Ruthenium-Catalyzed, Site-Selective C-H Allylation of Indoles with Allyl Alcohols as Coupling Partners

Article abstract of DOI:10.1021/acs.orglett.6b00217

A new ruthenium-catalyzed, heteroatom-directed strategy for C-H allylation of indoles is described. The use of allyl alcohols as coupling partners as well as pyridine as the removable directing group is highlighted. This methodology provides access to C2-allylated indoles by utilizing a strategy that does not require prefunctionalization of either of the coupling partners.

Full text of DOI:10.1021/acs.orglett.6b00217

Letter
pubs.acs.org/OrgLett
Ruthenium-Catalyzed, Site-Selective C−H Allylation of Indoles with
Allyl Alcohols as Coupling Partners
Gangam Srikanth Kumar and Manmohan Kapur*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066,
India
*
S Supporting Information
ABSTRACT: A new ruthenium-catalyzed, heteroatom-directed strategy for C−H allylation of indoles is described. The use of
allyl alcohols as coupling partners as well as pyridine as the removable directing group is highlighted. This methodology provides
access to C2-allylated indoles by utilizing a strategy that does not require prefunctionalization of either of the coupling partners.
he allylation reaction has been a well studied trans-
Scheme 1. C−H Allylation Reactions and Indole Allylations
T
formation with the palladium-catalyzed Tsuji−Trost
1
allylation bringing about a revolution in this field. Over the
years this reaction has undergone a metamorphosis and has been
extended to simple arenes as well. Earlier, the reaction was
limited to derivatives of allylic alcohols, and in most cases, allyl
acetates were used. Now, the reaction has been extended to allylic
2
,3
alcohols as well. With the advent of C−H functionalization
4
reactions, the C−H allylation reaction has generated great
interest in the scientific community. Selective functionalization
of proximal C−H bonds has been made feasible by the use of
5
suitable Lewis basic directing groups. Sometimes, these groups
are difficult to remove once the transformation has been carried
out. Those directing groups, which can be removed after their
utility is over, hold higher synthetic value than those that cannot
be done away with.
Nitrogen heterocycles, and in particular indoles, are of great
synthetic interest to chemists due to their relevance in biological
6
a−d
systems.
The functionalization of indoles has therefore
6e
merited considerable attention from organic chemists. In
recent years, the C2 C−H functionalization of indoles has proved
to be a worthy alternative to traditional functionalization
methods that were earlier employed. Considerable work has
been done in the area of allylation of arenes via C−H
7
−9
activation.
The allylation reaction with allyl alcohol
same N-pyrimidyl substrate was successfully allylated under
cobalt catalysis using allyl alcohols.
derivatives, such as acetates or allyl halides, usually leads to the
typical γ-allylated products. The use of allyl alcohols, in turn, can
Our group has also been interested in the C−H functionaliza-
12
10
tion as well as synthesis of indoles via transition metal catalysis.
lead to ketones or aldehydes (Scheme 1). The allylation of
11
To the best of our knowledge, the ruthenium catalyzed C2 C−H
allylation of indoles using allyl alcohols has not been reported.
We report herein, the N-pyridine directed ruthenium-catalyzed
site-selective C−H allylation of indoles and pyrroles, in which
unactivated allyl alcohols are employed as coupling partners. The
indoles also has been well studied. Of the four possible
11a−c
2a,b
8i
scenarios (N-allylation,
C-2, C-3,
or C7 allylation),
directed allylation at the C-2 position of indoles has recently
1
1e−h
gained importance.
Here too, the employment of activated
allyl alcohols such as allyl acetates or halides results in allylated
products, whereas Glorius et al. reported the use of allyl alcohols
in the Rh-catalyzed C−H functionalization of indoles, resulting
Received: January 22, 2016
1
0a
11f
in aldehydes. Notable is the work of Kanai et al., where the
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00217
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
,
a d
other highlight of this work is the scope of removal of the
directing group.
