Copper-Catalyzed Coupling of Triaryl- and Trialkylindium Reagents with Aryl Iodides and Bromides through Consecutive Transmetalations

Article abstract of DOI:10.1002/anie.201407586

An efficient copper(I)-catalyzed coupling of triaryl and trialkylindium reagents with aryl iodides and bromides is reported. The reaction proceeds at low catalyst loadings (2 mol %) and generally only requires 0.33 equivalents of the triorganoindium reagent with respect to the aryl halide as all three organic nucleophilic moieties of the reagent are transferred to the products through consecutive transmetalations. The reaction tolerates a variety of functional groups and sterically hindered substrates. Furthermore, preliminary mechanistic studies that entailed the synthesis and characterization of potential reaction intermediates offered a glimpse of the elementary steps that constitute the catalytic cycle.

Full text of DOI:10.1002/anie.201407586

Angewandte
Chemie
DOI: 10.1002/anie.201407586
Cross-Coupling
Copper-Catalyzed Coupling of Triaryl- and Trialkylindium Reagents
with Aryl Iodides and Bromides through Consecutive
Transmetalations**
Surendra Thapa, Santosh K. Gurung, Diane A. Dickie, and Ramesh Giri*
Abstract: An efficient copper(I)-catalyzed coupling of triaryl
and trialkylindium reagents with aryl iodides and bromides is
reported. The reaction proceeds at low catalyst loadings
(2 mol%) and generally only requires 0.33 equivalents of the
triorganoindium reagent with respect to the aryl halide as all
three organic nucleophilic moieties of the reagent are trans-
ferred to the products through consecutive transmetalations.
The reaction tolerates a variety of functional groups and
sterically hindered substrates. Furthermore, preliminary mech-
anistic studies that entailed the synthesis and characterization
of potential reaction intermediates offered a glimpse of the
elementary steps that constitute the catalytic cycle.
do not work well with ortho-substituted or sterically hindered
substrates.[12] Herein, we wish to report a CuI-catalyzed
coupling of triaryl- and trialkylindium reagents with aryl
iodides and bromides that proceeds through three consec-
utive transmetalations and tolerates sterically hindered sub-
strates. Furthermore, we have conducted preliminary mech-
anistic studies and propose a catalytic cycle that incorporates
the consecutive transmetalations.
We recently reported that a combination of the ligand PN
(Table 1) and CuI generated active catalysts that enabled the
cross-coupling of aryl silicon[8a] and aryl boron[9a] reagents
with aryl iodides. As part of our efforts to expand the scope of
I
À
Cu -catalyzed coupling processes for C C bond formation, we
C
ross-coupling reactions remain versatile synthetic methods
discovered that PN/CuI was an efficient catalyst for the
coupling of triphenylindium with para-iodotoluene and
afforded the product in 87% yield without requiring a base
(yield determined by GC; Table 1, entry 1). The reaction
provided only 10% of 3 in the absence of InCl3 (entry 2).
