Gold-catalyzed oxidative couplings of two indoles with one aryldiazo cyanide under oxidant-free conditions

Article abstract of DOI:10.1039/c7cc01304c

Gold-catalyzed oxidative couplings of two indoles and one α-cyano gold carbene to form bis(indolyl)methane derivatives are described. Two different indoles are compatible with these reactions to provide reasonable yields. A plausible mechanism is postulated to rationalize the experimental data including product distributions, D₂O labeling, and the significant effects of gold catalysts and cyano groups.

Full text of DOI:10.1039/c7cc01304c

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Gold-catalyzed Oxidative Couplings of Two Indoles with One
Aryldiazo Cyanide under Oxidant-free Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
RahulKumar Rajmani Singh, and Rai-Shung Liu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Gold-catalyzed oxidative couplings of two indoles and one -
cyano gold carbene to form bis(indolyl)methane derivatives and
hydrogen are described. Two different indoles are compatible
with these reactions to provide reasonable yields. A plausible
mechanism is postulated to rationalize experimental data
including product distributions, D2O labeling, and significant
effects of gold catalysts and cyano groups
Metal carbene species are versatile to react with small
molecules to form new C-X bonds (X = C, O, N).[1-2] In the
presence of alcohol, amines and electron-rich arenes or
heteroarenes, metal carbenes typically afford X-H insertion
products (X = C, O or N) over various metal catalysts including
Rh, Au, Ag, Ru, Pd or Cu;[3-4] asymmetric O-H insertions of
phenols have been satisfactorily achieved with chiral Cu and
Pd catalysts.[5] For some nucleophiles, Hu and coworkers
reported the reactions of carbon electrophiles with C-bound
well developed.[10] These natural products show diverse
biological activity ranging from cancer inhibition to
antibacterial effect,[11] thus inspiring a synthesis of new
derivatives bearing two different indoles or a bridged tertiary
carbon, but their synthetic procedures are quite long.[12]
Table 1 optimizes the reaction of compound 1a with diazo
rhodium enolates
I (EWG = CO2R) to give ,-addition
products.[6] In seeking new carbene chemistry, we report
herein gold-catalyzed oxidative couplings of two indoles with
one cyano gold carbene to form 2-substituted 2,2-
di(indolyl)acetonitriles efficiently. Particularly interesting is the
dication synthons[7] of gold carbenes to allow a double attack
of indole nucleophiles. In several instances, two different
indoles were also compatible with this catalytic reaction.
Oxidative coupling of two aryl C-H bonds to form a biaryl
species represents a recent advance in metal catalysis,[8] but
the oxidative coupling of two nucleophiles with one metal
carbene has no precedent in the literatures. In this system, no
external oxidant is required because of the evolution of
molecular hydrogen.[9].
cyanide 2a (1a/2a = 2.1) using various gold catalysts. Under
these conditions, diazo decomposition might occur, but to a
small extent. We tested the reactions of various gold catalysts
L’AuCl/AgNTf2 *L’ = P(t-Bu)2(o-biphenyl), IPr, PPh3, P(C6F5)3, P(-
OPh)3]; notably, electron-deficient gold complexes (L = P(C6F5)3,
P(OPh)3) tended to give double addition products 3a in 48-52%
yields (entries 4-5) whereas other gold species gave
hydroarylation product 4a in significant amount (18-60%,
entries 1-3). AgOTf alone was catalytically inactive, so leading
to recover initial 1a in 65% (entry 6). For P(OPh)3AuCl/AgOTf,
the yields of desired 3a in DCE, acetonitrile and acetone were
40%, 55% and 30%, respectively (entries 7-9). To our pleasure,
P(OPh)3AuCl/AgOTf at 10/50 mol % loading , increased the
yield of desired 3a up to 68% (entry 10). Other silver salts
AgNTf2 and AgSbF6, at 50 mol % loading, became inefficient for
these reactions; desired 3a were produced in only 25-38%
yields (entries 11-12). Under the conditions in entry 10,
Our new synthesis is accessible to naturally occurring
bis(indol-3-yl)methanes that are known as the family of indole
alkaloids (see Figure S1); their convenient synthesis has been
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phenyldiazo ester 2a’ and indole 1a were catalyzed by
Journal Name
The scope of this catalytic reaction is further expanded with
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DOI: 10.1039/C7CC01304C
P(OPh)3AuCl/AgOTf (10/50 mol %) in DCM to afford a typical its compatibility to diverse aryl- or alkyl-substituted diazo
hydroarylation product 3a’ in 71% yield (eq 1) [3a,b]
reflecting cyanides, as depicted in Table 3. We prepared various para-
,
the key role of a cyano group. In entry 10, we monitored the substituted phenyldiazo cyanides 2b
-2e to afford our targets
gaseous components with GC (MS 5Ǻ), detecting H2 in ca. 35%
yield; some H2 might be soluble in the solution.
