28
A. Majumder et al. / Journal of Organometallic Chemistry 781 (2015) 23e34
Table 6 (continued )
Entry
NuH
Time(h)a
24
Productb
Yield(%)c
86
O
O
O2N
O2N
O
O
N
N
H
a
Monitored using TLC, until all the amine was found consumed (sample not collected between 8 and 24 h).
All the N-arylated products gave satisfactory spectral data.
Isolated yield, after column chromatography of the crude product.
b
c
without any difficulties to result the corresponding cross coupling
corresponding diaryl ethers in good isolated yields (results are
shown in Table 8).
products in good to excellent yield. The system also permits the use
of sterically hindered aryl iodides such as 2,6-dimethoxy iodo-
benzene (Table 5, entry 7), 2,4,6-trimethyl iodobenzene, (Table 5,
entry 8) which gave good yields. Interestingly, in our study, we have
observed that the highest yield was obtained when p-methoxy
group was present on the aniline (Table 4, entry 3) as well as on
iodobenzene (Table 5, entry 5).
To check the overall compatibility of the reaction, aryl bromides
were also used as the aryl source. It was observed that this proce-
dure was not limited to aryl iodides, as aryl bromide also proved to
be the suitable partner and afforded similar results. Amination
reactions involving aryl bromide with the same catalyst could be
performed under the same reaction conditions and provided good
yields of the expected aryl amines with electronically diverse
substrate systems without increasing the reaction temperature
(Table 6).
Coupling of aryl iodides with phenol to produce diaryl ethers using
[Pd(pp3S4)(dba)]
A wide range of substituted iodobenzenes was also found to
be suitable substrates for CeO bond formation and the results
are summarized in Table 9. Under the optimized conditions,
phenol was compatible in coupling with electron-rich aryl io-
dides like 2-methyliodobenzene, 4-methyliodobenzene, 2,6-
dimethyliodobenzene and 4-methoxyiodobenzene (Table 9, en-
tries 1e3, 6) to furnish O-arylation products in excellent yields.
Though sterically hindered iodobenzene was less reactive, but
still gave good yield by prolonging the reaction time (Table 9,
entry 3). From the results it is evident that our catalysts work
well with both types of substituents (electron-deficient iodo-
benzenes and electron-rich ones) and gave excellent yields.
O-arylation of iodobenzene with different phenols to produce diaryl
ethers using [Pd(pp3S4)(dba)]
Coupling of iodobenzene with alcohols to produce alkyl aryl ethers
using [Pd(pp3S4)(dba)]
Next, we turned our attention to the exploration of the scope of
the diaryl ether formation process. O-arylation cross coupling re-
action followed the same trend as that of CeN coupling reactions.
Using the optimized conditions applied for CeN cross coupling
reactions, we initiated our investigations of the Pd(pp3S4)(dba)
catalyzed Ullmann-type CeO cross coupling reactions of phenols
with iodobenzene and the results are summarized in Table 7. We
found that iodobenzene reacted with phenols containing electron-
donating groups as well as electron-withdrawing groups to give the
corresponding diaryl ethers. Both the electron-donating groups
such as methyl, tert-butyl, phenyl and methoxy (Table 7, entries
1e2, 8, 12e13), and the electron-withdrawing groups like fluoride,
chloride, nitro (Table 7, entries 3e5, 10e11) on phenolic moiety
gave the desired product in good to excellent yield. Moreover, even
ortho substituents on the phenols (which are capable of providing
scope for steric bias) did not hamper the reaction and allowed the
reaction to proceed smoothly (Table 7, entries 1, 3, 7e9). This was
not the case reported in the literature [10k]. However, in case of
highly substituted phenols like pentafluorophenol and pentachlo-
rophenol, the reaction was hampered and yields of the products
decreased (Table 7, entries 14, 15). The presence of an electron-
donating methoxy group at the para position on phenol and
benzyl moieties increases the efficiency of the reaction (Table 7,
entry 13 and 17), whereas the presence of a nitro group at the para
position of phenol decreased the yield of the coupling reaction
(Table 7, entry 11). The highest yield was obtained when p-methoxy
phenol was reacted with iodobenzene (Table 7, entry 13).
After successful completion of diaryl ethers formation from
iodobenzene and the corresponding phenols in the presence of
Pd(pp3S4)(dba), the substrate scope was expanded to include the
synthesis of alkyl aryl ethers using aliphatic alcohols and iodo-
benzene and we applied the same protocol as above except using
5 mol % Cs2CO3 for this purpose (results are presented in Table 10).
This was done because using 2 mol % CsCO3, the reaction was found
to be very sluggish (keeping other reaction parameters consistent).
Simple alkyl, ethoxy, cyclohexyl and benzyl motifs all worked well
in the reaction, providing corresponding ethers in good to excellent
yields. It is noteworthy that, to date only a few Pd/ligand catalytic
systems are known for the intermolecular coupling reactions of
aliphatic alcohols [2a,14].
We have observed around 10% yield of the coupled products
corresponding to isopropanol (used as solvent) and the various aryl
iodides as substrates after 12 h under our reaction condition using
2 mol % Cs2CO3. This side product could easily be isolated by col-
umn chromatography. Since the average yield of the required
products was in the range between 80 and 85 %, formation of the
side products was not a major issue.
Recyclability of catalyst
In order to test the recyclability of the catalyst, aniline was
reacted with iodobenzene in the presence of the catalyst and
requisite amount of Cs2CO3 (entry 1, Table 4). After 6 h, the reaction
was found to be complete as ascertained from the TLC of the re-
action mixture. At this stage, 1 mmol of aniline and iodobenzene
We were pleased to note that under our optimized reaction
conditions, aryl bromides too reacted with phenols to provide the