Palladium-Catalyzed Cascade Double C—N Bond Activation: A New Strategy for Aminomethylation of 1,3-Dienes with Aminals

Article abstract of DOI:10.1002/cjoc.201800205

A new palladium-catalyzed selective aminomethylation of conjugated 1,3-dienes with aminals via double C—N bond activation is described. This simple method provides an effective and rapid approach for the synthesis of linear α,β-unsaturated allylic amines with perfect regioselectivity. Mechanistic studies disclosed that one palladium catalyst cleaved two distinct C—N bond to furnish a cascade double C—N bond activation, in which an allylic 1,3-diamine and allylic 1,2-diamine were initially formed as key intermediates through the palladium-catalyzed C—N bond activation of aminal and the α,β-unsaturated allylic amine was subsequently produced via palladium-catalyzed C—N bond activation of the allylic diamines.

Full text of DOI:10.1002/cjoc.201800205

Palladium-Catalyzed Cascade Double C-N Bond Activation: A New
Strategy for Aminomethylation of 1,3-Dienes with Aminals
Cuifang Qiao,a Anrong Chen,a Bingjian Gao,a Yang Liu,b and Hanmin Huang * a, b
ABSTRACT A new palladium-catalyzed selective aminomethylation of conjugated 1,3-dienes with aminals via double C-N bond activation is described. This
simple method provides an effective and rapid approach for the synthesis of linear α,β-unsaturated allylic amines with perfect regioselectivity.
Mechanistic studies disclosed that one palladium catalyst cleaved two distinct C-N bond to furnish a cascade double C-N bond activation, in which an
allylic 1,3-diamine and allylic 1,2-diamine were initial formed as key intermediates through the palladium-catalyzed C-N bond activation of aminal and the
α,β-unsaturated allylic amine was subsequently produced via palladium-catalyzed C-N bond activation of the allylic diamines.
KEYWORDS Aminomethylation, 1,3-diene, aminal, C-N bond activation
species can be utilized as a leading complex to establish a series
Introduction
of C-C and C-N bond formation reactions, offering a number of
exciting possibilities for reaction design and selective synthesis.[9]
Transition-metal catalysis has proved to be the most powerful
method in synthetic organic chemistry and has revolutionized
One reaction we have reported was a palladium-catalyzed
organic synthesis.[1] One common goal in this research area is to
enantioselective aminomethylamination reaction of 1,3-dienes
pursue high catalyst efficiency, which becomes more and more
with aminals at room temperature, in which the chiral allylic
important as reaction scale or catalyst cost increases. In this
1,3-diamines or 1,2-diamines were obtained at room temperature
with excellent regio- and enantioselectivities.[9d] Mechanistic
regard, one promising method can potentially achieve this goal by
using a single catalyst to conduct more than one chemical
studies disclosed that the reaction proceeded via migratory
reaction in a single synthetic operation.[2] Though these methods
have proven useful, relatively few catalytic system have been
insertion of terminal C-C double bond of the diene into the Pd-C
bond of the cyclopalladated complex to form a π-allylic-Pd
complex, which was then intercepted by R2N- to furnish the
utilized in developing new reactions based on inert chemical bond
activation.
