Suzuki–Miyaura, Mizoroki–Heck carbon–carbon coupling and hydrogenation reactions
139
coupling reactions between 4-bromoacetophenone yields (entries 5 and 6). The high catalytic efficiency
and phenylboronic acid in methanol using different of 2 at room temperature makes it a valuable cata-
catalysts were performed under very mild condi- lyst.
tions, viz. room temperature for 30 min (table 1).
The best results were obtained by using isolated pal- their coupling with phenylboronic acid gave modest
ladium(II) complex 2 or by generating it in situ by yields (table 2). Reactions with arylchlorides require
mixing 1 and Pd(COD)Cl
(entries 1 and 2). Use of higher catalyst loading. The reaction between 2-
the other palladium precursors such as Pd(OAc)
chlorobenzaldehyde and phenylboronic acid was
along with 1 also gave satisfactory yield (entry 4). used as test reaction at 110°C with K CO as a base
Under similar conditions, PdCl and Pd(SMe Cl
in DMF. The coupling proceeds for 20 h with cata-
supported by cyclodiphosphazane 1 afforded very lyst loading of 1 mol% and 2 mol% giving 25% and
poor yields (entries 5 and 6). When only [Pd(COD) 46% conversions respectively (entries 7 and 8). This
Cl
] was used, the yield was low (entry 3), which catalyst system is able to couple activated aryl chlo-
improved to 70% (entry 2) when ligand 1 was intro- rides with phenylboronic acid efficiently with
duced. These results suggest that 2 or its precursor 4 mol% of catalyst (entries 9, 10, 11 and 12).
As expected, aryl chlorides are more stubborn and
2
2
2
3
2
2
)
2
2
2
mixture is very effective in promoting the Suzuki
cross-coupling reactions.
By employing the same palladium complex 2,
Mizoroki–Heck reaction was performed for a range
of aryl bromides and the details are given in table 3.
Complex 2 is an active catalyst for Suzuki–
Miyaura coupling reactions (table 2). A number of
aryl bromides and chlorides have been successfully
coupled with phenylboronic acid. We choose as a
model reaction, the coupling between p-bromo-
acetophenone (0⋅5 mmol) and phenylboronic acid
3 4
All the reactions were examined with K PO as a
base and N,N-dimethyl acetamide (DMA) as solvent
at 130 °C. As expected, the more reactive 4-bromo-
benzaldehyde and 4-bromoacetophenone underwent
clean coupling with styrene giving essentially quan-
titative yields (entries 1 and 2). The inactivated
bromobenzene was coupled with styrene giving
excellent yield (entry 3). Reactions involving deac-
tivated arylbromides bearing a methoxy group
afforded fewer yields with 0⋅1 mol% of catalyst
(
0⋅75 mmol) in methanol at room temperature in the
presence of K CO (1 mmol) as a base with catalyst
2
3
loading of 0⋅06 mol%. In general, this coupling can
be satisfactorily carried out at room temperature for
all bromo precursors. Specifically, 2 is an effective
catalyst precursor for the coupling of activated
(
entry 4). On increasing the catalyst loading up to
0
⋅5 mol%, the yield increased to 72% (entry 5). Fur-
(
entries 1, 2 and 3) and deactivated (entry 4) aryl
bromides. Under similar reaction conditions, both
,3- and 1,4-dibromo benzene also afforded good
thermore, the activated terminal olefins such as
n-butylacrylate and t-butylacrylate efficiently cou-
pled with activated arylbromides (entries 6, 7 and 8).
The data given in table 3 shows the efficiency of this
new catalyst towards the Mizoroki–Heck coupling
of aryl bromides and terminal olefins. This catalyst
is more efficient than the Hermann’s palladacycle
which catalyses the reaction between bromobenzene
and styrene in DMA at 140 °C giving 77% yield of
1
Table 1. Effect of different palladium catalysts on
a
Suzuki-Miyaura cross coupling reaction .
the product.
t
Rhodium complex [RhCl(COD){( BuNP(OC
6 4
H
Entry
Catalyst
Yield (%)
2
OMe-o)) -κP}] (3) has been employed successfully
1
2
3
4
5
6
2
72
70
56
57
18
15
in the catalytic hydrogenation of styrene, α-methyl
styrene, etaconic acid and its derivative such as
dimethyl etaconate (table 4). For all substrates, the
hydrogenation reactions were performed at 80 °C
Pd(COD)Cl
Pd(COD)Cl
Pd(OAc) /1
Pd(SMe Cl
PdCl /1
2
/1
2
2
2
)
2
2
/1
2
under 4 atm of H in the presence of triethylamine as
2
a base and THF as solvent. The conversion of sty-
rene to ethyl benzene was observed at room tem-
perature with 0⋅5 mol% of catalyst loading (entry 1)
a
Reaction conditions: Catalyst (0⋅01 mol%), 4-bromo-
acetophenone (0⋅5 mmol), phenylboronic acid (0⋅75 mmol),
2 3
K CO (1 mmol) and MeOH (5 mL). Conversion to the
b
coupled product determined by GC-MS, based on aryl and the rate of the reaction was found to be tempera-
bromide; average of two runs
ture dependent. At 25 °C or room temperature, the