Dual Effects of Cyclopentadienyl Ligands on Rh(III)-Catalyzed Dehydrogenative Arylation of Electron-Rich Alkenes

Article abstract of DOI:10.1021/acscatal.8b01753

Despite extensive research on transition metal-catalyzed Fujiwara-Moritani type C-H olefinations, the alkenes used in these transformations are still mainly limited to active acrylate esters and styrenes. Selective aryl C-H olefination with electron-rich alkenes is recognized as a challenging issue. We herein report that simple and readily accessible electron-deficient [CpRh(III)] and [Cp^CF₃Rh(III)] (Cp^CF₃ = C₅Me₄CF₃) complexes are powerful catalysts for dehydrogenative arylation of electron-rich alkenes, including vinyl acetates, enamides, and vinyl ethers. Employing an electron-withdrawing Cp or Cp^CF₃ ligand instead of the privileged Cp? (C₅Me₅) ligand not only can facilitate the electrophilic aryl C-H rhodation but also can lower the olefin insertion barrier. Both electron-withdrawing and electron-donating directing groups such as -CONR₂ and -NHAc could be employed in these reactions, which provides convenient routes toward a series styryl acetates, N-acetylindoles, and aryl methyl ketones.

Full text of DOI:10.1021/acscatal.8b01753

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Article
Dual Effects of Cyclopentadienyl Ligands on Rh(III)-
Catalyzed Dehydrogenative Arylation of Electron-Rich Alkenes
Weidong Lin, Weiwei Li, Dandan Lu, Feng Su, Ting-Bin Wen, and Hui-Jun Zhang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01753 • Publication Date (Web): 19 Jul 2018
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Dual Effects of Cyclopentadienyl Ligands on
Rh(III)-Catalyzed Dehydrogenative Arylation of
Electron-Rich Alkenes
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Weidong Lin, Weiwei Li, Dandan Lu, Feng Su, Ting-Bin Wen* and Hui-Jun Zhang*
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, Fujian, P. R. China
ABSTRACT. Despite extensive research on transition-metal-catalyzed Fujiwara-Moritani-type
C-H olefinations, the alkenes used in these transformations are still mainly limited to active
acrylate esters and styrenes. Selective aryl C-H olefination with electron-rich alkenes is
recognized as a challenging issue. We herein report that simple and readily accessable electron-
deficient [CpRh(III)] and [CpCF3Rh(III)] (CpCF3 = C5Me4CF3) complexes are powerful catalysts
for dehydrogenative arylation of electron-rich alkenes including vinyl acetates, enamides and
vinyl ethers. Employing electron-withdrawing Cp or CpCF3 ligand instead of the privileged Cp*
(C5Me5) ligand can not only facilitate the electrophilic aryl C-H rhodation but also reduce the
olefin-insertion barrier. Both electron-withdrawing and electron-donating directing groups such
as -CONR2 and -NHAc could be employed in these reactions, which provides convenient routes
toward a series styryl acetates, N-acetylindoles and aryl methyl ketones.