corresponding allylic 1,3-diamine and regenerated the palladium
catalyst. This result together with the facts that the allylic C-N
The α,β-Unsaturated allylic amines and their derivatives are
important building blocks for the synthesis of various
bond could be cleaved with Pd(0) at higher temperature via
oxidative addition,[10] we envisioned that both the allylic
nitrogen-containing molecules and exist extensively in natural
products and other biologically active synthetics.[3-4] However, it is
1,3-diamine and allylic 1,2-diamine produced by the reaction of
surprising to find that there are only few methods available for
1,3-conjugate diene with aminal could be transferred to
constructing such kind of scaffolds.[5] Traditional approaches to
the synthesis of α,β-unsaturated allylic amines relied heavily on
multi-step reactions by the condensation of α,β-unsaturated
α,β-unsaturated allylic amine in the presence of Pd(0)
(Scheme-1b). As such, two mechanism different reactions could
be realized with one single catalyst. Herein, we report a new
aldehyde and an imine together with subsequent hydrolysis and
dehydration.[3a] Obviously, the tedious operation steps, difficult
preparation of raw materials as well as the narrow substrate
scope limited its application. Palladium-catalyzed Tsuji-Trost
reaction has long been recognized as a one of the most powerful
strategies for the formation of allylic compounds.[6] In this
context, the amination of conjugated dienyl substrates would give
rise to the desired α,β-unsaturated allylic amines. However,
compared to the reaction with simple allylic compounds, the use
of dienyl substrates in allylic amination was always complicated
by the issue of regioselectivity (Scheme-1a).[7] As such, to achieve
good regioselectivity, highly selective catalysts with special
ligands
were
generally
required.
Moreover,
the
prefunctionalization steps for synthesis of these conjugated
dienyl substrates were also required. Therefore, it is highly
desirable and challenging to explore greener and efficient
catalytic reactions to prepare α,β-unsaturated allylic amines.
An alternative strategy to synthesize α,β-unsaturated allylic
amines would be the direct aminomethylation of 1,3-conjugate
dienes, but no general approach of this type has been reported.
Recently, we have identified that aminals could serve as useful
electrophiles for undergoing the oxidative addition with Pd(0) to
form a cationic cyclopalladated complex.[8] This unique palladium
Scheme 1. Strategies for synthesis of α,β-unsaturated allylic
amines.
catalytic protocol that allows for the selective aminomethylation
of 1,3-dienes with aminals as well as insight into the dual role of
palladium catalyst in the process.
a Hefei National Laboratory for Physical Sciences at the Microscale and
Department of Chemistry, Center for Excellence in Molecular Synthesis,
University of Science and Technology of China, Chinese Academy of Sciences,
Hefei, 230026, People’s Republic of China
E-mail: hanmin@ustc.edu.cn
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000,
People’s Republic of China
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Results and Discussion
At the beginning of our study, (E)-buta-1,3-dienylbenzene (1a)
and N,N,N’,N’-tetrabenzylmethanediamine (2a) were chosen as
the benchmark substrates (Table 1). According to our previous
electron-donating
(Me,
t-Bu,
OMe,
OCF3,
Ph)
or
electron-withdrawing (F, CF3, NO2, Cl) groups on phenyl moiety
are tolerated well. This reaction also tolerates substituent at the
meta-position of the phenyl ring without attenuation of the yield
of the desired products (Table 2, 3fa and 3ka). While the reaction
does not sensitive to changes to the electronic nature of the
substrate, introducing substituent at the ortho-position resulted
in lower yield of the reaction. For instance, the 1,3-dienes bearing
substituents at meta- or para-position of the phenyl ring
proceeded well in this catalytic system and afforded the
corresponding products in good yields. Whereas the 1,3-dienes
with substituent at ortho-position afforded the corresponding
product in lower yields under the same condition (Scheme 2, 3ja
vs 3ka and 3la). Furthermore, naphthyl-substituted conjugated
dienes were also suitable substrates which worked well in the
reaction and provided the desired products in good yields with
high regioselectivities (Table 2, 3na and 3oa). However, the
reaction of 1p, containing a methyl group at the C=C double bond,
gave the desired product in only 29% yield with lower
regioselectivity. Furthermore, the aliphatic dienes 1q and 1r were
not well tolerated, in which the inseparable mixtures with double
bond migration were produced (Scheme 2, 3qa and 3ra). Finally,
the structure of 3aa was confirmed by single-crystal X-ray analysis
and the other products were tentatively assigned by analogy.[11]
work, Pd[(Xantphos)(CH3CN)2](OTf)2 was firstly utilized as
a
catalyst for the desired reaction. However, only trace amount of
the desired product was detected when the reaction was
conducted in i-PrOH at 120 oC (Table 1, entry 1). Screening of
solvent disclosed that the best result was obtained with CH2Cl2 as
solvent (Table 1, entry 4). With the best solvent in hand, we then
thoroughly screened the palladium catalyst and found that the
ligand and counterion play important roles for obtaining good
results (see SI). We observed that the Pd-complex ligated with
Xantphos and with OTf as counterion afforded a good yield of the
desired product. To our delight, good yield and the best E/Z ratio
was obtained when employing the catalyst produced by in-situ
reaction of [Pd(allyl)Cl]2, Xantphos and AgOTf (Table 1, entry 12).
