
chloroanisole 1 and para-toluidine 2 to afford the
amine 3 in 53% yield (Table 1, entry 1). Moreover, we
were encouraged to observe that the yield of 3 in-
creased to 66% upon increasing the phosphine con-
tent to 10 mol% (entry 2).[19] Unfortunately, little or
no product was obtained when 1 and 2 were reacted
in the presence of other monodentate and bidentate
phosphines including Ad2PBu, dppe, DavePhos and
XPhos. The proof of concept of a phosphite-based
ꢀ
Entry Ligand[a]
GC conv.[b]
[%]
ꢀ
catalyst for C N coupling reactions was further con-
firmed when dppf was used (entry 7), providing
amine 3 in 68% yield.
1
2
N
(10 mol%)
Given the success of the combination of Ni[P-
3
4
5
6
7
8
9
10
11
Ad2PBu (10 mol%)
XantPhos (5 mol%)
XPhos (10 mol%)
dppe (5 mol%)
<1
26
<1
<1
76
1
AHCTUNTGREN(NGUN OPh)3]4 and dppf (entry 7), and recent reports on the
[11]
ꢀ
use of dppf in C N cross-coupling reactions, we de-
cided to briefly investigate other bases and another
dppf (10 mol%)
phosphite complex, Ni[PACHTNUGTRNEUNG(OEt)3]4 (Table 2). Unfortu-
nately altering the type of base did not significantly
improve the yield. Additionally, the inactivity of Ni[P-
1,10-phen (10 mol%)
TMEDA (10 mol%)
DavePhos (10 mol%)
1
<1
AHCTUNTGREN(NGUN OEt)3]4 (entry 5) supported previous reports of low
ACHTUNGTRENNUNG(R,S)-BINAP (5 mol%) 70
lability of ethyl phosphites in zerovalent nickel com-
plexes.[38] Varying the amounts of phosphine and
nickel, and their ratio, seemed to have a slightly dele-
terious effect on the reaction (entries 8–10). Interest-
ingly, the presence of an additional phosphite seemed
ꢀ
Table 2. Catalytic conditions in the C N cross-coupling between para-chloroanisole 1 and para-toluidine 2.
Entry
Catalyst
Ni[P(OPh)3]4
Ligand
Base[a]
GC conv.[b] [%]
Yield [%]
1
2
3
4
5
6
7
8
G
dppf
dppf
dppf
dppf
dppf
NaO-t-Bu
NaH
76
20
78
7
68
18
30
n.d.
0
n.d.
52
47
n.d.
n.d.
0
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OEt)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P
(OPh)3]4 (10 mol%)[d]
Ni[P
(OPh)3]4 (2 mol%)[d]
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
Ni[P(OPh)3]4
ACHTUNGTRENNUNG
R
N
R
R
N
T
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
T
A
ACHTUNGTRENNUNG
NaH[c]
KHMDS
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu[c]
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
0
dppf+P
E
39
72
59
40
32
0
69
75
59
75
dppf
dppf (5 mol%)[d]
dppf
9
10
11
12
13
14
15
dppf (4 mol%)
D-t-BuPF
DCyPF
D-i-PrPF
p-CF3-dppf
p-OMe-dppf
66
65
50
61
ACHTUNGTRENNUNG
[a]
In each case 1.4 equiv. of NaO-t-Bu were used.
Monitored by GC only promising candidates were subjected to work-up and isolation.
3 equiv. of base were used.
Modified mol% used; DCyPF=1,1’-bis(dicyclohexylphosphino)ferrocene, D-t-BuPF=1,1’-bis(di-tert-butylphosphino)fer-
rocene; D-i-PrPF=1,1’-bis(diisopropylphosphino)ferrocene; p-CF3-dppf=1,1’-bis[di(4-trifluoromethylphenyl)phosphine]-
ferrocene; p-OMe-dppf=1,1’-bis[di(4-methoxyphenyl)phosphine]ferrocene.
[b]
[c]
[d]
1968
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Stable Nickel(0) Phosphites as Catalysts for C N Cross-Coupling
Table 3. Investigation of precatalysts and catalytic species
containing dppf.
Entry Catalyst
Ligand
GC Yield
[%] [%]
Scheme 1. Reactions of Ni(0) precursors with dppf.
