Ruthenium-Catalyzed Selective C?C Coupling of Allylic Alcohols with Free Indoles: Influence of the Metal Catalyst

Article abstract of DOI:10.1002/chem.201706080

Versatile reactive activities of allyl alcohols with free indoles in C?H functionalization reactions were investigated. Direct alkylation or cascade cyclization reactions could be selectively controlled based on the catalyst system: Ru(PPh₃)₃Cl₂ provided C3-substituted β-ketone indoles whereas [Ru(p-cymene)Cl₂]₂ yielded cyclized indoles.

Full text of DOI:10.1002/chem.201706080

DOI: 10.1002/chem.201706080
Communication
&
Coupling Reactions
Ruthenium-Catalyzed Selective CÀC Coupling of Allylic Alcohols
with Free Indoles: Influence of the Metal Catalyst
Ying-Qi Xia, Chao Li, Man Liu, and Lin Dong*[a]
Abstract: Versatile reactive activities of allyl alcohols with
free indoles in CÀH functionalization reactions were inves-
tigated. Direct alkylation or cascade cyclization reactions
could be selectively controlled based on the catalyst
system: Ru(PPh3)3Cl2 provided C3-substituted b-ketone in-
doles whereas [Ru(p-cymene)Cl2]2 yielded cyclized indoles.
Indole and many indole derivatives are currently explored as
privileged structures present in a large number of pharmaceut-
icals and biologically active compounds.[1] Among them, C3-
substituted indoles as important precursors have drawn the
most attention.[2] With the rapid development of transition-
metal-catalyzed CÀH activation in the past decade,[3] many
groups made significant contributions to functionalize the 3-
position of the indole structures through CÀH alkenylation, ar-
ylation, and alkylation reactions.[4] Owing to the continuing in-
creased interest in this field, the development of simple, effi-
cient, selective synthetic approaches for the preparation of C3-
substituted indole derivatives are still highly desirable.
In recent years, allyl alcohols have been widely employed as
coupling partners in CÀH functionalization reactions.[5–10] For
example, complex alkylation compounds could be obtained
via the oxidation of allylic alcohols to enones by transition
metal catalysts at the first step [Scheme 1, Eq. (1)].[6,7] Glorius,
Scheme 1. Transition-metal-catalyzed CÀH activation with allylic alcohol cou-
pling partners.
Jiang, and others independently developed CÀH bond oxida-
tive alkylation with allylic alcohols to afford the functional b-
aryl ketones through the b-hydride elimination and keto–enol
tautomerism pathway [Scheme 1, Eq. (2)].[8] Recently, metal-cat-
alyzed direct dehydrative CÀH allylation with allyl alcohols via
b-hydroxide elimination providing allylic alkylation products
were reported [Scheme 1, Eq. (3)].[8e,9] Allylic alkylation prod-
ucts could also be obtained through h3-allyl dication pathway
without the chelation assistance [Scheme 1, Eq. (4)].[10] Howev-
er, to our knowledge, non-activated free-allyl alcohols have
rarely been applied to the ruthenium-catalyzed selective CÀH
functionalization of free indoles. Herein we describe a catalyst-
controlled strategy to access various C3-selective substituted
indoles from free indoles and allylic alcohols, in which the se-
lectivity of direct C3 alkylation [Scheme 1, Eq. (5)] or cascade
cyclization [Scheme 1, Eq. (6)] is dependent on the particular
catalyst. These products were different from the CÀN cycliza-
tion products (annulation of 2-phenyl indoles with allyl carbo-
nates) shown in our previous work [Scheme 1, Eq. (7)].[11]
Our investigation was initiated by examining the selective
coupling reactions between 2-phenyl indole 1a and but-3-en-
2-ol 2a (Table 1). To our delight, C3-substituted b-ketone
indole 3aa could be isolated in 43% yield by using
Ru(PPh3)3Cl2 as the catalyst in the presence of Cu(OAc)2·H2O as
oxidant (entry 1). In addition, we obtained a reasonable yield
of 3aa when AgOAc was employed (entries 2–5). Other oxi-
dants such as Cu(OAc)2, Cu(acac)2 and CuOAc led to detrimen-
tal effects (entries 6–8). Notably, DCE is a suitable solvent in
the oxidative alkylation reaction (entries 9–12). Satisfyingly, the
yield of 3aa could be dramatically improved by adding base
to the reaction system (entries 13–15). To our surprise, when
[a] Y.-Q. Xia, C. Li, M. Liu, Prof. L. Dong
Key laboratory of Drug-Targeting and Drug Delivery System of the Ministry
of Education, West China School of Pharmacy
Sichuan University
610041 Chengdu (China)
E-mail: dongl@scu.edu.cn
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201706080.
