A simple and convenient method for the synthesis of N, N-diaryl tertiary amines

Article abstract of DOI:10.1055/s-0033-1340820

Direct preparation of tertiary amines in which two substituents are aromatic is described. In the presence of either the inorganic base sodium tert-butoxide or the sterically hindered organic base diisopropylethylamine, the alkylation of secondary diarylamines is achieved smoothly. In contrast to methods previously reported in the literature, this procedure is high-yielding and does not require the use of transition-metal catalysts or functional-group-intolerant hydride reductants. Georg Thieme Verlag Stuttgart ? New York.

Full text of DOI:10.1055/s-0033-1340820

1046▌
PRACTICAL SYNTHETIC PROCEDURES
pAracSticiaml sypnthleeticaprnocdeduCresonvenient Method for the Synthesis of N,N-Diaryl Tertiary
Amines
Synthesis of N,N-Diaryl Tertiary Amines
Francis S. Wekesa, Neha Phadke, Claire Jahier, David B. Cordes, Michael Findlater*
Texas Tech University, Department of Chemistry & Biochemistry, Lubbock, TX 79409, USA
Fax +1(806)7421289; E-mail: Michael.Findlater@ttu.edu
PSP
Received: 27.12.2013; Accepted after revision: 22.01.2014
No270
Abstract: Direct preparation of tertiary amines in which two substituents are aromatic is described. In the presence of either the inorganic
base sodium tert-butoxide or the sterically hindered organic base diisopropylethylamine, the alkylation of secondary diarylamines is
achieved smoothly. In contrast to methods previously reported in the literature, this procedure is high-yielding and does not require the use
of transition-metal catalysts or functional-group-intolerant hydride reductants.
Key words: tertiary amines, DIPEA, alkylation, benzyl halides
O
O
EtO
OEt
O
O
Me
N
H
Me
Ar
Ar
N
R
Ar
Charette
HNAr2
DIPEA
Ar
N
Br
R
R
Ar
noble metal
this work
N
catalyst/silanes
R
R
Ar
Brookhart/Beller
TMS
OTf
HNAr2, CsF (2.0 equiv )
R
Ar
Larock
N
Ar
Scheme 1 Selected routes to reduction of amides to tertiary amines
Amines are widely used in the chemical industry as basic amines. However, formation of quarternary ammonium
intermediates in the preparation of fine chemicals and salts, together with mixtures of starting material and de-
pharmaceuticals.1,2 Since amines are common structural sired tertiary amine limits the general application of this
features of most naturally occurring biologically active method.6 Reduction of amides using alkali metal or boron
compounds, they have been used in the design of chemo- hydrides is another common procedure. However, the use
therapeutic approaches to a variety of diseases.3 Thus, a of low functional group tolerant strong inorganic bases
great deal of sustained research interest in new synthetic coupled with the air and moisture sensitivity of such hy-
approaches to amines has been generated. Specifically, drides complicates their general use.7 Catalytic hydroge-
the preparation of tertiary amines (NR1R2R3) has been ap- nation of primary and secondary amides to amines has
proached in a variety of ways, each with unique benefits been reported under conditions of high pressures and tem-
and drawbacks. Buchwald4 and Hartwig5 have shown that peratures, but reductions of tertiary amides via hydroge-
the copper- or palladium-catalyzed N-arylation of a vari- nations has not been observed.8 Over the last few years,
ety of amines using aryl halides is a powerful method for metal-catalyzed reductions of amides employing silanes
the synthesis of arylamines. However, the requirement of have been extensively investigated. Recently, Brookhart
a transition-metal catalyst and often-poor yields of tertiary reported the use of an iridium pincer complex in the re-
amines are undesirable aspects of this approach. The di- duction of tertiary amides9 (Scheme 1). However, the use
rect alkylation of secondary amines with alkyl halides is of a precious metal catalyst and generation of stoichiomet-