1
2
3
4
Ni[P
G
dppf (10 mol%) 76
dppf (10 mol%) 99
68
84
59
61
82
0
Ni
Ni
ACHTUNGTRENNUNG
A
–
–
67
77
94
0
N
R
A
ACHTUNGTRNE(NUNG OPh)3]2
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
only to hinder the cross-coupling process (entry 6),
possibly because of an effect on the phosphine disso-
ciation equilibrium. Finally, inspired by previous re-
ports by Hartwig on dppf derivatives and palladium-
5
6[a]
[a]
Reaction was performed in non-inert conditions.
ꢀ
catalyzed C N cross-couplings, we examined small
changes to the dppf ligand that slightly altered the
electronics.[16,17] Use of the dppf derivatives, en- ditive, produced the amine product in a yield that was
tries 11–15 (Table 2), only moderately affected the comparable to that obtained using the combination of
yield of the amination reaction. However, given the Ni
additional synthetic requirements of these ligands,
this line of inquiry was concluded.
ACHTUNGRTEN(NUNG COD)2 with dppf (entry 2).
The higher yield obtained in the system with added
dppf (entry 5) compared to 4 alone (entry 4) suggests
The combination of Ni[PACHTUNTRGNEUNG(OPh)3]4 and dppf provid- that the additional dppf assists in a specific step in the
ꢀ
ed the best catalytic conditions, and hence we focused C N cross-coupling catalytic cycle. We found that 4
on identifying the exact catalytic species implicated in (Figure 1) is air-stable in the solid form, unlike its
this reaction. Moreover, we sought to compare our Ni(0) counterpart Ni
system with that studied in the seminal work of Buch- system was not effective under non-inert conditions.
wald and co-workers on an Ni(COD)2/dppf-catalyzed (entry 6). Very recently, during the final stages of this
reaction.[11] Our nickel phosphite example was lower study, a similar air-stable catalytic system [(dppf)Ni
(o-
yielding, however, the potential to form Ni(dppf)2 tolyl)Cl] was reported that also affords higher yields
through the use of either Ni(COD)2 or Ni[P
(OPh)3]4 if dppf is added.[39]
as pre-catalysts was evident. We targeted this species
from both sources, Ni(COD)2 and the nickel phos-
phite tetrakis complex, Ni[P(OPh)3]4. Ni(COD)2 re-
acted readily with dppf to afford Ni(dppf)2; however
the reaction of Ni[P(OPh)3]4 with dppf (toluene,
1008C) yielded only dppfNi[P(OPh)3]2 4 (Scheme 1).
(COD)2.[33] Unfortunately, this
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Attempts to force the substitution (toluene, 1008C)
of the remaining phosphite ligands proved fruitless.
ꢀ
The similarity of these conditions to our C N cross-
coupling conditions suggest that Ni
A
involved in the early stages of the reaction. Given
that either Ni(COD)2 or complex 4 could be consid-
ACHTUNGTRENNUNG
ered as putative catalysts or precatalysts in the amina-
tion reaction studied here, they were both subjected
to these reaction conditions (Table 3). The coupled
secondary amine was successfully produced in the
system catalyzed by the bis-dppf nickel complex
(entry 3) and likewise in the presence of the mono
dppf phosphite system 4 (entry 4).
These findings indicate that the steric hindrance
about the metal center in Ni
obstruct coordination of incoming coupling partners.
Interestingly, the (dppf)Ni[P(OPh)3]2 system (4) in
combination with additional dppf (5 mol%), as an ad- moved for clarity.
ACHTUNGTNER(NUGN dppf)2 is not sufficient to
Figure 1. Molecular representation of the ORTEP structure
of (dppf)Ni[P(OPh)3]2 4. Hydrogen atoms have been re-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Sven S. Kampmann et al.
ꢀ
Table 4. Catalyst 4 and C N cross-coupling of various func-
tionalized aryl species with para-toluidine 2.
Table 5. Scope of the aryl chloride coupling partner.
Entry
X
Yield [%]
Entry
X
Yield [%]
1
2
3
4
Cl
Br
I
99
5
6
7
8
OTs
47
0
0
100[a]
OCOCH3
OCOCF3
OCH3
100[a]
OTf 56
0
[a]
GC conversion.
