Palladium-catalyzed formation of secondary and tertiary amines from aryl dihalides with air-stable ferrocenyl tri- and diphosphines: Synthesis and X-ray structure of efficient catalysts beyond [PdCl2(DPPF)]

Article abstract of DOI:10.1016/j.catcom.2014.03.012

Robust, air-stable tridentate and bidentate ferrocenylphosphines 1,2-bis(diphenylphosphino)-1′-(diisopropylphosphino)-4-tert- butylferrocene, L5, and 1,1′-bis(diisopropylphosphino)-3,3′- bis(tert-butyl)ferrocene, L9, combined with 1 mol% of [PdCl(η³- C₃H₅)]₂ led to two new catalytic systems which allow the coupling of aniline derivatives with mono- and dihaloarenes to form functionalized diarylamines and triarylamines. The excellent selectivity of the reactions avoids the deleterious dehalogenation of the substrates and products. The X-ray structure characterization of the related complex [PdCl ₂(L9)] is reported in which ligand L9 in its meso form is significantly distorted.

Full text of DOI:10.1016/j.catcom.2014.03.012

Catalysis Communications 51 (2014) 10–14
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Palladium-catalyzed formation of secondary and tertiary amines from
aryl dihalides with air-stable ferrocenyl tri- and diphosphines: Synthesis
and X-ray structure of efficient catalysts beyond [PdCl2(DPPF)]
Sophal Mom a, Mélanie Platon a, Hélène Cattey a, Howard J. Spencer b, Paul J. Low b,c, , Jean-Cyrille Hierso
⁎
a,d,⁎⁎
a
Université de Bourgogne, Institut de Chimie Moléculaire de l'Université de Bourgogne, UMR-CNRS 6302, 9 avenue Alain Savary, 21078 Dijon, France
b
Durham University, Department of Chemistry, South Rd, Durham DH1 3LE, UK
University of Western Australia, School of Chemistry and Biochemistry, 35 Stirling Highway, Crawley, 6009 WA, Australia
c
d
Institut Universitaire de France (IUF), France
a r t i c l e i n f o
a b s t r a c t
Article history:
Robust, air-stable tridentate and bidentate ferrocenylphosphines 1,2-bis(diphenylphosphino)-1′-
(diisopropylphosphino)-4-tert-butylferrocene, L5, and 1,1′-bis(diisopropylphosphino)-3,3′-bis(tert-butyl)ferro-
cene, L9, combined with 1 mol% of [PdCl(η3-C3H5)]2 led to two new catalytic systems which allow the coupling
of aniline derivatives with mono- and dihaloarenes to form functionalized diarylamines and triarylamines. The
excellent selectivity of the reactions avoids the deleterious dehalogenation of the substrates and products. The
X-ray structure characterization of the related complex [PdCl2(L9)] is reported in which ligand L9 in its meso
form is significantly distorted.
Received 15 February 2014
Received in revised form 8 March 2014
Accepted 10 March 2014
Available online 18 March 2014
Keywords:
Palladium
Ferrocenylphosphine
Aniline
© 2014 Elsevier B.V. All rights reserved.
Dihaloarenes
C–N coupling
X-ray structure
1. Introduction
di-n-butylamine with 4-tert-butylbromobenzene was more difficult:
only 40% of coupling product was obtained employing 5% [PdCl2(DPPF)]
Arylamines are common building blocks of many biologically active
natural products and organic materials. The first syntheses of
arylamines were carried out by nitration of arenes followed by catalytic
hydrogenation or a reduction with a metal salt. These procedures that
employ strong acidic and oxidizing conditions are incompatible with
many functional groups, and thus require the use of protective groups
in multiple reaction step. The discovery of palladium-catalyzed aryl–
amine bond forming reactions procedures has enlarged the availability
of these important arylamine compounds [1–3]. The complex [PdCl2
(DPPF)] (DPPF = 1,1′-bis(diphenylphosphino)ferrocene) has been suc-
cessfully employed as a pre-catalyst in the coupling of aryl halides with
primary amine aniline derivatives [4]. This catalyst provided nearly
quantitative yields for electron-rich, electron-poor, hindered or unhin-
dered aryl bromides and iodides. The coupling of the secondary amine
and 15 mol% DPPF, a number of other systems giving better results
[5–7]. The air-stability of ferrocenyl diphosphines, the steric control
and chelating stabilization they provide, nevertheless renders this
class of ligands very convenient for handling and use off-the-shelf [8, 9].
