Palladium-catalyzed reductive coupling of phenols with anilines and amines: efficient conversion of phenolic lignin model monomers and analogues to cyclohexylamines

Article abstract of DOI:10.1039/c5sc00941c

Phenols, being readily available from naturally abundant lignins, are important future feedstocks for the renewable production of fuels, chemicals, and energy. Herein, a highly efficient Pd-catalyzed direct coupling of phenolic lignin model monomers and analogues with anilines to give cyclohexylamines using cheap and safe sodium formate as hydrogen donor is described. A variety of secondary and tertiary substituted cyclohexylamines can be synthesized under convenient conditions in moderate to excellent yields.

Full text of DOI:10.1039/c5sc00941c

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Palladium-catalyzed reductive coupling of phenols with
anilines and amines: efficient conversion of phenolic
lignin model monomers and analogues to
cyclohexylamines **
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Zhengwang Chen,a,b Huiying Zeng,a Hang Gong,a Haining Wanga and ChaoꢀJun Li*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Phenols, being readily available from naturally abundant lignins, convert lignin into high valued chemicals, herein we report a highly
are important future feedstocks for the renewable production of efficient palladium-catalyzed reaction between anilines and phenolic
fuels, chemicals, and energy. Herein, a highly efficient Pd- lignin model monomers and analogues.[15] It is noteworthy that the
catalyzed direct coupling of phenolic lignin model monomers transformation represents a new catalytic technique to promote a C-
and analogues with anilines to give cyclohexylamines using
O
bond cleavage.[16] Moreover, the direct use of phenolic
cheap and safe sodium formate as hydrogen donor is described. compounds as building blocks in this transformation provides a new
A variety of secondary and tertiary substituted cyclohexylamines non-fossil-approach towards cyclohexylamine derivatives, widely
can be synthesized under convenient conditions in moderate to explored structural units in the pharmaceutical industry.[17]
excellent yields.
Previous work
R2
nucleophilic
substitution
R2
R1
Lignocellulosic biomass represents the most abundant underꢀutilized
organic matter and provides potential renewable feedstock for the
production of fuels, chemicals, and energy in future.[1] However,
lignin (the key component of lignocellulose) is a complicated threeꢀ
dimensional amorphous polymer composed of substituted phenols.[2]
Owing to the irregularity and complexity, the lignin structure is often
represented by model compounds such as phenol, guaiacol, 4ꢀ
propylphenol, diphenylether, vanillin, guaiacylpropane, syringol, and
syringylpropane, for chemical studies.[3] Over the past decades,
important progress has been made in catalytic conversions of lignin
model compounds to valuable chemicals via oxidations,[4]
reductions,[5] redoxꢀneutral processes,[6] and others.[3,7]
Amines are important structural motifs and intermediates for fine
chemicals, pharmaceuticals, agrochemicals and natural products.[8]
Consequently, the development of general and efficient methods for
the derivatization of amines is an active research topic in modern
organic synthesis and medicinal chemistry.[9] There are three general
methods for direct amine alkylations: (1) via nucleophilic
substitution of alkyl halides with amines (Scheme 1, route I);[10] (2)
via Nꢀalkylation of amines with alcohols under catalytic conditions
(Scheme 1, route II);[11] and (3) via the reductive amination of
carbonyl compounds and amines (Scheme 1, route III).[12] Very
recently, Beller’s group presented a straight forward catalytic Nꢀ
alkylation of amines using carboxylic acids and silanes as reducing
agents.[13] An alternative method for synthesis of alkylamines was
Route I
Route II
Route III
R1
H2N
H2N
X
N
H
R2
catalytic
alkylation
R2
R1
R1OH
N
H
R1
reductive
amination
R3
R3
O
R2
H2N
X
N
R2
R1
H
R3
R3
Buchwald-Hartwig
R1
NH
R2
R1
Route IV
N
R2
+
This work
R1
R2
OH
R1
H
N
R3
N
R2
R3
Scheme 1. Methods for direct amine alkylations
To begin our research, we chose the reaction between phenol (1a)
with pꢀtoluidine (2a) as the prototype. Our initial investigations were
focused on attempts to achieve the transformation by using common
metal catalysts. Many transitionꢀmetal complexes such as Rh, Ru, Pt,
Ni, and Ir failed to produce any desired coupling products. To our
delight, Pd/C can efficiently catalyze the reaction to produce 2a in
94% yield (Table 1, entry 1). The control experiment showed that
the catalyst is essential for the reaction (Table 1, entry 2). Then, a
variety of palladium catalysts were tested and Pd/C provided the best
achieved by BuchwaldꢀHartwig amination via Nꢀarylation (Scheme
[14]
1, route IV).
