Rapid threefold cross-couplings with sterically bulky triarylbismuths under Pd-Cu dual catalysis

Article abstract of DOI:10.1039/c5ra23311a

The threefold cross-coupling reactivity of sterically highly demanding bulky triarylbismuths was addressed with task specific Pd-Cu dual catalytic conditions. In this study, an unprecedented hitherto unknown cross-coupling reactivity of sterically bulky triarylbismuths was demonstrated with a diverse range of aryl iodides and bromides. The intermediacy and in situ formation of arylcopper was probed with mesitylcopper in support of the proposed dual catalysis. This study opened up an easy way forward for application of sterically bulky bismuth reagents in threefold aryl-aryl coupling reactions.

Full text of DOI:10.1039/c5ra23311a

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Rapid threefold cross-couplings with sterically
bulky triarylbismuths under Pd–Cu dual catalysis†
Cite this: RSC Adv., 2016, 6, 1012
*
Maddali L. N. Rao and Ritesh J. Dhanorkar
The threefold cross-coupling reactivity of sterically highly demanding bulky triarylbismuths was addressed
with task specific Pd–Cu dual catalytic conditions. In this study, an unprecedented hitherto unknown cross-
coupling reactivity of sterically bulky triarylbismuths was demonstrated with a diverse range of aryl iodides
and bromides. The intermediacy and in situ formation of arylcopper was probed with mesitylcopper in
support of the proposed dual catalysis. This study opened up an easy way forward for application of
sterically bulky bismuth reagents in threefold aryl–aryl coupling reactions.
Received 5th November 2015
Accepted 1st December 2015
DOI: 10.1039/c5ra23311a
www.rsc.org/advances
The cross-couplings of organobismuth reagents demonstrated demanded by the difference in reactivity of several of these
remarkable reactivity under palladium-catalyzed condi- organometallic nucleophiles. Surprisingly, the corresponding
a
tions.1–5 The studies of 5,6,7,12-tetrahydrodibenz[c,f][1,5]aza- coupling reactivity using sterically bulky ortho substituted
bismocines and monoorganobismuth alkoxides reected the bismuth reagents such as 1a–1f (Fig. 1) was largely unknown.9,10
consistent and facile reactivity in aryl–aryl bond formations.2 As In that context, there is a desperate need to explore the
a step forward in this direction, triarylbismuths excelled in an couplings of sterically bulky triarylbismuth reagents for the
exemplary way with the use of sub-stoichiometric one-third development of viable protocol conditions.
amount delivering threefold cross-couplings in a one-pot
The threefold coupling reactivity of sterically demanding
operation.3–5 In this regard, the utilization of triarylbismuths triarylbismuth reagents is complicated due to unavoidable
in coupling reactions offer many synthetic advantages which sterics rendered with the presence of ortho functionalization. To
includes high coupling reactivity, sub-stoichiometric loading of circumvent this steric burden, it was reasoned that the devel-
the reagents, short reaction durations etc.
opment of a dual catalytic protocol comprising Pd–Cu system
Aryl–aryl bond formation is one of the most studied process would pave the way for the desired coupling process. Here the
in cross-coupling chemistry as biaryl scaffolds are high in plan was to generate monoarylcopper species in situ from
demand in various applications.6 The coupling reactivity of copper halide and arylbismuths involving transmetalation
sterically bulky organometallic nucleophiles has been a chal- during dual catalytic coupling process. The envisioned
lenging task. These studies using various organometallic supportive role of copper halide in Pd catalysis was favored by
reagents have been addressed consistently with rational design
of ligand and/or catalytic conditions.7 For example, recent
studies of Giri et al. revealed the facile reactivity of organo-
indiums under copper-catalyzed conditions involving consecu-
tive transmetalations.7a
Another recent report by Espinet et al. on Stille couplings
also showed novel couplings using bulky organotin reagents
under gold catalysis.7b Similarly, other coupling studies with
bulky organoboron,7c–e organozinc,7f organolithiums,8a organo-
magnesium,8b organotitanium8c and organoindium8d etc. have
been explored under various metal cross-coupling conditions
oen invoking the support from specialized ligand as
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, India.
E-mail: maddali@iitk.ac.in; Fax: +91-512-2597532; Tel: +91-512-2597532
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data and spectral data for all products was given. CCDC for 1a
(1005303), 1b (1005301), 1c (1005300), 1d (1005302) and 1f (1005299). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra23311a
Fig. 1 Structures of 1a–1d and 1f from single crystal XRD; 1e is DFT
optimized structure (by GAUSSIAN 09 at B3LYP/LANL2DZ level).