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ther decreasing the reaction temperature, or reducing the re-
action time could slightly decrease the yield of 3aa and 4aa.
Compounds 3aa or 4aa could not be detected in the absence
of Cu(OAc)2·H2O.[12]
Table 1. Reaction optimization.[a]
With the optimized reaction conditions in hand, we first ex-
plored the scope of indoles 1 with allylic alcohols 2a in direct
C3 alkylations (Table 2). C2-ester substituted indole 1b was a
Entry Catalyst
Additive (equiv)
Solvent Yield [%][b]
3aa 4aa
Table 2. The scope of indoles used in direct C3 alkylations.[a]
1
2
3
4
Ru(PPh3)3Cl2
–
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene 36
dioxane 10
CH3CN 15
CH3OH 13
43
65
20
27
17
60
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
32
43
20
–
17
36
24
15
0
36
44
58
60
–
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
Ru(PPh3)3Cl2
[Ru(p-cymene)Cl2]2
AgOAc
AgSbF6
AgBF4
AgNTf2
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc+AcOH (1)
AgOAc+NaOAc (1)
5
6[c]
7[d]
8[e]
9
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27[f]
DCE
DCE
57
76
92
–
–
–
–
–
–
–
AgOAc+LiOAc·2H2O (1) DCE
–
DCE
DCE
DCE
DCE
[Ru(p-cymene)Cl2]2 AgOAc
[Ru(p-cymene)Cl2]2 AgSbF6
[Ru(p-cymene)Cl2]2 AgBF4
[Ru(p-cymene)Cl2]2 AgNTf2
[Ru(p-cymene)Cl2]2 AgOAc
[Ru(p-cymene)Cl2]2 AgOAc
[Ru(p-cymene)Cl2]2 AgOAc
[Ru(p-cymene)Cl2]2 AgOAc
[Ru(p-cymene)Cl2]2 AgOAc+AcOH (1)
[Ru(p-cymene)Cl2]2 AgOAc+NaOAc (1)
[Ru(p-cymene)Cl2]2 AgOAc
DCE
toluene
dioxane
CH3CN
TFE
DCE
DCE
DCE
DCE
DCE
[a] Reaction conditions unless otherwise specified: 0.1 mmol of 1,
5.0 equiv 2a, 5 mol% Ru(PPh3)3Cl2, 2.2 equiv Cu(OAc)2·H2O, 0.3 equiv
AgOAc, 1.0 equiv LiOAc·2H2O, 1.0 mL of DCE, 1308C, 48 h, under Ar at-
mosphere. Isolated yield.
–
–
–
–
–
28[g,f] [Ru(p-cymene)Cl2]2 AgOAc
29[h]
Ru(PPh3)3Cl2
AgOAc+
90
LiOAc·2H2O (1)
suitable substrate but furnished 3ba in only 52% yield, proba-
bly owing to electronic effects. Free indole 1c could also react
well with 2a. Satisfactory, indoles with various electron-with-
drawing or electron-donating groups on the phenyl ring pro-
ceeded smoothly to give corresponding alkylation products
(3da–3na) in good-to-excellent yields. It is worth noting that
the catalytic systems have good tolerance to strong electron-
withdrawing groups (nitro) and highly active halogen (bromo
and chloro) groups. N-methyl substituted indoles could also ef-
ficiently participate in the reaction, giving 3oa in 60% yield.
