Rhodium(III)-catalyzed N-nitroso-directed C-H olefination of arenes. High-yield, versatile coupling under mild conditions

Article abstract of DOI:10.1021/ja3099245

N-Nitroso compounds are a versatile class of organic structures that allow expedient access to a diversity of synthetically useful architectures through demonstrated reactivities. We report herein the development of a Rh(III)-catalyzed N-nitroso-directed

Full text of DOI:10.1021/ja3099245

Article
pubs.acs.org/JACS
Rhodium(III)-Catalyzed N‑Nitroso-Directed C−H Olefination of
Arenes. High-Yield, Versatile Coupling under Mild Conditions
Baoqing Liu, Yang Fan, Yang Gao, Chao Sun, Cheng Xu, and Jin Zhu*
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of
Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China
S
* Supporting Information
ABSTRACT: N-Nitroso compounds are a versatile class of
organic structures that allow expedient access to a diversity of
synthetically useful architectures through demonstrated
reactivities. We report herein the development of a Rh(III)-
catalyzed N-nitroso-directed methodology for the ortho-
olefination of arenes. The heightened reactivity endowed by
the N-nitroso group translates to mild reaction conditions,
high reaction yields, and synthetic compatibility of otherwise
elusive substrates (e.g., an unactivated olefin, 1-octene).
Comprehensive mechanistic studies on the electronic effect,
deuterium exchange, kinetic isotope effect, kinetic profile, and numerous Rh(III) complexes have established [RhCp*]2+ as the
catalyst resting state, electrophilic C−H activation as the turnover-limiting step, and a five-membered rhodacycle as a catalytically
competent intermediate. The ability to elaborate the N-nitroso moiety to an amine functionality provides a seminal example of
the innumerable synthetic possibilities offered by this transformable directing group.
stabilized α-carbanion intermediates, an umpolung reactivity
compared to the parent amines),11 converted to diazonium
salts (extremely useful reagents for further functionalization
through Sandmeyer reactions, Schiemann reaction, hydrox-
ylation reaction, etc.),12 and rearranged to C-nitroso com-
pounds (Fischer−Hepp rearrangement).11 With an electron-
withdrawing group tethered (e.g., N-nitrosamides, N-nitro-
soureas, N-nitrosoguanidines, and N-nitrosocarbamates),13
distinct chemical transformations are also possible (e.g.,
conversion of N-nitrosamides to esters14). Importantly, N-
nitrosamines are known to interact with metal centers in a
variety of coordination modes (all involving the nitroso group
because of localized high-lying filled orbitals), including η1-O-
binding, η1-N-binding, bridged μ-η1:η1-N,O-binding, and cyclo-
metalated η2-N,C-binding.15 These diversified interaction
patterns should in principle allow regulation of chemical
reactivities of metalated intermediates in the target catalytic
cycle and therefore achievement of desired catalytic efficiency
in the respective transformation.3b Herein, we report on the
development of an N-nitroso directing group method for
Rh(III)-catalyzed C−H olefination of arenes. The utility of the
N-nitroso group has enabled the achievement of heightened
reactivity, which translates to mild conditions, high yields, and
synthetic versatility (broad substrate scope). Comprehensive
mechanistic studies have allowed the set of elementary steps to
be elucidated, thus providing key insights into this N-nitroso-
derived handy yet powerful synthetic system. In addition, as an
INTRODUCTION
■
Catalytic C−H functionalization has emerged as a valued atom-
and step-economical strategy for the elaboration of organic
molecules.1 In particular, transition-metal-based systems have
been the key driver of synthetic tool development by virtue of
their diversified reaction manifolds.2,3 In this regard, the
directing group approach is uniquely powerful for site-selective
C−H activation through docking-imparted high effective
molarity and therefore enables the kinetic bias-driven achieve-
ment of a desired reaction course.3 An illustrative example that
showcases the synthetic utility of such a regiocontrolled
approach is C−H olefination of arenes (oxidative Heck
coupling or the Fujiwara−Moritani reaction).4 Indeed, many
directing groups (e.g., amide,5 pyrazole,6 imine,7 ketone,8
carboxylic acid,9 and ester10) have been identified as effective
handles for the proximal site coupling of C−C bonds. However,
in spite of tremendous progress, further development is in
demand because essentially all the systems reported thus far are
synthetically restrictive, as, for example, manifested in the
following aspects: harsh reaction conditions (e.g., high reaction
temperature and highly acidic medium), challenge with respect
to the removal/transformation of undesired external directing
group, and limited substrate scope.
