Palladium-Catalyzed [3+3] Annulation of Vinyl Chromium(0) Carbene Complexes through Carbene Migratory Insertion/Tsuji–Trost Reaction

Article abstract of DOI:10.1002/anie.201707590

Vinyl chromium(0) Fischer carbene complexes were employed as the source of π-allylic palladium species for catalytic [3+3] annulation under palladium catalysis. Mechanistically, this transformation is proposed to involve carbene migratory insertion and intramolecular Tsuji–Trost reaction as the key steps. Substituted six-membered heterocyclic flavonones and quinolines are obtained, depending on the nucleophilic functional group on the coupling partners.

Full text of DOI:10.1002/anie.201707590

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International Edition: DOI: 10.1002/anie.201707590
German Edition: DOI: 10.1002/ange.201707590
Annulation
Palladium-Catalyzed [3+3] Annulation of Vinyl Chromium(0)
Carbene Complexes through Carbene Migratory Insertion/Tsuji–Trost
Reaction
Kang Wang, Yifan Ping, Taiwei Chang, and Jianbo Wang*
In memory of JosØ Barluenga
Abstract: Vinyl chromium(0) Fischer carbene complexes were
employed as the source of p-allylic palladium species for
catalytic [3+3] annulation under palladium catalysis. Mecha-
nistically, this transformation is proposed to involve carbene
migratory insertion and intramolecular Tsuji–Trost reaction as
the key steps. Substituted six-membered heterocyclic flavo-
nones and quinolines are obtained, depending on the nucleo-
philic functional group on the coupling partners.
I
n transition-metal-catalyzed reactions, p-allylic metal com-
plexes are one of the important intermediates.[1] Traditional
methods to access allylic metal complexes include oxidative
À
addition of allylic electrophiles, activation of allylic C H
bonds, and diene (or allene) insertion by organometallic
reagents.[2] Recently, we and others have developed transi-
tion-metal-catalyzed cross-coupling involving metal carbene
migratory insertion, which has been established as a powerful
[3,4]
À
strategy for C C bond constructions.
In particular, a,b-
Scheme 1. Transition-metal-catalyzed transformation of unsaturated
carbene species.
unsaturated diazo compounds or N-tosylhydrazones can be
employed as vinyl metal carbene precursors to generate p-
allylic metal species through carbene migratory insertions,
which can then participate in further transformations
(Scheme 1A).[5,6] For example, Liang et al. employed a,b-
unsaturated diazo compounds or N-tosylhydrazones as the
carbene precursors to generate p-allylic palladium intermedi-
ates, which were further trapped by internal nucleophiles to
afford various cyclic products.[6] However, a,b-unsaturated
diazo compounds and N-tosylhydrazones are relatively
unstable, which brings some limitations in applying them in
organic synthesis. It is thus desirable to explore other carbene
precursors that can generate p-allylic metal complexes
through metal carbene migratory insertion processes.
In this context, it is noteworthy that vinyl chromium(0)
Fischer carbenes are stable a,b-unsaturated metal carbene
complexes that have been extensively explored in organic
synthesis.[7,8] Particularly, in the presence of a transition-metal
catalyst, the vinyl chromium(0) carbenes may undergo
carbene transfer reactions to generate reactive vinyl metal
carbene species, which undergo further transformations such
as cyclization to afford synthetically relevant products
[Scheme 1B, Eq. (a)].[9] Sierra et al. have demonstrated that
chromium(0) carbene complexes, including vinyl chromium-
(0) carbenes, undergo transition-metal-catalyzed dimeriza-
tion to give alkenyl products.[10] In addition to the dimeriza-
tion reactions, the in situ generated transition-metal carbene
species may further insert into unsaturated bonds to afford
cyclic products. For example, Barluenga et al. and Aumann
et al. have reported nickel(0)- and rhodium(I)-catalyzed
cyclization of vinyl chromium(0) carbenes with alkynes and
allenes to produce five- and seven-membered cyclic prod-
ucts.[11] However, the transition-metal-catalyzed coupling
reaction of vinyl chromium(0) carbenes still has its limitations
where the types of transformation and the scope of the
coupling partners are concerned.