We then investigated the scope of substituted allylic alco-
hols with 2-phenyl indoles under Ru(PPh3)3Cl2 catalytic condi-
tions (Table 3). To our delight, the coupling of linear or cyclic
a-alkyl substituted allylic alcohols 2b–2e with 1a was smooth,
generating b-aryl ketones 3ab-3ae as the single direct alkyla-
tion products in good yields. However, primary alcohol 2 f did
not undergo coupling under these conditions. Moreover, a-aryl
substituted allylic alcohol 2g could also participate smoothly
with 1a and 1i in the reaction to yield the corresponding
products 3ag and 3ig in good yields.
30[f,g,h] [Ru(p-cymene)Cl2]2 AgOAc
DCE
–
55
[a] Reaction conditions: unless otherwise specified, 0.05 mmol of 1a,
5.0 equiv 2a, 2.2 equiv Cu(OAc)2·H2O, 5 mol% of catalyst,0.3 equiv silver
salt, 0.5 mL of solvent, 1308C, 48 h, Ar atmosphere. [b] Isolated yield.
[c] Cu(OAc)2 (2.2 equiv). [d] Cu(acac)2 (2.2 equiv). [e] CuOAc (2.2 equiv).
[f] Cu(OAc)2·H2O (3.2 equiv). [g] 1008C. [h] 0.5 mmol of 1a.
[Ru(p-cymene)Cl2]2 was used as the catalyst, cascade cyclization
product 4aa was detected as the sole product. In contrast, we
only obtained single CÀN cyclization products by using allyl
carbonates as coupling partners (entry 16).[11] Additives screen-
ing revealed that AgOAc could still promote the cyclization re-
action (entries 17–20). However, other solvents gave poorer re-
sults (entries 21–24). AcOH or NaOAc had a crucial effect on
the efficiency of this cascade reaction (entries 25 and 26). In-
creasing loading of Cu(OAc)2·H2O improved the catalytic activi-
ty to afford the desired seven-membered-ring product 4aa in
moderate yield (entry 27). Decreasing the temperature to
1008C could slightly increase the yield of 4aa (entry 28). The
reaction could be performed smoothly on a large scale to gen-
erate corresponding products in satisfying yields (entries 29
and 30). Significantly, the conditions are controllable in all
cases, in which C3 alkylation or cyclization products can be se-
lectively obtained. However, lowering the loading of 2a, fur-
We next evaluated the scope of 2-phenyl indoles with 2a in
the cascade cyclization reaction under the [Ru(p-cymene)Cl2]2
catalytic system (Table 4). When the C3-position of the indole
ring has a methyl group, only a single-substituted oxidative al-
kylation product 4pa was obtained. Formation of 4pa indi-
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Table 3. Substrate scope of allylic alcohols in direct C3 alkylations.[a]
Scheme 2. Synthetic applications of compounds 4ta and 4aa.
unique fused heterocyclic compound 5 in 83% yield via nucle-
ophillic addition and cyclization (Scheme 2).
[a] Reaction conditions unless otherwise specified: 0.1 mmol of 1a,
5.0 equiv 2, 5 mol% of Ru(PPh3)3Cl2, 2.2 equiv Cu(OAc)2·H2O, 0.3 equiv
AgOAc, 1.0 equiv LiOAc·2H2O, 1.0 mL of DCE, 1308C, 48 h, under Ar at-
mosphere. Isolated yield.