Scheme 2. Substrate Scope for the Reaction
The evolution of the work began with the quest for the best
combination of directing groups with the catalyst system. The
most important results are summarized in Table 1 (for the
a
Table 1. Optimization Studies
directing
group
yield
(%)
entry
1
catalyst/oxidant/additive/solvent/temp/time
Pd(OAc) /Cu(OTf) /AgSbF /dioxane/
d
2-pyridyl
<5
2
2
6
1
00 °C/12 h
2
3
4
5
6
7
8
9
acetyl
[RuCl (p-cym)] /Cu(OAc) /AgSbF /THF/
0
0
0
2
2
2
6
6
0 °C/12 h
2-pyrimidyl [RuCl (p-cym)] /Cu(OAc) /AgSbF /THF/
2
2
2
6
6
0 °C/12 h
-CONMe2
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
[RuCl (p-cym)] /Cu(OAc) /AgSbF /THF/
2
2
2
6
6
0 °C/12 h
b
[RuCl (p-cym)] /Cu(OAc) /AgSbF /
70
2
2
2
6
dioxane/100 °C/10 h
b
[RuCl (p-cym)] /Cu(OAc) /AgSbF /DCE/ 85
2
2
2
6
8
5 °C/12 h
c
[RuCl (p-cym)] /O Balloon/AgSbF /DCE/
<5
<5
2
2
2
6
8
0 °C/10 h
d
[RuCl (p-cym)] /Cu(OTf) /AgSbF /DCE/
2
2
2
6
8
5 °C/10 h
b
[RuCl (p-cym)] /NaOAc/AgSbF /DCE/85
70
67
0
2
2
6
°
C/10 h
b
10
11
12
[RuCl (p-cym)] /Cu(OAc) / AgBF
2 2 2 4
(0.5 equiv)/DCE/85 °C/10 h
[RuCl (p-cym)] /Cu(OAc) /DCE/80 °C/
2
2
2
1
0 h
Cu(OAc) /AgSbF /DCE/85 °C/12 h
0
2
6
a
Reaction conditions: unless otherwise mentioned all reactions
a
performed with 1a (0.3 mmol), 2a (0.45 mmol), 5 mol % catalyst, 2
All reactions were carried out with 0.3 mmol of 1 and 1.5 equiv of 2
b
equiv of oxidant, and 0.2 equiv of additive in 1.5 mL of solvent.
unless otherwise noted. All yields are isolated yields. 7 mol % catalyst
b
c
d
c
Isolated yield. GC yield. Complex mixture was obtained.
and 0.3 equiv additive were used. 2 equiv of allyl alcohol were used.
d
1
e
E:Z ratio determined by H NMR of reaction mixture. From 2-
f
g
methylprop-2-en-1-ol. From cyclohex-2-en-1-ol. From 1-(cyclohex-1-
en-1-yl)ethan-1-ol. Based on recovered starting material.
remaining attempts toward optimization, see the Supporting
Information (SI)). Initially, simple and readily available
palladium catalysts were screened. These efforts, however, did
not produce the desired results. The use of other directing groups
also did not help (Table 1). The best results were obtained with
the N-pyridyl directing group, when using ruthenium catalysis,
with AgSbF as the additive and Cu(OAc) as the oxidant. The
h
worked well with most of allyl alcohols screened for the substrate
scope. In almost all cases, the stereochemistry of the resulting
olefin was E. In some cases isomeric products were observed
(entries 3p−q, Scheme 2). The selectivity was excellent, with the
γ-selectivity predominating almost exclusively. With the site
selectivity too, only the C-2 C−H functionalization product was
observed. There were some notable failures, mainly due to steric
reasons (entries 3x−z, Scheme 2). Another reason could be the
lack of feasibility of the β-hydroxide elimination in 3y. Steric
reasons could also be attributed for the low yield in the case of 3s.
Substrates with halosubstituents too worked very well, yielding
products which have scope for further functionalization.
Interestingly, the pyrroles (3t−v, Scheme 2) were also
successfully synthesized from the corresponding N-pyridyl
pyrrole. Since the reaction was not monoselective, we used an
excess of the allyl alcohol to obtain the diallylated product
directly. Substrate 3w was also successfully synthesized in
moderate yield, proving the efficiency of the pyridyl directing
group. The same reaction, when carried out with N-pyridyl-2,3-
dimethylindole, was extremely sluggish and resulted in a complex
mixture, along with the recovered starting material. The N-
pyridyl indoline 4, however, reacted well to result in the C-7
6
2
best solvent for this transformation was dichloroethane. Under
some conditions (see the SI), we observed traces of the aldehyde
arising from the β-hydride elimination pathway. Under some of
the conditions (entry 8, Table 1), the C-3 allylated product was
also observed in minor quantities.
This probably arose from the copper salt acting as a Lewis acid
to activate the allyl alcohol, thereby resulting in an electrophilic
reaction at C-3 of the indole. Under the optimized reaction
conditions, the use of a pyrimidine directing group did not result
in the desired product; the starting material remained unreacted.