Table 2. Double indolylation with various indoles
Table 1. Optimization under various conditions
3p
-
3s (X = Cl, Br, CF3 and Me) in satisfactory yields (54-68%,
2h
entries 1-4). For meta-substituted phenyldiazo cyanides 2f
-
,
their resulting products were obtained in 54-60% yields (X = Cl,
Br or OMe, entries 5-7). Particularly notable is an alkyl-
substituted diazo cyanide 2i that delivered bis(indolyl)methane
species 3w-3x in 52-58% yields (entries 8-9), together with
cinnamontrile (E/Z = 1.3, 41-36 %), resulted from a 1,2-H shift
of gold carbene intermediates. The molecular structure of
bromo derivative 3q was characterized with X-ray
diffraction.[13]
To assess the scope of reactions, Table 2 shows the
compatibility of this gold catalysis with various indole Table 3. Double indolylations on various diazo cyanides
derivatives. We tested the reactions on various N-alkyl indole
derivatives 1b
-
1d
,
yielding
-phenyl bis(indolyl)methane
derivatives 3b
-
3d (R
=
n-propyl, n-butyl, isopropyl) in
reasonable yields (53-61%) whereas a hydroarylation product
4c was isolated in 15% for a n-butyl substituent (entries 1-3).
For unsubstituted and N-allyl indoles 1e and 1f (R = H and allyl),
their corresponding products were obtained in 51-53% yields
(entries 4-5). This reaction was extendible to N-methyl 2-
methylindole 1g to yield desired 3g in 63% yield (entry 6). We
prepared various phenyl-substituted indoles 1h
Cl, 6-Br and 6-Me) to produce compounds 3h
yields (entries 7-10). The reactions were extendible to various
N-aryl indoles 1l 1o, yielding the coupling products 3l 3o in 52-
62% (entries 11-14). We tested the reaction of N,3-dimethyl-
1H-indole S1 derivative with diazo nitrile 2a giving
-
1k (X = 5-Br, 5-
-
3k in 61-72%
-
-
,
a
complicated mixture of products (see SI, eq. s1). Other
electron-rich aromatic compounds such as furans and 1,3,5-
tromethoxybenzenes are inapplicable substrates. Whereas N-
Phenyl pyrrole gives hydroarylation product S3 in 45% yields
(eq. s2).
This new synthesis is further applicable to non-symmetric
bis(indolyl)methane derivatives which are synthetically
challenging. As depicted in Table 4, our new approach employs
5
N-aryl and N-alkyl indoles in comparable proportions, enabling
2 | J. Name., 2012, 00, 1-3
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the cross-coupling products 5a
(42-62%). Hydroarylation species
byproducts (12-18%) whereas other homo-coupling products
were present in small portions (< 6%, see Table S1). The product d-4a, a large deuterium content was found only at the
presence of two main products and suggests that N-aryl- C(CN)Ph carbon whereas its indolyl C(2) carbon was not
-
5f to become major products
In the presence of added D2O (3.0 equiv), the reaction of
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DOI: 10.1039/C7CC01304C
4
were present as the main indole 1a and phenyldiazo cyanide 2a afforded our target 3a
3
bearing no deuterium at all (eq 2). For its hydroarylation
4
5
indoles reacted first with gold carbenes whereas N-alkyl deuterated at all. This deuterium distribution helps to
indoles functioned as the second nucleophiles. In this reaction understand the reaction mechanism.
sequence, N,N-dimethylaniline was a suitable partner to react
with N-(4-methoxyphenyl)-1H-indole 1n and diazo nitrile 2a to
give a desired coupling product S3 in 51% yield (eq. s4).