Encouraged by these results, we next investigated the effects of
the temperature (Table 1, entries 13-14). Obviously, lowering the
temperature to 100 oC, the yield decreased to 64%. Further
increasing the reaction temperature to 130 oC (Table 1, entry 14),
however, did not improve the yield and E/Z ratio. The study of the
effect of reaction time demonstrated that the reaction was
performed completely in 8 h (Table 1, entry 15).
Table 2. Substrate scope of conjugated dienes.[a]
Table 1. Optimization of the reaction conditions.[a]
Yield
(%)
Entry
[Pd]
Solvent
E/Z
1
2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
Pd[(Xantphos)(CH3CN)2](OTf)2
[Pd(allyl)Cl]2
i-PrOH
n-hexane
toluene
DCM
trace
60
41
70
50
65
49
47
32
0
-
71:29
72:28
81:19
72:28
77:23
75:25
93:7
3
4
5
THF
6
Dioxane
CH3CN
DMF
7
8
9
DMSO
DCM
75:25
-
10
11
12
13[b]
[Pd(allyl)Cl]2/Xantphos
DCM
58
75
64
79:21
86:14
82:18
[Pd(allyl)Cl]2/Xantphos/AgOTf
[Pd(allyl)Cl]2/Xantphos/AgOTf
DCM
[a] Reaction conditions: 1 (0.6 mmol), 2a (0.4 mmol), [Pd(allyl)Cl]2
DCM
(0.01 mmol), Xantphos (0.022 mmol), AgOTf (0.022mmol), solvent
14[c]
15[d]
[Pd(allyl)Cl]2/Xantphos/AgOTf
[Pd(allyl)Cl]2/Xantphos/AgOTf
DCM
DCM
73
80
83:17
87:13
o
(1.5 mL), 120 C, 8 h, isolated yields. The E/Z ratio is determined
by GC or 1H NMR analysis of the crude product and given in
parenthesis. [b] 3pa:3pa' = 1:1 (see SI). [c] Mixture with double
bond migration.
[a] Reaction conditions: 1a (0.6 mmol), 2a (0.4 mmol),
[Pd(allyl)Cl]2 (0.01 mmol), Xantphos (0.022 mmol), AgOTf (0.022
mmol), solvent (1.5 mL), 120 oC, 12 h, isolated yields, the E/Z ratio
Next, the substrate scope was extended to a variety of
aminals with different substituents (Scheme 3). First, the
o
is determined by GC analysis of the crude product. [b] 100 C. [c]
130 oC. [d] 8 h.