that are capable of coupling a range of organometallic
reagents with organohalides and surrogates to construct
carbon–carbon bonds.[1,2] As a result, applications of these
coupling processes encompass a wide array of synthetic
targets, ranging from the manufacturing of materials and
pharmaceuticals to the synthesis of building blocks and
natural products.[3] Aside from traditional organometallic
reagents (such as RBX2, RSiX3, RZnX), triorganoindium
reagents (R3In) are increasingly gaining attention as efficient
partners for palladium-catalyzed cross-couplings.[4] Further-
more, organoindium reagents have also been shown to
undergo transmetalation with copper; this was first demon-
strated for allylic substitution reactions.[5] The popularity of
R3In rests on its ability to efficiently transfer all of its three
organic nucleophilic moieties (R) onto the products,[6]
thereby generating only 0.33 equivalents of indium halide as
a byproduct.[7]
Table 1: Cross-coupling of triphenylindium with para-iodotoluene.[a]
Entry
Modified conditions
Yield [%]
1
2
3
4
5
6
–
87
10
84
25
PhLi instead of Ph3In (without InCl3)
Ph3In[b]
Ph3In (0.33 equiv)
Ph3In (0.33 equiv), CsF (1 equiv)
Ph3In (0.33 equiv), NaOMe (1 equiv), 1008C
45
97 (92)[c]
Recently, we and others have shown that CuI catalysts
effect the cross-coupling of organosilicon[8] and organo-
boron[9] reagents with organohalides.[10] These transforma-
tions typically work for the coupling of aryl metal reagents
with aryl iodides.[11] However, the reported coupling reactions
[a] The reactions were run on a 0.10 mmol scale in DMF (0.5 mL). Ph3In
was generated in situ from the reaction of PhLi with InCl3. Yields
determined by GC analysis using 2-nitrobiphenyl as the internal
standard. [b] Partially purified to remove excess LiCl. [c] The value in
parentheses gives the yield of isolated product for a reaction on
a 1.0 mmol scale.
[*] S. Thapa, Dr. S. K. Gurung, Dr. D. A. Dickie, Prof. Dr. R. Giri
Department of Chemistry & Chemical Biology
University of New Mexico
Furthermore, the reaction with Ph3In that had been partially
purified to remove excess LiCl also afforded 3 in comparable
yields, suggesting that the halide salt does not play a role in
the cross-coupling (entry 3).
However, despite the potential to transfer all three phenyl
groups onto the products, the reaction of 0.33 equivalents of
Ph3In afforded the product only in 25% yield (entry 4).
Addition of CsF improved the yield only marginally (45%).
Investigations with in situ generated potential intermediates
that were likely to be formed after the first (Ph2InCl and
Albuquerque, NM 87131 (USA)
E-mail: rgiri@unm.edu
Homepage: https://sites.google.com/site/rgirichemistry/home
[**] We thank the University of New Mexico (UNM) for financial support
and upgrades of the NMR (NSF grants CHE08-40523 and CHE09-
46690) and mass spectrometry facilities. The Bruker X-ray diffrac-
tometer was purchased by an NSF CRIF:MU award to UNM
(CHE04-43580).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407586.
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Ph2InF) and the second (PhInCl2 and PhInF2) transmetala-
tions in the absence and presence of CsF indicated that these
latter species were inactive for further reactions (Scheme 1).[7
c,e,h]
Gratifyingly, studies with other anion sources (NaOMe,
MeLi, and Me2NLi) revealed that the phenylindium species
Ph2In(OMe) and PhIn(OMe)2, which were generated in situ
from the reaction of two or one equivalents of PhLi with InCl3
in the presence of one or two equivalents of NaOMe, were
reactive and provided the cross-coupled product in 75% and
63% yield, respectively (Scheme 1). After identifying MeOÀ
as an effective anion, we conducted the reaction with only
0.33 equivalents (33.3 mol%) of Ph3In with para-iodotoluene
in the presence of one equivalent of NaOMe and 2 mol% of
PN/CuI. The reaction proceeded at 1008C and afforded the
product in 97% yield in 24 hours (entry 6).
After optimizing the reaction conditions, we examined the
substrate scope of the current reaction. The transformation
displays a broad substrate scope, and the coupling of electron-
rich or electron-poor aryl iodides with electron-neutral or
electron-rich organoindium reagents proceeded well
(Table 2A). The coupling of aryl iodides that contain highly
sensitive functional groups, such as esters or nitriles, gave the
desired products in good yields (27–30). With heteroaryl
iodides, the reaction proceeded in the absence of PN to
Scheme 1. Reactions of various phenylindium species that can be
formed by consecutive transmetalations with para-iodotoluene. Reac-
tion conditions: 0.10 mmol scale, PN/CuI (2 mol%), 1208C, 24 h in
DMF (0.5 mL). The phenylindium reagents were prepared from the
reaction of InCl3 with the required amount of the corresponding base
(MeLi, LiNMe2, or NaOMe) and PhLi in THF at room temperature.