[*] K. Wang, Y. Ping, T. Chang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
E-mail: wangjb@pku.edu.cn
Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201707590.
As the continuation of our interest in catalytic carbene
transformation,[3b–e, 12] we conceived that vinyl chromium(0)
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Fischer carbene complexes may be considered as unsaturated
entries 2 and 3). The effect of base and temperature were then
transition-metal carbene precursors as well as a source of p-
allylic palladium species. As illustrated in Scheme 1B
[Eq. (b)], we have proposed that vinyl chromium(0) carbene
may serve as a three-carbon-unit carbene precursor to
participate in the migratory insertion/allylation sequence by
reacting with organometallic species (Ar’-[M]) under catalytic
conditions.[13] Thus, aryl iodides with ortho nucleophilic XH
substituent (XH = OH or NH2) were chosen as the coupling
partners under palladium catalysis (Scheme 1C). Upon the
formation of PdII–carbene species, the subsequent migratory
evaluated (Table 1, entries 4–6), which showed that reaction
afforded improved yield with K2CO3 as the base and could be
carried out at slightly low temperature. Furthermore, it was
found that by adding sodium borate salt NaBArF4 [ArF = 3,5-
(CF3)2C6H3] as an additive, the yield of the reaction could be
improved to 56%.[15] Adjusting the substrate ratio of 1a/2a
from 1:1 to 1.2:1 further promoted the transformation
(Table 1, entries 8–9). The effect of phosphine ligands was
further examined. Triphenyl phosphite L3 was found to
promote reaction efficiency (Table 1, entries 7 and 10).
Finally, we were able to obtain 3a in 80% yield by adding
water as the co-solvent (DCE/H2O = 50:1) (Table 1,
entries 11–12).
Having established the optimized reaction conditions, we
subsequently investigated the substrate scope of this annula-
tion reaction by varying the substituents of the vinyl
chromium(0) Fischer carbene complexes (Scheme 2). For
the aryl-substituted vinyl chromium(0) carbenes, electron-
rich functional groups, such as p-methyl and p-methoxy
groups, were tolerated well to give the respective flavonone
products 3b and 3c in excellent yields. However, moderate
yields were obtained when the substituted chromium(0)
carbenes bearing electron-rich groups, such as p-tert-butyl
group (1d) and p-methylthio group (1e), were employed. The
reaction of the substrates bearing electron-withdrawing
groups on the phenyl ring, such as fluoro (1 f), chloro (1g),
trifluoromethyl (1h), and ester groups (1i), all proceeded, but
affording the products 3 f–i in slightly diminished yields,
insertion/intramolecular
Tsuji–Trost
allylation[14]
are
À
expected to fulfill the C X bond formation to generate six-
membered heterocyclic structures. We report herein our
investigation along these lines.
At the outset of the investigation, we employed penta-
carbonyl [E-styryl(methoxy)carbene]chromium(0) (1a) and
2-iodophenol (2a) as the substrates. The reaction was carried
out in 1,2-dichloroethane at 708C with Pd(PPh3)4 as the
catalyst and K2CO3 as the base. Gratifyingly, we were able to
isolate the 4-methoxyl substituted 2H-chromene product 3a’
in 5% yield. 3a’ gave the flavanone product 3a after
hydrolysis in an aqueous solution of TsOH (Table 1,
entry 1). The yield of 3a were slightly improved by employing
Pd(OAc)2 as the catalyst and L1 as the ligand (Table 1,
Table 1: Optimization of the reaction conditions.[a]
Entry Cat. Pd
Ligand Base
Additive T [8C] 3a %[b]
1
2
3
4
5
6
7
Pd(PPh3)4
–
PPh3
L1
L1
L1
L1
L2
L1
L1
L3
L3
L3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
–
–
–
–
–
–
70
70
70
40
40
40
40
40
40
40
40
40
5
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
10
28
46
15
30
35
56
64
67
72
80
8[c]
9[c]
10[c]
11[c,d]
12[c,e]
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
4
4
4
4
4
[a] If not otherwise noted, the reaction conditions are as following:
a solution of 1a (0.10 mmol), 2a (0.10 mmol), Pd(OAc)2 (5 mol%),
trisphenylphosphine (10 mol%), K2CO3 (1.0 equiv) in 1 mL of DCE was
stirred at 708C for 8 h. When the reaction is completed, 30 mg of
TsOH·H2O was added together with 0.2 mL of H2O, then stirred at 508C
for 0.5 h. For entries 9–12, the ratio of 1a to 2a is 1.2:1. [b] Yield of
isolated product with silica-gel column chromatography. [c] 10 mol% of
NaBArF4 (sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate) was
added. [d] A mixed solvent of DCE/H2O=50:1 (1 mL DCE) was used.