In search for a directing group system with desired synthetic
features, we have turned our attention to the N-nitroso moiety.
N-Nitroso compounds are a versatile class of organic structures
that allow expedient access to a diversity of synthetically useful
architectures through demonstrated reactivities.11 For example,
N-nitrosamines could be reduced to hydrazines and amines,11
derivatized on α-carbons (electrophilic substitution through
Received: October 12, 2012
Published: December 11, 2012
© 2012 American Chemical Society
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Article
initial proof of directing group transformation, highly efficient
denitrosation has been demonstrated under conditions that
allow for either the retention or reduction of installed alkene
motif.
3−5, Table 1), a room temperature (rt) reaction in MeOH
(entries 6−8, Table 1) allows the achievement of a 92% isolated
yield for 3a. Note that the reaction time should be stringently
controlled because of the susceptibility of the product toward
further ortho-olefination. A control experiment indicates that no
olefination could be observed for N-ethylaniline under
otherwise identical reaction conditions, highlighting the
importance of the N-nitroso group.
RESULTS AND DISCUSSION
■
Reaction Development. Since anilines represent impor-
tant structural motifs/synthetic precursors in dye and
pharmaceutical industries, N-nitrosoanilines were selected for
reaction development. In particular, the first set of experiments
was aimed at the examination of the reaction between model
substrates N-ethyl-N-nitrosoaniline (1a) and methyl acrylate
(2a) using [RhCp*Cl2]2 as the catalyst precursor. A hypo-
thetically viable turnover of the Rh(III)/Rh(I) redox catalytic
cycle likely entails the participation of an oxidant (for
regeneration of Rh(III)), a base (for assistance in C−H bond
cleavage), and a metathesis reagent/chloride abstractor (for
enhancement of electrophilicity of Rh(III)). Initial screening of
the reaction conditions in the absence of AgSbF6 only led to the
low conversion of 1a (<20%, entries 1−4, Table S1, Supporting
Information) and low-yield production of a denitrosated
molecule dn-3a. With the participation of AgSbF6, the
conversion of 1a became substantially higher (>90%), but
still, dn-3a was predominantly formed in low yield under most
of the evaluated conditions (entries 5−9, Table S1, Supporting
Information). Only in one case (entry 10, Table S1, Supporting
Information), a nitroso-retained, ortho-olefinated molecule 3a
(E isomer) was furnished as the major product. Note that, in 1a
and 3a, as well as in other N-nitrosamines, contribution from
the polar resonance form of the N-nitroso group results in
hindered rotation around the N−N bond and therefore
potential existence of syn (defined here when the N-alkyl
group is cis to the N-nitroso oxygen atom) and anti isomers
(Scheme S1, Supporting Information).16 The relative ratio of
the two isomers is thermodynamically dictated exclusively by
their inherent stabilities. The equilibrium of two isomers will
not affect the synthetic utility of the olefination reaction
reported herein because typically the products after subsequent
N-nitroso transformations are the authentic synthetic targets.
Successful olefination of 1a with 2a provides the basis for
further optimization of the reaction conditions (Table 1). The
reaction proceeds optimally in the presence of both AgSbF6 and
AgOAc. Thus, whereas DMF, MeCN, and toluene are all
suitable solvents to effect the desired transformation (entries
Synthetic Scope. With a viable olefination reaction in
hand, we next examined the scope of N-nitrosoanilines by
employing 2a as the coupling partner (Table 2). A wide range
a b
,
Table 2. N-Nitrosamine Scope
a
b
Conditions: N-nitrosamine (1 equiv), olefin (1.5 equiv). Isolated
c
yields. The syn and anti notations refer to the isomers resulting from
the restricted rotation around the N-nitroso N−N bond; syn: when the
N-alkyl group (except for 3d, N-phenyl group) is cis to the N-nitroso
d
oxygen atom. For N-nitrosamine substrates, all exist predominantly in
the syn form with three notable exceptions (1b, syn/anti = 1:0.57; 1c,
e
anti; 1f, syn/anti = 1:0.1). All the products are in the E form with
a b
,
respect to the alkene group except 3g (E/Z = 1:0.08). All the syn/anti
Table 1. Reaction Development
f
g
ratios in the products are listed in the table. 3c is in the anti form. 3d
is in the syn form.
of N-nitrosoaniline derivatives are compatible with the
protocol, furnishing the majority of products (E isomers) in
high yields under mild synthetic conditions (rt to 35 °C).