Yields were determined by GC analysis using 2-nitrobiphenyl as the
internal standard.
provide the corresponding products in excellent yields, and
a variety of functional groups, such as alkoxy and chloride
moieties, were tolerated (Table 2B). This method is also
Table 2: Coupling of triarylindium reagents with aryl and heteroaryl iodides.[a]
[a] Reactions were run on a 1.0 mmol scale in DMF (5 mL) with PN/CuI (2 mol%) at 1008C for 24 hours. No PN was used for the coupling of
heterocyclic substrates. Ar’3In reagents (0.33 equiv) were prepared in situ from the reaction of InCl3 with the corresponding ArLi reagent in THF at
room temperature. Yields of isolated products are given. [b] Ar3In (0.5 equiv). [c] Ar3In (1.0 equiv). [d] 1208C.
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Table 4: Coupling of trialkylindium reagents with aryl and heteroaryl
applicable to the arylation of diiodoarenes; both iodo groups
were arylated with ease (Table 2C). Unlike previously
reported CuI-catalyzed cross-couplings with organoboron
and organosilicon reagents,[8,9] the current reaction works
very well for ortho-substituted and sterically hindered sub-
strates, even for tris(2-methoxyphenyl)indium and tris(2-
methylphenyl)indium reagents with 2-isopropyl- and 2,6-
dimethyliodobenzene, respectively (Table 2D). However,
these reactions required stoichiometric amounts of Ar3In
and a higher temperature (1208C).
iodides.[a]
The current reaction conditions also allow for the efficient
cross-coupling of triarylindium reagents with aryl bromides
(Table 3). Typically, the reaction proceeded well with neutral
and electron-poor aryl bromides, affording the corresponding
products in good yields. A variety of heteroaryl bromides
[a] Reactions were run on a 1.0 mmol scale in DMF (5 mL). R3In reagents
were prepared in situ from the reaction of InCl3 with RLi or RMgX
(3 equiv) in THF at room temperature. Yields of isolated products are
given. [b] The reaction of PhI with (iPr)3In provided the cross-coupled
product in only 30% yield based on 1H NMR spectroscopy. R3In reagents
for the synthesis of products 58–62, 64, and 65 were generated from the
reactions of the corresponding RMgX reagents with InCl3.
Table 3: Coupling of triarylindium reagents with aryl and heteroaryl
bromides.
Scheme 2. Proposed catalytic cycle incorporating consecutive trans-
metalations.
[(PN)CuOMe] (66) is readily generated from the reaction
of [(PN)CuI] with NaOMe at room temperature. The
formation of [(PN)CuOMe] was confirmed by comparison
[a] Reactions were run on a 1.0 mmol scale in DMF (5 mL). Ar’3In
reagents were prepared in situ from the reaction of InCl3 with the
corresponding ArLi reagent (3 equiv) in THFat room temperature. Yields
of isolated products are given. [b] Ar3In (0.5 equiv). [c] Ar3In (1.0 equiv).
1
of the H and 31P NMR signals of the reaction mixture with
those of NaOMe and independently synthesized and fully
characterized samples of [(PN)CuI][9a] and [(PN)CuOMe].
[(PN)CuOMe] was synthesized by stirring [(PN)CuOtBu],
which had been generated in situ by mixing a 1:1 ratio of PN
and CuOtBu, with MeOH in THF at room temperature
(Scheme 3) and characterized by NMR spectroscopy, ele-
mental analysis, and single-crystal X-ray diffraction. Based on
could also be coupled with different triarylindium reagents to
generate the desired products in excellent yields. Further-
more, the reaction can be extended to the coupling of
trialkylindium reagents with aryl iodides to afford the cross-
coupled products in good to excellent yields (Table 4).
