C-H Activation-Based Traceless Synthesis via Electrophilic Removal of a Directing Group. Rhodium(III)-Catalyzed Entry into Indoles from N-Nitroso and α-Diazo-β-keto Compounds

Article abstract of DOI:10.1021/acs.orglett.6b00310

A distinct C-H activation-based traceless synthetic protocol via electrophilic removal of a directing group is reported, complementing the currently exclusively used nucleophilic strategy. Rh(III)-catalyzed, N-nitroso-directed C-H activation allows the development of a traceless, atom- and step-economic, cascade approach for the synthesis of indole skeletons, starting from readily available N-nitroso and α-diazo-β-keto compounds. Importantly, the cyclization/denitrosation reaction represents a hitherto unobserved reactivity pattern for the N-nitroso group.

Full text of DOI:10.1021/acs.orglett.6b00310

Letter
pubs.acs.org/OrgLett
C−H Activation-Based Traceless Synthesis via Electrophilic Removal
of a Directing Group. Rhodium(III)-Catalyzed Entry into Indoles from
N‑Nitroso and α‑Diazo-β-keto Compounds
Jie Wang,† Mingyang Wang,† Kehao Chen, Shanke Zha, Chao Song, 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: A distinct C−H activation-based traceless
synthetic protocol via electrophilic removal of a directing
group is reported, complementing the currently exclusively
used nucleophilic strategy. Rh(III)-catalyzed, N-nitroso-
directed C−H activation allows the development of a traceless,
atom- and step-economic, cascade approach for the synthesis
of indole skeletons, starting from readily available N-nitroso
and α-diazo-β-keto compounds. Importantly, the cyclization/
denitrosation reaction represents a hitherto unobserved
reactivity pattern for the N-nitroso group.
ransition-metal-catalyzed C−H activation (CHA)1 pro-
vides a handy tool for the synthesis of heterocycles. A
Scheme 1. Schematic of CHA-Based Traceless Synthesis
Strategy via Electrophilic Removal of Directing Group
a
T
directing group2 is typically critical in the control of
regioselectivity, but can be, in part or as a whole, undesired
for intended applications. An ideal synthetic system should
allow the generation of target product in a traceless,3 atom- and
step-economic manner under CHA conditions (either on-
cycle4 or off-cycle,5 one-pot/cascade6). Thus, far, essentially all
of the unwanted portion of the directing group is removed as a
nucleophile, with a proton acting as the formal partner
electrophile. Mechanistically, a typical traceless CHA-based
synthetic protocol utilizes a 1,2-migratory insertion-generated
intermediate for cyclization and the initially extruded proton
remains as a freely available electrophile until being sequestered
at a later stage, by the removed group (Scheme 1A). This
reaction mode has enabled the synthesis of a variety of
heterocycles but can impose significant restriction on the
substrate scope. We envisioned that an umpolung,7 comple-
mentary reaction mode, with the directing group acting as an
electrophile at the removal stage, could be achieved through
forward reactivity analysis (Scheme 1B). In particular, electro-
philic removal can be a viable strategy by achieving the
following prerequisites: (1) freely available electrophilic proton
is sequestered at an early stage, prior to cyclization, by the
CHA-installed group through a proto-demetalation process;
(2) polarity-matched partners are positioned in a regiochemi-
cally allowed manner; (3) a sufficiently labile bond exists in the
directing group; (4) cyclization can proceed in a ring-strain-free
fashion.
a
L: Linkage group. DU: Directing unit. CU: Capping unit. [M]:
Transition metal. CHA: C−H activation. NP: Nucleophile. EP:
Electrophile.
stance, the final C−N bond formation is accomplished by the
nucleophilic attack of Rh(III)-bound C atom toward the amino
N atom, accompanied by simultaneous trapping of the nitroso
group with electrophilic proton. This is, however, not a typical
reactivity pattern observed for the N-nitroso group (under off-
cycle, metal-coordination-free conditions), where the amino N
atom is nucleophilic and the nitroso N atom is electrophilic.
We expect that this polarity can be matched by a cyclization
partner containing an electrophilic C atom and a nucleophilic
heteroatom. Herein, we report an unprecedented CHA-based
We have recently initiated a research program on N-nitroso-
directed CHA reactions.8 This labile directing group has
allowed the traceless synthesis of indoles by using alkynes as
the coupling partner.8a,9 In this on-cycle cyclization circum-
Received: January 30, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
traceless synthetic protocol through electrophilic removal of
directing group. In particular, we wish to disclose a traceless
indole synthetic protocol by using N-nitroso and α-diazo-β-
keto compounds10 as the reaction partner. Although an α-
diazo-β-keto compound has been previously employed in the
synthesis of indoles,11 the directing group cannot be removed,
as expected, due to mismatched polarity and/or low bond
lability. The proposed CHA/carbenoid 1,1-migratory insertion/
proto-demetalation allows subsequent tautomerization and
generation of an enolization intermediate containing an
electrophilic alkenyl C atom and a nucleophilic hydroxyl
group. The matching of polarity from the N-nitroso group and
lability of the N−N bond provides the possibility of
simultaneous C−N cyclization and N−N bond cleavage.
