Methoxy-directed aryl-to-aryl 1,3-rhodium migration

Article abstract of DOI:10.1021/ja409049t

Through-space metal/hydrogen shift is an important strategy for transition-metal-catalyzed C-H bond activation. Here we describe the synthesis and characterization of a Rh(I) 2,6-dimethoxybenzoate complex that underwent stoichiometric rearrangement via a highly unusual 1,3-rhodium migration. This aryl-to-aryl 1,3-Rh/H shift was also demonstrated in a Rh(I)-catalyzed decarboxylative conjugate addition to form a C-C bond at a meta position instead of the ipso-carboxyl position. A deuterium-labeling study under the conditions of Rh(I)-catalyzed protodecarboxylation revealed the involvement of an ortho-methoxy group in a multistep pathway of consecutive sp³ and sp² C-H bond activations.

Full text of DOI:10.1021/ja409049t

Communication
pubs.acs.org/JACS
Methoxy-Directed Aryl-to-Aryl 1,3-Rhodium Migration
Jing Zhang,† Jun-Feng Liu,‡ Angel Ugrinov,† Anthony F. X. Pillai,† Zhong-Ming Sun,*,‡
and Pinjing Zhao*,†
†Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102, United States
‡State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences
(CAS), Changchun, Jilin 130022, China
S
* Supporting Information
1,3-migration has not been reported with any transition-metal
ABSTRACT: Through-space metal/hydrogen shift is an
important strategy for transition-metal-catalyzed C−H bond
activation. Here we describe the synthesis and character-
ization of a Rh(I) 2,6-dimethoxybenzoate complex that under-
went stoichiometric rearrangement via a highly unusual
1,3-rhodium migration. This aryl-to-aryl 1,3-Rh/H shift
was also demonstrated in a Rh(I)-catalyzed decarboxylative
conjugate addition to form a C−C bond at a meta position
instead of the ipso-carboxyl position. A deuterium-labeling
study under the conditions of Rh(I)-catalyzed protodecar-
boxylation revealed the involvement of an ortho-methoxy
group in a multistep pathway of consecutive sp3 and sp2
C−H bond activations.
species. From the reaction mechanism perspective, 1,4- and 1,5-
migrations have been proposed to be facilitated by stabilized 5-
or 6-membered metallacycle intermediates and transition states
(Scheme 1).2 In comparison, DFT calculations suggest that
direct 1,3-metal/hydrogen shifts would require highly strained
4-member cyclic transition states with prohibitively high activa-
tion energies.6c,d,7 On the other hand, the classic 1,3-shifts of
transition-metal allyl species only involve migration of metal
centers but not hydrogen atoms.8 We herein describe stoichio-
metric and catalytic rearrangement processes that occur by a
formal aryl-to-aryl 1,3-rhodium migration in Rh(I)-mediated
decarboxylation. Mechanistic results from deuterium labeling
studies suggest a highly unusual, “double 1,4-Rh migration”
pathway that involves sp3 C−H bond activation at the methoxy
group.9
ransition-metal-catalyzed direct functionalization of C−H
Over the past decade, late transition-metal-mediated decar-
boxylation of benzoic acids has generated much interest as a
nonconventional approach toward reactive metal aryl inter-
mediates in catalysis.10−12 A very important structural motif
for decarboxylation is ortho-substitution of benzoic acids. In
particular, ortho-methoxy and ortho-fluorine groups have been
shown to significantly promote decarboxylation reactivity with
various transition-metal catalysts.10 We have previously
reported Rh(I)-catalyzed decarboxylative transformations of
2,6-difluorobenzoic acids including conjugate addition, oxida-
tive olefination,12a and protodecarboxylation.13 As part of our
efforts to gain mechanistic insights into Rh(I)-mediated decar-
boxylation, we have synthesized (bis)phosphine-ligated Rh(I)
benzoate complexes for direct observation of stoichiometric
decarboxylation. As described in Scheme 2, κ2-carboxylates 2a
and 2b were prepared by reactions between [(cod)Rh(μ-OH)]2
(cod: 1,4-cyclooctadiene), BIPHEP (2,2′-bis(diphenylphosphino)-
1,1′-biphenyl), and 2,6-difluorobenzoic acid (1a) or 2,6-dimeth-
oxybenzoic acid (1b), respectively. As we reported previously,13a
2,6-difluorobenzoate 2a underwent stoichiometric decarboxyla-
tion at 120 °C with 1 equiv of added pyridine in toluene, giving
the corresponding arylrhodium(I) complex 4a in quantitative
conversion.