the most straightforward way for the synthesis of tertiary ric siloxane by-products significantly limit the general use
of this approach. Beller and co-workers recently reported
an iron-catalyzed reduction procedure.10 Whereas this
method employs an earth-abundant transition metal, an
important improvement, stoichiometric amounts of si-
SYNTHESIS 2014, 46, 1046–1051
Advanced online publication: 25.02.2014
DOI: 10.1055/s-0033-1340820; Art ID: SS-2013-M0839-PSP
© Georg Thieme Verlag Stuttgart · New York
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

PRACTICAL SYNTHETIC PROCEDURES
Synthesis of N,N-Diaryl Tertiary Amines
1047
lanes are still required. Many methods such as those out- position (Table 1, entry 8). It is interesting to note, how-
lined above, are either ineffective or perform poorly in the ever, that an inseparable mixture was obtained in the case
case of tertiary amines where at least two of the substitu- of entry 4 (p-nitrobenzyl bromide). Although no trend is
ents are aromatic. For example, Beller’s iron-catalyzed re- readily evident in the variation of the electron density of
duction of N,N-diphenylbenzamide proceeds, under the electrophile, all yields are reproducible. Molecular
optimized conditions, in 50% yield.8
structures of 2a, 2b, and 2c are shown in Figure 1.
Charette and co-workers11 recently reported a highly che-
moselective and metal-free reduction of tertiary amides
(Scheme 1). However, the use of an expensive Hantzsch
ester as reductant and, once more, the low yields of tertia-
ry N,N-diphenylamine products hamper a more general
use of this method. A highly creative approach to N-ary-
lation of amines, discovered by Larock and co-workers12
involves the generation of intermediate benzynes. Whilst
high-yielding, such a method requires preparation of sily-
laryl triflate precursors and can lead to both a mixture of
isomers and unexpected products as a result of further
[4+2] cycloaddition reactions with benzyne. Thus, a con-
venient and high-yielding method of preparing N,N-di-
phenyl tertiary amines is still unknown. Herein we report
the conversion of a variety of substituted benzyl bromides
to the corresponding tertiary N,N-diphenylamines in a
convenient and high-yielding fashion (Scheme 1).
In the course of one of our research projects we needed an
expedient synthetic route for the preparation of N,N,N′,N′-
tetraaryl-m-xylylene diamines. A survey of the literature
led us to the work of Charette11 described above. Howev-
er, the use of excess Hantzsch ester precluded the use of
this method on a preparative scale across a wide range of
substrates. Moreover, yields of the targeted (diaryl) tertia-
ry amines never exceeded 50%.
Figure 1 Molecular structures of 2a, 2b and 2c as determined by
single-crystal X-ray diffraction studies; thermal ellipsoids are shown
at 50% probability levels
Next, the use of nonbenzylic substrates was probed in an
effort to explore the substrate scope of our reaction (Table
1, entries 9–11) and found no conversion had taken place
upon analysis of the reaction mixture. To determine if the
base were a limiting factor we turned to an inorganic base,
sodium tert-butoxide, and found qualitatively similar re-
sults (Table 1, all entries). However, we were pleasantly
surprised to discover that (in most cases) higher conver-
sions could be obtained under milder reaction conditions.
However, the use of diisopropylethylamine (DIPEA or
Hünig’s base) has been employed in the alkylation of sec-
ondary amines.13 Typically, the low nucleophilicity of
secondary N,N-diarylamines would preclude their use in
such a reaction. Thus, we set out to study the preparation
of N,N-diarylamines as a model system. Gratifyingly, in
the presence of a small excess of DIPEA, we obtained ex-
cellent yields of the desired N,N-diphenylbenzylamine in
6–14 hours. Given the lack of a generalized, high-yielding
method to access such species we decided to explore the
scope of this reaction.
