Diarylrhodates as promising active catalysts for the arylation of vinyl ethers with grignard reagents

Article abstract of DOI:10.1021/ja5043534

Anionic diarylrhodium complexes, generated by reacting [RhCl(cod)] ₂ with 2 equiv of aryl Grignard reagents, were found to be effective active catalysts in cross-coupling reactions of vinyl ethers with aryl Grignard reagents, giving rise to the production of vinyl arenes. In this catalytic system, vinyl-O bonds were preferably cleaved over Ar-O or Ar-Br bonds. A lithium rhodate complex was isolated, and its crystal structure was determined by X-ray crystallography.

Full text of DOI:10.1021/ja5043534

Communication
pubs.acs.org/JACS
Diarylrhodates as Promising Active Catalysts for the Arylation of
Vinyl Ethers with Grignard Reagents
Takanori Iwasaki, Yoshinori Miyata, Ryo Akimoto, Yuuki Fujii, Hitoshi Kuniyasu, and Nobuaki Kambe*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
In our previous work on anionic transition-metal-catalyzed C−
ABSTRACT: Anionic diarylrhodium complexes, gener-
ated by reacting [RhCl(cod)]2 with 2 equiv of aryl
Grignard reagents, were found to be effective active
catalysts in cross-coupling reactions of vinyl ethers with
aryl Grignard reagents, giving rise to the production of
vinyl arenes. In this catalytic system, vinyl-O bonds were
preferably cleaved over Ar−O or Ar−Br bonds. A lithium
rhodate complex was isolated, and its crystal structure was
determined by X-ray crystallography.
C bond formation, unsaturated hydrocarbon additives were
employed as the ligand or ligand precursor,8,9,14 to stabilize the
complexes by withdrawing electrons through π-back-donation
from the anionic metal center. Therefore, our initial efforts
involved reacting [RhCl(cod)]2 with PhMgBr to generate a
rhodate species, followed by its application to cross-coupling
reactions. When phenyl vinyl ether (1a) was reacted with
PhMgBr in the presence of 1 mol % of [RhCl(cod)]2 (2 mol %
based on metal), a coupling reaction via C−O bond cleavage
proceeded giving styrene (2a) in 64% yield, accompanied by a
3% yield of stilbene (3a) formed probably by the Heck-type
arylation of the coupling product 2a with PhMgBr (Table 1,
rylrhodium(I) complexes (B) are readily formed by the
Atransmetalation of the corresponding rhodium salts (A)
with various organometallic reagents1 and play important roles as
key catalytic species in C−C bond forming reactions.2 Some
useful examples include the addition of aromatic moieties to
carbonyl groups and enones (1,2- and 1,4-addition),3,4 alkenes
(also Heck-type reactions),5 and alkynes.6 Asymmetric addition
to carbonyl compounds provides a powerful tool for creating
chiral carbon centers.4 Another fundamental reaction course of
arylrhodium(I) complexes is oxidative addition with organo
(pseudo)halides to give Rh(III) complexes, which is involved as a
key process in cross-coupling reactions.7
In our previous studies, we reported that anionic transition-
metal species such as nickelate,8a−c palladate,8d and cobaltate9
complexes efficiently catalyze cross-coupling reactions of
unactivated alkyl (pseudo)halides with organometallic reagents.