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KEYWORDS. half-sandwich rhodium(III) complexes, cyclopentadienyl ligands, C-H activation,
electron-rich alkenes, homogeneous catalysis, ligand design
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Steric and electronic properties of ligands have significant effects on the reactivity of transition
metal complexes. In recent years, ligand development in C-H activation for pursuing highly
efficient catalysts has received growing attention.1,2 Besides promoting the cleavage of specific
C-H bonds, ideal ligands should also have positive effects on the following functionalization
steps and even on the poor reaction partners. Since the discovery of Fujiwara-Moritani reaction
in 1967,3 a wide range of transition-metal catalysts for oxidative dehydrogenative alkenylations
of arenes have been developed.4 However, the alkenes employed in most cases are still limited to
active acrylate esters and styrenes. Examples for C-H alkenylation with electron-rich alkenes,
such as vinyl acetates,5 vinyl ethers,6 and enamides7 are rare,8 which is mainly attributed to their
much higher insertion barrier into the (aryl)C-metal bond of metallacycle intermediate during the
catalytic cycle.9 Suitable ligands may facilitate the 1,2-migratory insertion of electron-rich
alkenes.10 In 2008, Xiao et al. discovered that in Pd-catalyzed classical Heck reactions, using
electron-deficient phosphine ligands could reduce the π back-donation from Pd center to the
coordinated electron-rich olefins and therefore reduce the olefin-insertion barrier.10a In 2001,
Svensson et al have found analogous results after they studied the electronic ligand effects on the
activity of cationic phenylpalladium(II)diimine complexes using DFT caculations.10b We then
reasoned that active cationic RhIII complexes11 featuring electron-deficient cyclopentadienyl (X-
Cp) ligands would be ideal catalyst candidates for Fujiwara-Moritani type arylation of electron
rich alkenes because the electron-withdrawing X-Cp ligands may promote both electrophilic
metalation of aromatic C-H bonds12 and 1,2-migratory insertion of electron-rich alkenes into
(aryl)C-metal bonds13,14 (Scheme 1).
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Scheme 1. Dual effects of X-Cp ligands on Rh(III)-catalyzed Fujiwara-Moritani type arylation
of electron rich alkenes.
Over the past decade, with pioneering contributions of Miura & Sato,15 Rovis,16 Cramer,17 and
Tanaka,18 a number of substituted cyclopentadienyl ligands, such as C5HPh4, Cpt, CpiPr, CpCF3
,
Cp*tBu, Cp*Cy, Ind*, chiral CpX,19 CpCy, CpE, CpA, have been identified as outstanding
alternatives for the privileged Cp* ligand in Rh(III)-catalyzed C-H functionalization
reactions.20,21 Nevertheless, the most simple and electron-deficient unsubstituted
cyclopentadienyl (Cp) ligand has not received adequate attention in this field.22,23 Herein, we
describe the facile synthesis of electron-deficient [X-CpRhI2]2/n complexes enabled us to develop
a powerful X-CpRh(III)-catalyzed dehydrogenative arylation of electron-rich alkenes.
To start our investigation, five simple and sterically different X-Cp ligands: Cp, CpCF3, CpiPr,
CpCO2Me and CpCOMe were chosen (Scheme 2). Much higher antisymmetric and symmetric CO
stretching frequencies of corresponding [X-CpRh(CO)2] complexes compared to [Cp*Rh(CO)2]
(2016/1948 cm-1) indicated the electron-withdrawing properties of these X-Cp ligands (Scheme 2
and Table S1).2a Notably, the introduction of -CO2Me or -COMe group as a strong electron-
withdrawing substituent on the Cp ring does not decrease its electron density dramatically. The
corresponding half-sandwich complexes [X-CpRhI2]2/n were then synthesized through a very
convenient and practical method starting from commercial available [Rh(cod)Cl]2 and X-CpNa
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(Scheme 2).24-26 Importantly, through this pathway the polymeric [CpRhI2]n could be isolated
cleanly.
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Scheme 2. Five X-Cp ligands and the preparation of [X-CpRhI2]2/n complexes.
Having the desired X-CpRh(III) complexes in hand, we firstly investigated their catalytic
activities in the coupling between rather electron-poor tertiary benzamide 1a and vinyl acetate 2a
(Table 1). For comparison purpose, the reaction catalyzed by [RhCp*Cl2]2 (2.5 mol%)/AgSbF6
(10 mol%) was performed, which afforded the corresponding alkenylation product 3aa in 21%
yield (entry 1). Only trace amount of ortho C-H vinylation product was observed.14 75% of the
benzamide 1a was recovered. Gratifyingly, employing the X-Cp-based rhodium(III) catalyst
precursors all resulted in much improved yields (37-86%, entries 2-6). The sterically different
but electronically similar Cp and CpCF3 ligands are both optimal for the alkenylation which
allowed the formation of 3aa in high yields (entries 2 and 3). Notably, as two electronically
similar precatalysts, [CpCO2MeRhI2]2 and [CpCOMeRhI2]2 exhibited rather different catalytic
activities, which is still not fully understood (entries 5 and 6). To exclude the halide effect,
preformed catalyst [CpRh(MeCN)3][SbF6]2 was employed affording 3aa in 87% yield (entry 7).
