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O
O
N
Pd(t-Bu
3
P)
2
(5 mol% )
LiCl, NaOAc, K
2
CO
3
DMF-H
2
O, 1 00°C, 18 h
+RCl
R
55-69%
ee > 90%
R=Ph;(o-Me, p-Me,m-NMe
2,
p-CHO, p-CN, p-Ac,
p-COPh)C
6
H
4
;1-C
10
H
7
; 2-ethylbutenyl, 1-cyclohexenyl
Scheme 9.41 Enantioselective arylation and alkenylation by the ligand-accelerated
Heck reaction.
O
O
Pd(O Ac)
2
(3 mo l%)
LiCl, NaOAc, K
2
CO
3
DMF-H
2
O, 100 °C, 18 h
N
N
A r
+A I
42-60%
de 94-98 %
Ar = Ph; (o-Me, p -Me, p-OMe, p-CHO, p-CN, p-Ac,
p-CO Ph, p-CF
3
)C
6
H
4
;1-C
10
H
7
Scheme 9.42 Enantioselctive arylation by aryl iodides in a ''phosphine-free'' cata-
lytic system.
HO
Br
CHO
O
O
Pd(OAc)
2
(0.01 mol% )
(t-Bu)
3
P, Na
2
CO
3
NMP-H
2
O, 130°C, 24 h
O
O
90%
Scheme 9.43 Example of a high TON reaction in the presence of (t-Bu)
3
P ligand.
The use of tris(tert-butyl)phosphine, however, does not guarantee that a
given reaction with some new olefin will run in the ligand-accelerated mode
(cf. Scheme 9.32). An interesting example of such behaviour can be seen in a
study of the arylation of methallyl alcohol by aryl bromides by Scrivanti
et al.
158
The initial attempt to use the standard Littke-Fu protocol using
(t-Bu)
3
P ligand and Cy
2
NEt base at temperatures up to 120 1C gave modest
yields only marginally better than those obtained with Ph
3
P ligand. A sharp
increase in yield was observed on increasing the temperature to 130-140 1C,
changing the base to Na
2
CO
3
and using highly polar aqueous N-methylpyr-
rolidone (NMP) solvent. Such an effect can probably be interpreted as that in
this case the Pd complex with (t-Bu)
3
P ligands behaves as a typical slow-re-
lease precatalyst, with phosphine ligands completely shed at the tempera-
ture of reaction. This is additionally manifested by the observation of high
TON and low Pd loading (TON up to 20 000, TOF exceeding 1000 h
1
)
(Scheme 9.43), which is a behaviour not so far observed for ligand-acceler-
ated Mizoroki-Heck reactions.
Among phosphine ligands that have been used for performing Mizoroki-
Heck reactions, one ligand - the tetrapodal phosphine Tedicyp,
159
developed
by Doucet, Santelli and co-workers, occupies a special place. Probably no
other ligand, including the classical Ph
3
P and (o-tol)
3
P, has been tested so
widely towards aryl and alkenyl bromides
160
and an overwhelming variety of
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