Combination of Cp*RhIII-Catalyzed C?H Activation and a Wagner–Meerwein-Type Rearrangement

Article abstract of DOI:10.1002/anie.201610117

A combination of Cp*Rh^III-catalyzed C?H activation and Wagner–Meerwein-type rearrangement was successfully achieved for the first time. Thus, bridged polycyclic molecules that are not readily accessible by other means were accessed under mild c

Full text of DOI:10.1002/anie.201610117

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610117
German Edition: DOI: 10.1002/ange.201610117
À
C H Activation
III
À
Combination of Cp*Rh -Catalyzed C H Activation and a Wagner–
Meerwein-Type Rearrangement
Xiaoming Wang, Andreas Lerchen, Tobias Gensch, Tobias Knecht, Constantin G. Daniliuc, and
Frank Glorius*
III
À
À
Abstract: A combination of Cp*Rh -catalyzed C H activa-
catalyzed C H activation with an efficient rearrangement of
bicyclic systems would be a highly enabling technology for
reaction discovery.
tion and Wagner–Meerwein-type rearrangement was success-
fully achieved for the first time. Thus, bridged polycyclic
molecules that are not readily accessible by other means were
accessed under mild conditions with high efficiency (as low as
0.5 mol% Rh catalyst) in the coupling of N-phenoxyacetamide
with 7-azabenzonorbornadiene.
A well-recognized feature of bicyclic systems is their
propensity to undergo Wagner–Meerwein rearrangements,
which are induced by the formation of electron deficient/
cationic centers.[6] In connection with the advantages of
III
[2]
À
Cp*Rh -catalyzed C H functionalization and the knowl-
edge that the RhIII C bond generated by C H activation
À
À
I
n recent years, significant advances in transition-metal-
[1]
À
=
catalyzed C H bond functionalization have been achieved.
could add to the C C bond of 7-oxa/azabenzonorborna-
Specifically, Cp*RhIII-catalyzed C H activation/coupling
dienes, which was pioneered by Li,[5] it would be attractive to
À
À
reactions have been widely explored and enabled the devel-
opment of methods for the synthesis of various structurally
diverse organic molecules.[2] Using unsaturated bonds as
coupling partners in this field represents an important
strategy to construct new frameworks. For example, the
utilize this C H activated species in a Wagner–Meerwein-
type rearrangement using norbornadiene derivatives (Sche-
me 1b). However, several key challenges needed to be
addressed. First, the alkyl metal intermediate generally
prefers to undergo b-elimination. Second, as the metal
would be reduced in the rearrangement step, an appropriate
oxidation process for the metal is needed to complete the
catalytic cycle. Finally, a suitable nucleophile is required to
capture the carbocation resulting from the rearrangement.
Described herein is the successful coupling of N-phenox-
yacetamide and 7-azabenzonorbornadiene for the efficient
synthesis of bridged polycyclic molecules, which combines
À
À
addition of C Rh bonds, generated by C H activation, to
strained olefins in heterobicyclic systems is an important
process because of the utility of the resulting organometallic
compounds (Scheme 1).[3–5] Both b-elimination and protona-
tion of the resulting C Rh bond have been well studied in
Cp*Rh -catalyzed C H activation, giving ring-opened and
protonated products, respectively (Scheme 1a; Cp* = h-
C5Me5). However, rearrangements of this key intermediate,
leading to more-complex structural motifs, have not been
reported to date. Owing to the facile generation of new cores
through rearrangements, a combination of the Cp*RhIII-
À
III
À
À
C H activation and Wagner–Meerwein-type rearrangement
using a Cp*RhIII catalyst (Scheme 2). The oxidative directing
group[7] used herein plays several important roles, acting both
as an internal oxidant and as an intramolecular nucleophile
À
after the O N bond cleavage. Significantly, the present
methodology affords a highly efficient way to construct the
rearranged products[8] under mild reaction conditions with
a high efficiency.
For the initial reactivity study, N-phenoxyacetamide 1a
was selected as the substrate.[9] 7-Azabenzonorbornadiene 2a
was considered to be an excellent bicyclic starting material as
Scheme 1. Wagner–Meerwein-type rearrangement in Cp*RhIII-catalyzed
À
C H activation. A directing group is included in Ar-H.
[*] Dr. X. Wang, A. Lerchen, T. Gensch, T. Knecht, Dr. C. G. Daniliuc,
Prof. F. Glorius
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Scheme 2. RhIII-catalyzed coupling of N-phenoxyacetamide and 7-aza-
benzonorbornadiene. [Rh]=[Cp*Rh(CH3CN)3](SbF6)2, here and in the
other Schemes.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610117.