These results suggested that the electronic effects of X-Cp ligands play key roles in this
transformation. Moreover, without the addition of AgSbF6, no desired product could be formed
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in the presence of polymeric [CpRhI2]n or dimeric [CpCF3RhCl2]2 (entry 8 and 9). Other silver
salts such as AgOAc, AgBF4 and AgNTf2 were also tested (entry 10-12). Only AgNTf2 proved to
be comparable with AgSbF6 (entry 12). These results disclosed that the real catalyst for this
alkenylation should be a cationic rhodium(III) species without halides. It is interesting to note
that the reaction performed in the absence of Cu(OAc)2⸱H2O also provided 3aa in 22% yield
(entry 13).
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Table 1. Catalyst Screening.a
Entry [X-CpRh(III)]
[Ag]
Yield (%)b
21 (13:1)
82 (14:1)
86 (10:1)
79 (6:1)
37 (19:1)
73 (15:1)
87 (10:1)
0
1
[Cp*RhCl2]2
[CpCF3RhCl2]2
[CpRhI2]n
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
-
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4
[CpiPrRhI2]2
[CpCO2MeRhI2]2
[CpCOMeRhI2]2
[CpRh(MeCN)3](SbF6)2
[CpRhI2]n
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-
9
[CpCF3RhCl2]2
-
0
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13c
[CpRhI2]n
AgOAc
AgBF4
AgNTf2
AgSbF6
0
[CpRhI2]n
10 (9:1)
88 (10:1)
22 (16:1)
[CpRhI2]n
[CpRhI2]n
aReaction Conditions: 1a (0.2 mmol), 2a (2.0 mmol), [X-CpRh(III)] (5 mol%), [Ag] (10 mol%),
o
b
Cu(OAc)2H2O (1.1 equiv), THF (1 mL), 80 C, in air for 12 h. Isolated yields (E/Z ratios are
shown in parentheses). cIn the absence of Cu(OAc)2H2O.
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After determining the optimal reaction conditions, we set out to investigate the scope of this
alkenylation reaction by examining various benzamides derivatives under the catalysis of
[CpRh(III)] and [CpCF3Rh(III)] respectively (Table 2). A wide range of amide groups containing
diverse substituents, such as -Me, -Et, -Cy, -Bn, -(CH2)5-, -OMe, and -2,6-Me2Ph, could be
employed as suitable directing groups. And the corresponding products 3ba-ia were all produced
in good yields (62-98%). Thereafter, the reactions of a series of ortho- and para-substituted
benzamides (1j-p) and especially several heteroaromatic substrates (1q-s) with vinyl acetate all
led to good yields of alkenylated products (3ja-sa). Both electron-donating (-Me, -OMe) and
electron-withdrawing groups (-F, -Cl, -Br, -Ph, -CO2Me) were well tolerated. In most cases, the
slightly more electron-deficient [CpRh(III)] catalyst delivered much better results than
[CpCF3Rh(III)] catalyst. Although it seems that the steric bulkiness of CpCF3 ligand may have
adverse effects on the reaction efficiency, it is beneficial to improving regioselectivity. For
instance, in the presence of [CpRh(III)], the alkenylation of N,N-diisopropyl-2-naphthamide 1t
with vinyl acetate afforded two regioisomeric products (87% yield, C2/C8 = 1:1). When using
bulky [CpCF3Rh(III)] as catalyst, only 71% yield of C8-alkenylation product was formed.
