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R
R
X
R
Pd
II
X
Pd(dba)
2
+
L (1 equiv)
X
PdL
2
L
THF or neat
25
THF or neat
70
R
R
R
40 equiv
°
C
°
C
> 10 equiv
7.11
R= Me
X = I, L = P
t
Bu
3
R= H
X = Br, L = AdP
t
Bu
2
X = Br, L = AdP
t
Bu
2
X = I, L = P
t
Bu
3
7.11a
7.11b
7.11c
7.11d
Scheme 7.5
Synthesis of the first three-coordinate 14-electron ArPd(II)X complexes
by Hartwig.
Adapted from Ref. 50. Copyright 2002 American Chemical Society.
Table 7.2 Roy and Hartwig's stoichiometric studies on reductive
elimination of aryl halides from monomeric ArPd(II)X
complexes.
X
P
t
Bu
3
X
Pd
7
0 °C
C
6
D
6
Pd[P
t
Bu
3
]
2
P
t
Bu
3
R
1
R
1
R
1
=Me
X=Cl:
X=Br:
X=I:
R
1
=H
X=Br:
X=I:
7.13a-e
7.12d
7.12e
7.12a
7.12b
7.12c
X
Yield of 7.13 (%)
K
eq
10.9
10
2
7.12a (X
¼
Cl)
76
32.7
10
1
7.12b (X
¼
Br)
98
1.79
10
1
7.12c (X
¼
I)
79
13.4
10
1
7.12d (X
¼
Br)
68
0.51
10
1
7.12e (X
¼
I)
60
Adapted from Ref. 51. Copyright 2003 American Chemical Society.
phosphine ligands are commonly proposed to occur via three-coordinate 14-
electron complexes such as 7.11
23,48,49
and yet no such species had been
isolated until a 2002 report by Hartwig and co-workers describing their
synthesis, characterization and reactivity (Scheme 7.5).
50
This pioneering
study led to a subsequent report in 2003 by Roy and Hartwig describing the
first reductive elimination of aryl halides from monomeric ArPd(II)X com-
plexes 7.12a-e, which are proposed to be directly involved in palladium
catalysis (Table 7.2).
51
This observation was a significant advance from their
2001 report,
46
where the species undergoing reductive elimination was not
directly observed.
Yields of the respective aryl halide products were higher than those pre-
viously reported for the dimeric ArPd(II)X complex and K
eq
values were ob-
tained by initiating the reaction in both directions. Reductive elimination
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