On the other hand, the use of air-stable robust ferrocenyl
polyphosphines (tri- and tetradentate) as ligands in palladium-cata-
lyzed C\C bond formation has led to a diverse range of highly efficient
catalytic systems, which offer activities several orders of magnitude
higher than DPPF-derived catalyst systems for cross-coupling reactions
using electronically and/or sterically demanding organic halides
[10–15]. The efficiency of ferrocenyl phosphines at low catalyst loading
was also recently confirmed in the palladium-catalyzed C\O bond cou-
pling for heteroarylether formation from functionalized phenols and
chloroheteroarenes [16]. We envisioned that ferrocenylphosphine li-
gands (Fig. 1) might be useful for the coupling of aniline derivatives
with bromoarenes, and especially concerning polyfunctionalized sub-
strates, as well as for thereafter applications to the more difficult forma-
tion of triarylamines [17–21]. The valuable targets we identified are
depicted in Fig. 2. The compounds of formula A may, for instance,
open the access to the formation of bioactive carbazoles [22–24],
while the compounds B and C are pertinent building blocks for con-
struction of redox-active polytriarylamine compounds and materials
⁎
Correspondence to: P. J. Low, University of Western Australia, School of
Chemistry and Biochemistry, 35 Stirling Highway, Crawley, 6009 WA, Australia.
Tel.: +61 8 648830 45.
⁎⁎ Correspondence to: J.-C. Hierso, Université de Bourgogne, Institut de Chimie
Moléculaire de l'Université de Bourgogne, UMR-CNRS 6302, 9 avenue Alain Savary,
21078 Dijon, France. Tel.: +33 3 80396107; fax: +33 3 80393682.
E-mail addresses: paul.low@uwa.edu.au (P.J. Low), jean-cyrille.hierso@u-bourgogne.fr
(J.-C. Hierso).
http://dx.doi.org/10.1016/j.catcom.2014.03.012
1566-7367/© 2014 Elsevier B.V. All rights reserved.

S. Mom et al. / Catalysis Communications 51 (2014) 10–14
11
Fig. 1. Ferrocenylphosphines tested in C\N bond formation from 1,2-dibromobenzene.
[25–27]. Therefore, finding efficient and easy to handle catalytic systems
for the synthesis of these species is timely.
significant amount of diphenylamine by dehalogenation of 3a. Further
optimization of the catalytic process was focused thus on L5 ligand as
summarized in Table 2. The optimization of temperature (entries 1–3)
indicated that reaction in refluxing toluene was beneficial. DBU as a
base was inefficient (entry 4). Cation dependence was observed for
the tert-butoxide bases, where t-BuONa (entries 5–8) was found to be
more suitable than t-BuOK. This behavior has been observed before
[34], and may result from the greater solubility of the sodium com-
pound which was reported to be in toluene at 25 °C as t-BuONa,
6 wt.% and t-BuOK, 2.3 wt.% [34]. With these conditions in hands it
was possible to significantly reduce the reaction time, which improves
the final isolated yield (98%) by diminishing the dehalogenation of 3a.
In the presence of an excess of aniline (3. equiv.), bis(amination) of
1,2-dibromobenzene takes place above 80% yields. GC/NMR monitoring
clearly show that the first amination is selectively achieved at 98 yield%
within 2 h, then followed by bis(amination) which is achieved at about
80% within 8 h. Interestingly, it appears thus that the coupling of the sec-
ond equivalent of aniline to the intermediate monobromodiarylamine is
much slower than the addition of the first equivalent of aniline to the
1,2-dibromobenzene. This is in contrast to previously reported results
using the air-sensitive electron-rich monophosphine P(t-Bu)3 [31].