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literature reports11 involving aryl/heteroaryl transfer from tin or our polar DMF solvent.11b This result unambiguously revealed
boron in the Stille11b,c and Suzuki11d coupling reactions. To our the effective and catalytic role of CuI in furnishing high cross-
surprise, this role of copper was never invoked so far in Pd- coupling reactivity under palladium-catalyzed conditions and
catalyzed bismuth coupling reactions. This made us to curi- it was considered as our standardized protocol (entry 7). In all,
ously investigate this reactivity for the advantage of couplings the above screening resulted in the development of a robust Pd–
with sterically demanding bismuth reagents. In fact, this Cu dual catalytic protocol for efficient coupling of sterically
approach lived up to its expectation to deliver an unprecedented congested bismuth reagent under mild reaction conditions.
aryl–aryl facile couplings of sterically bulky triarylbismuth
reagents under task specic Pd–Cu dual catalytic conditions as further explored with electronically diverse aryl iodides (Table
elaborated hereunder. 2). This with different aryl iodides delivered the facile threefold
To further elaborate, the cross-coupling reactivity of 1a was
We performed the study of aryl–aryl coupling employing reactivity of 1a to give ortho substituted biaryls 2.1–2.6 in 77–
sterically congested 1a under palladium-catalyzed conditions 87% yields. Our developed Pd–Cu protocol proved to be efficient
(Table 1). In general, electronically rich aryl halides are known with electronically rich aryl iodides to give high yields of biaryls
to be poor reactive.7,8 Hence, we explored our study with elec- 2.1 and 2.3. Our dual catalytic protocol tolerated sensitive
tronically richꢀ4-iodotoluene employing Pd(OAc)2/PPh3, K3PO4 formyl group (2.4) and also gave chemoselective iodo couplings
in DMF at 90 C conditions. This reaction proceeded poorly to in the presence of bromide terminus (2.7 and 2.8). Further,
give biaryl 2.1 in 9% yield (entry 1). This prompted us to test the chloro (2.5) and triuoromethyl (2.6) substituted phenyl iodides
envisioned role of copper halide in co-promoting the cross- also participated with excellent reactivity.
coupling process.
The fabulous coupling reactivity of 1a encouraged our
The initial attempt with stoichiometric amount of CuCl further examination with a few more sterically bulky BiAr3
delivered a dramatic improvement with 60% cross-coupling reagents 1b–1e (Fig. 1) to obtain ortho functionalized biaryls
yield (entry 2). Encouraged by this, further study was conduct- (Table 3). The coupling reactivity of tri-o-tolylbismuth reagent
ed with different copper halides (entries 3–7). This denoted the 1b with 2-iodobenzaldehyde furnished ortho disubstituted
benecial role of different copper halides and the best outcome biaryl 3.1 in 92% yield. The sterically more demanding trimes-
was obtained using CuI in 81% yield (entry 7). The inhibitory itylbismuth (1c) and tri(2,6-dimethylphenyl)bismuth (1d)
role of phosphine ligand during the transmetalation step of the reagents also coupled with functionalized aryl iodides to afford
catalytic cycle and CuI serving as a scavenger of the phosphine the corresponding tri- and tetrasubstituted biaryls (3.2–3.5) in
was reported in non-polar solvents.11b To check this possibility if 71–84% yields. The reaction of 3-bromophenyl iodide gave
any, we performed a control reaction with deliberate lowering of chemoselective iodo coupled product 3.4 in 76% yield. These
PPh3 from 0.4 equiv. (entry 7) to 0.2 equiv. amount (entry 8) and couplings with tri(naphthalen-1-yl)bismuth (1e) gave the cor-
this resulted in moderate coupling reactivity. This indicated responding 1-phenylnaphthalenes (3.6–3.9) in 85–98% yields.
that copper halide is not serving as a scavenger of phosphine in This evaluation demonstrated successful couplings of sterically
bulky BiAr3 reagents with electronically diverse aryl iodides
under the optimized Pd–Cu dual catalytic conditions.
Table 1 Screening conditionsa,b,c
Table 2 Couplings with aryl iodidesa
Entry
CuX
Time (h)
2.1 (%)
1
2
3
4
5
6
7
8d
—
2
2
2
2
2
2
4
4
09
60
56
64
70
29
81
57
CuCl (1 equiv.)
CuCl (0.2 equiv.)
