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PR
2
PR
2
CD
3
I
CH
3
I
Rh
III
I
CD
3
Rh
III
I
CH
3
PR
2
PR
2
7.15
-
d
; R =
t
Bu
7.16
-
d
; R =
i
Pr
7.15
; R =
t
Bu
7.16
; R =
i
Pr
Scheme 7.8 Labeling experiments probing the reversibility of MeI reductive elimin-
ation.
Adapted from Ref. 61. Copyright 2006 American Chemical Society.
of MeI, PCP pincer complexes 7.15 and 7.16 were obtained, respectively.
When complex 7.15 was treated with an excess of CO (~100 equiv), slow but
almost quantitative reductive elimination to form Rh(I) carbonyl complex
7.17 occurred. Conversely, when the
i
Pr analog 7.16 was treated under the
same reaction conditions, reductive elimination of MeI did not occur. In-
stead, adducts 7.18 and 7.19 were formed in 17 and 83% yield, respectively.
The authors ascribed this marked switch in reactivity to the decreased sterics
of the
i
Pr-containing P ligands. This interpretation echoes the discussion in
the previous sections concerning the influence of steric congestion on
carbon-halogen reductive elimination.
In order to gain insight into this rare process, the authors treated 7.15 and
7.16 with CD
3
I. Labeled complexes 7.15-d and 7.16-d were detected by
1
H
NMR spectroscopy, leading them to propose that a reversible process is
operative in both instances (Scheme 7.8), despite 7.16 not undergoing
thermodynamically favored reductive elimination.
The authors obtained further mechanistic insights by studying the effect
of additives on the overall reductive elimination process. Adding excess
n
Bu
4
NI to the reaction did not attenuate the rate of CH
3
-CD
3
exchange,
which contrasts with the work of Ettorre
32
by suggesting that iodide dis-
sociation is not involved in this process. Second, no rate retardation was
observed when excess MeI was added to the reaction of 7.15 to form 7.17.
The authors attributed this to the production of an Rh(I)(MeI) complex,
followed by associative substitution of the MeI ligand with one CO molecule
from the reaction atmosphere to yield 7.17. Recently, the same group ob-
served reductive elimination of MeBr and MeCl from analogous Rh(III)
complexes and conducted in-depth kinetic experiments as means to gain a
deeper mechanistic understanding.
62
7.5 Polyhalogenated Substrates in Cross-Coupling
Whereas the previous sections largely focused on stoichiometric reductive
elimination of carbon-halogen bonds from transition metal complexes, the
following sections review catalytic reductive elimination of carbon-halogen
bonds from Pd(II) centers, beginning with the seminal report on halogen
exchange by Buchwald (Section 7.5.4),
leading to the discovery and
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