The [Pd]/L5 catalytic system was also used to examine the influ-
ence of the nature of the halide on the cross-coupling partner in
the formation of the building block A (Fig. 3). We found that within
2 h the heterodihaloarenes 2b and 2c are efficiently coupled to ani-
line with high yield. From 2b the reaction is perfectly selective with
only arylation at the iodide position; this shows the influence of
the first substitution in ortho-position on the reactivity of the
2. Results and discussion
We hypothesized that backbone A may be the most difficult to syn-
thesize because of selectivity and reactivity reasons (functionalization
in ortho-position). Thus, we investigated the conditions of coupling of
aniline with 1,2-dibromobenzene. Preliminary screening tests showed
that no reaction proceeds in the absence of palladium, or in the presence
of palladium but without any auxiliary ligand. However, encouraging
screening experiments indicated that with 1 mol% of [PdCl(η3-C3H5)]2
and 2 mol% of DPPF, the use of toluene as solvent, at 100 °C, in the pres-
ence of 1.4 equiv. of t-BuOK as base, selectively led to the conversion of
1,2-dibromobenzene (2a) into 2-bromodiphenylamine (3a) in about
30% yield. This amount of palladium and ligand is five to ten times
lower, with conditions generally milder, than the current syntheses
for the coupling of comparable substrates [28–33]. Under these condi-
tions of base, solvent and temperature, the performances of phosphines
L2 to L9 were also examined and compared to DPPF (Table 1).
The ligand dependency of this reaction was clearly established. In-
deed, the ligands L4 and L7 were found fully inefficient (entries 4 and
7). While L2 gave only a modest yield in 3a (entry 2), the performances
of L3 match the moderate ones obtained with DPPF (entries 3 and 1, re-
spectively). The conversion of 2a was the most significant in reactions
promoted by [PdCl(η3-C3H5)]2 and ligands L5, L6 and L8; however
only L5 gave a satisfactory selectivity in 3a (entry 5) since the use of
the two other ligands (entries 6 and 8) induces the formation in
Fig. 2. Targeted diaryl and triaryl amines building blocks.

12
S. Mom et al. / Catalysis Communications 51 (2014) 10–14
Table 1
resulting compound. Gratifyingly, we observed that the system is even
Screening of [Pd/Ligand] catalyst activity.
suitable to selective coupling of 1,2-dichlorobenzene (2d), albeit in lon-
ger reaction time (16 h) with slightly lower yields (90%). The [Pd]/L5
catalyst system was then successfully used for the formation of com-
pounds 4a and 4b (type B). The compound bis(4-methylphenyl)amine
(4a) was synthesized in 84% yield by cross-coupling 4-methylaniline
with 4-bromotoluene (Fig. 3). In similar fashion, bis(4-methoxyphenyl)
amine (4b) was produced in 87% yield by coupling the p-anisidine with
4-bromoanisole. In comparison, by using Pd2(dba)3 (1 mol%) and DPPF
(3 mol%) the conversion obtained were 79% and 50%, respectively.
With the diarylamines 4a and 4b in hand we applied the same con-
ditions with the view of generating the triarylamines 5a and 5b by fur-
ther cross-coupling of the secondary amines with the dihalide 4-bromo-
4′-iodobiphenyl (Fig. 4). Unfortunately, while our optimized system
[Pd]/L5 was found very effective for formation of functionalized second-
ary amines (Fig. 3) it was inefficient for properly producing the tertiary
amines 5a (b5%) and 5b (0%) (Fig. 4). We then turned back to our li-
brary of ferrocenyl phosphine ligands considering some of the proper-
ties of L5, and in particular the combination of iso-propyl groups on
phosphorus and the control of conformation due to the presence of
tert-butyl groups on the ferrocene backbone, was identified as specific
to this ligand compared to L2–L4, L6–L8. Thus, the recently synthesized
diphosphine L9 [34] was tested in the synthesis of triarylamines (Fig. 4),
and was found to be appropriate to produce in good yield the electron-
rich triarylamine 5b (72%), in comparison, by using Pd2(dba)3 (1 mol%)
and DPPF (3 mol%) the conversion obtained of 5b were 29%. It also led
in moderate yield to 5a (27%).
