Catalytic Arylative Endo Cyclization of Gold Acetylides: Access to 3,4-Diphenyl Isoquinoline, 2,3-Diphenyl Indole, and Mesoionic Normal NHC–Gold Complex

Article abstract of DOI:10.1002/chem.202001502

3,4-Diphenyl isoquinoline and 2,3-diphenyl indole are readily accessed by catalytic selective bis-arylative endo cyclization of gold acetylides. The synthetic approach could be also extended to prepare six-membered mesoionic NHC complex, which could furth

Full text of DOI:10.1002/chem.202001502

Accepted Article
Accepted Article
Title: Catalytic arylative endo cyclization of gold acetylides: access to
3,4-diphenyl isoquinoline, 2,3-diphenyl indole, and mesoionic
normal NHC-gold complex
Authors: Jun Zhang, Jiwei Wang, Gendi Wang, Ye Liu, and Kemeng
Yuan
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202001502
Link to VoR: https://doi.org/10.1002/chem.202001502
01/2020

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Catalytic arylative endo cyclization of gold acetylides: access to
3,4-diphenyl isoquinoline, 2,3-diphenyl indole, and mesoionic
normal NHC-gold complex
Jiwei Wang,[a] Kemeng Yuan,[b] Gendi Wang,[b] Ye Liu,*[a] and Jun Zhang*[b]
[a]
J. Wang, Prof. Y. Liu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry & Molecular Engineering
East China Normal University
3663 North Zhongshan Road, Shanghai 200062 (China)
E-mail: yliu@chem.ecnu.edu.cn
[b]
K. Yuan, G, Wang, Prof. J. Zhang
Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel
Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (China)
E-mail: zhangj@ecust.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: 3,4-Diphenyl isoquinoline and 2,3-diphenyl indole are
readily accessed by catalytic selective bis-arylative endo cyclization
of gold acetylides. The synthetic approach could be also extended to
prepare six-membered mesoionic NHC complex, which could further
undergo deprotonation/complexation to afford 1,3-N-heterocyclic
dicarbene (NHDCs) Au2 and Au/Ag complexes. The key vicinal
diaurated alkene intermediates have been isolated and the studies on
their reactivities towards coupling partners reinforce the proposed
mechanisms.
Gold catalysis has raised considerable attention, and emerged as
efficient and versatile tool for C−C bond formation, mainly due to
the capability of gold to activate C−C multiple bonds towards
nucleophilic attack to initiate various novel transformations in
organic synthesis.[1] Vinyl gold species A are commonly proposed
as the key intermediates in gold-catalyzed intramolecular
nucleophilic attack of alkynes (Scheme 1a).[2] Gold complexes are
reluctant to undergo a redox process, due to the high redox
potential of the AuI/AuIII couple.[3] Therefore, cross-coupling of
vinyl gold intermediates A generally requires stoichiometric
amounts of strong external oxidants, such as PhI(OAc)2 or
Selectfluor, to promote AuI→AuIII oxidation.[4] Alternatively, by
combination of the powerful ability of gold to activate C−C multiple
bonds for cyclizations with the C−C bond forming and efficient
redox turnover of palladium, Au and Pd dual-catalyzed
cyclization/cross-coupling reactions have attracted much interests in
the past decade, wherein akey transmetalation of vinylgold with ArPd+
intermediate, followed by reductive elimination at Pd center is
involved.[5] In particular, such Au/Pd dual-catalyzed reactions could
utilize weak internal oxidants, such as aryl halides, avoiding the use of
strong external oxidants to avoid the low functional group tolerance
typically going along with strong oxidants like Selectfluor and
hypervalent iodine reagents.
Scheme 1. Gold-mediated arylative cyclization of alkynes.
general, the endo-dig cyclization of terminal alkynes upon an
anti-Markovnikov addition manner is kinetically unfavorable.
Recent studies on the combination of computational, theoretical,
and experimental data also show that nucleophilic exo-dig
cyclizations of alkynes are intrinsically more favourable.[6] Indeed,
we previously reported the Au-mediated cyclization of N-propiolic
formamidine F to exclusively form an exocyclic vinyl gold species
So far, most alkyne substrates used in such cyclization/cross-
coupling reactions had an internal alkynyl moiety, thus involving
endocyclic vinyl gold species A.[2] When terminal alkynes are
used, the intramolecular nucleophilic attack can process through
of type
B
through 5-exo-dig mode.[7] Later, we found that in
could
exo
-dig or endo-dig cyclization, corresponding to exocyclic (B)
the presence of base and two equivalents of Au salt,
F
or endocyclic vinyl gold species (C), respectively (Scheme 1b). In
1
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undergo 6-endo-dig cyclization to give six-membered vicinal
of type
E, which further undergoes cross-coupling with PhI
diaurated alkene species of type
involvement of the formation of a σ,π-digold acetylide species
D [8]
In this case, gold in the acetylide acts as a directing
E, indicating the
under Pd catalysis.
