Chemistry Reference
In-Depth Information
Ph
Pd(OAc)
2
(0.001 mol %)
28
(0.001 mol %)
K
2
CO
3
, H
2
O
100 °C, 2 h
Cl
(HO)
2
B
N
P
+
O
P
O
N
28
92%
Ph
Scheme 14.26
t-
Bu
Me
2
N
P
d
l
2
t-
Bu
2
CF
3
O
29
(2 mol%)
Zn dust, TMEDA, H
2
O, rt, 6 h
Br
CF
3
O
Cl
+
O
Cl
O
Cl
CF
3
88%
CF
3
Scheme 14.27
CO
2
Et
N
MeS
Br
EtO
2
C
N
Ph
Pd(P
t-
Bu
3
)
2
(2 m ol%)
K
3
PO
4
, H
2
O
MW 130 °C, 30 min
31
72%
MeS
+
Ph
30
Ph
Ph
Scheme 14.28
activated aryl chlorides. Suzuki coupling of aryl chlorides was achieved on
water using a Pd-IPr [IPr
¼
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine]
precatalyst at 80 1C.
85
The on-water protocol has been demonstrated for reactions that one
would not normally expect to be tolerant of moisture. For example, the
Negishi coupling of aryl halides with air- and moisture-sensitive organozinc
compounds would not be expected to proceed with water as the reaction
medium. Lipshutz and co-workers showed that under Barbier-type con-
ditions, benzylic halides can be coupled with aryl, heteroaryl and vinyl
halides using Pd/Amphos catalyst 29 on water at room temperature
(Scheme 14.27).
86
The intermediacy of organozinc reagents was demonstrated
by electrospray ionization mass spectrometric analysis of the reaction mix-
ture.
87
Arylation of protected glycine ester 30,withpK
a
values too high for it to
be significantly deprotonated in water, occurred eciently on water with a
Pd(Pt-Bu
3
)
2
precatalyst (Scheme 14.28).
88
On-water Buchwald-Hartwig ami-
nation was achieved with a Pd
2
(dba)
3
/XPhos [XPhos
¼
dicyclohexyl(2
0
,4
0
,6
0
-
triisopropylbiphen-2-yl)phosphine] precatalyst system at 110 1C.
89
14.2.2.2 Surfactant-Promoted Cross-Coupling Reactions
Although on-water reactions can occur eciently in some cases, the poor
interfacial mixing of water-suspended organic materials often results in poor
yields and selectivities. A variety of phase-transfer agents have been explored
to improve the interaction of hydrophobic materials in water. Quaternary
ammonium ions, such as TBAB, are commonly used to promote reactions in
Search WWH ::
Custom Search