The use of both Cu(OAc) and AgSbF were necessary, which
2
6
was proved by poor conversions in the absence of any one of
these. A control reaction, carried out in the absence of the Ru-
catalyst, did not yield the allylated product.
The substrate scope for this transformation is depicted in
Scheme 2. The reaction was well-tolerant of all functional groups
screened. The reaction worked well for electron-withdrawing
and -donating substituents on the indole skeleton. The reaction
B
DOI: 10.1021/acs.orglett.6b00217
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Letter
allylated product. Nonetheless, the reaction rate in this case was
much lower than that for the corresponding indole.
Scheme 4. Plausible Mechanism
To shed light on the probable mechanistic pathway involved in
the transformation, kinetic isotope effect studies were under-
1
4
taken (Scheme 3). A moderately low value of 1.40 was obtained
Scheme 3. Mechanistic Studies
postulated that the oxophilicity of the metal may drive the
reaction to choose the latter pathway. The regeneration of the
active catalyst by Cu(OAc) and AcOH completes the catalytic
2
7g,11f
cycle. Observation of the γ-selectivity and previous reports
seem to rule out the S 2′ pathway as well as the formation of the
N
π-allyl intermediate. The fact that considerable deuterium
incorporation was observed at the C7 position in our quenching
studies with D O indicated a reversible C−H activation and
2
ruthenacycle formation at C7. However, this six-membered
ruthenacycle (B) is expected to have a relatively lower stability
than the five-membered ruthenacycle (A) formed from the C2
C−H activation (Scheme 4). This would probably explain the
preferential formation of the C2 allylated product over the C7
allylated one.
We were also successful in the removal of the pyridyl directing
15
group. The reaction with MeOTf followed by alkali treatment
resulted in a clean conversion to the NH-indole (Scheme 5). In
some cases, isomerization of the olefin was also observed in the
deprotection reaction.
for individual experiments, whereas a value of 0.9 was obtained
via competitive studies, thereby indicating absence of a primary
kinetic isotope effect. To check for reversibility of the formation
of the ruthenacycle, quenching studies were undertaken, in both
the presence and absence of the coupling partner. In the presence
of the coupling partner, the recovered starting material (after 5
min) had a deuterium content of about 40% at C2. Surprisingly,
a
Scheme 5. Removal of the Pyridine Directing Group
in the presence of D O, the reaction seemed to accelerate
2
considerably. In the absence of the coupling partner, the reaction
resulted in about 66% deuterium incorporation into the starting
material at C2 (after 12 h, Scheme 3), thereby proving the
reversibility of the formation of the ruthenacycle. In both
experiments, the C7 position was also deuterated (Scheme 3).
We also attempted to explore the electronic effect of the C5
substituent on the rate of the reaction. The 5-nitro substrate
seemed to react faster whereas the 5-methyl substrate was
comparatively the slowest. The unsubstituted and the 5-methoxy
substrates had quite similar rates.
a
b
All yields are isolated yields. Ratio of terminal alkene/internal
alkene.
A plausible reaction pathway is proposed in Scheme 4, based
on our observations as well as literature reports. The first step is
the generation of the cationic Ru(II) catalyst, followed by
coordination of the metal to the pyridine nitrogen. This is then
followed by the proximal C−H activation at C-2, to result in the
ruthenacycle A. Insertion of the allyl alcohol via carboruthena-
tion results in C. The intermediate C now has the option of β-
hydride elimination or β-hydroxide elimination. It could be
In summary, we have developed a new Ru(II) catalyzed C−H
allylation method for indoles using unactivated allyl alcohols. A
removable N-pyridyl directing group was employed in the
reaction, thereby increasing the utility of the reaction. The site
selectivity for C−H functionalization was exclusive, and the
allylation reaction was highly γ-selective. The transformation was
C
DOI: 10.1021/acs.orglett.6b00217
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a clean reaction with very good yields for most substrates. This is
one of the few allylation methods in which unactivated allyl
alcohols are employed as coupling partners and the catalyst
system is simple and rather inexpensive.
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AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: mk@iiserb.ac.in.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Funding from CSIR-India (02(205)/14/EMR-II) and DST-
India (SB/S1/OC-10/2014) is gratefully acknowledged. G.S.K.
thanks CSIR for a Research Fellowship. We thank Mr. Lalit
Mohan Jha, IISERB, for the X-ray data. We also thank the
Director, IISERB for funding and infrastructural facilities. We
thank the Reviewers for insightful suggestions.
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