Entry
Ligand
Conversion of 2a (%)
Yield in 3a (%)
1
2
3
4
5
6
7
8
L1 (= DPPF)
28
14
32
b5
55
40
b5
50
25
12
30
b5
50
30
b5
25
L2
L3
L4
L5
L6
L7
L8
Conditions: [Pd]/L (0.02 equiv.), aniline 1a (1 equiv.), 1,2-dibromobenzene 2a (1 equiv.),
t-BuOK (1.4 equiv.), 100 °C, 18 h.
Table 2
Base, solvent and temperature optimization.
Entry
Base
Temperature
Time
Conversion of 2a (%)
Yield in 3a (%)
1
2
3
4
5
6
7
8
t-BuOK
t-BuOK
t-BuOK
DBU
t-BuONa
t-BuONa
t-BuONa
t-BuONa
100
85
18
20
18
18
16
8
55
30
92
50
30
79
b5
73
79
88
98
115
115
115
115
115
115
To better assess the structures of the ligand and palladium pre-
catalyst, single crystals of the [PdCl2L9] complex suitable for X-Ray char-
acterization were grown. The molecular structure of the complex is
depicted in Fig. 5 and confirmed the meso nature of the diphosphine
(Fig. 5).
b5
100
100
100
100
4
2
The palladium complex is embedded in a square planar environ-
ment, in which a large bite angle P1-Pd-P2 = 101.35° is observed;
Conditions: [Pd]/L (0.02 equiv.), aniline 1a (1 equiv.), 1,2-dibromobenzene 2a (1 equiv.),
base (1.4 equiv.).
Fig. 3. Synthesis of targets A and B from [Pd]/L5.

S. Mom et al. / Catalysis Communications 51 (2014) 10–14
13
Fig. 4. Ferrocenyl diphosphine L9 for halogenated triaryl amine formation.
this is likely favorable for reductive elimination of triarylated amines
4.1. Synthesis and characterization of [PdCl2L9]
at palladium. In [PdCl2L9] the deformation of the ferrocene back-
bone is noticeable with a Ct1-Fe-Ct2 = 173.63(15)°, which is signif-
icantly distorted from the expected ideal angle of 180°. As shown
in the view from above, despite the steric hindrance generated at
tert-butyl groups a small torsion angle (below 32°) is observed, i.e.
P1-Ct1-Ct2-P2 = −29.93(8).
In summary we disclosed herein two new catalytic systems based on
air-stable robust di- and tridentate ferrocenyl polyphosphines L5 and
L9, which allow the coupling of aniline derivatives with chloro or
bromoarenes to form diarylamines and triarylamines functionalized
with halides. The selectivity of the reactions allows the use of mild reac-
tion conditions and short reaction times, avoiding dehalogenation of the
substrates and products. The X-ray structure characterization of the re-
lated complex [PdCl2(L9)] was also reported.
A mixture of 1,1′-bis(diisopropylphosphino)-3,3′-di-tert-butyl fer-
rocene (304 mg, 0.573 mmol) and PdCl2 (110 mg, 0.619 mmol) was
heated in refluxing THF (10 mL) for 17 h. The brown solution was
filtrated through silica, and the column washed with dichloromethane.
The solvent was evaporated from the combined organic fractions under
vacuum to give 160 mg of the complex (39%). Fractional crystallization
allows recovering pure meso complex.
