Carbon-halide oxidative addition and carbon-carbon reductive elimination at a (PNP)Rh center

Article abstract of DOI:10.1021/ja057948j

An unsaturated (PNP)Rh fragment can be generated by means of C-C reductive elimination from (PNP)Rh(Me)(Ar) or (PNP)Rh(Ar)(Ar). This fragment undergoes carbon-halogen oxidative addition with aryl chlorides, bromides, and iodides at room temperature. The C-H oxidative addition products in reactions with haloarenes are not observed, and evidence is presented that carbon-halogen oxidative addition is thermodynamically preferred. C-C reductive elimination from (PNP)Rh(Me)(Ar) and (PNP)Rh(Ar)(Ar) proceeds near quantitatively as a clean, first-order reaction. Copyright

Full text of DOI:10.1021/ja057948j

Published on Web 02/09/2006
Carbon-Halide Oxidative Addition and Carbon-Carbon Reductive Elimination
at a (PNP)Rh Center
Sylvain Gatard, Remle C¸ elenligil-C¸ etin, Chengyun Guo, Bruce M. Foxman, and Oleg V. Ozerov*
Department of Chemistry, Brandeis UniVersity, MS015, 415 South Street, Waltham, Massachusetts 02454
Received November 22, 2005; E-mail: ozerov@brandeis.edu
Scheme 2
Involvement of haloarenes in transition-metal-catalyzed processes
often entails oxidative addition (OA) reactions.1 In this context, it
is of interest to discern the tendencies of transition metals to activate
either a C-halogen or a C-H bond of haloarene. Oxidative addition
of aromatic carbon-halogen bonds has been studied almost
exclusively in the context of the chemistry of group 10 metals
(especially Pd).2,3 Recently, several examples appeared where Rh
catalysts enable C-C coupling of aryl halides, the reactivity that
is typically achieved with Pd0 catalysts.4 These new Rh-catalyzed
processes probably involve aromatic carbon-halide OA and C-C
reductive elimination (RE). In the context of group 9 metals, the
requisite aromatic carbon-halide OA reactions (for heavier halides)5
and C-C RE reactions remain largely unexplored.6-8 Here we
present a system where the reactions of OA of unactivated aryl
Addition of MeMgCl to 7a in the presence of PhX cleanly and
halides (including Ar-Cl) to RhI and of C-C RE from RhIII are
rapidly produces 6a-c, as well (Scheme 2). Although concomitant
well-defined, easily studied, and occur at ambient conditions.
ethane formation was observed, (PNP)Rh(Me)2 was not detected.
The kinetics of the C-C elimination from 8 and 9 in the presence
of PhBr was investigated by 31P NMR.12-14 Both reactions followed
We have recently reported on the reactions of the (PNP)IrI
fragment with haloarenes.9 In that case, C-H OA was favored
kinetically, whereas the C-Cl (or C-Br) OA was preferred
thermodynamically (Scheme 1). Both types of products were
observed, and the C-H activation products were stable at e70 °C.
clean first-order kinetics. The rate of elimination of Ph-Ph (t1/2
)
7 min at 40 °C) was greater than that of the elimination of Ph-Me
(t1/2 ) 17 min at 40 °C) (Figure 1). The rate of both reactions was
independent of the concentration of PhBr (0.06-0.63 M). These
results are consistent with rate-limiting C-C elimination that
generates 5 (or its kinetic equivalent) followed by rapid C-X
oxidative addition to 5.
Scheme 1
Both 8 and 9 are presumably five-coordinate d6 complexes. C-C
RE from d6 PtIV complexes has been studied in detail and shown
to proceed exclusively via fiVe-coordinate PtIV intermediates.13
The structure of 6b was determined in a solid-state X-ray
diffraction study (Figure 2). The solid-state structural data for 6b
and the NMR solution data for 6a-c are very similar to that of
(PNP)Ir(Ph)(Cl), including the slow rotation about the M-C bond
Here we disclose that an analogous rhodium fragment (PNP)RhI
(5) reacts with haloarenes to produce exclusively C-X (in this
paper, X ) Cl, Br, I) OA products.
The (PNP)IrI fragment was accessed in situ via C-H RE. We