Having explored the scope of substrates available as the
electrophilic alkyl halide reaction partner, our attention
was next turned to the nucleophilic (amine) partner. Ini-
tially, our standard conditions were employed in the prep-
aration of the known 1-benzylpyrrolidine14 (Table 2, entry
1) and were once more pleased to see quantitative conver-
sion to the tertiary amine product. The heterocyclic sub-
strates pyrrole and 2,5-dimethylpyrrole were also
explored (Table 2, entries 2 and 3) under our optimized re-
action conditions. Surprisingly, only decomposition was
observed in the reaction, which employed DIPEA as the
base of choice, whereas sodium tert-butoxide afforded
quantitative yields of 1-benzylpyrrole (4b). This is a sig-
nificant finding given that, in the case of Larock and co-
workers,12 simple pyrrole substrates were not tolerated by
the benzyne intermediate generated by their reaction pro-
tocol. In the case of 2,5-dimethylpyrrole no reaction was
observed with either base.
Scope and Limitations
We chose to study the direct benzylation of the secondary
N,N-diarylamines. Initially, we chose the representative
examples of electron-poor benzyl bromides (Table 1, en-
tries 2–4) and electron-rich benzyl bromides (Table 1, en-
tries 5–7). In all cases, the reactions were monitored by
TLC and were complete in 6–24 hours in acetonitrile at 80
°C using ratios of diarylamine:benzyl bromide:DIPEA of
1.0:1.2:1.3. The introduction of either electron-releasing
or electron-withdrawing substituents did not affect the
overall yield of the alkylation product. The reaction also
tolerates steric bulk, introduced via substitution at the 2-
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1046–1051

1048
F. S. Wekesa et al.
PRACTICAL SYNTHETIC PROCEDURES
Table 1 Alkylation of N,N-Diphenylamines Using Alkyl Bromides
Ph
HNPh2, DIPEA
N
Br
R
R
MeCN, 80 °C
Ph
Entry
Substrate
Tertiary amine product
Base (% conversion)a
DIPEAb
t-BuONac
NPh2
Br
1
2
3
100
100
1a
2a
Br
NPh2
100
100
100
100
F
F
1b
2b
Br
Br
NPh2
F3C
F3C
2c
1c
NPh2
4
5
6
7
dec.
100
55
dec.
100
100
100
O2N
2d
O2N
1d
NPh2
Br
1e
2e
Br
Br
NPh2
MeO
MeO
1f
2f
MeO
MeO
NPh2
94
1g
1h
2g
Br
NPh2
8
90
100
2h
NPh2
Br
9
N.R.
N.R.
N.R.
3
1i
1j
2i
Br
NPh2
10
11
N.R.
N.R.
2j
NPh2
Br
1k
a Based upon GC-MS analysis.
2k
b General reaction conditions: diphenylamine (1 equiv), alkyl halide (1.2 equiv), DIPEA (1.3 equiv), MeCN (5 mL), 80 °C, 14–24 h.
c General reaction conditions: diphenylamine (1 equiv), alkyl halide (1.2 equiv), NaOt-Bu (1.2 equiv), MeCN (5 mL), r.t., 6–14 h.
Synthesis 2014, 46, 1046–1051
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PRACTICAL SYNTHETIC PROCEDURES
Synthesis of N,N-Diaryl Tertiary Amines
1049
Table 2 Alkylation of N,N-Diarylamines Using Benzyl Bromide
Ph
HNAr2, DIPEA
Ar
Ph
Br
N
MeCN, 80 °C
Ar
Entry
Substrate
Tertiary amine product
Base (% conversion)a
DIPEAb
t-BuONac
Ph
N
NH
1
2
100
100
3a
4a
Ph
N
NH
dec.
100
3b
4b
Ph
N
NH
3
N.R.
N.R.
3c
4c
NH
NH
N
4
5
100
100
81
2
2
Ph
3d
4d
F
N
F
2
100
2
Ph
3e
4e
NH
N
2
2
Ph
6
7
10
50
98
3f
4f
NH
N
2
2
Ph
N.R.
Br
Br
3g
4g
N
NH
2
8
N.R.
N.R.