T
bonds has become a powerful tool for organic synthesis.1
An important method for intramolecular C−H bond activation
is the “through-space” metal/hydrogen shift, most commonly
at 1,4- and 1,5-positions of a hydrocarbon backbone (1,4- and
1,5-migrations, Scheme 1).2 These rearrangement processes
Scheme 1. Intramolecular C−H Bond Activation via
Metal−Hydrogen Shifts
allow functionalization of C−H bonds that are difficult to activate
directly. Catalytic 1,4-rhodium migration was first reported in
2000 by Miura et al. in the reaction between arylboronic acids
and norbornenes.3a In the same year, Larock et al. reported the
first catalytic 1,4-palladium migration in coupling between aryl
iodides and alkynes.4a Since these pioneering studies, catalytic
1,4-migrations of various late transition-metal centers such
as Rh(I),3 Pd(II),4 Pt(II),5a,b and Ni(I)5c have been successfully
explored for selective functionalization of sp2 and sp3 C−H
bonds to form carbon−carbon and carbon−heteroatom bonds.
Several examples of catalytic 1,5-migrations have also been
reported for Rh(I)6a and Pd(II)6b−d intermediates.
We envisioned that the reaction between Rh(I) κ2-benzoates (2)
and pyridine would lead to the formation of pyridine-ligated
κ1-benzoate complexes (3). Indeed, we have observed clean
formation of 3a and 3b by 31P NMR (Scheme 2). The in situ
In contrast to the well-established 1,4- and 1,5-migrations,
other forms of metal/hydrogen shifts are very rare. In particular,
Received: September 1, 2013
Published: October 30, 2013
© 2013 American Chemical Society
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Communication
acid (1b) to form 2,4-dimethoxybenzoic acid (1c) (eq 1).
However, 1,3-dimethoxybenzene was formed as the major
Scheme 2. Synthesis and Thermal Transformation of
2,6-Disubstituted Rh(I) Benzoates
product by competitive protodecarboxylation. In com-
parison, a catalytic decarboxylative 1,4-addition13a of 1b with
t-butyl acrylate (6) was successfully carried out to give 1,3-migra-
tion product 8a in 71% yield and >20:1 selectivity over the
nonrearrangement product 7a (eq 2). This reaction was
promoted by 1.5 mol % [(cod)Rh(OH)]2, 3.0 mol % BINAP
(2,2′-bis(diphenylphosphino)-1,1′-binaphthyl), 1.0 equiv NaOH
additive, and 5:1 toluene/H2O mixed solvent at 120 °C.
Notably, this reaction occurred in good selectivity and without
the formation of corresponding Heck−Mizoroki olefination
products.12a,13a
formed 3a underwent quantitative decarboxylation that was con-
sistent with our previous observation.13a In sharp contrast,
thermolysis of in situ formed κ1-2,6-dimethoxybenzoate 3b at
120 °C in toluene did not generate the expected Rh(I) 2,6-
dimethoxyphenyl complex by decarboxylation.14 Instead, a
novel “1,3-carboxylate migration” appeared to occur, leading to
the formation of κ1-2,4-dimethoxybenzoate 4b in 34% yield as
the only detectable Rh(I) species by 31P NMR analysis.
Interestingly, the yield of 4b was improved to 71% when the
thermolysis was carried out under 1 atm of CO2 instead of N2.
Structures of isolated 2b, 3b, and 4b were determined by single
crystal X-ray diffraction (details in SI). In the solid state, the
chelating carboxylato ligand in 2b led to a significantly distorted
square planar geometry. In comparison, 3b and 4b adopt near
square-planar geometry with monodentate carboxylato ligands.