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Table 2: Scope of the 7-azabenzonorbornadienes.[a]
it is readily available and the N-atom could stabilize the
cation generated in the rearrangement (Scheme 2; for details,
see the Supporting Information). Using a catalyst system
consisting of [Cp*Rh(CH3CN)3](SbF6)2 (5 mol%) and
NaOAc (1.0 equiv) in CH3CN gave a promising 70% of the
product 3a. The structure of 3a was determined by single-
crystal X-ray diffraction,[16] which is consistent with the
involvement of a Wagner–Meerwein-type rearrangement in
the reaction sequence. Changing the solvent to CH2Cl2 led to
nearly quantitative yield. However, lowering the RhIII catalyst
loading to 0.5 mol% decreased the yield considerably owing
to low conversion. Next, a survey of possible additives showed
that the addition of AgOAc (20 mol%) was optimal, and the
product could be isolated in 82% yield, which highlights the
efficiency of the system.
With the optimized reaction conditions in hand, the
generality of this reaction was explored (Table 1, all cases
tested are shown).[10] To our delight, N-phenoxyacetamides
bearing both electron-withdrawing and electron-donating
substituents at different positions reacted smoothly with
substrate 2a. The corresponding products were isolated in 52–
95% yield using 0.5 mol% catalyst in majority of cases. For
unsymmetrical substrates, such as meta-substituted and meta,
para-disubstituted N-phenoxyacetamides, the reactions
[a] Reactions were carried out using 1a (0.24 mmol), 2 (0.2 mmol), [Rh]
(0.5 mol%), NaOAc (1.0 equiv), and AgOAc (20 mol%) in CH2Cl2
(0.5 mL) at 608C for 12 h. [b] 5.0 mol% catalyst loading.
dienes with a completely substituted benzene ring, as well as
benzannulated substrates were also all tolerated, affording
the corresponding products 3p–r. Moreover, changing the
protecting group to mesyl (Ms), and tert-butyloxycarbonyl
(Boc) gave the products 3s and 3t in good yield. However,
using the olefin with acetyl (Ac) on the N-atom only gave
trace product as a result of the low conversion. The reaction
with 7-oxabenzonorbornadiene as the partner gave a complex
reaction mixture, possibly owing to the reduced stabilization
of the rearranged cation by the oxygen rather than nitrogen
atom. Further, the tosyl group in 3a could be removed
smoothly, affording the product 4 in 75% yield [Eq. (1)].
À
showed excellent selectivity for the C H bond with less
steric hindrance, delivering products 3g–j in moderate to
good yields. Additionally, ortho-substituted and benzannu-
lated substrates were tolerated, giving the corresponding
products 3k and 3l.
On the basis of these results, we further investigated the
scope of the 7-azabenzonorbornadienes 2. As shown in
Table 2, 7-azabenzonorbornadienes with both electron-donat-
ing and electron-withdrawing groups on the arene performed
well, affording the corresponding products (3m–o) in yields
between 54% and 87%. In addition, 7-azabenzonorborna-
To gain insight into the mechanism, a series of experi-
ments was performed (Scheme 3). A stable cyclometalated
RhIII complex A was obtained upon treatment of substrate 1a
with [RhCp*Cl2]2. This species (2.0 mol%) was shown to be
an active catalyst for the coupling of 1a with 2a, giving 3a in
83% yield (Scheme 3a). However, a stoichiometric reaction
of complex A with 2a failed to afford any of the desired
product 3a, suggesting that product formation requires an
Table 1: Scope of the N-phenoxyacetamides.[a]
À
additional species generated from the C H activation step
(Scheme 3b). Indeed, the stoichiometric reaction with HOAc
as the additive afforded the product 3a in 71% GC yield.
These results suggested that the intermediate A is most likely
involved in the catalytic cycle, and that HOAc, generated in
the formation of rhodacycle, is essential for product forma-
tion. Next, 1a was subjected to the Rh-catalyzed reaction in
CH2Cl2/D2O (5:1) in the presence of 2a for a short time and
no deuterium incorporation was observed in either residual
1a or product 3a. This result illustrates that the cyclometa-
lation step is irreversible under the reaction conditions (see
the Supporting Information). No kinetic isotope effect was
observed (KIE, kH/kD = 1.0) for two separate parallel reac-
tions with 1a and 1a–d5 at room temperature, suggesting that
[a] Reactions were carried out using 1 (0.24 mmol), 2a (0.2 mmol), [Rh]
(0.5 mol%), NaOAc (1.0 equiv), and AgOAc (20 mol%) in CH2Cl2
(0.5 mL) at 608C for 12 h. [b] 2.0 or 5.0 mol% catalyst loading, see the
Supporting Information.
