Rhodium(III)-Catalyzed Oxidative ortho-Olefination of Phenyl Carbamates with Alkenes: Elucidation of Acceleration Mechanisms by Using an Unsubstituted Cyclopentadienyl Ligand

Article abstract of DOI:10.1021/acs.orglett.0c02499

It has been established that an unsubstituted cyclopentadienyl (Cp) Rh(III) complex is an effective catalyst for the oxidative ortho-olefination of phenyl carbamates with both acrylates and styrenes under mild conditions. In addition, diolefination of a p

Full text of DOI:10.1021/acs.orglett.0c02499

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Letter
Rhodium(III)-Catalyzed Oxidative ortho-Olefination of Phenyl
Carbamates with Alkenes: Elucidation of Acceleration Mechanisms
by Using an Unsubstituted Cyclopentadienyl Ligand
Jin Tanaka, Yuki Nagashima,* and Ken Tanaka*
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ABSTRACT: It has been established that an unsubstituted cyclopentadienyl (Cp) Rh(III) complex is an effective catalyst for the
oxidative ortho-olefination of phenyl carbamates with both acrylates and styrenes under mild conditions. In addition, diolefination of
a protected BINOL (1,1′-binaphthalene-2,2′-diol) proceeded in high yields and disubstituted acrylates could participate in this
catalysis. Experimental and theoretical mechanistic studies elucidated that an electron-deficient nature of the unsubstituted
CpRh(III) complex accelerates both the electrophilic aryl C−H rhodation and the rate-limiting alkene insertion steps.
he transition-metal-catalyzed directed oxidative C−H
olefination of arenes with alkenes is a useful method for
transition-metal-catalyzed oxidative C−H bond olefination,
but the reactions required harsh conditions, even with highly
reactive acrylates (Figure 1c).9 In 2011, Liu8a and Loh8b
reported the Cp*Rh(III) complex-catalyzed oxidative ortho-
olefination of phenyl carbamates with acrylates, while these
reactions required a stoichiometric amount of a Cu(II) oxidant
and a high temperature (110 °C). Subsequently, Ackermann8c
and Wang8d reported the Ru(II)/p-cymene complex-catalyzed
variants under the same reaction conditions that were used for
the Cp*Rh(III) catalysis. In 2013, Jeganmohan reported the
reactions using a catalytic amount of the Cu(II) oxidant, but a
high temperature (100 °C) was still required.8e Thus, the
aerobic oxidative ortho-olefination of phenyl carbamates with
alkenes under mild conditions remains a challenge. Herein, we
realize the ideal process described above with a broad substrate
scope by using the unsubstituted CpRh(III) complex.
Additionally, we clarify the acceleration mechanisms by using
the Cp ligand via experimental and theoretical studies.
T
the synthesis of substituted styrenes.1 Although numerous
directing groups have been used, the reactions generally
required harsh conditions,2 and the substrate scope was limited
when using conventional Pd(II), Ru(II)/p-cymene, and
Cp*Rh(III) (Figure 1a) catalysts.1 However, recent reports
revealed that the appropriate choice of the Cp ligand for the
Rh(III) complex made the reaction conditions milder and
expanded the substrate scope for both electron-rich and
-deficient arenes (anilides and benzamides, respectively).3−7
For anilides, an electron-deficient CpERh(III) complex (Figure
1a)3 catalyzed the aerobic oxidative ortho-olefination with
alkenes, including unactivated ones, at room temperature.4 For
benzamides, an amide-pendant electron-deficient CpA1Rh(III)
complex (Figure 1a) catalyzed the aerobic oxidative ortho-
olefination with alkenes, including disubstituted ones, from
room temperature to 40 °C.5 In contrast to the functionalized
Cps, an electron-deficient and sterically less demanding
unsubstituted CpRh(III) complex (Figure 1a) allowed the
use of electron-rich alkenes (Figure 1b, eq 1)6a and sterically
demanding benzamides (Figure 1b, eq 2),6b while the role of
the unsubstituted Cp ligand was not elucidated theoretically.6
Besides carboxylic acid and aniline derivatives, phenol
derivatives are important functionalized arenes. Phenyl
carbamates8 are the ones most frequently used in the
Received: July 29, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02499