Based on the yield improvement of 4b under CO2 atmo-
sphere, we propose a multistep pathway for the 1,3-carboxyl
migration as described in Scheme 3. Decarboxylation of 3b was
We have considered several possible pathways for the pro-
posed 1,3-Rh migration with arylrhodium(I) intermediate 5a
in decarboxylative transformations of 1b (Scheme 4). A direct
Scheme 4. Proposed Pathways for 1,3-Rh Migration
Scheme 3. Proposed Pathway for Isomerization of Rh(I)
Carboxylates 3b To Form 4b
expected to generate a Rh(I) 2,6-dimethoxyphenyl inter-
mediate 5a,14 which underwent rearrangement by 1,3-Rh/H shift
(1,3-Rh migration) to form Rh(I) 2,4-dimethoxyphenyl com-
plex 5b. With the reduced steric crowding around Rh center in
5b compared to 5a, the decarboxylation/carboxylation thermody-
namics was shifted to favor CO2 insertion into the Rh-aryl
linkage15 to give carboxylation product 4b as the most stable
Rh(I) species in the reaction system. With lower CO2 concentra-
tion in a non-CO2 atmosphere, 5b underwent competitive pro-
tonation of the Rh−C bond to generate 1,3-dimethoxybenzene
that was detected as the major byproduct.
1,3-Rh/H shift (path A) requires a 4-membered cyclometalated
Rh(III) hydrido intermediate A or a σ-bond metathesis
transition-state B.2 Both structures would be extremely strained
due to the inherent aromatic planarity and rigidity, making this
pathway a highly unlikely scenario.7 Path B involves protona-
tion of the Rh−C bond in 5a by hydrolysis to form 1,3-
dimethoxybenzene (9a) and a Rh(I) hydroxo intermediate. 5b
is then formed via aromatic C−H bond activation of 9a by
Rh(I) hydroxide,16 with the regioselectivity determined by
ortho- and para-directing methoxy groups in an electrophilic
aromatic substitution (SEAr) mechanism. In path C, 5a
undergoes cyclometalation to activate a methoxy sp3 C−H
bond at the ortho position and forms a Rh(III) hydrido
intermediate C.9 Subsequent C−H reductive elimination
at the original ipso position generates a Rh(I) aryloxyalkyl
We envisioned that the proposed 1,3-Rh migration could be
exploited catalytically to give novel rearrangement products. For
example, the 1,3-carboxyl migration of 3b (Scheme 2) could pro-
ceed catalytically to allow isomerization of 2,6-dimethoxybenzoic
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intermediate D, which undergoes further aromatic C−H bond
activation at the less hindered meta position to form another
cyclometalated Rh(III) intermediate E. E then undergoes C−H
reductive elimination at the methoxy position to form 5b.
Notably, the proposed transformations of 5a→D and D→5b
represent formal 1,4-Rh migrations and could also occur by
single-step σ-bond metathesis and without involvement of
Rh(III) hydrido intermediates.2 In all three possible path-
ways, the individual steps are possibly reversible and the
driving force for formation of 5b over 5a is most likely the
partly relieved steric hindrance with mono- vs dimethoxy
groups at ortho positions.
To evaluate the feasibility of path B, we have attempted coupl-
ing reaction with t-butyl acrylate (6) using 1,3-dimethox-
ybenzene (9a) in place of 1b under catalytic conditions shown
in eq 2. No reaction was observed, and 9a was fully recovered,
which strongly argues against path B. Regarding path C, our
efforts toward a direct observation of the proposed stoichio-
metric transformations were hampered by failed attempts for an
independent synthesis of intermediate 5a. However, the
proposed intramolecular transfer of H atoms (Ha and Hb)
provides a suitable target for deuterium labeling studies.3b,c,4g,h
Thus, path C was further evaluated by a catalytic deuterium
transfer process described below, using a modified procedure of
Rh(I)-catalyzed protodecarboxylation previously reported by
our group (Scheme 5).13b,17
derivatives of 2,6-dimethoxybenzoic acid (1b), and both results
supported the proposed intramolecular H-atom transfers by
path C: (1) Substrate d6-1b (fully deuterium-labeled methoxy
groups) underwent intramolecular deuterium transfer from a
OCD3 group to the original ipso position, forming hydrodecar-
boxylation product 9c in 67% yield. This result was consistent
with the proposed (ipso)aryl/methoxy 1,4-Rh/H shift in path C
(Scheme 4, 5a→D). (2) Substrate 3,5-d2-1b (deuterium label-
ing at both meta positions relative to the carboxyl group)
underwent deuterium transfer from one of the meta positions
to the nearby methoxy group, forming hydrodecarboxylation
product 9d in 59% yield. This result was consistent with the
proposed methoxy/(meta)aryl 1,4-Rh/H shift in path C
(Scheme 4, D→5b). It is noteworthy that the individual steps
of 5a→D and D→5b have been reported for Pd(II)-catalyzed
rearrangement processes by aryl-to-alkyl4d,h,i and alkyl-to-aryl4c
1,4-Pd migrations, respectively. However, a formal 1,3-migration
by two consecutive 1,4-migrations has not been reported. The
highly selective formation of 9b suggested that both steps of
1,4-migration were impressively rapid processes that effectively
prevented competitive protonation of intermediates 5a or D,
which would allow incorporation of external deuteriums at
ortho and methoxy positions. In addition, catalytic hydrodecar-
boxylation of 2,6-diethoxybenzoic acid (10) in toluene/D2O
did lead to exclusive ipso-deuteration to form 2-d-1,3-dieth-
oxybenzene (11) as the only detectable product. Thus, the
target 1,3-Rh migration process appears to rely on a delicate
balance on steric effects of the ortho-substituents: significant
steric crowding (OMe vs F) is needed to slow down ipso-
functionalization and promote rapid, consecutive Rh/H shifts,
whereas too much steric crowding (OEt vs OMe) inhibits the
first Rh/H shift step and shuts down the overall migration
process.