Subsequently, experiments were carried out to further inves-
tigate the catalytic mechanism (Scheme 3). When 1a was con-
ducted in DCE/D2O in the absence of 2a by using Ru(PPh3)3Cl2
as the catalyst, deuterium was observed by NMR spectroscopy
only at the C3-position of indole, which indicated that the alky-
Table 4. The scope of indoles in cascade reactions.[a]
[a] Reaction conditions unless otherwise specified: 0.1 mmol of 1,
5.0 equiv 2a, 5 mol% of [Ru(p-cymene)Cl2]2, 3.2 equiv Cu(OAc)2·H2O,
0.3 equiv AgOAc, 1.0 mL of DCE, 1008C, 48 h, under Ar atmosphere. Iso-
lated yield.
cates that dearomatization does not occur in preference to the
formation of heterocycles, and construction of carbocycles is
more favorable than heterocycles. The cascade reactions
worked well for the groups on the phenyl ring of indole skele-
tons (1q–1s). However, when ortho-ethyl-substituted substrate
1t was used, only single-substituted alkylation product 4ta
was isolated and no cyclization product was observed, proba-
bly owing to the influence of steric hindrance. Conversely,
direct cyclization of 4ta with TsOH could give the CÀC bond
formation product 4ta’ in 98% yield (Scheme 2). The sub-
strates bearing different substituents at the para positions of
the phenyl ring were tolerated in the catalyst systems (1u and
1v). However, when the methyl group of allylic alcohol was
changed to other substituents, such as H, phenyl, and cyclo-
hexyl, no desired product was obtained. Surprisingly, the cycli-
zation product 4aa could be further converted into the quite
Scheme 3. Preliminary mechanistic investigations.
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lation is a direct CÀH bond activation without chelation assis-
tance [Scheme 3, Eq. (8)]. In contrast, ortho-positions of the
phenyl ring were observed with 89% deuterium, which implies
that the cascade cyclization underwent coordination of nitro-
gen [Scheme 3, Eq. (9)]. Deuteration at the a-position of car-
bonyl groups was observed, which might proceed by deutera-
Under the Ru(PPh3)3Cl2 catalyst system (direct C3 alkylation),
the ruthenium catalyst first inserts into C(3)-H of an indole to
form complex I’, which could be stabilized by oxygen and adja-
cent system to afford intermediate II’. Then intermediate II’ un-
dergoes olefin insertion to give ruthenium species III’. Finally,
b-H elimination forms alkylation product 3aa. However, the al-
lylic alcohol 2a converting into a,b-unsaturated enone in the
presence of the ruthenium catalyst at the first step might be
also possible, which is similar to Equation (1), Scheme 1.[7]
In summary, we have realized ruthenium(II)-catalyzed selec-
tive CÀC coupling of allylic alcohols with free indoles for the
synthesis of diverse C3-substituted indole derivatives.
Ru(PPh3)3Cl2 provides C3-substituted b-ketone indoles and
[Ru(p-cymene)Cl2]2 yields 5,12-dihydrobenzo[6,7] cyclohepta
[1,2-b] indoles. The selective pathway may be attributed the
difference in binding affinity of a metal center with but-3-en-2-
ol. The presented conversions extend the diversity of heterocy-
clic scaffolds accessible from 2-phenyl indoles with allyl carbo-
nates as shown previously.[11] Given the iniquitousness of the
indoles, the reactions may find broader applications in the syn-
thesis of related useful products.
