Pd-Catalyzed Cascade Metallo-Ene Cyclization/Metallo-Carbene Coupling of Allenamides

Article abstract of DOI:10.1002/ejoc.202001625

A highly efficient palladium-catalyzed cascade metallo-ene/metallo-carbene coupling reaction was developed to produce 2,3-dihydropyrrole derivatives in high yields. In this transformation, two new Csp³?Csp² and Csp²?Csp

Full text of DOI:10.1002/ejoc.202001625

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Pd-Catalyzed Cascade Metallo-Ene Cyclization/Metallo-
Carbene Coupling of Allenamides
Chengqiang Cao,[a] Yi Yang,[a] Xin Li,[a] Yunxia Liu,[a] Hui Liu,*[a] Zengdian Zhao,*[a] and
Lei Chen*[a]
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A highly efficient palladium-catalyzed cascade metallo-ene/
metallo-carbene coupling reaction was developed to produce
2,3-dihydropyrrole derivatives in high yields. In this trans-
formation, two new Csp3À Csp2 and Csp2À Csp2 bonds were
constructed in one-pot. The alkene was one of the most easily
functionalized groups, making it possible for these molecules to
be transformed into more complex molecules. More impor-
tantly, the final product possessed an attractive 1,3,8-trienes
scaffold, which was difficult to be synthesized.
Scheme 1. Synthesis of polyfunctionalized 2,3-dihydropyrrole derivates.
Multifunctional pyrrole has attracted much attention due to its
widespread presence in natural products, bioactive molecules,
material chemistry, and organic chemical intermediates.[1] A lot
of drugs and bioactivated compounds bearing pyrrole scaffold
have been reported, such as an irreversible pan-ErbB inhibitor
pyrotinib,[2] a serotonin reuptake inhibitor from Eli Lilly,[3]
farnesyltransferase inhibitors (LB-42708),[4] and drugs with anti-
juvenile hormone, antiviral,[5] or antibiotic activity (Figure 1).
Therefore, it is important to develop practical and efficient
methods for the construction of pyrrole scaffolds.
Until now, numerous efficient methods of constructing
pyrrole motifs have been reported.[6–11] The method using
transition metal-catalysis has become the most powerful tool,[6]
such as Ni,[7] Pd,[8] Pt,[9] Rh,[10] and Ru.[11] Due to the unique
reactivity, selectivity, availability, and stability of allenamides,
they have been widely used in the synthesis of nitrogen-
containing heterocyclic derivatives.[12] In the past few‘s years,
our group has developed a variety of methodologies towards
valuable heterocycles using allenamide derivatives.[13] As shown
in Scheme 1, in the presence of palladium(0) species, the
allenamides A could generate two π-allyl palladium intermedi-
ates B and B’ with Csp3À Csp2 bond formation via Metallo-ene
transformation. Based on intermediates B and B’, we have
applied Suzuki coupling and Sonogashira coupling to capture
the intermediates to construct 2,3-2H-pyrrole derivates (I) and
(II) bearing aromatic or alkynyl groups.[13b–c] To further explore,
novel methodologies to construct functionalized pyrroles, we
continued to consider the transformation of intermediate B and
B’. So far, numerous classic methodologies have been devel-
oped in the construction of alkenes.[14] Among them, metal-
carbene strategy is quite effective in the synthesis of special
double bonds in the target position.[15] So we tried to apply
metal-carbene strategy to capture the intermediate B to furnish
the 2,3-dihydropyrrole derivatives bearing triene which is
difficult to synthesis via the other method.
Diazo compounds have been well investigated in metal-
carbene process.[16–21] The pioneering work of Van Vranken
showed that the palladium-catalyzed system promoted diazo
compounds to form novel carbon-carbon double bonds.[16]
Since Aggarwal’s work, the use of tosylhydrazone as diazo
compounds precursor promoted palladium-carbene coupling
reaction into a new realm with a remarkably wide scope.[17]
Various substituted alkenes were synthesized efficiently via
palladium-carbene process by Van Vranken,[18] Wang,[19] Barluen-
ga and Valdés,[20] and others.[21]
Considering the pyrrole intermediate B’, we assumed that it
may form palladium-carbene pyrrole intermediate with diazo
compounds to deliver 2,3-2H-pyrroles (III) bearing a triene
scaffold as shown in Scheme 1. Initially, our study commenced
with allenamides (1) and N-tosyl hydrazones (2a) in the
presence of 10 mol% PdCl2(PPh3)2, 2.5 e
°
quiv. tBuOLi at 90 C in anhydrous dioxane under nitrogen
atmosphere. To our delight, the target product 3a was obtained
in 56% yield, and excellent E-configuration was obtained which
was determined via 1H NMR spectroscopy (the coupling
constant of the proton on C=C bond is 16 Hz) (Entry 1, Table 1).
However, besides the target product 3a, we speculate that the
reaction generated the side-product 3a’ by GC-MS data. Next,
the effect of catalyst, ligand, and solvent was examined in the
reaction. When PdCl2(PPh3)2 was replaced by PdCl2(dppf), the
yield was increased to 73%, however, there were still reductive
[a] C. Cao, Y. Yang, X. Li, Y. Liu, Prof. H. Liu, Prof. Z. Zhao, Prof. L. Chen
School of Chemistry and Chemical Engineering
Shandong University of Technology
266 West Xincun Road
Zibo 255049, P. R. China
E-mail: chstone@163.com
huiliu1030@163.com
zhaozengdian@sdut.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202001625
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Figure 1. Representative bioactive molecules bearing a 2,3-dihydro-pyrrole scaffolds.
Table 1. Condition optimization screening.[a]
products present (Entry 2, Table 1). Then toluene, iPrOH, and 2-
MeÀ THF were used as a solvent, indicating that 2-MeÀ THF were
used as solvent could provide the corresponding product in
