Pd-catalyzed Oxidative Cross-coupling of Alkyl Chromium(0) Fischer Carbene Complexes with Organoboronic Acids

Article abstract of DOI:10.1002/asia.201801278

Alkyl chromium(0) carbene complexes have been explored as the cross-coupling partners in the palladium-catalyzed reaction with aryl or alkenyl boronic acids. This coupling reaction displays the versatile reactivities of alkyl chromium(0) carbenes under palladium catalysis. Mechanistically, this transformation is proposed to involve deprotonation of the alkyl chromium carbene substrate to generate a vinyl chromium anion intermediate that undergoes transmetalation to organopalladium species and reductive elimination.

Full text of DOI:10.1002/asia.201801278

CHEMISTRY
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Accepted Article
Title: Pd-catalyzed oxidative cross-coupling of alkyl chromium(0)
Fischer carbene complexes with organoboronic acids
Authors: Jianbo Wang, Kang Wang, Jinghui Yang, and Xingqi Yao
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10.1002/asia.201801278
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Pd-catalyzed oxidative cross-coupling of alkyl chromium(0)
Fischer carbene complexes with organoboronic acids**
Kang Wang, Jinghui Yang, Xingqi Yao and Jianbo Wang*
Abstract: Alkyl chromium(0) carbene complexes have been
explored as the cross-coupling partners in the palladium-catalyzed
reaction with aryl or alkenyl boronic acids. This coupling reaction
demonstrates versatile reactivities of alkyl chromium(0) carbenes
under palladium-catalysis. Mechanistically, this transformation is
proposed to involve deprotonation of alkyl chromium carbene
substrate to generate vinyl chromium anion intermediate, which
undergoes transmetalation to organopalladium species and
reductive elimination.
catalyzed cross-couplings.[13] We have reported the Pd-
catalyzed coupling of terminal alkynes with chromium(0) Fischer
carbenes,[13a] Pd-catalyzed [3+3] annulation of vinyl chromium(0)
carbenes,[13b] and Pd-catalyzed reductive cross-coupling
reaction of aryl chromium(0) Fischer carbene complexes with
aryl iodides.[13c] As the continuation of our efforts, we then turn
our attention to the Pd-catalyzed cross coupling reactions of
alkyl chromium(0) carbenes with organoboronic acids (Scheme
1c). We have reasoned that similar carbene transfer occurs to
generate organopalladium(II)-carbene intermediate, which will
be followed by migratory insertion/β-H elimination to give the
coupling products. Herein we report our study along this line.
Fischer carbene complexes, especially chromium(0) Fischer
carbenes, are proven to be highly valuable in organometallics
and organic synthesis.[1] As the extension of classic
stoichiometric application of Fischer carbene complexes, the
transition metal catalyzed carbene-transfer reaction recently
emerged as an attracting arena.[2] In general, the carbenic ligand
of Fischer carbene transfers to catalytic metal species to
generate new carbene complexes.[3] For examples, Sierra et al.
have developed the self-dimerization of Fischer carbenes under
palladium catalysis (Scheme 1a).[4] Aumann,[5] Barluenga,[6]
Narasaka[7] and others have reported transition-metal-catalyzed
cyclopropanations, cycloadditions and cross-coupling reactions
of Fischer carbene complexes. However, despite the recent
developments, the scope of transition-metal-catalyzed carbene
transfer reactions with Fischer carbenes is still limited.
On the other hand, transition metal catalyzed carbene
coupling reactions based on migratory insertion have been
emerged as efficient strategies for C-C bonds construction in the
[8]
past decade. Diazo compounds, including those generated in
Scheme 1. Palladium-catalyzed transformation of chromium(0) Fischer
situ from N-tosylhydrazones, were established as the effective
coupling partners in these transformations.[9] For example, we
have reported the palladium-catalyzed oxidative cross-coupling
of diazocarbonyl compounds with organoboronic acids (Scheme
1b).[10,11] As the continuation of our interest in carbene
transformation based on migratory insertion, we have also
explored non-diazo carbene processors, such as ene-yne-
ketones, allenyl ketones and cyclopropenes, for these cross-
coupling reactions.[12] More recently, we have explored Fischer
carbene complexes as the reaction partners in palladium-
carbene complexes.