With an endeavor to develop novel methods to
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DOI: 10.1039/C5SC00941C
results (Table 1, entries 3ꢀ7). The reaction showed a strong groups were all effective for the transformation. In addition, the
dependence on the solvent, among which THF, ethanol, dioxane, and product yields were only slightly affected by the location of the
water were also effective besides toluene (Table 1, entries 8ꢀ13). The substituents (on orthoꢀ, metaꢀ, or paraꢀposition of the benzene ring)
reaction was the most effective when conducted at 100°C; whereas a (Table 2, 3a−e and 3h−j). However, when both ortho positions were
higher or lower temperature resulted in a lower yield (Table 1, substituted, a moderate yield was obtained due to the increased steric
entries 14 and 15). A 7 mol% of Pd/C appeared to be the optimal hindrance (Table 2, 3g). Furthermore, both 2,4ꢀdimethoxy
catalyst loading (Table 1, entries 16 and 17). It was found that the substituted and 2,5ꢀdimethoxy substituted anilines afforded the
product was obtained in 66% yield when H2 was used as an corresponding products efficiently, with the latter being slightly
alternative hydrogen source (Table 1, entry 18).
more effective (Table 2, 3k and 3l). For the aniline bearing a vinyl
substituent, the product was formed in 75% yield in which the vinyl
group was also reduced to an ethyl group (Table 2, 3m). Challenging
Table 1: Optimization of reaction conditions for cyclohexylation of pꢀ
toluidine with phenola
Table 2: Pdꢀcatalyzed cyclohexylation of various aniline derivatives
and amines with phenola
Entry
Catalyst
Pd/C
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
T/℃
100
100
100
100
100
100
100
100
100
100
100
100
100
120
80
Yieldb/%
94
1
-
n.r.
n.r.
n.r.
34
2
PdCl2
Pd(PPh3)4
Pd(dba)2
Pd(PPh3)2Cl2
PdCl2(dtbpf)
Pd/C
3
4
5
n.r.
n.r.
89
6
7
8
Pd/C
EtOH
47
9
Pd/C
dioxane
H2O
86
10
11
12
13
14
15
16[c]
17[d]
18[e]
Pd/C
80
Pd/C
MeCN
DMF
n.p.
n.r.
86
Pd/C
Pd/C
toluene
toluene
toluene
toluene
toluene
Pd/C
73
Pd/C
100
100
100
95(92)
83
Pd/C
Pd/C
66
aReaction conditions: phenol (0.2 mmol), pꢀtoluidine (0.2 mmol), catalyst
(10 mol%), sodium formate (6 equiv) and solvent (0.8 mL) under an argon
atmosphere. bYields were determined by GC analysis with mesitylene as
internal standard; isolated yields in brackets. cPd/C (7 mol%) was used.
dPd/C (5 mol%) was used. eH2 (1 atm) was used instead of sodium formate.
With the optimized reaction conditions in hand, we set out to test
the generality of this reaction with regard to both reaction partners
(Tables 2 and 3). We first investigated the scope of anilines by
reacting with phenol under the optimized standard conditions. The
reaction furnished the corresponding secondary or tertiary aniline
derivatives in moderate to excellent yields (Table 2, 3a−u). Various
anilines bearing electronꢀwithdrawing and/or electronꢀdonating
aReaction conditions: phenol (0.2 mmol), arylamine (0.2 mmol), Pd/C
(7 mol%), sodium formate (6 equiv) and toluene (0.8 mL) at 100°C for
24 h under an argon atmosphere; yields of isolated products are given.
bThe reaction was run at 11 mmol scale. cReacted for 36 h. d3ꢀ
Vinylaniline was used as the substrate. Reacted for 16 h. Reacted for
36 h. gReacted at 80°C.