Despite the importance and usefulness of Rh catalysts in
organometallic chemistry and in organic synthesis, anionic
diarylrhodium complexes C and their potent catalytic activities
remained undeveloped in sharp contrast with neutral and
cationic rhodium species.1 Garcia et al. synthesized anionic
diarylrhodium complexes and revealed a crystal structure of a
bimetallic complex having Ag within the sum of covalent radii of
Rh and Ag but their catalytic activities were not examined.10 Here
we report on the first examples in which anionic rhodium(I)
species C, generated by the addition of arylmetal reagents to Rh
salts A via arylrhodium(I) B (Scheme 1, A to B to C), play
important roles as an active catalytic intermediate in cross-
coupling reactions of vinyl ethers with aryl Grignard
reagents.11−13
a
Table 1. Cross-Coupling of Vinyl Ethers with PhMgBr
b
b
entry
1
catalyst (mol %)
2a (%)
3a (%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1a
1a
1a
1a
[RhCl(cod)]2 (1)
[RhCl(cod)]2 (1)
[RhCl(cod)]2 (1)
[RhCl(cod)]2 (1)
RhCl(PPh3)3 (2)
[Rh(OAc)2]2 (1)
Rh(acac)3 (2)
64
9
3
<1
<1
<1
1
6
<1
49
65
65
48
2
3
[Cp*RhCl2]2 (1)
2
a
Reaction conditions: 1 (0.5 mmol), PhMgBr (in THF, 1.0 mmol),
b
catalyst (0.01 mmol on metal), rt, 1 h. Determined by GC. Cp*:
pentamethylcyclopentadienyl.
entry 1).5 The fact that a biphenyl derivative was not formed
suggests that the reaction proceeded via the cleavage of the
vinylic carbon−oxygen bond in 1a rather than the aryl-oxygen
bond.15 Reactions using vinyl ethers having a primary, secondary,
or tertiary alkyl group resulted in poor yields, presumably due to
β-hydrogen elimination of alkoxyrhodium intermediates and/or
steric hindrance (entries 2−4). When a diaryl ether or an aryl
methyl ether was employed, no coupling reaction occurred and
the starting compounds were recovered. Other rhodium salts
[Rh(OAc)2]2 and Rh(acac)3 also gave similar results (entries 6,
7) although Rh complexes carrying PPh3 or Cp* ligands afforded
Scheme 1. Organorhodiums (L = ligand)
Received: April 30, 2014
Published: June 17, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
a
slightly lower yields (entries 5, 8). The use of Co, Ru, or Pd salts
resulted in poor yields, and Ir was completely ineffective.16
The scope of the reaction with respect to Grignard reagents is
shown in Table 2. Aryl Grignard reagents having either an
Table 3. Arylation of β-Styryl Ethers
a
Table 2. Scope of Grignard Reagents
a
Yields were determined by GC. Isolated yields are in parentheses.
0.10 mol % of [RhCl(cod)]2. 0.05 mol % of [RhCl(cod)]2. 24 h.
b
c
d
e
NMR yield.
a
4a (E/Z = 93/7), 4b (E/Z = >99/1), 4c (E/Z = 95/5), or 4d (E/Z =
72/28) was used unless otherwise stated. Yields were determined by
electron-withdrawing or -donating substituent at the p-position
afforded the coupling products 2b,e,h in excellent yields. The
present catalyst showed high catalytic performance with TONs
of up to 662. A substituent at the m-position had no effect on the
reaction giving 2c,f in good yields, although the reaction was
sluggish when o-tolyl and o-anisyl Grignard reagents were
employed affording 2d,g in low yields. A 2-naphthyl group was
vinylated efficiently, and an acetal functionality survived under
the present conditions giving rise to 2i and 2j, respectively.
As shown in Table 3, the trans enriched β-styryl phenyl ethers
4 afforded trans-stilbenes as the major products in all cases. The
stereochemistry of these products was found to be kinetically
controlled because cis-stilbene was recovered with only 1%
isomerization when a reaction of 4a with p-tolMgBr was
conducted for 4 h forming 79% of 3b in the presence of cis-
stilbene. The net retention of stereochemistry of the present
coupling reaction was confirmed using a cis-rich β-d1-vinyl ether
(DHCCHOAr, E/Z = 19/81) giving rise to the corresponding
cis-rich β-d1-styrene (E/Z = 27/73) in 69% yield.16 When cis
enriched 4a (E/Z = 13/87) was employed, however, the reaction
was slow and 3a was obtained only in a poor yield (26%, E/Z =
38/62) after 12 h. The use of the naphthyl Grignard reagent
required a longer reaction time but gave the desired product 3i in
nearly quantitative yield. It is noteworthy that cross-coupling
took place exclusively at the vinylic carbon, when a β-styryl ether
having a Br substituent on the aromatic ring was employed. Styryl
ethers having CF3 or an amide substituent 4c,d also coupled with
aryl Grignard reagents to give the coupling products 3m−o.17
It is of great synthetic importance that this present cross-
coupling reaction could be successfully applied to silyl vinyl
ethers which can be readily prepared from the corresponding
aldehydes. For example, when trans-silyl enol ethers having TES
(4e) and TBDMS (4f) were employed, the corresponding
coupling products were obtained in 68% and 63% yields,
respectively, with complete stereoretention (eq 1). Styryl-OTs
(E/Z = 66/34) and β-bromostyrene (E/Z = 84/16) also gave
stilbene 3a in 51% yield (E/Z = 66/34) and 71% yield (E/Z =
89/11), respectively.