At the outset of the investigation, we employed methyl
chromium(0) Fischer carbene 1a and biphenyl boronic acids 2a
as the model substrates. The reaction was carried out in toluene
at 80 oC with Pd(OAc)2 as the catalyst, triphenylphosphine (L1)
as the ligand and benzoquinone (BQ) as the oxidant. Gratifyingly,
we could isolate the desired biphenyl ketone product 3a in 52 %
yield after hydrolysis in aqueous HCl condition (Table 1, entry 1).
The effect of temperature and solvent were then examined, we
found that 90 oC and toluene were the best choices for this
transformation (entry 2). By simply changing the ratio of 1a to 2a
to 1.5:1, we could isolate the product 4-phenyl acetophenone in
71 % (entry 6). Different base conditions were further examined
(entry 7-10). No ketone product 3a was obtained when
potassium carbonate was employed, probably due to its
stronger basicity (entry 7). Meanwhile different amines were also
[*]
Dr. K. Wang, J. Yang, X. Yao, 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
Dr. K. Wang
i
i
tested (entry 8-10), such as Pr2NEt, Cy2NH, PrNHBn and Et3N,
which reveals that Pr2NH was the suitable choice. Increasing
Research Institute of Aerospace Special Materials and Processing
Technology, Beijing 100074 (China)
i
the loading of BQ from 1.5 equiv to 2.0 equiv could further
improve the yield to 79 % (entry 11). The tri(2-furyl)phosphine
(L3) was proved to be the best choice after the screening of
different ligands (entry 12). Finally, the yield of product 3a was
improved to 85 % by increasing the loading of Pr2NH to 4 equiv
(entry 14). When 4-biphenylboronic acid pinacol ester was
Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032 (China)
i
Supporting information for this article is given via a link at the end of
the document.
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10.1002/asia.201801278
Chemistry - An Asian Journal
COMMUNICATION
employed as substrate in standard condition, no coupling
products 3a was obtained, probably due to the different
transmetalation reactivities between boronic acid 2a and its
pinacol ester.
With the optimized reaction conditions in hand, we then
explored the scope of the reaction by using a series of
arylboronic acids (Scheme 2). We chose both methyl
chromium(0) carbene 1a and n-butyl chromium(0) carbene 1b
meta- and ortho-position could also give the corresponding
coupling products 3l-o in moderate to good yields by reacting
with n-butyl chromium carbene complexes 1b. Moreover, the
reaction was found to tolerate the 3-trifluoromethyl and 3-
trifluoromethoxyl substituents, giving the products 3h and 3n.
Next, we expanded the scope of boronic acids to vinylboronic
substrates 4a-e (Scheme 3). A series of substituted trans-2-
styrene boronic acids were found to react with alkyl chromium(0)
as the substrates to react with substituted arylboronic acids 2b-o. carbene complexes 1b-c smoothly, affording the corresponding
Arylboronic acids, which have substituents of different electronic
properties at the para-position, were well tolerated. The 4-vinyl-,
α,β-unsaturated ketone products 5a-d in moderate yields. The
reaction with trans-2-hexylvinyl boronic acid gave the product 5e
in slightly diminished yields.