To ascertain the mechanism, we attempted to isolate the
proposed vicinal diaurated alkene intermediate. Delightfully,
.
group to realize the regioselective nucleophilic attack
at the α-carbon in gold acetylide moiety. In addition, gold
acetylides are generally quite stable and have been used as
alternative substrates in C-C coupling reactions in the case that
the corresponding terminal alkynes are unstable.[9] To date,
studies on the reactivity of the vicinal diaurated alkene
reacting gold acetylide
1
with one equivalent of IPrAuOTf led
in 92 % yield
to form a vicinal diaurated alkene complex
3
(Scheme 3). As expected, under catalysis of Pd(PPh3)4,
o
digold
diarylation product
at room temperature,
to give under the reaction conditions. Additionally, in the
presence of excess PhI, no reaction was observed between
and stoichiometric amounts of Pd(PPh3)4 in CH3CN at room
temperature after 24 hours. These results suggest that the
vicinal diaurated alkene complex might be the reaction
3
reacted with excess PhI in CH3CN at 50 C to give
2
in a moderate yield of 62%. However,
cannot undergo such transformation
3
species
derived from
vinyl complexes.[8] We wonder whether
E
remain very rare. We previously reported
could undergo homocoupling to afford gold
could undergo
E
2
F
E
1
cross-couplings either with aid of either strong oxidative
reagent or under Pd catalysis. Herein, firstly we reported
catalytic bis-arylative endo cyclization of gold acetylides for the
formation of 3,4-diphenyl isoquinoline and 2,3-diphenyl indole
under Pd catalysis. Secondly, we have realized catalytic domino
cyclization and oxidative coupling of in-situ prepared gold
acetylide for the facile synthesis of six-membered NHC complex,
a precursor for the synthesis of 1,3-NHDC (N-heterocyclic
dicarbene) Au2 and Au/Ag complexes. Moreover, two key active
vicinal diaurated alkene intermediates have been isolated and
fully characterized, and the studies on their reactivities towards
coupling partners confirm the proposed mechanism.
3
intermediate. Notably, the mechanism involving a vicinal
Pd/Au alkene intermediate formed from the cyclization of π-
Pd activated gold acetylide species cannot be completely
ruled out. The structure of
crystal X-ray diffraction analysis (Figure 1). The Au-C bond
distances [Au1-C2, 2.043(6) and Au2-C3, 2.046(6) Å] in
3 was established by the single
3
are slightly longer than the gold carbene complexes (e.g.,
2.015(3) Å[11a] and 2.010(10) Å[11b]), and similar with that of a
C(sp2)-Au single bond (e.g., 2.045(6) Å)[11c]
.
Scheme 2. Cyclization/crossing coupling reaction of gold acetylide 1.
The isoquinoline ring is present in many natural alkaloids,
and 3,4-disubstituted isoquinolines could be synthesized by
Pd-catalyzed cross-coupling of 2-(1-alkynyl)benzaldimines
containing a internal alkynyl moiety with organic halides.[10]
Scheme 3. Isolation of vicinal diaurated alkene 3 and its reactivity toward PhI.
Firstly, we chose gold acetylide
1 (IPr = 2,6-bis(diisopropyl-
phenyl)imidazol-2-ylidene) bearing a benzaldimine moiety
as the model substrate for Pd-catalyzed cyclization/coupling
with organic halide (Eqn 1, Scheme 2). In the presence of
excess PhI, by using Pd(PPh3)4 as the catalyst, gold
acetylide
1 underwent endo cyclization/crossing coupling to
exclusively give diarylation product, 3,4-diarylisoquinoline
2
in a high yield of 96% at 50 °C in 12 h. However, at room
temperature, the transformation didn’t work (Eqn 2, Scheme
2). Interestingly, introduction of catalytic amount of IPrAuOTf
(10 mol%) led to form the desired
h at room temperature. The structure of
X-ray diffraction analysis (please see Figure S1). These
results indicate that the formation of at room temperature
probably involves a key vicinal diaurated alkene intermediate
2
in 90% yield within 100
Figure 1. X-ray crystal structure of vicinal diaurated alkene 3. The counter anion
and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å):
Au1−C1 2.033(6), Au1−C2 2.043(6), Au2−C3 2.046(6), Au2−C4 2.042(6),
C2−C3 1.398(8).