b
GC. E/Z ratios were determined by 1H NMR. 4a (E/Z = 13/87) was
c
used. Isolated yield.
An interesting chemoselectivity of the Rh catalyst was noted,
compared to Ni and Pd which have been utilized for cross-
coupling reactions of ethers and esters with organometallic
reagents.11,12a−d Results for some competitive reactions of vinyl
ether 1a and p-bromotoluene with PhMgBr in the presence of
Rh, Ni, or Pd catalysts are summarized in eq 2. In the case of Rh,
the vinyl ether 1a was selectively coupled with PhMgBr to give
the styrene 2a in 60% yield accompanied by only a trace amount
of the biaryl derivative 5, as expected from the successful site-
selective cross-coupling of 4b leading to 3l in Table 3. In contrast,
the Ni catalyst gave a mixture of 2a and 5 in 40% and 15% yields,
respectively, and the Pd catalyst selected the bromoarene in
preference to the vinyl ether to give 5 exclusively in 71% yield.
To determine the actual catalytic species in this reaction, we
conducted stoichiometric reactions using [RhCl(cod)]2, 1a, and
1 to 4 equiv of PhMgBr (Table 4). When 1 equiv of PhMgBr was
added to an equimolar mixture of [RhCl(cod)]2 (based on Rh)
and 1a, the styrene (2a) and stilbene (3a) were both produced in
small amounts and 79% of 1a was recovered (entry 1), suggesting
that the arylrhodium complex B is not an active catalyst for the
arylation of vinyl ethers under the present conditions.13a,b When
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Table 4. Stoichiometric Reactions Using 1 to 4 equiv of
PhMgBr
a
a
a
entry
PhMgBr
conv. 1a (%)
2a (%)
3a (%)
1
2
3
1 equiv
2 equiv
4 equiv
21
68
61
1
1
n.d.
61
48
n.d.
Figure 1. ORTEP drawing of RhPh2(cod)·Li(dme)3 (6) with thermal
ellipsoids at the 50% probability level. H atoms and disordered DME are
omitted for clarity.
a
Determined by GC. n.d.: not detected.
