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t
Bu
N
N
O
N
N
N
N
t
Bu
CN
t
Bu
NH
N
N
N
N
H
N
OH
O
Br
Pd(OAc)
2
/
x
=0.0001mol%
85%, 850 000 TON
Milrinone
Irbesartan
17
Figure 4.11
17 as a ligand for Suzuki coupling: preparation of arylheterocycles at
extremely low catalyst loadings and the total synthesis of milrinone and
irbesartan.
Pd(OAc)
2
(1mol%)
or [Pd(allyl)Cl]
2
(0.5 mol%)
18
(2 mol% )
Cs
2
CO
3,
dioxane
90 - 100°C, 1 8 h
Cl
He tAr
C l
+(Het)Ar'
B(OH)
2
He tAr
(H et) Ar'
NN
COMe
PhO
F
MeOC
18
S
N
N
89%
81%
96%
O
N
Bn
2
N
N
Me OC
S
N
NPh
X
N
X=O:80%
X=S:92%
82%
85%
99%
Scheme 4.9 Reactivity of ligand 18 in Suzuki coupling.
NN
R
R
t
Bu
NN
R
R
R
R
Cl
Pd
Cl
Pd
Cl
R=
N
Ph
[Pd( NHC)(cin)Cl]
6
:R =H
19a
:R=Ph
3
C
19b
:R=(4-MePh)
3
C
19c
:R=(4-
t
Bu Ph)
3
C
19d
: R = 1-adamantyl
t
Bu
[Pd(IPr**)(pyridinyl)Cl
2
]
20
Figure 4.12
Structures of the [Pd(NHC)(cin)Cl] complexes 6, 19a-d and [Pd(IPr**)-
(pyridinyl)Cl
2
](20).
Another example showing that the size of the ligand is not an absolutely
vital factor was reported by Trzeciak and co-workers in 2013.
57
They showed
that small NHCs in complexes of formula [Pd(NHC)
2
X
2
] (Figure 4.13) gave
results comparable to those with IMes for the coupling between 2-bromo-
benzene and phenylboronic acid (Table 4.5). The use of mokt-y as the NHC
ligand even allowed a TON as high as 760 000 to be reached.
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