Palladium-Catalyzed Allyl-Allyl Reductive Coupling of Allylamines or Allylic Alcohols with H2as Sole Reductant

Article abstract of DOI:10.1021/acs.orglett.0c03865

Catalytic carbon-carbon bond formation building on reductive coupling is a powerful method for the preparation of organic compounds. The identification of environmentally benign reductants is key for establishing an efficient reductive coupling reaction. Herein an efficient strategy enabling H2 as the sole reductant for the palladium-catalyzed allyl-allyl reductive coupling reaction is described. A wide range of allylamines and allylic alcohols as well as allylic ethers proceed smoothly to deliver the C-C coupling products under 1 atm of H2. Kinetic studies suggested that the dinuclear palladium species was involved in the catalytic cycle.

Full text of DOI:10.1021/acs.orglett.0c03865

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Letter
Palladium-Catalyzed Allyl−Allyl Reductive Coupling of Allylamines
or Allylic Alcohols with H2 as Sole Reductant
Xibing Zhou, Guoying Zhang, Renbin Huang, and Hanmin Huang*
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ABSTRACT: Catalytic carbon−carbon bond formation building
on reductive coupling is a powerful method for the preparation of
organic compounds. The identification of environmentally benign
reductants is key for establishing an efficient reductive coupling
reaction. Herein an efficient strategy enabling H2 as the sole
reductant for the palladium-catalyzed allyl−allyl reductive coupling
reaction is described. A wide range of allylamines and allylic
alcohols as well as allylic ethers proceed smoothly to deliver the
C−C coupling products under 1 atm of H2. Kinetic studies suggested that the dinuclear palladium species was involved in the
catalytic cycle.
Scheme 1. Palladium-Catalyzed Allyl−Allyl Reductive
he transition-metal-catalyzed reductive coupling reaction,
which avoids the individual preparation and intricate
Coupling with H2 as the Sole Reductant
T
handling of hazardous organometallics, has emerged as a
powerful strategy for the synthesis of organic compounds.1 As
a result, the chemistry of these transformations has been the
subject of intense scrutiny in recent years.2−6 Two major
thrusts have dominated the research in this area. The first
encompasses the expansion of the usefulness of C−C bond-
forming reactions and other transformations.2−6 The second
major thrust involves the pursuit of effective and environ-
mentally benign reductants, as stoichiometric amounts of
reductants are generally required to maintain the catalytic
cycle. However, the reductants employed in these reactions are
critically limited to a large amount of reducing metals,4
organometallics,5 and other reducing agents that are sensitive
to air and moisture.6 These undoubtedly suffer from some fatal
drawbacks including the formation of a large amount of
undesirable waste, catalyst poisoning, and lower atom
efficiency.
process is thought to be initiated by the metal-hydride species,
and the hydrogenolysis reaction might result from the
reductive elimination of allylmetal hydride species (Scheme
1b). To chemoselectively generate the metal hydride from H2
and facilitate the key C−C bond-forming reductive elimination
process, a catalyst that not only could activate H2 but also does
not display hydrogenative reactivity toward the CC bond
and could facilitate the self-transmetalation of the allylmetal
hydride to form the diallylmetal species is required. Herein we
report an unprecedented palladium-catalyzed allyl−allyl
reductive coupling of allylamines, allylic alcohols, and allylic
The catalytic reductive coupling of allylic electrophiles is an
important synthetic method for the construction of 1,5-dienes,
which are common structural motifs in natural products and
synthetic intermediates.7,8 Despite the fact that substantial
efforts have been devoted to such Wurtz-type reductive
̈
coupling reactions,9 a superstoichiometric amount of metal,
such as Mn, Mg, or Zn, is still required as the reductant in
most of these reactions (Figure 1A).10 One attractive approach
to circumvent this problem is to use H2, the cleanest and least
expensive renewable chemical feedstock, as the sole reductant.