Moreover, the reactions of meta-substituted benzamides 1u-1w with 2a in the presence of
[CpRh(III)] all gave both C2- and C6-alkenylated products 3ua-wa, whereas using
[CpCF3Rh(III)] as the catalyst the C6-alkenylation of 1v and 1w were preferred. For 3-
methoxybenzamide 1u, due to the coordination ability of -OMe group to Rh(III),8e employing
[CpCF3Rh(III)] instead of [CpRh(III)] only resulted in increased C6-alkenylated product 3ua
(C2/C6= 2:1 to 1:1). When the meta-substituent is bulky enough as in 1x, the C6-alkenylation
product 3xa could be formed selectively no matter which catalyst was employed.
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Table 2. Scope of Benzamides 1.a,b
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aReaction Conditions: 1 (0.2 mmol), 2a (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % of
o
[Rh]), AgSbF6 (10 mol %), Cu(OAc)2H2O (1.1 equiv), THF (1 mL), 80 C, in air for 12 h.
bIsolated yields of 3 under the catalysis of [CpRh] and [CpCF3Rh] respectively (ratios refer to the
amount of C2-alkenylation product versus C6- or C8-alkenylation product are shown in
parentheses). c100 oC, 24 h. d2a (2 equiv).
Next, we investigated the scope of alkenes in this Rh-catalyzed C-H alkenylation reaction (Table
3). A series of vinyl esters 2b-e as well as geminal-substituted vinyl acetates 2f-j were suitable
substrates for this transformation. In most cases, the sterically less demanding [CpRh(III)]
catalyst displayed higher activities than [CpCF3Rh(III)], which is consistent with the results
shown in Table 2. As a sterically bulky substrate, 1,2-disubstituted alkene 2k reacted with N,N-
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dimethylbenzamide 1b in the presence of [CpRh(III)] providing not only the expected β-
arylation product 3bk (23%) but also a 1,1-diphenylethylene derivative 3bk’ (59%).
Furthermore, both catalysts could be employed in the alkenylation of 1a with 2-vinylisoindoline-
1,3-dione 2l, which afforded the desired product 3al in good yields ([CpRh]: 89%; [CpCF3Rh]:
88%). The selective α-arylation of vinyl ether 2m with 1a in the presence of [CpCF3Rh(III)] also
led to the formation of product 3am’ in moderate yield.27 The reaction of 2l or 2m with 1a could
also be catalysed by [Cp*Rh(III)], albeit affording 3ak and 3al’ in much lower yields (28% and
29%).
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Table 3. Scope of Alkenes.a,b
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aReaction Conditions: 1 (0.2 mmol), 2 (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % of
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[Rh]), AgSbF6 (10 mol %), Cu(OAc)2H2O (1.1 equiv), THF (1 mL), 80 C, in air for 12 h.
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bIsolated yields of 3 under the catalysis of [CpRh] and [CpCF3Rh] respectively (yields for the
reactions catalyzed by [Cp*RhCl2]2 were shown in parentheses). c2 (2 equiv). d2 (5 equiv). e100
oC, 24 h. fAcetone instead of THF.
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The much higher catalytic activities of [CpRh(III)] and [CpCF3RhIII] compared with [Cp*Rh(III)]
was further indicated in the direct couplings between anilides 4 and electron-rich alkenes (Table
4). The reactions of N-(m-tolyl)acetamide 4b with vinyl acetate 2a as well as geminal-substituted
vinyl acetates 2f-h all afforded the corresponding products 5ba and 5bf-bh in good yields,
whereas the cationic [Cp*Rh(III)] catalyst is ineffective for the dehydrogenative arylation of
geminal-substituted vinyl acetates. For example, the reaction of 4b with 2f in the presence of 5
mol % of [CpRh(III)] afforded the corresponding 5bf in 79% yield. No vinylation and double
olefination products were observed. Also, the alkenylation of 1,1-dimethyl-3-phenylurea 4c with
2a could not be catalysed by [Cp*Rh(III)], but it works smoothly in the presence of [CpRh(III)]
or [CpCF3Rh(III)] catalyst and provides 5ca in good yield. Then the reactions of acetamide 4a
with other two representative electron-rich alkenes 2l and 2m were performed. Both of the
desired products 5al and 5am' could be formed in good yields.28 Moreover, under [CpCF3Rh(III)]
catalysis, the reaction of 4b with 2m afforded the acetylation product 5bm' in 90% yield.