2
was confirmed by
2
2
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Scheme 6. Cyclization/cross-coupling reactions of 7.
Scheme 4. Cyclization/crossing coupling reaction of 4.
Recently, we have reported the catalytic cyclization/oxidative
homo-coupling reaction of gold acetylide, which was prepared in
situ from terminal alkyne 7 (Scheme 5).[8] The vicinal diaurated
alkene species 8 was isolated and identified as the key reactive
intermediate via σ,π-digold-activated mechanism. Vicinal
diaurated alkene species 8 also could be considered as the
ditopic NHC digold 8’, containing a gold-carbene bond at C4
position. However, neither 7 nor 8 could undergo cross-coupling
with PhI under Pd catalysis. Inspired by the catalytic homo-
coupling of 7, we further investigated the cross-coupling reaction
of 7 using PhB(OH)2 as cross-coupling partner in the presence of
strong external oxidant. Interestingly, when using Selectfluor as
The increasing attention in outstanding biomedical
applications of substituted indoles led to the recent development
of methods for the access to 2,3-disubstituted indoles.[12]
Therefore, we next tested the reactivity of gold acetylide 4 bearing
a
2-N,N-dimethylaniline moiety in such cyclization/cross-
coupling reaction. Under catalysis of Pd(PPh3)4, in the
presence of PPh3AuOTf as co-catalyst, cyclization/cross-coupling
of 4 with excess PhI followed by a N-demethylation reaction led
to the diarylation product 2,3-diphenyl indole 5 in a high yield
92%~94% either at elevated temperature (50 oC) within 1 h or at
room temperature within 5 h (Eqn 1, Scheme 4). Without
PPh3AuOTf as co-catalyst, a decrease of reaction rate was
observed. Under identical reaction condition, 5 was obtained in a
76% yield at 50 oC within 1 h or in a 59% yield at room temperature
within 5 h (Eqn 1, Scheme 4). Again, addition of catalytic amounts
of PPh3AuOTf could accelerate the reaction rate in the same way
as observed for the reaction of 1. In the presence of stoichiometric
PPh3AuOTf, vicinal diaurated alkene intermediate 6 could also be
obtained by the cyclization of 4 (Eqn 2, Scheme 4). A similar tetra-
aurated indole complex, which was previously reported by Zhao
group, has been synthesized through the cyclization of o-
(trimethylsilylethynyl)aniline with [(PPh3Au)3(μ3-O)](BF4).[13] As
expected, in the presence of a catalytic amount of Pd(PPh3)4, the
digold 6 could also undergo cross-coupling with PhI to form 5 in
87% yield at 50 OC or in 35% yield at room temperature (Eqn 2,
Scheme 4), suggesting the role of 6 as a reaction intermediate.
o
an oxidant, reacting of digold species 8 with PhB(OH)2 at 25 C
led to monoarylated product 9, instead of the diarylation product
(Scheme 6). Finally, in the presence of 1.1 equivalent of
AuCl.Me2S, excess PhB(OH)2 and Selectfluor, 9 could also be
directly prepared from 7. Similar to the homo-coupling reaction,
the cross-coupling also occurred at Au-C5 bond adjacent to the
carbonyl moiety in 8, whereas another Au–C4 bond adjacent to
the N-substituent in 8 kept intact during cross-coupling step. The
outcome is probably due to the stronger Au–carbene bond at C4
position, compared to the Au–C5 one adjacent to the carbonyl
moiety.
Scheme 5. Our previous work on cyclization/homo-coupling reaction of terminal
alkyne 7.8
Figure 2. X-ray crystal structure of 9. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å): Au1−C4 1.985(4), Au1−Cl1 2.2909(15),
C3−C4 1.368(6), C2−O1 1.221(5).
3
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The structure of 9 was established by the single crystal X-ray
diffraction analysis (Figure 2). The Au-C bond distance (1.985(4)
Å) is similar to those in the abnormal carbene (aNHC) Au
complexes (1.981(5) Å[14a]) and in the normal NHC Au complex
IPrAuCl (1.942(3) Å[14b]). The 13C resonance of the C4 carbon in
9 at 184.4 (s) ppm is deshielded compared to the CNHC resonance
in AuCl(IPr) (175.1 ppm).[14b] These observations indicate Au
complex 9 can be better described as mesoionic normal NHC-
gold complex.