Scheme 5. Deuterium-Labeling Study on Rh(I)-Catalyzed
Hydrodecarboxylation
Based on the proposed mechanism, we envisioned that 1,3-
Rh migration is not limited to decarboxylation process and
could occur with analogous Rh(I) aryl species generated by
other transformations. Indeed, preliminary results showed that
methoxy-directed 1,3-migration also occurred in Rh(I)-
catalyzed coupling of arylboronic acids with olefins (eq 3),
where arylrhodium(I) species were formed by B-to-Rh
transmetalation.18,19 A catalyst system of [(cod)Rh(OH)]2
precursor and racemic BINAP ligand promoted the reaction
between 2,6-dimethoxyphenylboronic acid (12) and t-butyl
acrylate (6) at 120 °C to form a mixture of ipso products (7a,
7b) and meta products (8a, 8b) by competitive 1,4-addition
and Heck−Mizoroki olefination. The tandem 1,3-migration/
1,4-addition product 8a was isolated as the major component in
45% yield. Despite the moderate selectivity, this result serves as
a proof-of-concept for methoxy-directed 1,3-Rh migration in
general coupling reactions that may be exploited for site-
selective arene functionalization.
Protodecarboxyation of 2,6-dimethoxybenzoic acid (1b) was
effectively promoted by a catalyst system of 1.5 mol % [(cod)-
Rh(OH)]2, 3.0 mol % DPPP ligand (1,2-bis(diphenyl-phosphino)-
propane), 1 equiv of Na2CO3 additive in 6:1 toluene/H2O at
120 °C to give 1,3-dimethoxybenzene (9a) in 64% isolated
yield. Using D2O in place of H2O in the solvent system led to
the exclusive formation of 4-d-1,3-dimethoxybenzene (9b) in
61% yield. Such regioselective deuterium incorporation confirmed
the involvement of 1,3-Rh migration to form intermediate 5b
(Scheme 3), which underwent subsequent deuteration of the
Rh-aryl bond with D2O. The catalytic protodecarboxylation
was then studied with two site-selective deuterium-labeled
In summary, we report a novel 1,3-migration of rhodium that
was demonstrated in several stoichiometric and catalytic
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Huang, Q.; Zhao, J.; Jenks, W. S.; Larock, R. C. J. Am. Chem. Soc. 2007,
129, 6298. (h) Kesharwani, T.; Larock, R. C. Tetrahedron 2008, 64,
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50, 8647.
(5) Selected studies on 1,4-migration of other transition metals: (a)
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(b) (Pt) Crosby, S. H.; Clarkson, G. J.; Rourke, J. P. J. Am. Chem. Soc.
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Am. Chem. Soc. 2007, 129, 810.
isomerization processes involving proposed Rh(I) 2,6-dime-
thoxyphenyl intermediates. Mechanistic results from a
deuterium-labeling study support a highly unusual, “consecutive
1,4-migration” pathway via sp3 C−H bond activation of the
methoxy group. With ongoing studies on further mechanistic
details, we aim to better understand structure−reactivity
correlations in this novel isomerization process and seek
broader applications in synthetic chemistry.
(6) Representative studies on 1,5-Rh and Pd migrations: (a) Tobisu,
M.; Hyodo, I.; Onoe, M.; Chatani, N. Chem. Commun. 2008, 6013.