Notably, this cyclization/denitrosation reaction mode has
never been observed for the N-nitroso group.
S2). No further improvement in the yield was observed with
the change of catalyst amount, reaction temperature, or
reaction time (Table S3). No reaction occurs in the absence
of either [Cp*RhCl2]2 or AgSbF6 (entries 12, 13), suggesting,
most likely, the participation of a cationic Rh(III) complex in
the catalytic cycle. The replacement of [Cp*RhCl2]2 by
[Cp*CoI2]2, [Ru(p-cymene)Cl2]2, or [Cp*IrCl2]2 under
otherwise identical conditions proves to be detrimental (entries
14−16).
With the optimized conditions established, the scope of N-
nitrosoanilines was then examined by employing 2 as the
coupling partner (Table 2). A variety of N-nitrosoaniline
a b
,
Table 2. Reaction Scope for N-Nitroanilines
We commenced the initial condition-screening experiments
by using N-nitrosoaniline 1a (0.4 mmol) and diazo compound
2 (0.6 mmol) as the model substrates. An initial test with
[Cp*RhCl2]2 (2 mol %) and AgSbF6 (8 mol %) furnished a
desired target cyclized indole derivative 3a in a promising 35%
yield after 5 h of reaction at 80 °C in MeOH (entry 1, Table 1).
ab
,
Table 1. Optimization of Reaction Conditions
additive
yield
(%)
a
Reaction conditions: 1a−1s (0.4 mmol), 2 (0.6 mmol), [Cp*RhCI2]2
entry
catalyst system
(mmol %)
solvent
(2 mol %), AgSbF6 (8 mol %), HOAc (20 mol %), DCE (2 mL).
b
1
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2/AgSbF6
[RhCp*Cl2]2
MeOH
DCE
THF
MeCN
DMF
DMSO
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
35
52
30
42
0
Isolated yields.
2
3
derivatives react smoothly to furnish the corresponding indole
derivatives in good to excellent yields. The bulkiness of the N-
alkyl substituent apparently exerts an effect on the reactivity.
Substrates containing methyl and ethyl groups (1a and 1b)
react equally well, and an isopropyl group (1c) affords a slightly
lower 61% yield. The reaction is also sensitive to the steric
features in the arenes. Both para and meta derivatives show
higher reactivity toward the sterically less hindered C−H bond.
For an ortho substitution, only small-sized fluorine (1q) allows
the generation of product in an acceptable 54% yield. The
electronic property of arene is another factor that can affect the
reaction outcome. An electron-rich substituent (Me, OMe)
shows a better reactivity compared with an electron-poor
substituent (halogen, CO2Me, NO2, phenyl). Thus, methyl and
methoxy groups (3g, 3h, 3o, and 3p) give the respective
product in excellent yields, ranging from 76% to 89%, whereas
halogen atoms (3d−3f and 3l−3n) and methyl ester (3i) afford
apparently lower yields. Significantly, a synthetically demanding
reaction can proceed for a substrate (3j) bearing a very
electron-withdrawing group (NO2). In addition, the reaction
also works well for N-nitroso-tetrahydroquinoline and gives the
product 3r in good yield. Subsequently, the scope of the diazo
compounds was investigated using N-nitrosoaniline 1a as one
of the substrates. As shown in Table 3, a diversified range of
indole derivatives could be synthesized in good to excellent
yields. Specifically, ethyl 2-diazo-3-oxo-3-phenylpropanoate
bearing electron-donating or -withdrawing groups at a variety
of different positions of the phenyl ring (2a−2o) react
smoothly with 1a to give the desired products 4a−4o (51−
88%). Methyl 3-alkyl-2-diazo-3-oxopropanoate (2p, 2q), 2-
4
5
6
0
7
AgOAc (100)
Na2CO3 (100)
NaOAc (100)
HOAc (100)
HOAc (20)
HOAc (20)
HOAc (20)
HOAc (20)
HOAc (20)
35
33
42
68
88
0
8
9
10
11
12
13
14
15
AgSbF6
0
[Cp*Col2]2/AgSbF6
[(p-cymene)RuCl2]2/
AgSbF6
20
0
16
[Cp*IrCl2]2/AgSbF6
HOAc (20)
DCE
50
a
Reaction conditions: 1a (0.4 mmol), 2 (0.6 mmol), [Cp*RhCI2]2 (2
mol %), AgSbF6 (8 mol %), additive, solvent (2 mL). Isolated yields.
b
The switching of solvent from MeOH (entries 1−6) to various
other reaction media proves DCE as the optimum solvent,
where the yield of 3a could be increased to 52% (entry 2).