CuBr (0.2 equiv.)
CuI (0.2 equiv.)
CuI (0.1 equiv.)
CuI (0.2 equiv.)
CuI (0.2 equiv.)
a
a
Reaction conditions: tri(o-anisyl)bismuth (0.25 mmol, 1 equiv.), aryl
Reaction conditions: 1a (0.25 mmol, 1 equiv.), 4-iodotoluene (0.875
mmol, 3.5 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2 (0.025 mmol,
0.1 equiv.), PPh3 (0.1 mmol, 0.4 equiv.), CuX, DMF (3 mL), 90 ꢀC,
iodide (0.875 mmol, 3.5 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2
(0.025 mmol, 0.1 equiv.), PPh3 (0.1 mmol, 0.4 equiv.), CuI (0.05 mmol,
0.2 equiv.), DMF (3 mL), 90 ꢀC, 2–4 h; isolated yields; homo-coupled
biaryls formed in minor amounts.
b
c
time.
Isolated yields. Homo-coupled biaryl formed in minor
amount. d With 0.2 equiv. of PPh3.
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Table 3 Couplings of aryl iodides with 1b–1e reagentsa
cross-coupling ability of 1a was tested with electronically poor
aryl bromides under Pd–Cu dual catalytic conditions (Table 4).
Simultaneously, these couplings were addressed under CuI free
palladium catalysis. This was to establish the difference in
coupling reactivity with and without copper conditions. In this
endeavour, we have witnessed effective couplings of o-, m- and
p-substituted functionalized aryl bromides in high yields.
The corresponding couplings attempted with CuI free Pd
conditions invariably gave poor yields indicating denitive role
of CuI in the coupling process. To note, electronically different
and ortho-substituted aryl bromides reacted with equal ease,
rendering high product yields (2.4 and 4.1–4.4). Again, the
diverse meta- and para- substituted aryl bromides also involved
with excellent reactivity (4.5–4.12). In this investigation, 1a was
effectively cross-coupled with various electronically decient
aryl bromides functionalized with formyl, cyano, acetate,
chloro, uoro, triuoromethyl and nitro groups.
Entry
1
BiAr3
Biaryls
Yield (%)
92
2
73
Encouraged by this magnicent coupling reactivity of 1a,
this study was extended to other bismuths 1b–1f with varied
steric demand (Table 5).
3
4
1c
1c
71
76
The coupling reactivity of 1b with different aryl bromides
resulted in high prole couplings giving ortho substituted
biaryls 3.1 and 5.1–5.5 in 75–96% yields. Importantly, bismuth
reagent 1d with sterically demanding ortho disubstituted 2,6-
dimethylphenyl group participated superbly to afford tri- and
tetrasubstituted biaryls 5.6–5.8 and 3.5 in 80–96% yields.
Additionally, couplings with tri(naphthalene-1-yl)bismuth 1e
also gave high yields (3.7, 3.8, 5.13 and 5.14). Similarly, sterically
bulky trimesitylbismuth 1c furnished tetrasubstituted biaryls
5.9–5.11 and 3.3 in 60–83% yields. More so, sterically
5
6
84
85
Table 4 Couplings of electronically poor aryl bromides with 1aa,b
7
8
1e
1e
1e
98
96
85
9
a
Reaction conditions: BiAr3 (0.25 mmol, 1 equiv.), aryl iodide (0.875
mmol, 3.5 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2 (0.025 mmol,
0.1 equiv.), PPh3 (0.1 mmol, 0.4 equiv.), CuI (0.05 mmol, 0.2 equiv.),
DMF (3 mL), 90 ꢀC, 3 h; isolated yields; biaryls formed in minor
amounts.
a
Reaction conditions: tri(o-anisyl)bismuth (0.25 mmol, 1 equiv.), aryl
bromide (0.875 mmol, 3.5 equiv.), K3PO4 (1.5 mmol,
6 equiv.),
To further showcase the efficacy of our Pd–Cu dual catalysis,
cross-couplings of sterically bulky BiAr3 reagents with aryl
bromides were planned. It was studied elaborately by employing
both electronically rich and poor aryl bromides. Initially, the
Pd(OAc)2 (0.025 mmol, 0.1 equiv.), PPh3 (0.1 mmol, 0.4 equiv.), CuI
(0.05 mmol, 0.2 equiv.), DMF (3 mL), 90 ꢀC, 3–4 h; isolated yields;
b
biaryls formed in minor amounts.
conditions given in parenthesis.