2
Ph
3h
4h
a Based upon GC-MS analysis.
b General reaction conditions: diphenylamine (1 equiv), alkyl halide (1.2 equiv), DIPEA (1.3 equiv), MeCN (5 mL), 80 °C, 14–24 h.
c General reaction conditions: diphenylamine (1 equiv), alkyl halide (1.2 equiv), NaOt-Bu (1.2 equiv), MeCN (5 mL), r.t., 6–14 h.
Both electron-donating and electron-withdrawing diaryl- tions is a result of the increase steric encumbrance intro-
amine substrates (Table 2, entries 4 and 5) are tolerated al- duced by the methyl substituents. Interestingly,
though the introduction of steric bulk into the ortho- quantitative conversion to product 4g can be effected us-
position of the diarylamine significantly retards conver- ing sodium tert-butoxide as the base of choice. The mo-
sion (Table 2, entries 6–8).14,15 This suggests the failure of lecular structure of 4g is shown in Figure 2.
2,5-dimethylpyrrole to react under our optimized condi-
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1046–1051

1050
F. S. Wekesa et al.
PRACTICAL SYNTHETIC PROCEDURES
recorded on a 400 MHz spectrometer. High-resolution mass spec-
trometry (HRMS) was performed on an electron ionization time-of-
flight (EI-TOF) mass spectrometer. Melting points were recorded
using a capillary melting point apparatus and are uncorrected.
X-ray diffraction data for compounds 2a, 2c, and 4g were obtained
on a Bruker Smart Apex II CCD diffractometer, and data for com-
pounds 2b and 6 were obtained on a Rigaku SCXMini CCD diffrac-
tometer.17
N,N-Diaryl Tertiary Amines Using Hünig’s Base; N-Benzyldi-
phenylamine (2a); Typical Procedure A
To a stirred solution of diphenylamine (2.95 mmol, 1.0 equiv) in
MeCN (10 mL) were added benzyl bromide (3.54 mmol, 1.2 equiv)
and i-Pr2EtNH (DIPEA, 3.84 mmol, 1.3 equiv). The mixture was
heated to 80 °C and stirred until the reaction was complete, as mon-
itored by TLC. Solvent was removed under vacuum and the product
was extracted with CH2Cl2 (2 × 25 mL). The combined CH2Cl2 lay-
ers were washed with H2O (2 × 10 mL), and dried (MgSO4). After
removal of the solvent, the product was purified by crystallization
from hexanes to give compound 2a.
Figure 2 (a) Molecular structure of 4g as determined by single-
crystal X-ray diffraction studies. Thermal ellipsoids are shown at 50%
probability levels. (b) Molecular structure of 6 as determined by
single-crystal X-ray diffraction studies; thermal ellipsoids are shown
at 50% probability levels, hydrogen atoms omitted for clarity.
Returning to our stated goal, preparation of compounds of
N,N,N′,N′-tetraarylamines, we decided to apply our opti-
mized conditions in the preparation of N,N′-[(4,6-dimeth-
yl-1,3-phenylene)bis(methylene)]bis(N-phenylaniline)
(6). Gratifyingly, treatment of 1,5-bis(bromomethyl)-2,4-
dimethylbenzene (5) with two equivalents of diphenyl-
amine in the presence of DIPEA as the base cleanly af-
forded the desired tetraarylamine in moderate yield (45%)
(see also Supporting Information). There is only one prior
literature example of a tetraarylamine of this type, the un-
N,N-Diaryl Tertiary Amines Using Sodium tert-Butoxide;
N-Benzyldiphenylamine (2a); Typical Procedure B
NaOt-Bu (340 mg, 3.54 mmol) was added slowly to a solution of di-
phenylamine (550 mg, 2.95 mmol) dissolved in MeCN (10 mL).