However, a significant hurdle preventing the development of
such a process is that hydrogenation of the unsaturated CC
bond11 or hydrogenolysis of the C−X bond would compete
with the desired C−C bond-forming reaction (Scheme 1b).12
Mechanistically, the undesired competitive hydrogenation
Received: November 22, 2020
Published: January 5, 2021
https://dx.doi.org/10.1021/acs.orglett.0c03865
© 2021 American Chemical Society
Org. Lett. 2021, 23, 365−369
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ethers with H2 as the sole reductant under mild reaction
conditions. Our strategy relies on the heterolytic cleavage of
the H−H bond by the in situ formed Pd−N or Pd−O bond
from allylamines or allylic alcohols via the cleavage of the C−N
bond or C−O bond.
optimized reaction conditions. As summarized in Scheme 3,
substrates containing different leaving groups exhibited distinct
a
Scheme 3. Effect of the Leaving Groups
We commenced our studies by treating (E)-N,N-diethyl-3-
phenylprop-2-en-1-amine with 1 atm of H2 and a catalytic
amount of palladium catalyst at 80 °C in N-methyl-2-
pyrrolidone (NMP). The commonly used PdCl2/PPh3 was
found to be capable of promoting the C−C bond-forming
reaction, giving the 1,5-dienes 3a and 4a in 39% combined
yield in a 1:1 ratio. The branched 1,5-diene 5a was not
detected at all. Further optimized reaction conditions by the
screening of phosphine ligands disclosed that the monodentate
phosphine ligands such as Cy3P did not give the desired
products (Scheme 2, entry 2). Bidentate phosphine ligands
a
Scheme 2. Optimization of the Reaction Conditions
a
Reaction conditions: allylic compound (0.5 mmol), PdCl2 (2 mol
%), DPPB (2.5 mol %), NMP (0.5 mL), H2 (1 atm), 80 °C, 12 h. The
ratio (3a/4a) was determined via analysis of the crude reaction
mixture with GC and GC-MS. Isolated yield. NEt3 (20 mol %).
reactivities. For the allylamines with NEt2, N(i-Pr)2, or NHBn
as leaving groups, the desired products could be produced in
moderate to excellent yields, whereas no reaction occurred
when NBnTs was utilized as the leaving group. Interestingly,
no desired reaction was observed under these conditions when
allylic alcohol and allyl acetate were utilized as substrates.
Consequently, to solve this problem, the modified reaction
conditions were identified by introducing 20 mol % of Et3N
into the reaction system. Thereby, using the newly optimized
reaction conditions, the desired C−C bond coupling products
were obtained in 82 and 68% yield, respectively. However, the
allylic chloride was not applicable for the present reaction, even
with Et3N as an additive.
With the optimized conditions established, we first
investigated the scope of the allylamines. Aromatic allylamines
bearing either an electron-donating or an electron-withdrawing
group on the phenyl ring exhibited good reactivity and gave
the 1,5-diene products 3 and 4 in good yields in an almost 1:1
ratio in all cases (Scheme 4). It was even more intriguing that
generally no hydrogenation or hydrogenolysis product was
observed. Substituents at different positions on the arenes
could be tolerated. In addition to phenyl-substituted allyl-
amines, naphthyl-substituted allylamine was also a suitable
substrate for this reaction, generating the corresponding
coupling products (3h and 4h) in high yields with moderate
regioselectivity, although a prolonged reaction time was
required. Moreover, the reaction of heteroaryl-substituted
allylamine N-benzyl-3-(furan-2-yl)prop-2-en-1-amine (1i) also
proceeded smoothly to provide the desired products in good
yields. In addition, the substrate with substituted groups on the
allylic skeleton was examined. For the reactions of three-
substituted aryl allylamines (1j−m), we could obtain similar
good results to give the linear 1,5-dienes as the major products.