80% yield (Entry 3–5, Table 1). Surprisingly, the use of MeTHF as
solvent could inhibit the formation of byproducts, which was
helpful to the purification. Subsequently, we investigated a
large number of ligands, including both monodentate and
bidentate phosphine ligands might play an important role in
the transformation. To further understand the influence of
bidentate ligands, the angle size of PÀ PdÀ P in PdLn was
investigated. It was found that the yields showed a correspond-
ing relationship with the angle of bidentate ligands such as
dppm, dppe, dppp and dppb, and dppf[20c–l] (the PÀ PdÀ P angle
°
°
°
°
bond of dppm is 72.2 , dppe is 85.8 , dppf is 99.1 , dppp is
90.6 , dppb is slightly greater than 99.1 , L4 is 108 , L5 is 113 .)
(Figure 2). When PÀ PdÀ P bond angle closed to 99.1 (such as
°
°
°
°
dppf), the yield was closed to reach a high level (Entries 13–17).
Furthermore, we investigated the temperature and reaction
time, and it indicated that higher temperature and longer
reaction time led to the yield decreasing (Entries 18 and 19). In
addition, when tBuOLi was replaced by CH3OLi, the yield was
sharply decreased to 10% (Table. 1, Entry 20). Finally, 10 mol%
°
PdCl2(dppf), 2.5 equiv. tBuOLi at 90 C in 2-MeÀ THF for 3 hours
was employed as the standard conditions.
Based on the optimized conditions, the substrate scope of
N-p-tosyl hydrazone was investigated (Table 2). We examined
both N-p-tosyl aldehyde and ketone hydrazones bearing differ-
ent functional groups under the optimal conditions. When
phenyl substitution of aldehyde hydrazone bearing electron-
donating group such as À Me, À Et, -iPr, or -tBu, the correspond-
ing products were obtained in good yields (3b, 3c, 3d, 3e). On
the other hand, substrates with electron-withdrawing groups
such as À CF3, À F, and À Cl could also be effectively transformed
to the corresponding products (3f, 3g, and 3h). In addition, N-
p-tosyl-furfural hydrazone could also produce the correspond-
ing coupling product 3i in 77% yield. Besides N-p-tosyl-
aldehyde hydrazone derivatives, the representative N-p-tosyl-
[a] Reation conditions:1 (0.2 mmol), 2a (0.4 mmol), [Pd] (0.02 mmol), base
(0.5 mmol), solvent (2.0 mL). [b] Isolated. [c] The threaded tube reaction. [d]
When other bases were used, the yield was reduced. [e] [Pd] (0.01 mmol).
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Table 2. The scope of substrates.
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Figure 2. Relationship between bond angle of PÀ PdÀ P and yield.
ketone hydrazone derivatives were also investigated. The
desired product 3j from the symmetrical diphenylmethyl
ketone derivatives was isolated in 75%. When the substitutions
are in the ortho or meta position of phenyl group of aldehyde
hydrazone, both electron-donating groups (m-Me, o-Me,m-
OMe, and o-OMe) and electron-withdrawing groups (m-Cl and
o-Cl) could give the corresponding products in good yields(3k–
3p). We found the corresponding product 3q was provided in
excellent yield. When phenyl substitution of aldehyde hydra-
zone bearing two identical substituents such as À Me, À OMe, À F,
the corresponding product 3r–3t, could also be obtained in
good yields.
A possible pathway for this palladium-catalyzed cascade
metallo-ene cyclization/metallo-carbene coupling reaction of
allenamides is proposed in Scheme 2. Oxidative addition of
Pd(0) with allyl acetate motif generates π-allyl palladium
intermediate 1-III and the intermediate 1-IV. During this
process, Csp3À Csp2 bond and alkenyl pyrrole skeleton were
constructed. Diazonium substrates 2a-I is formed in situ from
N-p-tosyl hydrazone derivatives with the assistance of tBuOLi,
[a] The condition: 1 (0.2 mmol), 2 (0.4 mmol), PdCl2(dppf) (0.02 mmol),
tBuOLi (0.5 mmol), 2-MeTHF (2.0 mL).
which then reacts with intermediate 1-IV to form the
palladium-carbene complex 2a-II. Subsequent migratory inser-
Scheme 2. Proposal mechanism for the Pd-carbene reaction system.
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In conclusion, we developed a highly efficient palladium-
catalyzed cascade metallo-ene cyclization/ metallo-carbene
coupling reaction, and 2,3-dihydropyrrole derivatives were
obtained in high yields. During this transformation, two new
Csp3À Csp2 and Csp2À Csp2 bonds were constructed in one-pot.
Notably, the final product contained an attractive 1,3,8-trienes
scaffold, which is challenging to be synthesized and easily to be
transformed into more valuable molecules. Further application
of this methodology was will be reported in due course.
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We gratefully acknowledge the National Natural Science Founda-
tion of China (22078178, 21702126), the Shandong Provincial
Natural Science Foundation (ZR2018MB008, ZR2018MB012), Youth
Innovative Talents Attracting and Cultivating Plan of Colleges and
Universities in Shandong Province, the Shandong Provincal Key
Research and Development program of China (2017GGX20131)
and the computational resources from Shandong University of
Technology. We thank Prof. Di Sun at Shandong University for
assistance with the X-ray crystal structure analysis. Technical
support from SDU SC&PP research facilities was acknowledged.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Allenamides · Carbenes · Homogeneous catalysis ·
Nitrogen heterocycles · Palladium
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Manuscript received: December 16, 2020
Revised manuscript received: February 4, 2021
Accepted manuscript online: February 9, 2021
Eur. J. Org. Chem. 2021, 1538–1542
www.eurjoc.org
1542
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