The reaction mixture was stirred for 5 min before benzyl bromide
(3.54 mmol, 1.2 equiv) was added dropwise. A white solid formed
immediately. The mixture was transferred to a Schlenk line and stir-
ring was continued overnight. Solvent was removed under vacuum
and the product was extracted with CH2Cl2 (2 × 25 mL). The com-
bined CH2Cl2 layers were washed with H2O (2 × 10 mL), and dried
(MgSO4). Solvent was removed and the product was obtained in an-
alytically pure form by crystallization from hexanes to give 2a.
substituted
phenylaniline).16 However, extremely forcing conditions
were required to convert the m-xylylene dichloride to the N-Benzyldiphenylamine (2a)
N,N′-[1,3-phenylenebis(methylene)]bis(N-
Typical Procedure A; yield: 612 mg (80%); colorless crystals; mp
desired compound. Crystallization from hexanes afforded
crystals of suitable quality for study using single-crystal
X-ray diffraction experiments. The molecular structure of
6 is shown in Figure 2.
89–91 °C (Lit.7 mp 88–90 °C).
The spectroscopic data (1H and 13C NMR) were in good agreement
with those reported for the authentic sample.7,18
N-(4-Flourobenzyl)diphenylamine (2b)
In conclusion, we have developed a highly practical and
convenient synthesis of tertiary N,N-diarylamines by the
reaction of secondary N,N-diarylamines with benzyl ha-
lides in the presence of either DIPEA or sodium tert-bu-
toxide as base. The differences observed in reactivity
between the two bases may result from differing mecha-
nisms. In the case of DIPEA, the reaction mechanism like-
ly involves generation of a quaternary salt upon exposure
of the benzyl halide to the amine base. The reaction is tol-
erant of a variety of arylamines and benzyl halides under
relatively mild conditions. Significantly, these conditions
are amenable to preparatory scale synthesis using cheap
and readily available starting materials. Furthermore, this
method avoids the use of costly transition metals or harsh
alkali metal reductants. We are currently investigating
these molecules as ligand supports in the metal-catalyzed
oxidation of hydrocarbons and will report on these studies
in due course.
Typical Procedure A; yield: 695 mg (85%); colorless crystals; mp
96–98 °C (Lit.18 mp 97–101 °C).
Spectroscopic data (1H and 13C NMR) were in good agreement with
those reported for the authentic sample.18
N-(4-Triflouromethylbenzyl)diphenylamine (2c)
Typical Procedure A; yield: 628 mg (65%); white solid; mp 62–63
°C.
1H NMR (400 MHz, CDCl3): δ = 5.05 (s, 2 H), 6.95–6.97 (m, 2 H),
7.04–7.06 (m, 4 H), 7.24–7.28 (m, 4 H), 7.47 (d, J = 7.79 Hz, 2 H),
7.57 (d, J = 7.79 Hz, 2 H).
13C NMR (125 MHz, CDCl3): δ = 56.0, 120.6, 121.7, 125.5, 125.7,
126.8, 129.4, 143.5, 147.7.
HRMS: (ESI, +): m/z [M + H]+ calcd for C20H16F3N: 328.1313;
found: 328.1308.
Anal. Calcd for C20H16F3N: C, 73.38; H, 4.93; N, 4.28. Found: C,
73.44; H, 4.99; N, 4.42.
N-(4-Methylbenzyl)diphenylamine (2e)
Typical Procedure A; yield: 605 mg (75%); grey solid; mp 55–
57 °C.
All reagents were purchased from commercial vendors, and were
used without further purification, unless otherwise noted. MeCN
was distilled from CaH2. All manipulations of oxygen and mois-
ture-sensitive materials were conducted with a standard Schlenk
technique or in a glovebox. Flash column chromatography was per-
formed using silica gel (230–400 mesh). Analytical TLC was per-
formed on 60 F254 (0.25 mm) plates. 1H and 13C NMR spectra were
Spectroscopic data (1H and 13C NMR) were in good agreement with
those reported for the authentic sample.19
N-(2-Methylbenzyl)diphenylamine (2h)
Typical Procedure A; yield: 636 mg (75%); off-white crystals; mp
97–98 °C.