The amination compatibility of various substituted
aryl substrates containing diverse leaving groups has
previously been reported for a range of simple to
complex nickel catalysts.[5,40] To test the catalytic abili-
ꢀ
ty of (dppf)Ni[PACHTUNGTRENNUNG(OPh)3]2 (4), we examined the C N
cross-couplings of a range of halides and pseudoha-
lides with para-toluidine (2) (Table 4). Interestingly,
there seemed to be no drop off in reactivity across
the halides surveyed, possibly due to the excellent ox-
idative addition ability of the complex. By contrast,
the yields obtained for the aryl triflate and less com-
monly reported aryl tosylate were both modest. How-
ever, more elaborate leaving groups used successfully
in previous work, were not reactive in this
system.[2,41–44]
Next we surveyed the catalytic ability of
(dppf)Ni[PACHTUNGTRENNUNG(OPh)3]2 (4) in the cross-coupling reactions
of a range of aryl chlorides with 2 (Table 5). No corre-
lation could be discerned between the electronics of
the aryl chloride and the yield, with reactions involv-
ing chloroanisole (leading to 6g) and chlorobenzophe-
none, (producing 6p) both providing high yields. The
substitution pattern of a methyl substituent in the
coupling partner was determined to affect the yield
[a]
[b]
40 h, an aliquot was taken after 18 h and showed 21%
GC conversion.
NMR yield.
slightly, with lower yields obtained for products with mides (Table 6). In this case there seemed to be no
the methyl group in the ortho position (6f). The reac- discernable reactivity pattern or predictable change in
tion did not proceed in the presence of a nitro sub- activity when compared to the corresponding aryl
stituent (6d), a phenomenon also recently observed in chlorides. Compound 6b and 6f were produced in
nickel-mediated Heck cross-coupling reactions.[9] In higher yields (99% and 91%, respectively), but the
the two pyridyl-derived substrates tested moderate to methoxy-substituted compound 6g was prepared in
good results were observed, depending on the posi- a slightly lower yield of 63%. Interestingly, the reac-
ꢀ
tion of the nitrogen (6l and 6n). The quinoline deriva- tion leading to 6n underwent a second C N coupling
tive 6m could also be prepared successfully in 93% to afford a bis-pyridyl compound. The differences in
yield.[45] To determine if the catalyst would function the aryl halide leading to product 6q had no effect on
on more complex ring systems, a thalidomide deriva- the yield. The remaining aryl bromides resulted in
tive was also prepared (6o). Although this conversion good conversions to the secondary amines, except in
was achieved in low yields, large amounts of the start- the case of the fluorinated derivative 6r. The reaction
ing aryl chloride were also found remaining.[46]
of 4-bromochlorobenzene with aniline 2 resulted
Following the success of the aryl chlorides a small mainly in the formation of the 1,4-benzenediamine
reaction trial was performed with a range of aryl bro- derivative 6u (81%), whearas the monocoupled deriv-
1970
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Stable Nickel(0) Phosphites as Catalysts for C N Cross-Coupling
Table 6. Scope of the aryl bromide coupling partner.
Table 7. Scope of the amine coupling partner for the amina-
tion reaction.
[a]
In this instance a large amount of the bis-pyridyl aniline
was also produced (83%, NMR yield), please see the
Supporting Information.
Compound resulted from competing experiment with 4-
[b]
bromochlorobenzene at 608C for 22 h.
[a]
5 mol% (R,S)-BINAP was used.
atives were only isolated in a total of 7% yield (3:2
favoring the Ar Br coupled product).
by 31P NMR spectroscopy (see the Supporting Infor-
ꢀ
The range of amines suitable for this reaction was mation). In the early stages of the reaction, it is clear
also explored (Table 7). Using (dppf)Ni[PACTHNUGTRNEG(UN OPh)3]2 (4) the catalyst 4 is consumed with the disappearance of
as a catalyst, the slightly hindered aniline 8b bearing the phosphine (d=23.3 ppm) and phosphite (d=
two methyl substituents in an ortho configuration was 127.3 ppm) resonances (Figure 2). Also evident is
successfully obtained, although in moderate yield. a new resonance appearing at d=139.8 ppm in the
The two electron-rich anilines 8c and 8e were each
obtained in a high yield, as was the naphthylamine
substrate 8d. The secondary aniline leading to 8f was
coupled in 63% yield. However, coupling of the ben-
zylamine was poorer even when the more reactive
aryl bromide was used to afford 8g. The coupling of
morpholine was successful in 76% yield, but more so
when (rac)-BINAP was used as an additive instead of
dppf or when additional catalyst was used. This alter-
nate phosphine additive was also preferentially used
for the coupling of other secondary amines leading to
products 8k, 8l and 8m. The coupling of the primary
amine to produce compound 8j was also moderately
successful. Such amines have very recently been
2
ꢀ
formed through a (BINAP)Ni
G
catalyzed reaction.[47]
An early insight into the reactionꢁs catalytic cycle
was attempted through the monitoring of the reaction Figure 2. Proposed catalytic cycle.