Table 1. Optimization of the reaction conditions.[a]
yield[b
Base
(x equiv)
Temp
(oC)
]
Entry
1a:2a
solvent
Ligand
(%)
1
2
1.0:1
1.0:1
1.0:1
1.0:1
1.0:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
2.0:1
2.0:1
2.0:1
2.0:1
toluene
toluene
toluene
THF
iPr2NH (3)
iPr2NH (3)
iPr2NH (3)
iPr2NH (3)
iPr2NH (3)
iPr2NH (3)
iPr2NEt (3)
Cy2NH (3)
iPrNHBn (3)
Et3N (3)
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L3
80
90
100
90
90
90
90
90
90
90
90
90
90
90
52
62
47
34
52
71
40
51
48
46
79
74
82
85
3
Scheme 2. The reaction scope of arylboronic acids. Reaction conditions: a
solution of 1a-b (0.40 mmol), 2b-o (0.20 mmol), Pd(OAc)2 (5 mol%), L3 (20
mol%), iPr2NH (4 equiv), BQ (1.5 equiv) in 2 mL toluene stirred at 90 °C for 3 h.
After filtered through the short silica gel column with diethyl ether, the crude
product was dissolved in THF (1 mL) and HCl (0.5 mL, 2 M) at 60 °C for 2 h.
Isolated yield with silica gel column chromatography.
4
5
dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
6
7
8
9
10
11
12
13
14
iPr2NH (3)
iPr2NH (3)[c]
iPr2NH (3)
iPr2NH (4)
Scheme 3. The reaction scope of vinylboronic acids. Reaction conditions: a
solution of 1b-c (0.40 mmol), 4a-e (0.20 mmol), Pd(OAc)2 (5 mol%), L3 (20
mol%), iPr2NH (4 equiv), BQ (1.5 equiv) in 2 mL toluene stirred at 90 °C for 3 h.
After filtered through the short silica gel column with diethyl ether, the crude
product was dissolved in THF (1 mL) and HCl (0.5 mL, 2 M) at 60 °C for 2 h.
Isolated yield with silica gel column chromatography.
[a] If not otherwise noted, reaction conditions are as follows: 1a (0.20 mmol),
i
2a (0.20 mmol), Pd(OAc)2 (5 mol%), ligand (20 mol%), Pr2NH (3 equiv) and
BQ (1.5 equiv) in a solvent stirred at 80 oC for 3 h. After filtered through short
silica gel column with diethyl ether, the crude product was dissolved in THF (1
mL) and HCl (0.5 mL, 2 M) at 60 °C for 2 h. [b] Isolated product with silica gel
column chromatography. [c] 20 mol% excess of iPr2NH were added to produce
PCy3 from L2.
Finally, the scope of -substituted alkyl chromium carbenes
1d-l was evaluated by reacting with 4-methoxylphenylboronic
acid 2d (Scheme 4). The reaction proceeded smoothly in the
presence of both primary and secondary alkyl chains, affording
the ketones 6a-i. It is worth mentioning that various functional
groups on the alkyl chromium carbene species ban be tolerated.
As shown in the Scheme 4, heterocycles 1i, unsaturated carbon-
carbon double and triple bonds 1j and 1k as well as trimethylsilyl
group 1l are compatible with the reaction.
4-thiomethyl-, 4-chloro- and 4-bromo-substituted arylboronic
acids were all suitable substrates, affording the corresponding
substituted acetophenone products 3b-h in excellent yields. The
reaction with arylboronic acids bearing various substituents at
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Based on the previous studies,[13] we have proposed a
mechanism for this coupling reaction (Scheme 5, path a). The
catalytic cycle is initiated via oxidation of Pd(0) species A by BQ
to produce catalytic Pd(II) species B. Transmetallation with
arylboronic acid then takes place to generate arylpalladium(II)
intermediate C. The alkyl carbene ligand then transfers from
alkyl chromium(0) carbenes to intermediate C to produce the
palladium carbene intermediate D. Migratory insertion of the aryl
group from palladium to the carbenic carbon gives alkyl-
palladium intermediate E. Finally, β-hydride elimination of E
generates the vinyl ether F which could transform to the final
ketone products after hydrolysis. The resulted H-Pd(II)-X
However, an alternative pathway cannot be ruled out (Scheme
5, path b). Alkyl chromium carbenes is first deprotonated by
iPr2NH to generate vinyl chromium species D.[1b,14] Transfer of
the vinyl ether moiety from D to aryl palladium intermediate C
takes place to give aryl vinyl palladium intermediate E, followed
by reductive elimination and hydrolysis to afford the final product.