2 equiv of Grignard reagent were used, 68% of 1a was consumed,
but no 2a was detected, with 3a being produced in 48% yield,
probably via a further Heck type arylation of 2a (entry 2).5 In
sharp contrast, 2a was produced as the sole product when 4 equiv
of Grignard reagent were used (entry 3). Taking these results
into account, RhAr(cod) generated by the transmetalation of
“RhCl(cod)” with ArMgBr is catalytically active not for arylation
of vinyl ethers but for styrenes.16 The arylation of vinyl ethers is
catalyzed by a diarylrhodate C, RhAr2(cod)−·MgBr+, generated
by the reaction of RhAr(cod) with ArMgBr. In the presence of 2
equiv of PhMgBr, the RhAr2(cod)−·MgBr+ was initially
generated and reacted with 1a to give 2a and RhAr(cod),
which react with each other to give 3a. This was supported by the
fact that when p-methylstyrene 2b, in place of 1a, was subjected
to the same conditions as entry 1, 3b was produced in 56% yield,
but 2b was recovered unreacted when the same reaction was
conducted in the presence of 3 equiv of PhMgBr.16
Scheme 2. A Possible Mechanism
frequently encountered active catalytic species in Rh-catalyzed
C−C bond formation, this is not the case in the present reaction
system. Thus, the formed arylrhodium B reacts with ArMgBr to
give the diarylrhodate C which adds to the CC bond of vinyl
ethers leading to D,13c,d and the following β-oxygen elimination
affords the product and regenerates B, a process that would be
more likely to occur than an oxidative addition/reductive
elimination pathway.13a,b The stereochemistry of net retention
of the present reaction can be explained by syn-addition followed
by anti-elimination via D.20 The latter process may be facilitated
by the coordination of MgBr+ to the oxygen of the intermediate
D to activate the C−O bond. Although complex B functions as a
catalyst for the Heck-type arylation of 2 to give stilbene
derivatives 3,5 it readily reacts with Grignard reagents to form the
complex C resulting in the suppression of double arylation to
form 3.16
When [RhCl(cod)]2 was treated with p-F-C6H4MgBr (10
equiv) in THF-d8, the three signals for COD were completely
1
shifted and new peaks appeared in the aromatic region on H
NMR, where the aryl group and COD were observed in a 2:1
ratio being assignable to the diarylrhodate C.16 When 1 equiv of
the vinyl ether 1a was added to the solution, the corresponding
coupling product 2h and complex C were detected in 73% and
91% yields, respectively (eq 3). These observations along with
the results in Table 4 strongly support that a rhodate complex C
is an active catalytic species.
In conclusion, we report on the isolation of an anionic
diarylrhodium complex and the determination of its structure by
X-ray diffraction analysis, confirming that it exists as a separated
ion pair. The diarylrhodates C efficiently catalyzed the cross-
coupling of vinylic ethers with aryl Grignard reagents through
vinylic C−O bond cleavage with a high degree of chemo-
selectivity even in the presence of aromatic C−O as well as C−Br
bonds. The present study demonstrates the high synthetic
potential of anionic Rh complexes, in that they are capable of
performing unique catalytic reactions, in comparison with widely
used neutral or cationic organorhodiums.
Although all attempts to crystallize the diarylrhodates,
generated by Grignard reagents, were fruitless, the structure of
the unique anionic diarylrhodium 6 was unambiguously
determined by X-ray diffraction analysis using PhLi instead of a
Grignard reagent.17 As shown in Figure 1, the crystal structure of
RhPh2(cod)·Li(dme)3 (6) contains anionic Rh and the Li cation.
The Rh center contains one COD and two phenyl groups in a
square planar geometry with the sum of the angles around Rh
being 360.0°. The rhodium−carbon bond distance (2.062 Å) of
6 is slightly shorter than that of a silver rhodate bimetallic
complex (2.093−2.098 Å).10b,18 The lithium cation is coordi-
nated by three DME ligands. To the best of our knowledge,
complex 6 is the first example of a structurally well-defined ion
separated anionic diarylrhodium complex.19
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental results, procedures, characterization data,
and crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
kambe@chem.eng.osaka-u.ac.jp
■
Scheme 2 shows a proposed reaction pathway for this reaction.
Transmetalation between [RhCl(cod)]2 and ArMgBr gives the
arylrhodium B. Although arylrhodium B complexes are
Notes
The authors declare no competing financial interest.
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Synth. Catal. 2011, 353, 1923 and references cited therein for arylation of
activated CC bonds. Rh-catalyzed Me−OAr bond cleavage:
(e) Arisawa, M.; Nihei, Y.; Suzuki, T.; Yamaguchi, M. Org. Lett. 2012,
14, 855.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Scientific
Research (S) (20225004), a Grant-in-Aid for Scientific Research
(A) (25248025) and JSPS Japanese-German Graduate Extern-
ship from JSPS, and a Grant-in-Aid for Scientific Research on
Innovative Area “Molecular Activation Directed toward
Straightforward Synthesis” (2204) from MEXT. T.I. thanks
The UBE Foundation.
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