Yields from CuI free Pd
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Table 5 Couplings of electronically poor aryl bromides with 1b–1fa,b
Table 6 Screening with o-tolyl bromidea,b
Entry
Ligand
6.1 (%)
1
2
3
PPh3 (0.4 equiv.)
P(p-tolyl)3 (0.4 equiv.)
dppf (0.2 equiv.)
dppf (0.1 equiv.)
dppf (0.2 equiv.)
25
67
78
64
9
4
5b
a
Reaction conditions: o-tolyl bromide (0.875 mmol, 3.5 equiv.), 1a (0.25
mmol, 1 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2 (0.025 mmol, 0.1
equiv.), phosphine ligand, CuI (0.05 mmol, 0.2 equiv.), DMF (3 mL), 90
ꢀC, 4 h; isolated yields; biaryls formed in minor amounts. b Without CuI.
This scrutiny produced high coupling performance as the
reactivity of 1a with simple phenyl bromide or its electronically
rich variants gave biaryls (2.2, 2.3, 6.1 and 6.2) in 62–91% yields.
Similar cross-couplings with other bismuth reagents 1b–1f also
afforded high yields (3.2, 3.6, 6.3–6.5) demonstrating enduring
efficacy of the established Pd–Cu dual protocol.
a
Furthermore, the cross-coupling of ortho disubstituted and
electronically rich 2-bromo-1,3-dimethylbenzene was tested
with 1a using XPhos ligand (Scheme 1) to obtain ortho trisub-
stituted biaryl system. In fact, this reaction successfully fur-
nished biaryl 7.1 in 64% yield. Under similar conditions,
couplings of 1a and 1f with pentauorophenyl bromide gave
biaryls 7.2 and 7.3 in 66% and 79% yields (Scheme 1).
Reaction conditions: BiAr3 (0.25 mmol, 1 equiv.), aryl bromide (0.875
mmol, 3.5 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2 (0.025 mmol,
0.1 equiv.), PPh3 (0.1 mmol, 0.4 equiv.), CuI (0.05 mmol, 0.2 equiv.),
DMF (3 mL), 90 ꢀC, 3–4 h; isolated yields; biaryls formed in minor
amounts. b Yields under CuI free Pd protocol given in parenthesis.
demanding 1f with 2,4-dimethoxyphenyl gave biaryl 5.12 in 96%
yield. A random check of the corresponding cross-couplings
using CuI free Pd conditions gave poor yields (given in paren-
thesis in Table 5). This evaluation with sterically bulky triar-
ylbismuths delivered facile threefold reactivity with diverse aryl
iodides and electronically poor aryl bromides under the estab-
lished Pd–Cu dual catalytic conditions.
To highlight, our established three Pd–Cu dual catalytic
conditions elegantly reacted with exemplary ease and utility in
Table 7 Couplings with electronically rich aryl bromidesab
However, the corresponding reactivity was found to be poor
with electronically rich aryl bromides (Table 6). For example,
reaction of o-tolyl bromide with 1a using Pd–Cu catalytic
conditions delivered 6.1 in 25% yield (Table 6, entry 1). To
improve this reactivity, a brief screening was conducted with
P(p-tolyl)3 and dppf ligands (entries 2–4). From this, catalytic
protocol with dppf ligand was proved to be effective for
couplings with electronically rich aryl bromides (entry 3).
This protocol but without CuI gave very poor yield (entry 5)
indicating the important role of CuI in this coupling process.
This screening afforded an effective protocol involving with
dppf (0.2 equiv.), Pd(OAc)2 (0.1 equiv.), K3PO4 (6 equiv.), CuI (0.2
a
Reaction conditions: BiAr3 (0.25 mmol, 1 equiv.), aryl bromide (0.875
equiv.), DMF, 90 ꢀC and 4 h conditions (entry 3). With this, the mmol, 3.5 equiv.), K3PO4 (1.5 mmol, 6 equiv.), Pd(OAc)2 (0.025 mmol,
0.1 equiv.), dppf (0.05 mmol, 0.2 equiv.), CuI (0.05 mmol, 0.2 equiv.),
reactivity of various electronically rich aryl bromides was tested
using bulky triarylbismuth reagents, 1a–1f (Table 7).
DMF (3 mL), 90 ꢀC, 4 h; isolated yields; biaryls formed in minor
amounts. b Yield under CuI free Pd protocol given in parenthesis.