Apparently, compared with the classical Friedel-Crafts acylation reactions, this direct C-H
acetylation with vinyl ethers may provide a much cleaner route towards aryl methyl ketones.
Surprisingly, the reaction of 1,2-disubstituted alkene 2k with 4a selectively led to the formation
of a C-H/C-O cross-coupling product 6ak ([CpRh(III)]: 72%; [Cp*Rh(III)]: 12%). The expected
β-arylation product 5ak was not observed.
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Table 4. Reaction of Anilides with Electron-Rich Alkenes.a,b
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aReaction Conditions: 4 (0.2 mmol), 2 (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % of
o
[Rh]), AgSbF6 (10 mol %), Cu(OAc)2H2O (1.1 equiv), THF (1 mL), 80 C, in air for 12 h.
bIsolated yields of 5 under the catalysis of [CpRh] and [CpCF3Rh] respectively (yields for the
c
d
reactions catalyzed by [Cp*RhCl2]2 were shown in parentheses). Cu(OAc)2H2O (0.3 equiv). 2
(5 equiv). eAcetone instead of THF. fAgSbF6 (5 mol %). g2 (4 equiv), 120 oC.
The significance of [CpRh(III)]-catalyzed dehydrogenative arylation of electron-rich alkenes was
demonstrated by the facile construction of 1,2-diarylethene and indole skeletons (Scheme 3). (E)-
1,2-Diarylethene product 7 was formed in 58% yield through [CpRh]-catalyzed selective C-H/C-
O cross-coupling between styryl acetate product 3aa (E:Z = 14/1) and acetanilide 4a (entry 1).
For further applications, the amide group could be chemoselectively reduced to aldehyde29 or
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transformaed to ketone.30 Direct regioselective ortho-alkenylation of acetanilide 4d with 2f
followed by hydrolysis and cyclization under acidic conditions led to the formation of N-
acetylindoles 8 in 69% yield (entry 2).
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Scheme 3 Facile Synthesis of 1,2-Diarylethene 7 and Indole 8.
To gain insight into the mechanism, we conducted several experiments (Scheme 4). Competition
experiment between 1-OMe and 1-F under the standard reaction conditions highlighted the
higher reactivity of more electron-rich benzamide (Scheme 4a), which indicated an electrophilic-
type C-H activation mode. It is also implied that electron-deficient [X-CpRh] should be more
suitable than electron-rich [Cp*Rh] for this C-H rhodation step. Then, we explored the
hydrogen/deuterium (H/D) exchange reactions of 1a in the presence of acetic acid-D4 (Scheme
4b; for more details, see the Supporting Information). When the reaction is performed under the
catalysis of [CpRh] and in the absence of 2a for 6 hours, 59% deuterium incorporation at both
the ortho positions of the phenyl ring was observed. Still under the catalysis of [CpRh], if the
reaction is conducted in the presence of 2 equiv of 2a, there was almost no deuterium
incorporation observed in the recovered 1a. These results suggest that the C-H rhodation with
[CpRh] is irreversible in the presence of an alkene. In contrast, under the catalysis of [Cp*Rh],
the H/D exchange at the two ortho positions of 1a was independent of the alkene, which
indicated that the migratory insertion of 2a might be accelerated by using the electron-deficient
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Cp ligand instead of the Cp* ligand. Thereafter, the stoichiometric reaction of 1b with
[CpRhI2]n/AgSbF6 and vinyl acetate in acetone was performed at room temperature, which
afforded the expected rhodacycle 9 in 80% yield (Scheme 4c). The structure of 9 was confirmed
by single crystal X-ray diffraction (Scheme 4c).31 The formation of rhodacycle 9 was also
observed under the catalytic conditions by using 10 mol % of [CpRh] (see the Supporting
Information). The alkenylation of 1b with 2a in the presence of 5 mol% 9 was then conducted,
which afforded the desired 3ba in 86% yield (Scheme 4d). Therefore, the Rh(III)-catalyzed
dehydrogenative arylation of vinyl acetate may indeed proceed through a generally proposed
cycle including directed electrophilic C-H rhodation, olefin insertion and subsequent β-H
elimination.9,13
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Scheme 4. Mechanistic Studies (For clarity, hydrogens and SbF6 counterion have been omitted
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from the ORTEP view of rhodacycle 9.)