reactions
of
a
variety
of
aminals
with
(E)-buta-1,3-dien-1-ylbenzene 1a were conducted. As shown in
Table 3, the benzyl aminals with both electron-withdrawing and
electron-donating group underwent the desired reaction
smoothly in good to excellent yields (67-89% yields, 3ab-3af) with
good excellent stereoselectivity. Aminals derived from simple
alkyl amines, such as diethyl amine, dipropylamine, could react
smoothly with the diene 1a to give the corresponding products
(3ag-3ah) in 64-89% yields. Moreover, the fluorine-containing
(E)-1-(buta-1,3-dien-1-yl)-4-(trifluoromethoxy)benzene 1g could
With the optimized reaction conditions in hand, various
1,3-dienes 1 were tested to examine the generality of this
process. The results are summarized in Table 2. Reactions of
aromatic conjugated dienes containing different substitutents on
the phenyl-ring all gave their corresponding products in good
yields
with
completely
regioselectivities
and
good
stereoselectivities. This reaction appears insensitive to the
electronic nature of the substitutents, thus either
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also react with aminals derived from simple aliphatic amines
smoothly to give the corresponding products (3gg-3gi) in good to
excellent yields. Although aminals derived from cyclic amines
could also react with conjugated 1,3-diene, only moderate yields
To gain insight into the possible mechanism of the present
reaction, we conducted a set of experiments (Scheme 3). As we
have mentioned above, 1,3-diamine was formed at room
temperature. When the reaction was catalyzed by a catalytic
were obtained (3gj-3gm).
amount of cyclopalladated complex A under the standard
Table 3. Substrate scope of aminal.[a]
reaction conditions for 12 hours, the desired product was
obtained in 71% yield, which suggested that the cyclopalladated
complex A might be involved in the present reaction (Scheme
3a).[8] However, the stoichiometric reaction of cyclopalladated
complex A with (E)-buta-1,3-dienylbenzene 1a did not take place
at all in DCM at 120 oC (Scheme 3b). This result indicated that the
stable π-allylic-Pd species B, formed via the insertion of the
double bond into the C-Pd bond of the cyclopalladated complex
A, could not undergo β-hydride elimination to generate the
desired product. Moreover, it was pleased to find that the allylic
1,3-diamine 4aa and allylic 1,2-diamine 5aa could be converted
into the desired α,β-Unsaturated allylic amine 3aa in good yields
with high stereoselectivity under the standard reaction conditions
(Scheme 3c and 3d). Moreover, we studied the reaction profile of
the standard reaction and observed the formation and decay of
the allylic 1,3-diamine and allylic 1,2-diamine with reaction time
lasting (see Supporting Information). As shown in Figure 1, the
diamine (allylic 1,3-diamine and allylic 1,2-diamine) is the major
products during the first two hours. As the reaction time went on,
the two allylic diamines converted smoothly into the desired
α,β-unsaturated allylic amine gradually, which further confirmed
that the allylic 1,3-diamine and 1,2-diamine were the key
intermediates of the present reaction and the desired product
was most likely formed via a unique double C-N bond activation.
[a] Reaction conditions: 1 (0.6 mmol), 2 (0.4 mmol), [Pd(allyl)Cl]2
(0.01 mmol), Xantphos (0.022 mmol), AgOTf (0.022mmol), solvent
o
(1.5 mL), 120 C, 8 h, isolated yields. The E/Z ratio is determined
by GC or 1H NMR analysis of the crude product and given in
parenthesis.
Scheme 3. Preliminary mechanism studies.
Scheme 2. Synthetic application of the Pd-catalyzed
aminomethylation of conjugated 1,3-dienes.
The synthetic versatility of the present catalytic system was
demonstrated through a large-scale reaction. As demonstrated in
Scheme 2, the reaction proceeded smoothly on a gram scale even
in the present of a lower catalyst loading and afforded the
product in good yield (81%). Carbon monoxide (CO) could insert
into the C-N bond of the α,β-unsaturated allylic amine 3aa to
afford the amide 6 throug the palladium-catalyzed carbonylation
reaction.[10f] Moreover, with diethyl maleate as a dienophile, 3aa
could be smoothly converted into the tetra-substituted
cyclohexene 7 in good yield via Diels-Alder reaction, which might
be used for the synthesis of unnatural aminoacids.
Figure 1. Reaction profile of 1,3-dienes 1a with aminal 2a.