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Optimization of Reaction Conditions
b
2a
(equiv)
Cu(OAc)2 % yield of 3aa + 4aa
entry
CpX
X
(mol %)
(3aa/4aa)
1
Cp*
CpE
CpA1
CpA2
CpA3
CpA4
CpA5
Cp
Cp
Cp
Cp
Cp
2
2
2
2
2
2
2
2
SbF6
SbF6
SbF6
SbF6
SbF6
SbF6
SbF6
SbF6
SbF6
SbF6
NTf2
OAc
NTf2
NTf2
NTf2
200
200
200
200
200
200
200
200
200
200
200
200
20
17 (17/0)
28 (28/0)
14 (14/0)
14 (14/0)
15 (15/0)
35 (35/0)
19 (19/0)
58 (58/0)
91 (69/22)
85 (71/14)
92 (79/13)
0
2
3
4
5
6
7
8
9
c
2
c
c
c
c
c
c
10
11
12
13
14
15
1.1
1.1
1.1
1.1
1.1
1.5
,d
,d
,d
Cp
Cp
Cp
73 (65/8)
84 (71/13)
98 (63/35)
,
,
e
e
20
20
a
[CpXRhCl2]2 or [CpRhI2]n (0.0050 mmol of Rh), AgX (0.010
mmol), Cu(OAc)2 (0.020−0.200 mmol), 1a (0.100 mmol), 2a
(0.110−0.200 mmol), and (CH2Cl)2 (1.0 mL) were used under Ar.
b
c
d
e
1
Determined by H NMR. For 72 h. Under air. At 40 °C.
Thus, the substrate scope was examined by using 0.2 mmol
Figure 1. Research background.
of phenyl carbamates 1 and slight excess amounts (1.5 or 1.1
equiv) of alkenes 2 at 40−60 °C (Figure 2a). Both butyl (2a)
and methyl (2b) acrylates reacted with 1a to give monoenes
3aa and 3ab in good yields along with dienes 4aa and 4ab,
respectively. When using ortho-substituted phenyl carbamates
(1b−f), monoenes 3ba−fa were obtained at 60 °C in high
yields regardless of the electronic and steric nature of
substituents. As with 1a, para-substituted phenyl carbamates
1g and 1h reacted with 2a to give monoenes 3ga and 3ha in
good yields along with dienes 4ga and 4ha, respectively. For
meta-substituted phenyl carbamates, meta-methyl one 1i was
olefinated with 2a to give sterically less demanding isomer 3ia
with high selectivity, but meta-chloro one 1j reacted with 2a to
give sterically demanding isomer 3ja as a major product. The
CpRh(III) complex could also catalyze the olefination with
styrenes. Styrene (2c) reacted with 1a to give monoene 3ac in
moderate yield, although diene 4ac was also generated in
moderate yield. When using ortho-substituted phenyl carba-
mates 1b−e, monoenes 3bc−3ec, respectively, were obtained
at 40−60 °C in high yields. para-substituted phenyl carbamates
1g and 1k reacted with 2c to give monoenes 3gc and 3kc in
moderate yields along with dienes 4gc and 4kc, respectively.
For meta-substituted phenyl carbamates, meta-methyl one 1i
was olefinated with 2c to give sterically less demanding isomer
3ic as a sole product. However, meta-chloro one 1j reacted
with 2c at 60 °C to give 3jc, 3jc′, and 4jc in an almost 1:1:1
ratio. For styrenes, electronically and sterically diverse
substituted styrenes 2d−j reacted with 1a or 1d to give
olefinated products in good to high yields.
We first screened the Rh(III) complexes for the oxidative
olefination of phenyl carbamate (1a) with butyl acrylate (2a)
in the presence of Cu(OAc)2 (200 mol %) and AgSbF6 (10
mol %) at room temperature under an argon atmosphere
(entries 1−8 of Table 1). The use of the Cp*, CpE, and
CpA1Rh(III) complexes afforded the desired monoene 3aa in
low yields of 14−28% (entries 1−3, respectively). Previously
reported CpA2−5Rh(III) complexes (Figure 1a)10,11 were also
tested (entries 4−7, respectively), which revealed that the use
of the CpA4Rh(III) complex bearing the highly acidic
secondary amide moiety11 afforded 3aa in an improved yield
of 35% (entry 6). Pleasingly, the use of the unsubstituted
CpRh(III) complex6 further improved the yield of 3aa to 58%
(entry 8). A prolonged reaction time (72 h) increased the yield
of 3aa to 69%, while a significant amount of diene 4aa (22%)
was generated (entry 9). Decreasing the amount of 2a to 1.1
equiv slightly increased the yield of 3aa (71%) and significantly
decreased the yield of 4aa (14%) (entry 10). Screening of
silver(I) salts (entries 10−12) revealed that the use of AgNTf2
afforded the highest yield of 3aa/4aa (entry 11). The use of air
as the terminal oxidant with a catalytic amount of Cu(OAc)2
decreased the yield of 3aa/4aa to 73% due to the incomplete
conversion of 1a (entry 13). Increasing the reaction temper-
ature to 40 °C increased the conversion of 1a (entry 14), and
complete conversion of 1a could be realized by using 1.5 equiv
of 2a, although the yield of 4aa significantly increased to 35%
(entry 15).