(b) Bour, C.; Suffert, J. Org. Lett. 2005, 7, 653. (c) Mota, A. J.; Dedieu,
A.; Bour, C.; Suffert, J. J. Am. Chem. Soc. 2005, 127, 7171. (d) Mota, A.
J.; Dedieu, A. J. Org. Chem. 2007, 72, 9669.
(7) Mota, A. J.; Dedieu, A. Inorg. Chem. 2009, 48, 11131.
(8) Organotransition Metal Chemistry: From Bonding to Catalysis;
Hartwig, J. F., Eds.; University Science Books: Herndon, VA, 2010,
Section 3.5.
(9) Pd-catalyzed C-H activation of methoxy groups by aryl-to-
methoxy 1,4-migration: (a) Dyker, G. Angew. Chem., Int. Ed. 1992, 31,
1023. (b) Dyker, G. J. Org. Chem. 1993, 58, 6426.
(10) Leading reviews on transition-metal-catalyzed decarboxylation:
(a) Gooßen, L. J.; Rodríguez, N.; Gooßen, K. Angew. Chem., Int. Ed.
2008, 47, 3100. (b) Rodríguez, N.; Gooßen, L. J. Chem. Soc. Rev. 2011,
40, 5030.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
szm@ciac.ac.cn
■
pinjing.zhao@ndsu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(11) Transition metal-catalyzed decarboxylative cross-coupling was
pioneered almost a half-century ago by Nilsson et al. with studies on
Cu(I)-catalyzed decarboxylative biaryl synthesis using arene- and
heteroarenecarboxylic acids and aryl iodides: (a) Nilsson, M. Acta
Chem. Scand. 1966, 20, 423. (b) Nilsson, M.; Ullenius, C. Acta Chem.
Scand. 1968, 22, 1998.
(12) Representative recent studies on transition-metal-catalyzed
decarboxylative couplings for C−C bond formation: (a) Pd(II)-
catalyzed decarboxylative Heck−Mizoroki olefination by Myers et al.:
Myers, A. G.; Tanaka, D.; Mannion, M. R. J. Am. Chem. Soc. 2002, 124,
11250. (b) Pd(II)-catalyzed decarboxylative biaryl synthesis by
Gooßen et al.: Gooßen, L. J.; Deng, G.; Levy, L. M. Science 2006, 313,
662.
(13) (a) Sun, Z.-M.; Zhao, P. Angew. Chem., Int. Ed. 2009, 48, 6726.
(b) Sun, Z.-M.; Zhang, J.; Zhao, P. Org. Lett. 2010, 12, 992.
(14) An Au(I) 2,6-dimethoxyphenyl complex with IPr carbene ligand
has been reported to form by stoichiometric decarboxylation of 2,6-
dimethoxybenzoic acid: Dupuy, S.; Lazreg, F.; Slawin, A. M. Z.; Cazin,
C. S. J.; Nolan, S. P. Chem. Commun. 2011, 47, 5455.
(15) Leading studies on stoichiometric and catalytic carboxylation of
Rh(I) aryl species: (a) Darensbourg, D. J.; Groetsch, G.; Wiegreffe, P.;
Rheingold, A. L. Inorg. Chem. 1987, 26, 3827. (b) Ukai, K.; Aoki, M.;
Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706.
(16) Examples of Rh(I) hydroxide-mediated stoichiometric and
catalytic C−H bond activation: (a) Kloek, S. M.; Heinekey, D. M.;
Goldberg, K. I. Angew. Chem., Int. Ed. 2007, 46, 4736. (b) Sun, Z.-M.;
Zhang, J.; Manan, R. S.; Zhao, P. J. Am. Chem. Soc. 2010, 132, 6935.
(17) Recent report on Ag- and Cu-catalyzed decarboxylative
deuteration, see: Rudzki, M.; Alcalde-Aragones, A.; Dzik, W. I.;
Rodriguez, N.; Gooßen, L. J. Synthesis 2012, 44, 184.
■
This work was supported by ND EPSCoR (EPS-0447679), the
NSF (CHE-1301409 to P.Z.), and the NIH National Center for
Research Resources (grant 2P20 RR015566). Z.-M.S. thanks
Nature Science Fund of China (21171162), Jilin Province
Youth Foundation (20130522132JH), and SRF for ROCS
(State Education Ministry of China) for financial support. We
also thank NSF-CRIF (CHE-0946990) for funding the
purchase of departmental XRD instrumentation.
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