Synthesis 2014, 46, 1046–1051
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Synthesis of N,N-Diaryl Tertiary Amines
1051
Spectroscopic data (1H and 13C NMR) were in good agreement with
those reported for the authentic sample.20
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N-Benzyl-4-methyl-N-(p-tolyl)aniline (4d)
Typical Procedure A; yield: 565 mg (70%); white crystals; mp 86–
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Spectroscopic data (1H and 13C NMR) were in good agreement with
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those reported for the authentic sample.21
N,N-Bis(4-fluorophenyl)benzylamine (4e)
Typical Procedure A; yield: 348 mg (40%); white solid; mp 41–
43 °C.
1H NMR (400 MHz, CDCl3): δ = 4.88 (s, 2 H), 6.91–6.96 (m, 2 H),
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6.71–7.24 (m, 1 H), 7.30–7.31 (m, 4 H).
13C NMR (125 MHz, CDCl3): δ = 56.9, 115.8, 116.0, 121.9, 122.0,
126.5, 127.0, 128.6, 138.7, 144.5, 156.7, 159.1.
HRMS (ESI, +): m/z [M + H]+ calcd for C19H15F2N: 296.1251;
found: 296.1253.
Anal Calcd for C19H15F2N: C, 77.27; H, 5.12; N, 4.74. Found: C,
76.83; H, 5.17; N, 4.70.
N,N-Bis(2-bromo-4-methylphenyl)benzylamine (4g)
Typical Procedure B; yield: 357 mg (90%); white crystals; mp 97–
101 °C.
1H NMR (400 MHz, CDCl3): δ = 2.25 (s, 6 H), 4.77 (s, 2 H), 6.83
(d, J = 8.24 Hz, 2 H), 6.94, (d, J = 8.24 Hz, 2 H), 7.15–7.19 (m, 1
H), 7.26–7.28 (m, 2 H), 7.39 (s, 2 H), 7.51 (d, J = 7.79 Hz, 2 H).
13C NMR (125 MHz, CDCl3): δ = 20.4, 56.8, 121.0, 124.8, 126.8,
127.6, 128.2, 128.4, 134.6, 134.7, 138.0, 144.5.
HRMS: (ESI, +): m/z [M + H]+ calcd for C21H19Br2N: 443.9962;
found: 443.9957.
Anal. Calcd for C21H19Br2N: C, 56.66; H, 4.30; N, 3.15. Found: C,
56.57; H, 4.12; N, 3.23.
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N,N′-[(4,6-Dimethyl-1,3-phenylene)bis(methylene)]bis(N-
phenylaniline) (6)
Typical Procedure A; yield: 311 mg (45%); white crystals; mp 191–
193 °C.
1H NMR (400 MHz, CDCl3): δ = 2.22 (s, 6 H), 4.78 (s, 4 H), 6.68–
6.70 (m, 8 H), 6.85 (m, 4 H), 6.95 (s, 1 H), 7.02–7.06, (m, 8 H), 7.48
(s, 1 H).
13C NMR (125 MHz, CDCl3): δ = 18.3, 54.0, 120.5, 121.0, 124.5,
129.2, 132.3, 132.8, 133.7, 147.7.
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free of charge from the Cambridge Crystallographic Data
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from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +44(1223)336033;
e-mail: deposit@ccdc.cam.ac.uk.
HRMS (ESI, +): m/z [M + H]+ calcd for C34H32N2: 469.2565; found:
469.2642.
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Acknowledgment
(19) Park, S.-E.; Kang, S. B.; Jung, K.-J.; Won, J.-E.; Lee, S.-G.;
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The authors gratefully acknowledge the Robert A. Welch Founda-
tion (Grant D1807) and the National Science Foundation (CRIF
MU grant CHE-1048553) for financial support, Dr. Kazimerz
Surowiec for help in obtaining mass spectra and elemental analyses,
and Dr. Dimitri Pappas for useful discussions.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
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nfomartit
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1046–1051