B
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Figure 2. Reaction scope. [CpRhI2]n (0.010 mmol of Rh), AgNTf2 (0.020 mmol), Cu(OAc)2 (0.040 mmol), 1 (0.200 mmol), 2 (0.300 mmol), and
a
b
(CH2Cl)2 (2.0 mL) were used. Cited yields were of isolated products. 2 (0.220 mmol). 2 (0.600 mmol).
In the previous examples using the Cp*Rh(III)8a,12 and
9e
Pd(OAc)2 catalysts, the diolefination of the synthetically
useful protected BINOL (1,1′-binaphthalene-2,2′-diol)13 was
inefficient and required a large excess amount of the acrylate or
styrene (5−6 equiv) and a high temperature of 90−160 °C.
Pleasingly, the CpRh(III) complex catalyzed the diolefination
of 1,1′-binaphthyl-2,2′-dicarbamate (1l) with slight excess
amounts (3 equiv) of both 2a and 2c to give dienes 3la and
3lc, respectively, in excellent yields at 40 °C under air (Figure
2b). This diolefination could be conducted on a preparative
scale, although the incomplete conversion of 1l was
observed.14 Additionally, substituted acrylates could also be
used at an elevated reaction temperature of 80 °C (Figure
2c).15 Crotonate 2k reacted with 1e to give monoene 3ek in
moderate yield. Methacrylate 2l also reacted with 1e to give
allylated product 3el′ as a major product and olefinated
Figure 3. Plausible reaction mechanism for formation of 3ab.
products 3el as minor products.
A plausible reaction mechanism is shown in Figure 3. First,
Rh(III) acetate A, generated from [CpRhI2]n, AgNTf2, and an
acetate anion, reacts with 1a to form rhodacycle B via an
electrophilic concerted metalation−deprotonation pathway.
Subsequent coordination of alkene 2b and insertion afford
rhodacycle D via C. β-Hydride elimination and oxidation with
a Cu(II) oxidant afford product 3ab-coordinating Rh(III)
acetate E. Finally, dissociation of product 3ab regenerates
catalytically active Rh(III) species A. To explore the effect of
the Cp ligand, deuterium kinetic isotope effects (DKIEs) of the
olefination of 1a with 2a were measured by the two parallel
C
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Figure 4. Theoretical mechanistic studies for formation of 3ab. Energy changes and bond lengths are shown in kilocalories per mole and angstroms,
respectively, and represent the relative free energies calculated at the M06/6-311+G** (C, H, O, and N) and SDD (Rh) levels using PCM
[(CH2Cl)2]. The values given in parentheses are the relative free energies calculated at the M06/6-31G** (C, H, O, and N) and LANL2DZ (Rh)
levels in the gas phase.
reactions and the intermolecular competition reactions16,17
using the Cp*Rh(III) and CpRh(III) catalysts. We found that
the reactions using the Cp*Rh(III) catalyst resulted in the
significantly different DKIE values between the two measure-
ments (1.8 vs 5.4). This observation suggests that not only the
C−H cleavage step but also other rate-limiting steps may exist
in the Cp*Rh(III)-based catalysis. In contrast, in the
CpRh(III) catalysis, the DKIE difference between the two
measurements (1.9 vs 3.8) was small, which suggests that not
only the C−H bond cleavage step but also the other rate-
limiting steps may be accelerated.
To gain detailed insight into the difference between the Cp
and Cp* ligands, we then performed a model calculation using
1a, 2b, and a CpRh(III)(OAc) or Cp*Rh(III)(OAc) cation
complex (Figure 4a,b). The calculated pathways indicate that
the alkene insertion step (IM3 to IM4), not the C−H bond
cleavage step (IM1 to IM2), is the rate-limiting step on both
reaction pathways, which is consistent with the different KIE
values between the two measurements, as shown in Figure S1.