On the basis of the above mechanistic studies and precedent
results,[8-9] we proposed a cascade catalytic cycle for the present
reaction (Figure 2). Initially, a catalytically active Pd (0) species
was formed, which underwent oxidative addition with aminal to
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Catalysis, University Science Books, Sausalito California, 2010; (b) van
Leeuwen, P. W. N. M. Homogeneous Catalysis: Undersatanding the
Art, Kluwer Academic Publishers, 2004.
generate the cyclopalladated complex A via C-N bond activation.
Subsequently, migratory insertion of the terminal carbon-carbon
double bond into the Pd-C bond of the cyclopalladated complex A
was taken place to generate π-allylic-Pd complex B, which was
nucleophilic attacked by R2N- to form allylic 1,3-diamine C and
allylic 1,2-diamine C’ (minor intermediate, which was formed
from intermediate B via β-hydride elimination and reinsertion)[9d]
together with regenerated the Pd(0) species. Afterwards, the
oxidative addition of C or C’ to the Pd(0) species took place to give
intermediate D or D’ via the second time C-N bond cleavage.[10]
The reactive species D or D’ would undergo syn-β-hydride
elimination to deliver the desired product 3aa together with
intermediate E. Eventually, reductive elimination of intermediate
E regenerated the active Pd(0) species to furnish the catalytic
cycle.
[2] For selected reviews on tandem catalysis, see: (a) Malacria, M.;
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[6] (a) Trost, B. M.; Bunt, R. C. J. Am. Chem. Soc. 1998, 120, 70; (b) Trost,
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Figure 2. Plausible reaction mechanism.
Conclusions
In conclusion, we have developed a novel palladium catalyzed
selective aminomethylation of conjugated dienes with aminals
through carbon-carbon bond forming process. The reaction
[8] Xie, Y.; Hu, J.; Wang, Y.; Xia, C.; Huang, H. J. Am. Chem. Soc. 2012,
134, 20613.
provided
a
practical approach to synthesize linear
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α,β-unsaturated allylic amines with perfect regioselectivities and
good stereoselectivities. Primary mechanistic studies disclosed
that one palladium-catalyst cleaved two distinct C-N bonds to
furnish a cascade double C-N bond activation, in which both allylic
1,3-diamine and allylic 1,2-diamine were initially formed through
the palladium-catalyzed C-N bond activation of aminal and the
α,β-unsaturated allylic amine was then produced via the
palladium-catalyzed C-N bond activation of the allylic 1,3-diamine
and 1,2-diamine. Further investigation will be focused on the
understanding the detailed mechanism and the applications
based on this chemistry
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
[11] CCDC 1823156 (3aa) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The
Cambridge
Crystallographic
Data
Centre
via
Acknowledgement
www.ccdc.cam.ac.uk/conts/data request/cif.
This research was supported by the National Natural Science
Foundation of China (21672199 and 21790333) and CAS
Interdisciplinary Innovation Team.
(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2017
Revised manuscript received: XXXX, 2017
References
[1] (a) Hartwig,J. F.; Organotransition Metal Chemistry: From Bonding to
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Entry for the Table of Contents
Page No.
Palladium-Catalyzed Cascade Double C-N
Bond Activation:
A New Strategy for
Aminomethylation of 1,3-Dienes
Double C-N Bond Activation:
A new palladium-catalyzed selective aminomethylation of
conjugated 1,3-dienes with aminals via double C-N bond activation is described. This simple
method provides an effective and rapid approach for the synthesis of linear α,β-unsaturated allylic
amines with perfect regioselectivity. Mechanistic studies disclosed that the reaction proceeds via a
cascade double C-N bond activation, in which one sigle palladium-catalyst realized two distinct C-N
bond activation.
Cuifang Qiao, Anrong Chen, Bingjian Gao,
Yang Liu, and Hanmin Huang *
aDepartment, Institution, Address 1
cDepartment, Institution, Address 3
E-mail:
E-mail:
bDepartment, Institution, Address 2
E-mail:
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