Regardless of the electronic nature of the aromatic substituent,
olefination proceeds selectively by N-nitroso-directed ortho
reactivity. Thus, the reaction works very well for N-methyl-N-
nitrosoanilines possessing either electron-donating methyl and
methoxy groups (1g, 1k, 1h, and 1m) or electron-withdrawing
halogen atoms (1e, 1i, 1l, 1f, 1n, and 1o). The tolerance of the
bromo substituent by our synthetic scheme (1o) diverges from
the reaction involving a Pd(II)/Pd(0) catalytic cycle,17 thus
offering a valuable handle for further synthetic elaborations.
The nitro group is also tolerated (1j), albeit affording the
product in a reduced yield. Substrates bearing m-methyl and m-
entry
additive
solvent
temp
yield
1
2
3
4
5
6
7
8
Cu(OAc)2·H2O
AgSbF6/Cu(OAc)2·H2O
AgSbF6/AgOAc
AgSbF6/AgOAc
AgSbF6/AgOAc
AgSbF6/AgOAc
AgSbF6/AgOAc
AgSbF6/AgOAc
MeOH
MeOH
DMF
60 °C
60 °C
100 °C
60 °C
80 °C
60 °C
30 °C
rt
68%
78%
83%
84%
68%
86%
88%
92%
MeCN
toluene
MeOH
MeOH
MeOH
a
Conditions: N-nitrosamine (1 equiv), olefin (1.5 equiv), all the
additives (2.2 equiv) except AgSbF6 (8 mol%). Isolated yields.
b
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Article
fluoro substituents provide the respective products in a
regioselective manner (1k and 1f). Regiocontrolled olefination
is not observed for m-methoxy-, m-chloro-, and m-bromo-
derivatized substrates (1m, 1n, and 1o). Complications in
olefination patterns for meta-substituted N-nitrosoanilines
reflect a subtle interplay between electronic (e.g., as a secondary
chelation site18) and steric effects exerted by the meta-
substituents. An array of N-alkyl substituents is tolerated,
with the bulkiness of the substituent dictating the reactivity.
Thus, substrates containing ethyl and isopropyl groups (1a and
1b) react equally well, but a tert-butyl group (1c) significantly
retards the reaction. The product yield is also low for N-phenyl-
N-nitrosoaniline (1d), due to competitive multiple-olefination
side reactions.
nitroso-based synthetic method has prompted us to gain a
detailed understanding of the reaction mechanism. The role of
the electronic effect was probed by performing a competition
olefination experiment in an equimolar mixture of electronically
different N-nitrosoaniline derivatives, 1m and 1l (eq S1,
Supporting Information). The electron-rich 1m is more
reactive, suggesting that electrophilic aromatic substitution
(EAS), rather than concerted metalation−deprotonation
(CMD), is responsible for C−H activation.2g We next
performed deuterium labeling experiments to further inves-
tigate the catalytic C−H activation and subsequent steps.
Through studies on various substrates (1a, 1g, 1h, 1m, 1l, and
1i) (eqs S2−S16, Supporting Information), three scenarios
apparently emerged depending on the electronic property of
the aromatic substituent: irreversible ortho-rhodation (electron-
withdrawing, 1i); quasi-irreversible ortho-rhodation (1a); and
reversible ortho-rhodation followed by irreversible migratory
insertion (electron-donating, 1m). Thus, in the absence of
olefins, although virtually no deuterium was incorporated into
1i under either catalytic (eq S11, Supporting Information) or
stoichiometric (eq S12, Supporting Information) conditions,
deuterium could be apparently identified in 1a through
prolonged catalysis (eq S6, Supporting Information) or under
stoichiometric conditions (eq S13, Supporting Information).
For 1m, deuterium labeling could be observed only in the
absence of 2a (eqs S9 and S15, Supporting Information); in the
presence of such a coupling partner, no deuterium incorpo-
ration in either the products or the remaining 1m was
discernible (eq S16, Supporting Information). The intermo-
lecular kinetic isotope effect (KIE) was measured to be 4.5 (eq
S17, Supporting Information), consistent with the turnover-
limiting C−H activation step.20
We next proceeded to establish the scope of the olefin
coupling partner by reacting with 1a (Table 3). A variety of
a b
,
Table 3. Olefin Scope
Rh(III) Complexes and Catalytically Competent
Rhodacycle. We further sought to probe the nature of the
C−H activation process by the isolation of a catalytically
competent rhodacycle. Thus, 1i reacts stoichiometically with
the Rh(III) catalytic species to form a complex c1 (with weakly
coordinated HOAc, Scheme 1 and eq S18, Supporting
a
b
Scheme 1. Synthesis and Interconversions of Rh(III)
Complexes
Conditions: N-nitrosamine (1 equiv), olefin (1.5 equiv). Isolated
c
yields. The products are in the E form with respect to the alkene
group for 3p−3s and 3u. 3vb is assigned as the E isomer based on the
nuclear Overhauser effect spectroscopy experiment.