1H NMR (CDCl3, rac + meso): δ(ppm) = 0.58 (dd, 6H, 3JPH = 7 Hz, i-
Pr-CH3), 0.85 (p-t, 6 H, 3JPH = 6 Hz, i-Pr-CH3), 1.11 (s, 18 H, t-Bu, minor
isomer), 1.16 (s, 18H, t-Bu, major isomer), 1.60 (m, 12 H, i-Pr-CH3), 2.87
(hept, 2H, 3JHH = 7 Hz, i-Pr-CH), 3.07 (hept, 2H, 3JHH = 7 Hz, i-Pr-CH),
4.25, 4.33, 4.38 (m, 2H each, H-Cp, major isomer), 4.15, 4.44 (m, H-Cp,
minor isomer). 31P{1H} NMR (CDCl3): δ (ppm) = 62.15 (s, 2 P-i-Pr2,
minor isomer 17%, racemic compound), 59.35 (s, 2 P-i-Pr2, major isomer
83%, meso compound). 13C NMR (CDCl3): δ (ppm) = 108.3 (s, 2C, CpC-t-
Bu, major isomer 83%), 106.8 (s, 2C, CpC-t-Bu, minor isomer 17%), 74.0
(s, 2C, CpC-Pi-Pr2), 73.4 (s, 2C, Cp-CH), 70.8 (s, 2C, Cp-CH, major iso-
mer), 69.9 (s, 2C, Cp-CH, minor isomer), 67.9 (s, 2C, Cp-CH, major iso-
mer), 67.3 (s, 2C, Cp-CH, minor isomer), 30.6 (s, 6C, t-Bu-(CH3)3,
major isomer), 30.5 (s, 6C, t-Bu-(CH3)3, minor isomer), 29.6 (s, 2C,
C(CH3)3, minor isomer), 29.4 (s, 2C, C(CH3)3, major isomer), 27.8 (m,
2C, CH-i-Pr), 27.7 (m, 2C, CH-i-Pr), 27.4, 27.2 (m, 2C each, CH-i-Pr,
minor isomer), 20.5 (s, 2C, CH3-i-Pr), 20.3 (s, 2C, CH3-i-Pr), 19.4 (s, 2C,
CH3-i-Pr), 18.9 (s, 2C, CH3-i-Pr), 20.8, 20.6, 19.10 (m, 2C each, CH3-i-Pr,
minor isomer). ESI-MS: [M − Cl]+: m/z = 671.16259, simulated =
671.16225, δ = 2.140 ppm; [M + Na]+: m/z = 729.12005, simulat-
ed = 729.12062, δ = 0.375 ppm.
3. Experimental
The reactions were carried out in oven-dried (115 °C) glassware
under an argon atmosphere using Schlenk and vacuum-line techniques.
The solvents were distilled over appropriate drying and deoxygenating
agents prior to use. Commercial aryl halides and aniline derivatives
were used without further purification. 1H, 31P and 13C NMR spectra
were recorded in CDCl3. All the ferrocenylphosphine ligands were syn-
thesized by methods reported in the literature [10–16, 35], and are
stored and weighed under air without special precautions. The
ferrocenyl ligand L5 is commercially available from STREM Chemicals
under the name HiersoPHOS-4.
Fig. 5. Ortep molecular views of palladium dichloride stabilized by the coordination of diphosphine L9 (hydrogen atoms are omitted for clarity). Selected distances (Å) and
angles (°): Fe-Ct1 = 1.663(3); Fe-Ct2 = 1.668(3); Ct1-Fe-Ct2 = 173.63(15); P1-Pd-Cl1 = 85.85(3); P2-Pd-Cl2 = 85.76(3); P1-Pd-P2 = 101.35(3); Cl1-Pd-Cl2 = 87.19(3); P1-Ct1-Ct2-
P2 = −29.93(8); and P1…P2 = 3.5318(11).

14
S. Mom et al. / Catalysis Communications 51 (2014) 10–14
[9] Biarylphosphines are another important family of ligands for such C\N bond
4.2. Catalytic cross-coupling reactions
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In a Schlenk tube equipped with a stirring bar, the aniline derivative
(2 mmol, 1 equiv.), the haloarene (2 mmol, 1 equiv.), the complex
[PdCl(η3-C3H5)]2 (0.02 mmol), L5 or L9 (0.04 mmol), and t-BuONa
(2.8 mmol, 1.4 equiv.) were introduced. The tube was purged several
times with argon. Toluene (9 mL) was added to the mixture. The
Schlenk tube was placed in an oil bath at 115 °C and reactants were
allowed to stir under reflux for 2 to 20 h. After filtration, the solvent
was evaporated and the residue was charged onto a silica gel column
for chromatography.
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Acknowledgments
Support provided by the “Université de Bourgogne”, CNRS (PhD
grant awarded to M. P. in the 3MIM program), and the ANR (program
CAMELOT-2009 granted the PhD of S. M.) are acknowledged. Thanks
are due to Dr. J. Roger for post-reviewing additional experiments.
Thanks are due to Johnson-Matthey for a generous gift of palladium.
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