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Scheme 1 Other couplings.
couplings of sterically bulky BiAr3 reagents with electronically
diverse aryl iodides and bromides. Very importantly, with the
variation of phosphine alone (PPh3, dppf and XPhos), we could
derive facile couplings with different electrophilic coupling
partners employing mild Pd–Cu dual catalytic conditions. The
straightforward impact of the nature of phosphines facilitating
the couplings of different aryl halides under Pd–Cu dual catal-
ysis demonstrated the independent role being played by both
Pd and Cu reagents in the coupling process. To understand this,
a few control reactions have been performed as given in Scheme
2. Firstly, the cross-coupling of 1c using CuI free Pd catalytic
conditions furnished 5.9 in 15% yield (eqn (1)). Whereas, it gave
5.9 in 60% yield using Pd–Cu catalytic conditions (Table 5). The
intermediacy and effective role of arylcopper was further eval-
uated. The required mesitylcopper was separately prepared12
and employed in catalytic amount in lieu of CuI in our estab-
lished conditions (eqn (2)). This reaction gave the desired
product 5.9 in 63% yield and thus established the catalytic role
of arylcopper in the coupling process. Further coupling of 4-
bromoacetophenone and 1c without Pd catalyst and with cata-
lytic CuI did not give the cross-coupling product (eqn (3)). This
conrmed the specic role of Pd in Pd–Cu dual catalytic cycle.
Furthermore, a stepwise process involving in situ formation of
oxidative addition product and its coupling with mesitylcopper
using the established conditions gave cross-coupled product 5.9
in 59% yield (eqn (4)).
Alternative preparation of the oxidative addition product13
employing Pd(PPh3)4 and its reaction with mesitylcopper gave
cross-coupling product in 41% yield (eqn (5)). These two reac-
tions (eqn (4) and (5)) collectively demonstrated the involvement
of both oxidative addition product and mesitylcopper in our
Pd–Cu dual catalytic conditions. Further check with mesi-
tylcopper as co-catalyst in couplings of different bismuths 1a
and 1b and 4-bromoacetophenone gave corresponding coupling
products 4.10 and 5.5 in 59% and 62% yields respectively (eqn
(6) and (7)) demonstrating the general catalytic role of arylcopper
under palladium coupling conditions. Additionally, reaction of
1c under dual catalytic conditions but with the absence of
4-bromoacetophenone gave homo-coupled bimesityl (eqn (8)).
This investigation employing different bismuth reagents, inter-
mediate mesitylcopper and oxidative products thus emphatically
established the specic role of arylpalladium (2a) and arylcopper
(2b) as proposed in the following catalytic cycle (Scheme 3).
The proposed mechanism exemplies the simultaneous
Scheme 2 Mechanistic investigations.
their task specic role in delivering the cross-coupling product.
The catalytic role of palladium in the oxidative addition,
transmetalation and reductive elimination steps forms the
main catalytic cycle. Additional involvement of catalytic CuI for
in situ generation of arylcopper11d from arylbismuth and its
participation in aryl exchange during transmetalation step of
involvement of both palladium and copper catalytic cycles with Scheme 3 Proposed Pd–Cu dual catalytic cycle.
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the palladium catalytic cycle was proposed. The efficient reac-
tivity witnessed with the use of catalytic CuX under dual cata-
lytic conditions supports the proposed regeneration of the CuX
during the catalytic process. Further, the facile couplings of
diverse aryl halides obtained under task specic three different
palladium protocols comprising PPh3, dppf and XPhos ligands
and uniformly co-catalyzed by CuI signies the independent
nature of both Pd and Cu catalytic cycles as part of the dual
catalytic process.
In summary, we have unravelled an unprecedented cross-
coupling reactivity of sterically congested BiAr3 reagents with
a diverse range of aryl iodides and bromides under task specic
Pd–Cu dual catalytic conditions. The three Pd–Cu dual proto-
cols with PPh3, dppf or XPhos ligands and CuI signify the ex-
ible nature of the established dual catalytic conditions besides
their versatility and broad synthetic utility. With this, we have
unravelled the outstanding reactivity of various sterically con-
gested BiAr3 reagents in aryl–aryl couplings under mild Pd–Cu
dual catalytic conditions using routinely used metal and ligand
systems. This study is expected to open up an easy way forward
for further applications of these bismuth reagents in organic
synthesis.
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Acknowledgements
We thank the Council of Industrial and Scientic Research
(CSIR), India for funding this work (Project No. 02(0091)/12/
EMR-II). RJD acknowledges the research fellowship received
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