For the rate-limiting alkene insertion step, transition state
TS2a bearing the Cp ligand (ΔG⧧ = +12.3 kcal/mol; total of
+13.2 kcal/mol from IM1) is more stable than TS2b bearing
the Cp* ligand (ΔG⧧ = +14.3 kcal/mol; total of +23.4 kcal/
mol from IM1), which allows this olefination reaction to
proceed at a lower temperature with the CpRh(III) complex
than with the Cp*Rh(III) complex.
The energy decomposition analysis (EDA)18 (Figure 4c) of
these transition-state structures indicates that large values of
the interaction energy (INT, −41.8 kcal/mol) are mainly
responsible for the low activation barrier. Indeed, the two C−
Rh bonds in four-membered transition state TS2a are shorter
than in TS2b; i.e., the “compact TS” is formed. Natural bond
orbital (NBO) analysis19 (Figure 4d) showed greater donation
of electrons from the CC σ bond to the CpRh(III) 4d*
orbital in TS2a (10.2 kcal/mol) than in TS2b (3.1 kcal/mol),
which might facilitate the formation of the new C−C bond.
Additionally, the Cp ligand lowers the activation energy
(ΔΔG⧧ = −6.2 kcal/mol) on the initial concerted deproto-
nation step (IM1 to IM2). This is also due to the formation of
the “compact TS” in the Cp complex because (1) C−Rh, C−
H, and Rh−O bond lengths in the six-membered ring of TS1a
are shorter than those of TS1b, (2) the increase in the
interaction energy [−66.4 (TS1b) to −78.2 (TS1a)] is
observed by EDA (Figure 4c), and (3) the greater donation
of electrons from the C−H σ bond to the CpRh(III) 4d*
orbital in TS1a (163.5 kcal/mol) than in TS1b (26.2 kcal/
mol) is shown by the NBO analysis (Figure 4d). Finally, it is
also revealed that the β-hydride elimination giving a cis-
D
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ideal synthesis: Alkenylation of aryl C−H bonds by a Fujiwara-
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necessary for the cis-product and stable coordination of the
directing group to the Rh center are not compatible in the
transition state [ΔG⧧ = +27.3 kcal/mol, IM4a to TS3a (cis)],
while the β-hydride elimination giving a trans-product
proceeds smoothly with a small activation energy of +12.2
kcal/mol [8.2 + 4.0, IM4a to TS3a (trans)].
(2) In some specific arene substrates, the transition-metal-catalyzed
directed oxidative C−H bond olefination with alkenes proceeded
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Pd(II)-Catalyzed Fujiwara-Moritani Reactions at Room Temperature
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C.; Besset, T.; Glorius, F. Rh(III)-Catalyzed Directed C−H
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X.; Huang, J.; Du, C.; Zhang, X.; Song, F.; You, J. N-Oxide as a
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ikandan, R.; Madasamy, P.; Jeganmohan, M. Ruthenium-Catalyzed
ortho Alkenylation of Aromatics with Alkenes at Room Temperature
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(3) Shibata, Y.; Tanaka, K. Catalytic [2+2+1] Cross-Cyclotrimeriza-
tion of Silylacetylenes and Two Alkynyl Esters To Produce
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(4) (a) Takahama, Y.; Shibata, Y.; Tanaka, K. Oxidative Olefination
of Anilides with Unactivated Alkenes Catalyzed by an (Electron-
Deficient η5-Cyclopentadienyl)Rhodium(III) Complex Under Am-
bient Conditions. Chem. - Eur. J. 2015, 21, 9053−9056. (b) Takahama,
Y.; Shibata, Y.; Tanaka, K. Concise Synthesis of Fungal Metabolite
(+)-Fusarochromanone via Rhodium(III)-catalyzed Oxidative sp2 C−
H Bond Olefination. Chem. Lett. 2016, 45, 1177−1179.
In conclusion, we have achieved the oxidative ortho-
olefination of phenyl carbamates with both acrylates and
styrenes under mild conditions by using an unsubstituted
CpRh(III) catalyst and elucidated the acceleration mechanisms
by using the unsubstituted cyclopentadienyl ligand with
experimental and theoretical studies.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02499.