Further screening of additives shows that acidic conditions are
more favored in promoting the transformation (entries 7−10).
Investigation on an array of representative acids proves HOAc
as the best additive. With the addition of 100 mol % HOAc, the
yield of 3a could be further increased to 68% (entry 10 and
Table S1). Systematic optimization with respect to the amount
of HOAc, within the 5−200 mol % range, enables the
pinpointing of its optimum quantity, 20 mol %, under which
conditions 3a was obtained in 88% yield (entry 11 and Table
B
DOI: 10.1021/acs.orglett.6b00310
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Organic Letters
Letter
a b
,
experiments were performed using substrates with different
electronic characteristics, and the electron-rich substrate (1h)
reacts 2.7 times faster than the electron-poor substrate (1d).
This is consistent with a slightly asynchronous C−H insertion
and an electrophilic aromatic substitution mechanism, with Ar−
Rh interaction, rather than the acidity of an ortho-C−H bond,
as the predominant factor dictating the efficiency of the CHA
step. The ability to stall the reaction at an intermediate stage
with a less reactive dimethyl 2-diazo malonate is consistent with
an initial CHA/carbenoid 1,1-migratory insertion/proto-
demetalation step and a subsequent separate off-cycle intra-
molecular cyclization process.
Table 3. Reaction Scope for Diazo Compounds
On the basis of these experiments and literature precedents, a
plausible mechanism for the indole synthesis process is
proposed in Scheme 3: generation of a cationic Rh(III) species
Scheme 3. Proposed Reaction Pathway
a
Reaction conditions: 1a (0.4 mmol), 2a−w (0.6 mmol),
[Cp*RhCl2]2 (2 mol %), AgSbF6 (8 mol %), HOAc (20 mol %),
b
DCE (2 mL). Isolated yields.
diazocycloalkane-1,3-diones (2r−2t), and furyl, thiophenyl, and
naphthyl diazo esters (2u, 2v, and 2w) also provide the
corresponding products in good yields.
We further performed several experiments to gain insight
into the reaction mechanism (Scheme 2). The ready
Scheme 2. Mechanistic Experiments
by [RhCp*Cl2]2 and AgSbF6, coordination of OAc−, CHA of
N-nitrosoaniline with Rh(III) to afford rhodacycle I, coordina-
tion with a diazo compound to form diazonium intermediate II,
extrusion of N2 from II to afford Rh(III)-carbene III, 1,1-
migratory insertion to generate Rh(III) species IV, proto-
demetalation of IV to give intermediate V, and release of the
Rh(III) catalyst simultaneously for a new catalytic cycle. Then
tautomerization of intermediate V generates in situ an enol
intermediate VI, which undergoes cyclization/denitrosation to
afford the indole product 3a.
In conclusion, a distinct electrophilic directing group removal
strategy has been proposed and demonstrated for a CHA-based
synthetic protocol. The protocol allows for the synthesis of
indole derivatives from readily available N-nitroso and α-diazo-
β-keto compounds. The annulation reaction, involving tandem
CHA and a hitherto elusive cyclization/denitrosation step for
the N-nitroso group, proceeds in a traceless, atom- and step-
economic manner, and the corresponding indole derivatives
could be obtained with a broad range of substitution patterns.
observation of H/D scrambling with the addition of a standard
amount of CD3CO2D (20 mol %) suggests that the CHA and
ortho-rhodation step is reversible. The reversibility is further
highlighted by an elevated degree of scrambling upon the use of
an increased amount of CD3CO2D or when in combined use
with CD3OD. Similarly as expected, D/H scrambling occurs for
1a-d3 with the addition of HOAc. A kinetic isotope effect value
of 3.8 was obtained when using an equimolar mixture of 1a and
1a-d3 in the coupling with 2 at a low degree of conversion,
supporting CHA as the rate-limiting step. Competition
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00310.
Experimental procedure, characterization of the products
(PDF)
C
DOI: 10.1021/acs.orglett.6b00310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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products (PDF)
Crystallographic data for complex 4k (CIF)
Crystallographic data for complex 5 (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jinz@nju.edu.cn.
Author Contributions
†J.W. and M.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support from the National Natural
Science Foundation of China (21425415, 21274058) and the
National Basic Research Program of China (2015CB856303,
2011CB935801).
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Org. Lett. XXXX, XXX, XXX−XXX