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2.5 mol% Pd(PPh
3
)
4
K
2
CO
3
MBM
6 h
2
R
R
R
R = C
6
H
5
, 3-pyridyl, 4-BrC
6
H
4
, butyl
Scheme 2.13 Glaser-coupling catalyzed by [Pd(PPh
3
)
4
].
4.5 mol% Pd(OAc)
2
3 equiv. AgNO
3
3 equiv. NEt
3
NaCl
MBM
30 s
-1
, 30 min
B(OH)
2
R
R
R
R = H, 4-CH
3
,
2-CH
3
,
4-OCH
3
/ 1-naphthylboronic acid
Scheme 2.14 Homo-coupling of boronic acids in a mixer ball mill (MBM).
1 equiv. DDQ
10 mol% PyBox-1
MBM
30 s
-1
,
30-90 min
Ar = 4-(OCH
3
)C
6
H
4
,
C
6
H
5
, 2-(OCH
3
)C
6
H
4
,
4-(CH
3
)C
6
H
4
R = C
6
H
5
, 4-(CH
3
)C
6
H
4
, 4-FC
6
H
4
, CO
2
CH
3
, 2-Py
N
+
Ar
H
R
N
Ar
R
Scheme 2.15
Synthesis
of
optically
active
1-alkynyltetrahydroisoquinoline
derivatives.
2.3.3 Homo-coupling of Boronic Acids
The Suzuki reaction described by Nielsen and co-workers revealed a sig-
nificant amount of the arylboronic acid homo-coupling product (13-28%
analytical yield) after 30 min.
11
Similar results were detected by Cravotto
et al. using Pd/CS-HDMI.
18
Higher yields of the biphenyl were obtained by
Klingensmith and Leadbeater.
13
Biphenyls were synthesized from two
molecules of phenylboronic acid using AgNO
3
, NEt
3
and Pd(OAc)
2
as catalyst
and base (Scheme 2.14). NaCl was added to make the reaction mixture less
fluid. The substrates were treated in a mixer ball mill (MM200, Retsch
GmbH) for 30 min and 19-72% of the desired product was obtained.
2.4 Miscellaneous
The syntheses of optically active 1-alkynyltetrahydroisoquinoline derivatives
was realized with the ball milling technique using recoverable copper balls
(Scheme 2.15).
29
In the investigations of chiral ligands PyBox led to good
yields and the highest enantiomeric excess (74%). In the presence of 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) product yields of 60-77%
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