Ditopic NHC ligands incorporating two carbene centers in a
same heterocycle ring have recently emerged as the tuneable
molecular scaffolds that can bridge transition metals.[16] Some
ditopic NHC homo- and heterodimetallic complexes, such H and
I, have found applications in a range of challenging catalytic
tandem reactions.[17] Recently, we have developed coinage metal
complexes J and K, based on 1,2- or 1,4-ditopic NHC.[8,18] So far,
the known 1,3-ditopic NHC ligands are those having a five-
membered heterocyclic ring. Since gold complex 9 has an H
substituent at the C2-position, we suppose that the deprotonation
of the C2-position of 9 followed by metal coordination, would offer
a desirable six-membered 1,3-ditopic NHC digold complex L.
Different from six-membered 1,2- and 1,4-ditopic NHCs, both
containing one normal NHC centre, six-membered 1,3-ditopic
NHC in L, could be described as anionic NHDC containing two
normal NHC centres in the same heterocycle. As expected, upon
deprotonation of
9
by potassium bis(trimethylsilyl)amide
(KHMDS) and the subsequent complexation with PPh3AuCl, a
1,3- ditopic NHC digold complex 10 was obtained (Scheme 8).
Notably, upon treatment of 9 with KHMDS and PPh3AuCl, the
chloride ligand is exchanged by a PPh3 in the resulting complex
10. Ditopic NHC-based heterodinuclaer Au/Ag complex 11 could
also be prepared by this synthetic approach. The single crystal X-
ray crystallographic studies established the molecular structure of
10 (Figure 4) and 11 (please see Figure S2). In 10, the Au–C4
bond [i.e., Au2–C2, 2.057(6) Å] is somewhat longer than the Au–
C2 bond [i.e., Au1–C1, 1.994(6) Å], and the two Au-C bonds are
marginally longer than those in the related five-membered NHDC
digold complex [ClAu:C{[N(2,6-iPr2C6H3)]2CHCAu(PPh3)}] [i.e.,
Au–C4, 2.022(5) Å; Au–C2, 1.975(5) Å],[19] respectively. The Au–
C4 bond [i.e., Au1–C4, 2.046(4) Å] in 11 is similar to that in 10.
The Ag-C2 bond [i.e., Ag1–C1, 2.084(4) Å] in 11 is in the typical
range for NHC-Ag complexes.[20]
Scheme 7. Gold complex 9 catalyzed addition of aryl amine into phenyl
acetylene. Reaction conditon: 1.1 mmol of amine, 1 mmol of phenylacetylene,
1 mL of CD3CN. Yields were determined by 1H NMR spectroscopy on the basis
of the amount of phenyl acetylene remaining in solution.
It’s well-established that NHC-transition metal complexes
bearing sterically bulky N-aryl substituents on the NHCs
often exhibit high stability and catalytic performance in various
organic reactions. Considering 9 contain bulky substituents at
both nitrogen atom and at the α position of the carbene carbon
atom, we were highlighted that 9 could be used as a potential
candidate in gold catalysis (Scheme 7). The preliminary tests
proved 9 an efficient catalyst in the addition of arylamine into
alkyne with a comparable activity to IPrAuCl.[15]
Figure 3. Known ditopic NHC complexes and the targeted six-membered ditopic
NHC complex L.
Figure 4. X-ray crystal structure of 10. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å): Au1−C1 1.994(6), Au2−C2 2.057(6), O1−C5
1.221(7), N2−C1 1.341(7), N2−C2 1.399(7), C2−C6 1.373(8), C5−C6 1.454(9).
Scheme 8. Synthesis of ditopic NHC metal complexes 10 and 11 from 9.
Figure 5. Various mesomeric structures for 10 and 11.
4
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In summary, we have developed Pd-catalyzed or Au-
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synthesize 3,4-diphenyl isoquinoline and 2,3-diphenyl indole, as
well as six-membered mesoionic NHC complex. The mesoionic
NHC complex could further undergo deprotonation/complexation
to prepare 1,3-NHDC Au2 and Au/Ag complexes. Two key vicinal
diaurated alkene intermediates have been isolated and fully
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Financial support from the National Natural Science Foundation
of China (No. 21671066, 21673077, 21972045) is greatly
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10.1002/chem.202001502
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3,4-Diphenyl isoquinoline and 2,3-diphenyl indole have been synthesized by catalytic bis-arylative endo cyclization of gold acetylides.
By using the synthetic approach six-membered mesoionic NHC complex has also been obtained, which could be used as precursor
for the synthesis of 1,3-NHDC Au2 and Au/Ag complexes. Catalytically relevant vicinal diaurated alkene intermediates have been
isolated and fully characterized.
6
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