Adv. Synth. Catal. 2014, 356, 1967 – 1973
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1971

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Sven S. Kampmann et al.
28.8 Hz), 23.3 (t, J=28.8 Hz); IR (ATR) n=3056, 1590,
1486, 1435, 1188, 1161, 1093, 1025, 958, 871, 745, 722, 686,
566, 524 cmꢀ1.
phosphite region of the spectra, however, not corre-
sponding to free phosphite (d=127.9 ppm). An in-
crease in the intensity of the free dppf phosphine res-
onance d=ꢀ16.4 ppm is also clear. Both these factors
suggest that a bis-phosphite nickel complex 4a is pos-
sibly formed in the early stages of the reaction. Fol-
lowing other Ni(0)/Ni(II) catalytic cycles proposed for
Crystals suitable for X-ray diffraction were obtained by
placing a solution of the complex in dry and degassed tolu-
ene/n-hexane (1:1) in the freezer for one day. Please see
supporting information for more details. CCDC 983669
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[47,48]
ꢀ
nickel catalyzed C N coupling reactions,
an oxi-
dative addition to Ar(X)Ni[P
A
promoted amination to Ar
N
ACHTUNGTRENNUNG
estimated. The final, and possibly slow, reductive
elimination is suggested to be assisted through the co-
ordination of dppf. The role of the additional dppf,
however, cannot be confirmed at this stage. The GC
analysis of the reaction mixture (see the Supporting
Information) highlights an impressive conversion
(4%!56%) for this reaction between the sampling at
the 15 and 30 min time periods. We are currently pur-
ꢀ
General Procedure for C N Cross-Coupling
Reactions
A flame-dried Schlenk tube was loaded sequentially with
NaO-t-Bu (201 mg, 2.10 mmol, 1.40 equiv.), (dppf)Ni-
AHCTUNGRTENN[GUN (OPh)3]2 (4) (93 mg, 75 mmol, 5 mol%), dppf (42 mg,
75 mmol, 5 mol%), the corresponding amine (1.80 mmol,
1.20 equiv.), and aryl chloride (1.50 mmol, 1.00 equiv.). The
suing a more detailed kinetic study of this reaction to mixture was dissolved in toluene (6 mL), placed into a pre-
heated oil bath at 1008C and stirred for 18 h. Upon cooling,
the reaction mixture was fused onto silica and purified via
flash column chromatography (hexanes/EtOAc or DCM/
MeOH) to give the desired product.
determine; the rate-limiting step of the reaction, the
roles of dppf and the solvent.
In conclusion, we have identified nickel phosphites
that can serve as more stable replacements for
NiACHTUNGTRENNUNG(COD)2. Additionally, the nickel phosphite/phos-
ꢀ
phine complex 4 serves as an excellent catalyst for C
N amination reactions of various aryl chlorides and
anilines or amines. Interestingly, the addition of
a second phosphine (dppf or rac-BINAP) assists in
the coupling process. However, further studies are re-
quired to identify and confirm the structures of inter-
mediates from the proposed catalytic cycle for this
coupling process.[23,47,49]
Acknowledgements
The authors would like to thank Assoc. Professor Lindsay
Byrne at UWA for NMR assistance. We are indebted to the
Danish National Research Foundation (DNRF93) Centre for
Materials Crystallography.
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ACHTUNGTRENNUNG
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1
158–1608C (decomposition). H NMR (600 MHz, C6D6): d=
7.59 (t, J=8.1 Hz, 8H), 7.09 (d, J=7.2 Hz, 3H), 7.05 (t, J=
7.1 Hz, 8H), 7.00–6.89 (m, 24H), 6.80–6.75 (m, 7H), 4.26 (s,
4H, Cp), 3.93 (s, 4H, Cp); 13C NMR (150 MHz, C6D6): d=
153.8, 152.4, 151.2 (d, J=7.1 Hz), 141.3 (d, J=29.1 Hz),
134.8 (d, J=14.4 Hz), 131.7, 130.03 (d, J=2.3 Hz), 129.2,
128.4, 125.6, 124.5, 123.2, 122.2, 121.2, 120.6 (d, J=5.0 Hz),
74.8 (d, J=9.4 Hz, Cp), 73.9 (d, J=33.1 Hz, Cp), 71.2 (d, J=
3.7 Hz, Cp); 31P NMR (243 MHz, C6D6): d=127.3 (t, J=
1972
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Adv. Synth. Catal. 2014, 356, 1967 – 1973
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