Chemistry Reference
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carbon-carbon bonds). This challenging variation of reductive elimination
has required the development of new catalysts and ligands.
7.4.1 Carbon-Halogen Bond Reductive Elimination
Despite the myriad of examples involving oxidative addition of transition
metals to carbon-halogen bonds,
22-28
reductive elimination to form new or
reform existing carbon-halogen bonds remains a rare and challenging
transformation.
29
Accordingly, the oxidative addition of transition metals to
carbon-halogen bonds has traditionally been considered irreversible, a
supposition rooted in its high exothermicity.
30,31
As counter evidence, Ettorre
reported the reductive elimination of iodobenzene from the octahedral Pt(IV)
complex 7.1 to form trans-Pt(II) complex 7.3 [eqn (7.1)].
32
The author stated
that the addition of NaI to the reaction slowed the rate of reductive elimin-
ation, suggesting that a solvolytic process involving the formation of an
iodide ion occurred during the reaction coordinate to form complex 7.2.
þ
solvent
!
I
½
Pt
ð
PEt
3
Þ
2
Ph
2
I
2
7
:
1
½
trans-
½
Pt
ð
PEt
3
Þ
2
Ph
2
I
ð
solvent
Þ
7
:
2
PhI
trans-
½
Pt
ð
PEt
3
Þ
2
PhI
7
:
3
!
(7
:
1)
In 1987, Echavarren and Stille reported the Pd-catalyzed coupling of aryl
triflates with organostannanes. The formation of aryl chloride 7.6 was ob-
served as a by-product during the reaction between 7.4 and sterically hin-
dered tetrakis(trimethylsilyl)methylstannane (Scheme 7.2).
33
This result was
proposed to be an artifact of the inability of the bulky stannane to trans-
metallate to the resulting ArPd(II)Cl species 7.5. The exact pathway to 7.6 was
not proposed; however, an S
N
Ar mechanism on either 7.4 or 7.5, as well as
reductive elimination from 7.5 are all possibilities.
Numerous reports of Pd(IV) intermediates in catalysis have appeared over the
past few decades
34-36
and, despite resistance towards the idea of these high-
valent species in certain cases, there isanactiveinterestinbothelucidating
PdCl
2
(PPh
3
)
2
(2 mol%)
LiCl (3 equiv)
OTf
Cl
TMS
(TMSCH
2
)
3
Sn
DMF, 110 °C, 50 h
O
2
N
O
2
N
7.4
7.6
25% yield
via:
L
L
Pd
Cl
O
2
N
7.5
Scheme 7.2 Ar-Cl reductive elimination as a by-product in Pd-catalyzed Stille
coupling of bulky stannanes.
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