However, these reactions required stoichiometric amounts
of the organoindium reagents. The reaction proceeded with
both primary and secondary alkylindium reagents. Unlike
with trialkylindium species that had been prepared from
alkyllithium reagents (63), the reactions of alkylindium
reagents that had been generated from Grignard reagents
and InCl3 proceeded in the absence of NaOMe (58–62, 64,
and 65).
Scheme 3. Synthesis of [{(PN)CuOMe}2] (66).
its X-ray structure, [(PN)CuOMe] exists as a methoxy-
bridged dimer in the solid state (Figure 1). Complex 66 is
also catalytically active and afforded the cross-coupled
product 3 in 92% yield (determined by GC) from the
reaction of Ph3In (0.33 equiv) with para-iodotoluene under
the standard reaction conditions. Therefore, we anticipate
that R3In and the intermediate species R2In(OMe) and
We further conducted preliminary mechanistic studies,
and a possible catalytic cycle is presented in Scheme 2. We
showed by 1H and 31P NMR studies in [D7]DMF that
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stoichiometric reactions by NMR spectroscopy to elucidate
the sequence of elementary steps in the catalytic cycle.
Received: July 24, 2014
Revised: August 20, 2014
Published online: && &&, &&&&
Keywords: copper · cross-coupling · N,P ligands ·
organoindium reagents · transmetalation
.
Figure 1. X-ray crystal structure of 66. Selected bond lengths [ꢀ] and
angles [8]: Cu(1)–O(1) 1.995(5), Cu(1)–P(1) 2.1234(19), Cu(1)–N(1)
2.421(6); O(1)-Cu(1)-P(1) 140.78(14), O(1)-Cu(1)-N(1) 100.1(2), P(1)-
Cu(1)-N(1) 84.02(14).
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(Scheme 4). The reaction of Ph3In (0.33 equiv) with para-
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intermediates.
of TEMPO (93% yield as determined by GC). Furthermore,
the radical probe 67 reacted with Ph3In (0.33 equiv) under the
standard reaction conditions to afford the arylated product 68
in 75% yield. However, the cyclized product 69, which was
expected to arise from the presence of the corresponding
ortho-(3-butenyl)phenyl free radical, was not detected. These
experiments suggest that the transformation is unlikely to
proceed via free aryl radical intermediates,[13] implying that
the reaction likely proceeds by an oxidative addition/reduc-
tive elimination pathway.[14]
In summary, we have developed an efficient copper(I)-
catalyzed coupling of triorganoindium reagents with aryl
iodides and bromides. All of the three organic nucleophilic
moieties of the reagent were transferred to the products
through three consecutive transmetalations. Aside from
triarylindium reagents, the reaction also proceeded well
with trialkylindium compounds for alkyl–aryl cross-couplings.
The transformation tolerates sterically hindered aryl halides
and ortho-substituted triarylindium reagents. Preliminary
mechanistic studies were conducted by independently syn-
thesizing possible reaction intermediates and by following the
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[13] The ortho-(3-butenyl)phenyl radical generated from the radical
probe 67 is known to cyclize in DMF at 508C to give the cyclized
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A. L. J. Beckwith, J. Chem. Soc. Chem. Commun. 1986, 464.
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radicals, caged aryl radicals stabilized by RCuII species that can
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Chem. 2014, 79, 2771.
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Communications
Cross-Coupling
S. Thapa, S. K. Gurung, D. A. Dickie,
R. Giri*
&&&& — &&&&
Copper-Catalyzed Coupling of Triaryl- and
Trialkylindium Reagents with Aryl Iodides
and Bromides through Consecutive
Transmetalations
All-out cross-coupling: All three nucleo-
philic moieties of triorganoindium
reagents can be transmetalated onto
copper(I) catalysts in the presence of
NaOMe, which enables the coupling with
aryl iodides and bromides in a carbon–
carbon bond-forming process. The
transformation tolerates a wide range of
functional groups and proceeds well with
sterically hindered substrates and tri-
alkylindium reagents.
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