Table 4. Double indolylations with two different indoles
As shown in Scheme 2, we postulate that N-aryl indoles
react with gold carbenes
analogues. The indolyl C(3)-H protons of species
acidic and become readily deprotonated by OTf- to form gold-
containing indole species that can react with HOTf to cleave
the Au-C bonds to form unsaturated iminiums and LAuH.
A
more rapidly than their N-alkyl
B
are highly
C
D
This process will be favourable with electron-deficient ligand L.
A subsequent attack of N-alkyl indoles at these electrophilic
iminiums
E affords double indolylation products 5 ultimately;
the released proton herein reacts with LAu-H to form H2 and
LAu+. To rationalize D2O experiments (eq 2), we postulate that
the cyano groups of species
fragment is an electron-rich group. Such a protonation leads to
formation of nitrilium species
, to induce a release of LAu+ to
Cyano can be elaborated into various functional groups to generate 3-ketenimines and hydroarylation products . In
C can be protonated if gold
F
G
4
expand synthetic utility; for instance, the treatment of species the case of phenyldiazo ester, protonation of an ester is more
3e and 3y with MeMgCl[14a,b] in toluene led to their facile relative to a cyano group, thus yielding only an
methylation products 6a and 6b with yields > 84%, as depicted hydroarylation products 3a’ exclusively. This mechanism
in Scheme 1. The same reactions using n-C4H9MgBr yielded n- rationalizes well the preference of these reactions with
alkylation products 6c and 6d efficiently. Species 3e was electron-deficient gold catalysts and
subjected to LiAlH4 reduction in THF to give natural occurring The success of this non-symmetric bis(indol-3-yl)methane
in 65% yield. For benzyl-substituted synthesis relies on the varied rates of two independent gold-
derivative 3y, this LiAlH4 reaction led to an elimination of catalyzed reactions, i.e. and . We envisage that N-
HCN[14c] to afford compound 6f in 70% yield. A subsequent aryl indoles stabilize the key iminium ion
better than their N-
hydrogenation of resulting 6f with Pd/C/H2 yielded another alkyl analogues thus becoming the dominant species of
natural compound 1,1-bis(1H-indol-3-yl)-2-phenylethane intermediate . The initial sequence presumably proceeds
in 88% yield. Notably, these reaction patterns may more rapidly than the latter process to consume most N-aryl
-cyano gold carbenes.
[10d, h]
turbomycin B (6e
)
1
+A
→
D
D→5
B
,
D
[10g, 11e]
(6g)
vary with N-substituted indole derivatives. Treatment of 3a indoles.
with MeMgCl in toluene at 135 oC delivered corresponding
decynated product in 72% yields (eq. s3)
Scheme 1. Various functionalizations of cyano groups.
Scheme 2. A plausible reaction mechanism
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7
8
Metal carbenes as dication synthons was previous reported
in a stoichiometric fashion whereas this systemViisewpArretifcolerOmnleinde
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In summary, gold-catalyzed oxidative couplings of two
indoles with one 
-cyano gold carbene[15] under oxidant-free
conditions are described; these transformations manifest new
roles of gold carbenes as dication synthons. These new
reactions are preferable with electron-deficient gold catalysts
and diazo cyanides, over a wide range of substrate scope. Two
different indoles were also compatible with this reaction with
reasonable yields. A plausible mechanism is postulated to
rationalize D2O result, product distributions, and significant
effects of gold catalysts and cyano groups.
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Table of Content:
In the absence of external oxidants, gold-catalyzed oxidative couplings of two indoles with one phenyldiazo cyanide is
described; these annulations work well with a good range of diazo and indole species.