It is pointing out that the regioselectivity varied swiftly with the
variation of the substituent. For example, the reaction
proceeded with excellent regioselectivity for substrates
containing a phenyl group or chloride at the γ-position
(1l,m). A lower selectivity was observed when a methyl group
was installed into the γ-position (1j,k). β-Substituted
a
Reaction conditions: 1 (0.5 mmol), [Pd] (2 mol %), ligand (2.5 mol
%), solvent (0.5 mL), H2 (1 atm), 80 °C, 24 h. The ratio (3a/4a) was
determined via analysis of the crude reaction mixture with GC and
GC-MS. Isolated yield. Ligand (4.4 mol %). 12 h.
b
c
with different bite angles, like BINAP, DPPF, DPPE, DPPH,
and Xantphos, were then examined, and it was found that
DPPB was the most efficient for the present reaction to deliver
the desired products in 81% combined yield with excellent E/Z
selectivity (>99:1). We could reduce the reaction time to 12 h
(Scheme 2, entry 9). Control experiments demonstrated that
no desired products were obtained in the absence of ligand
(Scheme 2, entry 10). Moreover, the reactivity was also
strongly dependent on the palladium precursor. The simple
PdCl2 proved to be the optimal precursor (Scheme 2, entries
11−13). The solvent examination demonstrated that the polar
solvents including dimethylformamide (DMF), dimethylaceta-
mide (DMA), and tetrahydrofuran (THF) were suitable for
the present reaction, and NMP was the best solvent.
Various allylic compounds with different leaving groups were
then evaluated for the reductive coupling reactions under the
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a
Scheme 4. Substrate Scope for Allylic Compounds
Scheme 5. Substrate Scope for Allylic Alcohols
a
a
Reaction conditions: 1 (0.5 mmol), PdCl2 (2 mol %), DPPB (2.5
Reaction conditions: 2 (0.5 mmol), PdCl2 (2 mol %), DPPB (2.5
mol %), H2 (1 atm), NMP (0.5 mL), 80 °C, 12 h. Isolated yield. The
ratios of 3/4 within parentheses were determined by analysis of the
crude reaction mixture with GC and GC-MS. 24 h. 120 °C. PdCl2
(5 mol %), DPPB (6.25 mol %). 18 h. 1r (5 mmol), PdCl2 (0.1 mol
%), DPPB (0.125 mol %), H2 (1 atm), NMP (5 mL), 80 °C, 72 h.
mol %), NEt3 (0.1 mmol), H2 (1 atm), NMP (0.5 mL), 80 °C, 18 h.
Isolated yield. The ratios of 3/4 within parentheses were determined
b
c
d
b
by analysis of the crude reaction mixture with GC and GC-MS. 24 h.
e
f
c
d
e
120 °C. PdCl2 (5 mol %). Diastereoselectivity was determined by
1H NMR analysis.
a
Scheme 6. Substrate Scope for Multiallylic Compounds
substrates (1n,o) exhibited lower reactivity to give the 1,5-
dienes in moderate yields under slightly modified conditions.
In contrast, the α-substituted allylamines (1p−s) could
produce the corresponding dienes in 78−94% yields as a
single regioisomer (3q−t). However, the aliphatic-substituted
allylamines were not applicable in this reaction because the
hydrogenolysis reaction took place to significantly inhibit the
coupling reaction. Notably, the reductive coupling on a 5
mmol scale of 1r was found to be completed in 72 h in the
presence of 0.1 mol % of catalyst, yielding 1.07 g of diene 3r
(96% yield). Most of the products 3 and 4 could be isolated
through flash chromatography, although the regioselectivities
of these reactions were not satisfying.
The catalytic protocol was also successfully amenable to a
wide range of allylic alcohols (2a−s) in the presence of 20 mol
% of NEt3, which acted as a base for promoting the H2 to
reduce the Pd(II) to the Pd(0) species (Scheme 5).13 Similar
to allylamines, the reactions proceeded well, allowing the
preparation of the corresponding 1,5-dienes in good to
excellent yields (Scheme 5). In addition, the tertiary alcohol
2t could be smoothly transferred to hexa-1,5-diene-1,1,3,4,6,6-
hexaylhexabenzene in 93% yield with excellent regioselectivity
and 65:35 diastereoselectivity. Having demonstrated that the
process is compatible with a wide range of allylamines and
allylic alcohols, the investigation of the scope with respect to
the compounds containing multiallylic backbones was also
undertaken. As summarized in Scheme 6, tertiary diallylamines
a
Reaction conditions: 6 (0.5 mmol), PdCl2 (2 mol %), DPPB (2.5
mol %), H2 (1 atm), NMP (0.5 mL), 80 °C, 12 h, isolated yield. The
ratio of 3/4 within parentheses was determined by analysis of the
b
crude reaction mixture with GC and GC-MS. NEt3 (0.1 mmol), 120
°C, 18 h.