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In summary, electron-deficient [CpRh(III)] and [CpCF3Rh(III)] complexes exhibit much higher
catalytic activities than the privileged [Cp*Rh(III)] in dehydrogenative arylation of electron rich
alkenes. The arylation of vinyl acetates provides a facile and convenient route towards various
styryl acetates and N-acylindoles. Synthetically useful aryl methyl ketones are prepared readily
through the direct arylation of vinyl ether under mild conditions. [CpRh(III)] could also be used
to catalyze the chemoselective C-H/C-O cross-coupling between styryl acetates and acetanilides,
which may provide an atom- and step-economic approach towards (E)-1,2-diarylethenes. The
cheap and readily available features of electron-rich alkenes and their versatile reactivities as
well as the convenient synthetic method towards dimeric or polymeric [X-CpRhI2]2/n make this
alkenylation reaction a valuable tool for C-C bond formation. More importantly, the successful
use of dual effects of electron-withdrawing Cp and CpCF3 ligands in this reaction provides new
insights for catalyst and ligand design in C-H functionalizations.
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ASSOCIATED CONTENT
Supporting Information. General and experimental information on preparation of [X-
CpRhI2]2/n complexes, optimization studies and procedures for dehydrogenative arylation of
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electron-rich alkenes, details for mechanism studies, characterization data (1H NMR, C NMR,
19F NMR, HRMS) for all new compounds. This material is available free of charge on the ACS
Publications website.
AUTHOR INFORMATION
Corresponding Author
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*E-mail for H.-J. Zhang: meghjzhang@xmu.edu.cn
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*E-mail for T.-B. Wen: chwtb@xmu.edu.cn
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ORCID
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H.-J. Zhang: 0000-0001-9567-3010
T.-B. Wen: 0000-0002-2212-9943
ACKNOWLEDGMENT
We thank the financial support from Natural Science Foundation of China (No. 21572188,
21772162, 21302157) and Fundamental Research Funds for the Central Universities (No.
20720160049). H.-J. Zhang thank Prof. Pierre H. Dixneuf for valuable discussions and
corrections.
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(27) The electrical properties of two olefin carbons in vinyl ether highly influence the olefin
insertion step, which results in the regioselective α-arylation of 2m. The acetylation product
3am’ is probably derived from in situ hydrolysis of the α-arylation intermediate. Also see
reference 6.
(28) For the reaction between 4a of 2m, a N-acetylindole 5am'' was formed as a byproduct
([CpRh]: 6%; [CpCF3Rh]: 7%), which may be derived from the hydrolysis of a β-arylation
intermediate followed by cyclization.
(29) (a) Xiao, P.; Tang, Z.; Wang, K.; Chen, H.; Guo, Q.; Chu, Y.; Gao, L.; Song, Z.
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(30) Huang, P.-Q.; Wang, Y.; Xiao, K.-J.; Huang, Y.-H. A General Method for the Direct
Transformation of Common Tertiary Amides into Ketones and Amines by Addition of Grignard
Reagents. Tetrahedron, 2015, 4248-4254.
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(31) For X-ray single crystal structure analysis of 9 CCDC 1829538, see the SI.
Table of Contents.
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