tion
through
keto–enol
tautomerization
[Scheme 3,
Eq. (10)].[5e,8c,e] The reaction of 1a with but-3-en-2-one could
be conducted smoothly when Ru(PPh3)3Cl2 was used as the
catalyst, which implies that the oxidation of allylic alcohols to
enones by Ru(PPh3)3Cl2 at the first step is likely to occur.[7]
However, the reaction was dramatically affected in the absence
of Cu(OAc)2·H2O,[12] which indicates that oxidant is very impor-
tant in the catalytic system and the b-hydride elimination path-
way might also exist [Scheme 3, Eq. (11)].[8] On the contrary,
the reaction was very non-selective when 1a was reacted di-
rectly with but-3-en-2-one in the [Ru(p-cymene)Cl2]2 catalytic
conditions, which implies that the reaction does not proceed
through oxidation of allylic alcohols to enones [Scheme 3,
Eq. (12)]. The deuterium kinetic isotopic effects were deter-
mined to be 1.1 and 2.7 in the Ru(PPh3)3Cl2 and [Ru(p-cy-
mene)Cl2]2 catalytic conditions, respectively, which indicates
that CÀH bond cleavage might be involved in the rate-deter-
mining step [Scheme 3, Eqs. (13) and (14)].[12]
Experimental Section
A plausible mechanism is proposed in Scheme 4. Under the
[Ru(p-cymene)Cl2]2 catalyst system (the cascade cyclization),
ruthenium coordinates to the nitrogen and followed by CÀH
bond activation forms the RuII complex intermediate I in the
first step. Then olefin insertion yields seven-membered com-
plex II, which then undergoes b-H elimination and keto–enol
tautomerism pathway to give alkylation intermediate III and a
Ru0 species; the latter is reoxidized by CuII to complete the cat-
alytic cycle. Subsequently, the second alkylation process occurs
to generate intermediate IV. Finally, dehydrative cyclization fur-
nishes the final annulation product 4aa.[13]
General procedure for synthesis of the C3-substituted b-ketone
indoles: 2-Phenyl indole 1a (0.05 mmol, 9.7 mg), but-3-en-2-ol 2a
(5.0 equiv, 22 mL), Ru(PPh3)3Cl2 (5 mol%, 2.4 mg), AgOAc (0.3 equiv,
2.5 mg), Cu(OAc)2·H2O (2.2 equiv, 22 mg) and LiOAc·2H2O
(1.0 equiv, 5.1 mg) were stirred in DCE (0.5 mL) at 1308C for 48 h
under Ar atmosphere. After completion, the reaction mixture was
purified by flash chromatography eluting with petroleum ether
and ethyl acetate (PE/EA=100:1) to give the product 3aa as
yellow liquid (12.1 mg, 92%).
General procedure for synthesis of the annulated 2-phenyl in-
doles: 2-Phenyl indole 1a (0.05 mmol, 9.7 mg), but-3-en-2-ol 2a
(5.0 equiv, 22 mL), [Ru(p-cymene)Cl2]2 (5 mol%, 1.5 mg), AgOAc
(0.3 equiv, 2.5 mg) and Cu(OAc)2·H2O (3.2 equiv, 32 mg) were stirred
in DCE (0.5 mL) at 1008C for 48 h under Ar atmosphere.
After completion, the reaction mixture purified by flash
chromatography eluting with petroleum ether and ethyl
acetate (PE/EA=100:1) to give the product 4aa as
yellow liquid (9.5 mg, 60%).
Acknowledgements
The authors are grateful for the financial support
from the NSFC (21572138).
Conflict of interest
The authors declare no conflict of interest.
Keywords: allylic alcohols
·
free indoles
·
ruthenium
indoles
·
selective CÀC coupling
·
b-ketone
Scheme 4. Plausible reaction mechanism.
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Manuscript received: December 22, 2017
Revised manuscript received: January 29, 2018
Version of record online: && &&, 0000
Chem. Eur. J. 2018, 24, 1 – 6
www.chemeurj.org
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Coupling Reactions
Y.-Q. Xia, C. Li, M. Liu, L. Dong*
&& – &&
Ruthenium-Catalyzed Selective CÀC
Coupling of Allylic Alcohols with Free
Indoles: Influence of the Metal
Catalyst
R u C’ing this? Versatile reactive activi-
ties of allyl alcohols with free indoles in
CÀH functionalization reactions were in-
vestigated. Direct alkylation or cascade
cyclization reactions were selectively
controlled based on the particular cata-
lyst system: Ru(PPh3)3Cl2 provided C3-
substituted b-ketone indoles, whereas
[Ru(p-cymene)Cl2]2 gave cyclized in-
doles.
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&
Chem. Eur. J. 2018, 24, 1 – 6
www.chemeurj.org
6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!