[e] A solvent of DCE/H2O=50:1 (2 mL DCE) was used and the reaction
was carried out in 0.2 mmol scale.
Scheme 2. The reaction scope with respect to substituted vinyl
chromium(0) carbenes. Reaction conditions: a solution of 1a–o
(0.24 mmol), 2a (0.20 mmol), Pd(OAc)2 (5 mol%), triphenyl phosphite
(10 mol%), NaBArF (10 mol%), K2CO3 (1.0 equiv) in a mixed solvent
4
of DCE/H2O=50:1 (2 mL DCE) was stirred at 408C for 8 h. When
completed, 30 mg of TsOH·H2O was added together with 0.2 mL of
H2O, then stirred at 508C for 0.5 h. All yields refer to the isolated
products. [a] Pentacarbonyl [E-styryl(methoxy)carbene] tungsten (0)
was employed as the substrate.
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presumably due to the slow carbene-transfer steps caused by
these electron-withdrawing substituents. Additionally, styryl
tungsten(0) carbene complexes produced the same product
3a, albeit in a diminished yield of 25%. The reaction of
substrates bearing electron-donating heterocycles, such as the
furyl and thienyl ring substituted vinyl chromium(0) carbenes
1j and 1k, gave the corresponding products 3j and 3k in
excellent yields. Chromium(0) carbene substrates bearing
naphthyl and ferrocenyl aromatic substituents were also
suitable for the reaction (3l and 3m). Furthermore, we were
also able to obtain the vinyl sp2 and alkyl sp3 carbon
substituted flavonone products 3n and 3o in good yields by
employing the corresponding substrates.
Moreover, the reaction with the multisubstituted substrate 5-
iodovanillin (2j), which has methoxyl and formyl groups, also
proceeded well to afford the corresponding products.
Finally, this annulation reaction was expanded to 2-
iodoaniline (5a), and 2,4-disubstituted quinoline 6a was
isolated as the product due to the dehydrogenative rear-
omatization of the intermediate generated through Tsuji–
Trost allylation (Scheme 4).[17] It was observed that the yield
of 6a could be improved by carrying out the reaction at 608C
in the absence of NaBArF salt. Diminished yield was
4
obtained when E-styryl W0 carbene was employed. Under
Next, we studied the scope with respect to substituted 2-
iodophenols (2b–k, Scheme 3). The electron-donating groups
on the 2-iodophenol derivatives were shown to have a bene-
ficial effect on reaction efficiency. For example, 5-methyl-, 4-
Scheme 4. The reaction scope with respect to substituted 2-iodoani-
lines. Reaction conditions: a solution of 1a or 1p (0.24 mmol), 5a–d
(0.20 mmol), Pd(OAc)2 (5 mol%), triphenyl phosphite (10 mol%),
K2CO3 (1.0 equiv) in a mixed solvent of DCE/H2O=50:1 (2 mL DCE)
was stirred at 608C for 4 h. All yields refer to the isolated products.
[a] The yield in parenthesis refers to the reaction where pentacarbonyl
[E-styryl(methoxy)carbene]tungsten (0) was employed as the substrate.
the modified conditions, the reactions with substrates bearing
methyl, chloro, and fluoro substituents all proceeded well to
give the quinoline products 6b–d in good yields. Moreover,
the natural alkaloid graveoline (6e), which shows antitumor
activity,[18] could be obtained in 58% yield. Thus, the trans-
formation is potentially useful for the synthesis of multi-
substituted quinolines, which are important structure units of
natural products and pharmaceuticals.[19]
A proposed reaction mechanism for this palladium-
catalyzed annulation transformation is shown in Scheme 5.