À
the C H bond cleavage is likely not involved in the rate-
determining step.[11] Moreover, the reaction of 1a with 2a was
2
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ment. Subsequently, an intramolecular attack of the pheno-
late to the iminium cation would occur to release the product
3a and regenerate the catalyst by a final ligand exchange. In
an alternative scenario, the Wagner–Meerwein-type rear-
rangement would occur directly from the RhIII species B to
afford the rearranged RhI intermediate G, followed by the
I
À
oxidiative addition of Rh to the O N bond in the presence of
HOAc to produce E (Scheme 4, Path b). Another possibility
is that the reaction proceeds through acyclic alkene carboa-
mination intermediate 5 (Scheme 3c), formed by the amido
group transfer from intermediate B or C,[15] followed by
a rearrangement with acetamide as a leaving group. Indeed,
trace amounts of 5 could be observed as a by-product in the
reaction. However, when 5 was submitted to the standard
reaction conditions, no desired product was formed (Sche-
me 3c). Thus, the Wagner–Meerwein-type rearrangement
most likely involves an organo-Rh species.
Scheme 3. Selected mechanistic studies.
tested in the presence of radical scavengers (TEMPO and
BHT), and the reactions still afforded the product 3a in 82%
and 95% yield, respectively, suggesting that the reaction is
most likely not based on a radical pathway.[6]
Natural population analyses of intermediates B, C, and D
by DFT computations revealed an increase in positive charge
at rhodium and a decrease in electron density at the rhodium-
bound carbon atom (see the Supporting Information). More-
On the basis of these observations and previous reports,[5]
a plausible catalytic cycle is shown in Scheme 4. The first step
is presumed to be a facile arene rhodation generating A with
concomitant formation of acetic acid, which is followed by the
insertion of alkene 2a to afford the seven-membered rhoda-
over, the C Rh bond is a nucleophilic species and b-
III
À
elimination or protonation of B could occur as was reported.[5]
On the basis of these considerations, we favor Path a with
a RhIII–RhV–RhIII process as a mechanistic model for our
current reaction as an electron-deficient metal center is more
likely to induce the Wagner–Meerwein-type rearrange-
ment.[6,14] According to this proposal, we concluded that
alternative nucleophiles may competitively attack the rear-
ranged iminium cation to capture the intermediate E. Using
methanol as the solvent, a by-product 6 was isolated in 26%
yield (Scheme 3d). The structure of 6 was confirmed by
XRD,[16] further supporting the involvement of the rearrange-
ment in the reaction.
À
cycle intermediate B. Then, a subsequent cleavage of the O
N bond by RhIII would lead to the RhV nitrene complex C
(Scheme 4; Path a). The formation of RhV nitrene complexes
has also been proposed in previous studies.[12,13] Based on the
importance of HOAc for the formation of the product
observed in our experiments and related DFT calculations
from Houk and Wu,[13a] the coordination of HOAc to the RhV
nitrene intermediate, generating the intermediate D is
proposed to be the next step. This high oxidation state RhV
intermediate D then undergoes rearrangement to form
intermediate E, in which rhodium is reduced to RhIII.[14]
Furthermore, the stabilization of the resulting carbocation
by the nitrogen lone pair and the release of the ring strain
probably also contribute to the driving force of the rearrange-
In summary, an efficient and conceptually new RhIII-
À
catalyzed combined C H activation/Wagner–Meerwein-type
rearrangement provides bridged polycyclic molecules that are
not readily accessible by other means. This mechanistic
proposal should be taken into consideration in the future
design of rhodium-catalyzed reactions involving bicyclic
systems. Further mechanistic studies and attempts to convert
the products of these reactions into targets of biological
interest are under investigation.
Acknowledgements
This work was generously supported the Alexander von
Humboldt Foundation (X.W.) and the Deutsche Forschungs-
gemeinschaft (F.G.). We are grateful to Frederik Sandfort for
experimental assistance. We thank Dr. Manuel van Gemme-
ren, Dr. Kathryn M. Chepiga, and Suhelen Vꢀsquez Cꢁspedes
for helpful discussions (all WWU Mꢂnster).
Conflict of interest
The authors declare no conflict of interest.
Scheme 4. Proposed reaction mechanism.
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Manuscript received: October 15, 2016
Final Article published: && &&, &&&&
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Communications
À
C H Activation
X. Wang, A. Lerchen, T. Gensch, T. Knecht,
C. G. Daniliuc,
F. Glorius*
&&&& — &&&&
III
À
Combination of Cp*Rh -Catalyzed C H
Activation and a Wagner–Meerwein-Type
Rearrangement
Nicely (re)arranged: A combination of
means were accessed under mild condi-
III
À
Cp*Rh -catalyzed C H activation and
tions with high efficiency (as low as
0.5 mol% Rh catalyst) in the coupling of
N-phenoxyacetamide with 7-azabenzo-
norbornadiene.
Wagner–Meerwein-type rearrangement
was successfully achieved for the first
time. Thus, bridged polycyclic molecules
that are not readily accessible by other
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5
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