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[Pd(IPent)(PEPPSI)] (4)
(4mol%)
R''
X
R''
N
+
H
N
R'
R
R
R'
KOtBu or 98 (1.5 equiv)
DME, 80°C
X=Cl,Br
N
N
N
O
COMe
CN
MeOOC
OK
KOtBu: 0%
98:89%
KOtBu: 0%
98:58%
KOtBu: 24%
98: 59%
98
N
EtOC
N
O
COOMe
KOtBu: 23%
98: 56%
KOtBu: 40%
98 (3 equ iv): 90%
Scheme 4.68 Amination using potassium 2,2,5,7,8-pentamethylchroman-6-oxide
(98) as the base.
Et
Cl
NN
Et
N
Et
Et
NN
Et
Et
Pd
Cl
Cl
Pd
Cl
N
Cl
Pd
Cl
N
Ph
3
P
Pd
Cl
Cl
Et
Et
N
Et
N
N
Et
99a
99b
99c
Figure 4.42 Pd-NHC complexes for C-S bond formation.
base and solvent, respectively, and the scope of the reaction was studied
using 2 mol% of complex 99c at 110 1C. A range of bromides including ac-
tivated and deactivated congeners was engaged in the coupling with various
aryl sulfides and the corresponding C-S bonds were formed in good yields
(Table 4.29). Unfortunately, aryl chlorides and aliphatic thiols were
unreactive.
A year later, Shi et al. reported [Pd(SIPr)(pyridinyl)Cl
2
](100) as a precatalyst
capable of promoting C-S coupling using both bromides and chlorides as
coupling partners.
188
Optimized reaction conditions utilized KOtBu as the
base in toluene at 100 1C using 2 mol% of the Pd-NHC precatalyst. Under
these conditions, a wide library of sulfide products could be prepared using
phenyl and benzyl thiols as coupling partners (Scheme 4.69).
Sayah and Organ developed a low-temperature protocol for the formation
of C-S bonds.
189
The system was composed of [Pd(IPent)(PEPPSI)] (4)
(2 mol%) as the precatalyst, KOtBu as the base and LiOiPr (20 mol%) as an
additive in toluene at 40 1C. In this case, LiOiPr was found to be necessary to
facilitate catalyst activation at low temperature. The system performed well
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