reasoned that the Rh analogue 5 can be accessed via RE, as well.
C-C reductive elimination10 serves well in this regard.
Addition of PhLi or PhMgBr to (PNP)Rh(Me)(Cl) (7a) in the
presence of Ph-X (Scheme 2) cleanly produces (PNP)Rh(Ph)(X)
(6a-c) in <24 h at 22 °C (Scheme 2).11 Compound 8 is observed
as an intermediate, formed upon mixing. Addition of MeMgCl to
6b in the presence of PhX also results in the formation of 8 and
subsequently, in the presence of PhX, 6a-c. Addition of PhLi to
6b rapidly produces (PNP)Rh(Ph)2 (9) which evolves biphenyl and,
in the presence of PhX, into 6a-c. Owing to the instability of 8
and 9, they were only characterized in situ. The resonances arising
from the Rh-CH3 moiety in 8 are particularly characteristic (13C
NMR: δ 3.3 (dt, JC-Rh ) 33 Hz, JC-P ) 6 Hz); 1H NMR: δ 1.95
(ddt, JH-C ) 160 Hz, JH-Rh ) 6 Hz, JH-P ) 3 Hz)). 9 is C2V-
symmetric (1H NMR); the chemical shift of its 31P NMR resonance
(δ 36.8 ppm) and the JRh-P (116 Hz) are near identical to those
of 8.
1
on the NMR time scale (five H NMR resonances for Ph).
Compounds 10-12 were prepared analogously to 6a-c from
the corresponding aryl halides (Scheme 3) to assist in elucidating
certain aspects of the presented reactivity. 10 was chosen for the
Figure 1. Plots illustrating the first-order reductive elimination of Ph-
Me from 8 and of Ph-Ph from 9 at different concentrations of PhBr
(shown).
9
2808
J. AM. CHEM. SOC. 2006, 128, 2808-2809
10.1021/ja057948j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
observations augur that in this PNP system at ambient temperature
catalytic processes (a) that involve C-H OA in haloarenes are more
likely to be successful with Ir than Rh, and (b) that involve C-X
OA are more likely to be successful with Rh than Ir. An intriguing
question is whether our findings reflect a broader trend of the
behavior of 4d versus 5d metals in C-H versus C-X OA.
Acknowledgment. We thank the Donors of the American
Chemical Society Petroleum Research Fund, Research Corporation,
NSF (CHE-0517798), and Brandeis University for support of this
research.
Figure 2. Rendered ORTEP drawing of the solid-state structure of 6b
(thermal ellipsoids set at 50% probability).15 On the left, all Me groups, all
H atoms, the benzene solvent molecule are omitted, disorder not shown
for clarity. On the right, the coordination environment about Rh is shown.
Supporting Information Available: Experimental details and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Scheme 3
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of p-PhC6H4CF3 (93 ( 5%) and p-MeC6H4CF3 (95 ( 5%),
respectively, as measured by 19F NMR.
In sharp contrast to the (PNP)Ir chemistry, we did not observe
any products arising from the C-H OA of haloarenes. If the C-C
elimination was performed in the absence of PhX, the Rh fragment
was found ostensibly to attack the ethereal solvent (from the solution
of organolithium or Grignard reagents). In the presence of Et2O,
the major product was (PNP)Rh(η2-C2H4) (14).
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C-Cl OA product (6a) in the Rh system is thermodynamically
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In summary, we report a Rh system where the fundamental steps
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can be observed at ambient conditions. We also show that in
reactions of haloarenes with (PNP)Rh the C-X OA is thermody-
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thermodynamically preferred, as well.9 For Ir (5d metal), the C-H
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5 on Et3B or a related species present in solution.
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OA to Ir(I),17a C-H versus C-F to Ni(0) and Pt(0),17b and Os(0) and
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