To differentiate the two possible pathways, mechanistic
experiments were conceived by using methyl chromium(0)
carbene 1a and benzyl bromide 5a as the reactants (Scheme 6,
a). The oxidative addition of Pd(0) to benzyl bromide would
generate the benzyl Pd(II) species that resembles the
organopalladium intermediate C in Scheme 5. Enol ether 7 and
8 would be the main products via Pd-carbene pathway upon
migratory insertion/β-H elimination steps because of the
stabilization of the phenyl group.14 However, if the vinyl
chromium species is generated to react with benzyl Pd(II)
species (path b), formation of terminal vinyl product 6 should be
favored through the deprotonation/transmetalation sequence.
The experiment showed that terminal enol ether 6 was the
main product, along with internal enol ethers 7 (E-isomer) and 8
(Z-isomer). The three products were in a ratio of 6:7:8 = 4: 1.7:1
(Scheme 6, b). The formation of internal ether products 7 and 8
may be attributed to the tautomerization of 6. To confirm such
possibility, the mixture of three products was allowed to react
under the standard conditions. The ratio of the terminal enol
ether 6 indeed decreased and that of internal enol ethers 7 and
8 increased, while the ratio of 7 to 8 remained unchanged.
Moreover, we also carried out further experiment to confirm the
tautomerism by isolating the three vinyl ether products (See
Schemes S3 and S4 in the Supporting Information). These
results indicate that path b is more likely the mechanism for this
coupling reaction.
i
species regenerates the Pd(0) species with the aid of Pr2NH to
complete the catalytic cycle.
Scheme 4. The reaction scope of -substituted methyl chromium(0) carbenes.
Reaction conditions: a solution of 1d-l (0.40 mmol), 2d (0.20 mmol), Pd(OAc)2
i
(5 mol%), L3 (20 mol%), Pr2NH (4 equiv), BQ (1.5 equiv) in 2 mL toluene
stirred at 90 °C for 3 h. The crude product was dissolved in THF (1 mL) and
HCl (0.5 mL, 2 M) at 60 °C for 2 h. Isolated yield with silica gel column
chromatography.
Scheme 6. Mechanistic experiments.
In summary, we have developed a novel oxidative cross-
coupling reaction of alkyl chromium(0) Fischer carbene
complexes and organoboronic acids under palladium catalysis.
Mechanistic study indicates that the coupling follows a pathway
involving generation of chromium anion species, transmetalation
and reductive elimination. This work may open up new
possibilities for developing novel cross-coupling reactions based
on alkyl chromium(0) Fischer carbene complexes. However, the
palladium(II) carbene process (path a) may also partially
Scheme 5. Possible reaction pathways.
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Acknowledgements
The project is supported by National Basic Research Program of
China (973 Program, No. 2015CB856600) and NSFC
(21332002).
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Keywords: oxidative cross-coupling · chromium(0) carbene ·
organoboronic acids · palladium catalysis · transmetalation
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Entry for the Table of Contents
COMMUNICATION
Kang Wang, Jinghui Yang, Xingqi Yao,
Jianbo Wang*
Page No. – Page No.
Palladium-catalyzed oxidative cross-
coupling reaction of alkyl
chromium(0) Fischer carbene
complexes with organoboronic acids
Cross-coupling with chromium(0) Fischer carbene：Palladium-catalyzed oxidative
cross-coupling of alkyl chromium(0) Fischer carbene complexes with organoboronic
acids has been realized. The work further demonstrates the generality of
chromium(0) Fischer carbene complexes as a new type of reaction partners in
cross-coupling reactions.
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