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containing different groups on the N atom (6a−c) gave the
desired product in good yields under standard reaction
conditions. The reaction of allylic ether 6f provided products
3a and 4a in a 65% combined yield.
To shed light on the mechanism of this reductive coupling
reaction, kinetic studies of the standard reaction of 1a were
Figure 2. Proposed reaction mechanism.
might be transferred to E′ through a σ−π−σ transformation.15
Reductive elimination takes place with the intermediate E or
E′ to form the C−C coupling product 3 or 4 and regenerate
Pd(0). The palladium dihydride species F would also undergo
reductive elimination to regenerate Pd(0) while releasing H2.
In summary, for the first time, the most environmentally
benign H2 has been identified, which is an effective reducing
agent for the palladium-catalyzed allyl−allyl reductive coupling
reactions. Allylamines, allylic alcohols, and allylic ethers were
applied as coupling partners, which enabled the reductive
coupling reaction to proceed under 1 atm of a H2 atmosphere.
This newly designed transformation tolerates a broad range of
allylic compounds, leading to 1,5-dienes in good yields. Kinetic
studies suggest that the formation of the dinuclear palladium
species from self-transmetalation is the rate-determining step.
Further research is currently under way to obtain a detailed
understanding of the reaction mechanism and the application
of this strategy in other reactions.
Figure 1. (A) Kinetic plots of the reaction in eq 1 and the reaction in
eq 2. (B) Plot of initial rates with respect to 1a showing zeroth-order
dependence. (C,D) Plot of initial rates with respect to [Cat.] or
[Cat]2 showing second-order dependence.
ASSOCIATED CONTENT
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* Supporting Information
conducted (Figure 1). The reaction profile showed that a
relatively long induction phase was observed under the
standard reaction conditions. However, the induction period
disappeared when the palladium catalyst was aged in the
presence of 10 mol % of Et3N and 1 atm of H2 for 15 min
(Figure 1, eq 2). This result revealed that the catalytic active
Pd(0) was most likely involved in and generated from the
reduction of PdCl2 with H2 under basic conditions. Initial rates
for the reaction were then investigated by changing the
concentrations of 1a and the catalyst. The result disclosed a
zero-order dependence of the rate on the concentration of 1a.
A more marvelous occurrence is the observed second-order
dependence of the rate on the concentration of the palladium
catalyst, indicating that two palladium species are involved in
the rate-determining step.
On the basis of the above results, we propose the following
mechanism (Figure 2). The reduction of PdCl2 by H2 via σ-
bond metathesis and the reductive elimination sequence in the
presence of amine affords Pd(0). Oxidative addition of Pd(0)
to the allylamine generates the π-allyl palladium A. H2
coordinates to the palladium center and undergoes σ-bond
metathesis to form the allylic palladium hydride species C.13
The hydride-bridged dinuclear palladium species D14 is
formed, followed by rearrangement to generate the palladium
dihydride species and the intermediate E. The complex E
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03865.
Experimental procedures and characterization data
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Hanmin Huang − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, P. R. China; Center for Excellence in Molecular
Synthesis of CAS, Hefei 230026, P. R. China; State Key
Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, P. R. China; orcid.org/0000-
0002-0108-6542; Email: hanmin@ustc.edu.cn
Authors
Xibing Zhou − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, P. R. China
Guoying Zhang − State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou Institute of Chemical
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Letter
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Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R.
China
Renbin Huang − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, P. R. China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03865
́
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (21925111, 21790333, 21702197, and
21672199).
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