d
monosubstituted olefins, ethyl acrylate (2p), styrene (2q), p-F-
styrene (2r), and p-methoxystyrene (2s) are suitable substrates
for the reaction. 1,1-Disubstitution could also be tolerated, and
with the engagement of methyl methacrylate (2t), a product
bearing an alkene group not in conjugation with the aromatic
ring is generated. Most significantly, an unactivated olefin, for
example, 1-octene (2u), could be employed in the trans-
formation to afford a nonconjugated product. β-Hydride
elimination from the benzylic hydrogen atoms is apparently
disfavored for the reactions involving 2t and 2u owing to the
restriction of conformation. Therefore, our synthetic protocol
enables the construction of distinct olefinated products that
could not be accessed by the conventional Heck-type
methods.19 For a 1,2-disubstituted olefin, dimethyl maleate
(2v), an additional amide bond is formed, allowing the
generation of two ring-closed isomeric products in good
combined yield.
Information). The five-membered rhodacycle structure of c1
was unambiguously determined by single-crystal analysis of a
chloride complex (c2) (Scheme 1, eq S19, and Figure S19-2,
Supporting Information). Complex c2 could be facilely
converted back to complex c1 by the abstraction of chloride
(Scheme 1 and eq S20, Supporting Information). In addition, a
variety of transformations are possible starting from c2
(Scheme 1 and eqs S21−24, Supporting Information), leading
to c3 (with weakly coordinated CD3OD) and c4 (with strongly
Electronic Effect, Deuterium Exchange, and Kinetic
Isotope Effect. The broad substrate scope offered by our N-
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coordinated OAc−). Among those complexes, c1 could react
stoichiometrically with 2a at rt to afford 3a in high yield within
5 min (eq 1). This is substantially faster than the c1 formation
Scheme 2. Proposed Catalytic Cycle
coordination of OAc− (zero-order kinetics on AgOAc because
of the essentially constant supply of a fixed concentration of
OAc− by the precipitate) and N-nitrosoaniline derivative,
turnover-limiting electrophilic ortho-rhodation (C−H bond
cleavage assisted by OAc− and olefin), migratory insertion of
olefin, β-hydride elimination and reductive elimination to
release olefinated product and Rh(I), and reoxidation of Rh(I)
to Rh(III) by Ag+ to close the catalytic cycle. The slower
catalytic reaction rate in the presence of excess NaOAc is
consistent with the intermediacy of [RhCp*(OAc)]+ (with or
without weakly coordinated neutral species), rather than
RhCp*(OAc)2, in the catalytic cycle. The ortho-rhodation
step is facilitated by olefin (positive-order kinetics before
saturation) by virtue of its role in the displacement of HOAc.
The saturation kinetics for olefin could be rationalized by either
the thermodynamically uphill formation of a low-concentration
metalated intermediate right prior to the turnover-limiting step
and/or the accumulation of an off-cycle olefin-coordinated
(e.g., with catalyst resting state) species reservoir.
reaction, which provides further support for the proposal of C−
H activation as the turnover-limiting step. c1 and c3 are also
capable of effecting the catalytic olefinatio with an approx-
imately identical efficiency (eqs 2, 3, and S25, Supporting
Information), suggesting the intermediacy of these complexes
(with or without weakly coordinated neutral species) in the
catalytic cycle.