e
f
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substrates such as 2n (sterically hindered), 2o (diamine), 2p (free between 9:1 to 1:1 (Table 3, 4a−m and 4q). In general, both
diamine, giving monocyclohexylation product selectively), 2q electronꢀrich and electronꢀdeficient substituents on the phenol were
(quinoline), and 2r (an electronꢀwithdrawing group) were all compatible with the reaction (Table 3, 4a−m). For example, 4ꢀ
compatible with the standard conditions. It is noteworthy that the propylphenol (as the most prevalent structural unit in lignin) gave
carbonyl-containing substrate gave the desired product in 51% yield the corresponding product in good yield (Table 3, 4d). The phenol
in 16 h, in which the carbonyl survived and could be further reduced bearing a bulky tertꢀbutyl group afforded the product in a moderate
to an alkyl group with a prolonged reaction time (Table 2, 3s). yield (Table 3, 4g). Furthermore, both αꢀ and βꢀnaphthols also
Moreover, the standard conditions were compatible with secondary reacted smoothly with pꢀtoluidine to give the corresponding products
amines and afforded the tertiary amine products in good yields in moderate yields (Table 3, 4n and 4o). Finally, the
(Table 2, 3t and 3u). Aliphatic amines could also give the dihydroxybenzenes, catechol and hydroquinone, could also react to
corresponding products in good yields (Table 2, 3v and 3w). provide the mono and diꢀsubstituted products in 77% and 69% yields,
Importantly, the product could be obtained in 84% yield when we respectively (Table 3, 4p and 4q).
scaled up the reaction to 11 mmol (1.03 g) (Table 2, 3a). As
expected, the corresponding dehalogenated cyclohexyl amines were
obtained when haloꢀsubstituted phenols or anilines were used as
substrates.
Table 3: Reaction of various substituted phenols and aniline derivativesa
Scheme 2. Pdꢀcatalyzed formation of secondary amines from 3ꢀsubstituted
phenols and anilines
Interestingly, when 3ꢀmethoxyphenol was reacted with pꢀtoluidine
and aniline under the standard conditions at 120°C, 3a and 3b were
obtained in 84 and 90% yields, respectively, most likely resulted
from
the
sequential
reductionꢀeliminationꢀreductionꢀimine
formationꢀreduction processes (Scheme 2a). When 3ꢀphenoxyphenol
was used, similar transformation occurred and 3a was obtained in 70%
yield (Scheme 2b), which shows promise of this work for direct
lignin utilizations.
aReaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), Pd/C (7 mol%),
sodium formate (6 equiv) and toluene (0.8 mL) at 120°C for 24 h under
an argon atmosphere; isolated yield and the ratio of cis/trans isomers
1
b
was determined by crude H NMR analysis. Naphthalenꢀ1ꢀol was used
as substrate. cNaphthalenꢀ2ꢀol was used as substrate. dCatechol was used
as substrate. eHydroquinone was used as substrate with 2 equiv aniline.
Scheme 3. Tentative mechanism for the reaction between phenol and aniline
Continuing investigations of the reaction scope, various
substituted phenols were explored (Table 3). With the presence of an
additional substituent, diastereoisomers will be formed. Gratifyingly,
in all cases, the reaction proceeded effectively at a slightly elevated
temperature (120°C), with the ratios of the cis/trans isomers ranging
A tentative mechanism for the reaction was proposed in Scheme
3: Firstly, phenol was reduced into cyclohexanone[18] under the
[Pd]/[H]ꢀcatalyzed reductive conditions.[19] Then, the imine
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DOI: 10.1039/C5SC00941C
L. Marcum, L. J. Haupert, L. M. Amundson, H. I. Kenttämaa, F.
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intermediate B was formed through the standard condensation
between aniline and cyclohexanone. Finally, reduction of the imine
intermediate under [Pd]/[H] generated the cyclohexylaniline
derivative and regenerated the active palladium catalyst.[20]
6
Conclusions
In conclusion, we have developed a simple, efficient, and novel
Pdꢀcatalyzed direct reductive coupling of phenols with amines for
the formation of secondary and tertiary cyclohexylamine derivatives.
A wide range of substituted phenols and aniline derivatives could be
coupled effectively by this method. Aliphatic amines are also
effective by this method. In addition, the reaction employed cheap
sodium formate as the hydrogen donor, avoiding the use of a
potentially hazardous pressurized H2 atmosphere. Furthermore, the
reaction of the phenolic lignin model compounds provides great
opportunities of using lignin as a renewable feedstock for chemical
synthesis. Further studies on the reaction mechanism and
applications of this methodology are currently underway in our
laboratory.
7
8
9
10
11
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Acknowledgements
We thank the CSC (China Scholarship Council) for a
postdoctoral fellowship (to Z.W.C.) and NSERC, FQRNT, CFI,
and the Canada Research Chair (to C.J.L.) for their support of
our research.
Notes
aDepartment of Chemistry and FQRNT Centre for Green Chemistry and
Catalysis, McGill University, 801 Sherbrooke St. W., Montreal, Quebec
H3A 0B8 (Canada).
12
bSchool of Chemistry and Chemical Engineering, Gannan Normal
University, Ganzhou 341000, PR China.
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