Synthetic experiments, data for compound character-
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AUTHOR INFORMATION
Corresponding Authors
■
Ken Tanaka − Department of Chemical Science and Engineering,
Tokyo Institute of Technology, Tokyo 152-8550, Japan;
orcid.org/0000-0003-0534-7559; Email: ktanaka@
apc.titech.ac.jp
Yuki Nagashima − Department of Chemical Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8550,
Japan; orcid.org/0000-0001-8470-5638;
Email: nagashima.y.ae@m.titech.ac.jp
(5) Yoshimura, R.; Shibata, Y.; Yamada, T.; Tanaka, K. Aerobic
Oxidative Olefination of Benzamides with Styrenes Catalyzed by a
Moderately Electron-Deficient CpRh(III) Complex with a Pendant
Amide. J. Org. Chem. 2019, 84, 2501−2511.
Author
(6) (a) Lin, W.; Li, W.; Lu, D.; Su, F.; Wen, T.-B.; Zhang, H.-J. Dual
Effects of Cyclopentadienyl Ligands on Rh(III)-Catalyzed Dehydro-
genative Arylation of Electron-Rich Alkenes. ACS Catal. 2018, 8,
8070−8076. (b) Yoshimura, R.; Tanaka, K. Rhodium-Catalyzed
ortho-Olefination of Sterically Demanding Benzamides: Application to
the Asymmetric Synthesis of Axially Chiral Benzamides. Chem. - Eur.
J. 2020, 26, 4969−4973.
Jin Tanaka − Department of Chemical Science and Engineering,
Tokyo Institute of Technology, Tokyo 152-8550, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02499
Notes
(7) For the effects of steric and electronic tuning of the Cp ligands
on the Rh(III)-catalyzed C−H bond functionalization, see: (a) Piou,
T.; Romanov-Michailidis, F.; Romanova-Michaelides, M.; Jackson, K.
E.; Semakul, N.; Taggart, T. D.; Newell, B. S.; Rithner, C. D.; Paton,
R. S.; Rovis, T. Correlating Reactivity and Selectivity to Cyclo-
pentadienyl Ligand Properties in Rh(III)-Catalyzed C-H Activation
Reactions: An Experimental and Computational Study. J. Am. Chem.
Soc. 2017, 139, 1296−1310. (b) Piou, T.; Rovis, T. Electronic and
Steric Tuning of a Prototypical Piano Stool Complex: Rh(III)
Catalysis for C−H Functionalization. Acc. Chem. Res. 2018, 51, 170−
180. (c) Romanov-Michailidis, F.; Phipps, E. J. T.; Rovis, T. Sterically
and Electronically Tuned Cp Ligands for Rhodium(III)-Catalyzed
Carbon-Hydrogen Bond Functionalization. In Rhodium Catalysis in
Organic Synthesis: Methods and Reactions; Tanaka, K., Ed.; Wiley-
VCH: Weinheim, Germany, 2019; pp 629−644.
(8) (a) Gong, T.-J.; Xiao, B.; Liu, Z.-J.; Wan, J.; Xu, J.; Luo, D.-F.;
Fu, Y.; Liu, L. Rhodium-Catalyzed Selective C−H Activation/
Olefination of Phenol Carbamates. Org. Lett. 2011, 13, 3235−3237.
(b) Feng, C.; Loh, T.-P. Rhodium-catalyzed direct ortho C−H
olefination of phenol derivatives. Chem. Commun. 2011, 47, 10458−
10460. (c) Li, J.; Kornhaaß, C.; Ackermann, L. Ruthenium-catalyzed
oxidative C−H alkenylation of aryl carbamates. Chem. Commun. 2012,
48, 11343−11345. (d) Li, B.; Ma, J.; Liang, Y.; Wang, N.; Xu, S.;
Song, H.; Wang, B. Regio- and Stereoselective Olefination of Phenol
Carbamates through C−H Bond Functionalization. Eur. J. Org. Chem.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are thankful for a Grant-in-Aid for Scientific
Research (JP19H00893) from JSPS and Umicore for generous
support in supplying the rhodium complexes. A generous
allotment of computational resources from TSUBAME (Tokyo
Institute of Technology) is gratefully acknowledged.
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(14) Interestingly, a mono-olefination product was not observed,
which indicates that the olefination of the mono-olefination product
might be faster than that of 1l.
F
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