Kinetic Profile and Catalyst Resting State. Kinetic
studies were then performed for the establishment of a
plausible set of elementary steps in the catalytic cycle. The
reaction is first order in [RhCp*Cl2]2, first order in AgSbF6,
zero order in AgOAc, first order in the N-nitrosoaniline
derivative (1a), and exhibits a saturation kinetics for olefin (2a)
(Figures S27-1 to S31-2, Supporting Information, and eq 4):
1
1
1
rate = k·[[RhCp*Cl2]2] ·[AgSbF] ·[AgOAc]0·[1a] ·[2a]s
6
(4)
Importantly, during the kinetic studies, a high-concentration,
persistent Rh(III)-based species (δ ∼1.688 for Cp*, Figure S32,
Supporting Information; the exact chemical shift is dependent
on ionic strength and therefore slightly varied during a catalytic
run, Figures S32 and S33, Supporting Information) was
Transformation of Directing Group. As a further check
of the synthetic versatility associated with our protocol, we next
examined the amenability of the N-nitroso group to further
elaborations. Indeed, the N-nitroso group could be transformed
to an amine group highly efficiently under either mild CuCl/
HCl,22 NiCl2·6H2O/NaBH4,23 Fe(CO)5,24 or more forced
1
identified through H NMR in the catalytic system. In accord
with the kinetic profile, and through an independent synthesis
of RhCp*(OAc)2 (c5)21 and study of its ligand dissociation
behavior in CD3OD (δ ∼1.668 for Cp* of [RhCp*]2+ without
coordinated OAc− but shifted to ∼1.537 for Cp* of [RhCp*]2+
with coordinated OAc−), the catalyst resting state is assigned as
[RhCp*]2+ (with or without weakly coordinated neutral species
but without coordinated OAc−) (Figures S34 and S35, eq S26,
and Tables S2 and S3, Supporting Information). The
assignment is further supported by titration studies of
[RhCp*Cl2]2/AgSbF6 with AgOAc (Figure S36, Supporting
Information) and NaOAc (Figure S37, Supporting Informa-
tion).
25
Raney Ni/H2 conditions (Tables 4 and 5). The CuCl/HCl
conditions privilege denitrosation as the exclusive mode of
reaction without affecting alkene and other functionalities,
including the nitro group (Table 4). Both NiCl2·6H2O/NaBH4
and Fe(CO)5 could in general be used to supplant CuCl/HCl
(Table 4), thus offering reaction condition versatility for varied
synthetic contexts. The Raney Ni/H2 conditions enable the
simultaneous reduction of both N-nitroso and alkene groups
(Table 5). Thus, ortho-alkylation is the ultimate observed
pattern of reaction for ester-free olefinated substrates. For ester-
containing substrates, further ring closure from intramolecular
secondary coupling occurs if sterically allowed. Halogen atoms
could be retained or removed depending on the substitution
feature.
Proposed Catalytic Mechanism. Taken together, the
above results point toward the following reaction sequence in
the catalytic cycle (Scheme 2): generation of catalyst resting
state [RhCp*]2+ by [RhCp*Cl2]2 and AgSbF6, reversible
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a
Table 4. Denitrosation of Olefinated N-Nitrosamines
Table 5. Simultaneous Denitrosation and Alkene Reduction
of Olefinated N-Nitrosamines
a
a
Isolated yields.
ASSOCIATED CONTENT
* Supporting Information
■
S
General methods and reaction development; synthesis and
characterization of N-nitrosamine substrates, olefination
products, denitrosation products, and Rh(III) complexes;
competition, deuterium exchange, kinetic isotope effect, kinetic,
and resting state assigment experiments; reversibility studies;
NMR spectra for all compounds; ORTEP drawing of c2; and
crystallographic data as a CIF file. This material is available free
of charge via the Internet at http://pubs.acs.org.
a
b
Isolated yields. A mixed solvent of THF/MeOH (10:1) instead of
c
pure THF is used for condition ii. Not efficient for the formation of
the depicted target product under condition iii. Not efficient for the
formation of the depicted target product under condition ii. Reaction
time: 5−10 min. Simultaneous removal of fluorine under condition iii,
affording 4riii (65%). Simultaneous reduction of alkene group under
condition ii, affording 4tii (80%) and 4uii (55%), respectively.
d
e
f
g
AUTHOR INFORMATION
■
Corresponding Author
jinz@nju.edu.cn
Notes
CONCLUSION
■
The authors declare no competing financial interest.
In summary, a mild, high-yield, and versatile Rh(III)-catalyzed
N-nitroso-directed aryl ortho-olefination method has been
developed. Notably, the unique coordination characteristics of
the N-nitroso group have enabled the synthetic utility of
otherwise elusive substrates (e.g., an unactivated olefin, 1-
octene). Comprehensive mechanistic studies support a reaction
pathway involving [RhCp*]2+ as the catalyst resting state and
electrophilic C−H activation as the turnover-limiting step. A
catalytically competent five-membered rhodacycle has been
structurally characterized, revealing a key intermediate in the
catalytic cycle. The ability to elaborate the N-nitroso moiety to
an amine functionality (with aniline derivative as the product)
provides a seminal example of the innumerable synthetic
possibilities offered by this transformable directing group.
ACKNOWLEDGMENTS
■
J.Z. gratefully acknowledges support from the National Natural
Science Foundation of China (21274058) and the National
Basic Research Program of China (2013CB922101,
2011CB935801)
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