Firstly, substrate 2-iodophenol (1a) undergoes oxidative
addition to Pd0 species A to give the aryl-palladium(II)
species B. The chromium(0) carbene complex then undergoes
carbene-ligand transfer from chromium(0) to palladium
intermediate B, thereby generating the vinyl palladium
carbene intermediate C.[9] However, we could not rule out
an alternative pathway in which the sequence of oxidative
addition and carbene transfer is inverted.[10d,17] Subsequently,
migratory insertion of the phenol group from palladium to the
carbenic carbon gives h1-allylic palladium(II) intermediate
D,[3] which isomerizes to the more stable h3-allylpalladium E.
The intermediate E serves as the key intermediate in an
intramolecular Tsuji–Trost reaction to produce the 4-
Scheme 3. The reaction scope with respect to substituted 2-idophe-
nols. Reaction conditions: a solution of 1a (0.24 mmol), 2b–k
(0.20 mmol), Pd(OAc)2 (5 mol%), triphenyl phosphite (10 mol%),
NaBArF4 (10 mol%), K2CO3 (1.0 equiv) in a mixed solvent of DCE/
H2O=50:1 (2 mL DCE) was stirred at 408C for 8 h. When completed,
30 mg of TsOH·H2O was added together with 0.2 mL of H2O, then the
mixture was stirred at 508C for 0.5 h. [a] Yields of isolated products
with silica-gel column chromatography. Thermal ellipsoids shown at
30% probability for the X-ray structure of 4e.
methoxy-, and 4-tert-butyl-substituted phenols gave the
products in excellent yields (4a–c). The reaction also toler-
ated a free hydroxy group (4d). The reaction with 2-
iodonaphthol gave the corresponding product 4e in moderate
yield, and the structure was unambiguously assigned through
X-ray crystallographic analysis.[16] Various halogenated 2-
iodophenols are also suitable substrates for the transforma-
tion, giving the products 4 f–i in moderate to good yields.
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Scheme 5. Proposed mechanism.
methoxyl-substituted 2H-chromene product F, which is fur-
ther hydrolyzed to flavonone product G. The palladium(0)
catalytic species is regenerated through intramolecular ally-
lation to participate the next catalytic cycle.
In summary, we have explored vinyl chromium
chromium(0) Fischer carbene complexes as precursors of
palladium(II) carbenes through transmetallation. The reac-
tion represents a novel access to p-allylic palladium(II)
species. In contrast to the traditional cyclization of vinyl
chromium(0) carbenes (Dçtz cycloaddition), which consists
of the formation of a ketene intermediate and subsequent
cyclization,[8] this reaction involves a cascade of transmetal-
lation, palladium carbene migratory insertion, and intramo-
lecular nucleophilic addition to an p-allylic palladium(II)
intermediate. The reaction demonstrates the potential of
vinyl chromium(0) Fischer carbene complexes as three-
carbon units in carbene-based cross-coupling.
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Acknowledgements
The project is supported by National Basic Research Program
of China (973 Program, No. 2015CB856600) and NSFC
(Grant 21332002).
Conflict of interest
The authors declare no conflict of interest.
Keywords: allylation ·annulation ·chromium(0) carbenes ·
migratory insertions ·palladium catalysis
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[13] AlbØniz and Espinet el al. reported that a series of tungsten(0)
carbenes could react with stoichiometric amount of (pentafluor-
ophenyl)palladium species through carbene-ligand transmetala-
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rique, A. PØrez-Mateo, Angew. Chem. Int. Ed. 2002, 41, 2363 –
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[14] a) J. Tsuji, Tetrahedron 1986, 42, 4361 –4609; b) B. M. Trost,
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[15] The NaBArF4 salts were reported to be capable of improving the
electrophilic properties of allylic palladium species and accel-
erating the Tsuji –Trost reaction: L. A. Evans, N. Fey, J. N.
Harvey, D. Hose, G. C. Lloyd-Jones, P. Murray, A. G. Orpen, R.
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[16] CCDC 1541745 contains the supplementary crystallographic
data for 4e. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
Manuscript received: July 25, 2017
Accepted manuscript online: August 28, 2017
Version of record online: September 12, 2017
13144 www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 13140 –13144