C-H and C-O oxidative addition in reactions of aryl carboxylates with a PNP pincer-ligated Rh(i) fragment

Article abstract of DOI:10.1039/c1cc15845g

Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO₂CPh only underwent C-H oxidative addition (OA). PhO ₂CCF₃ chiefly underwent acyl-oxygen OA. PhO ₂CBu^t and PhO₂CNEt₂ initially underwent OA of an ortho-C-H bond of the phenyl group but continued thermolysis led to the phenyl-oxygen OA products.

Full text of DOI:10.1039/c1cc15845g

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COMMUNICATION
C–H and C–O oxidative addition in reactions of aryl carboxylates with a
PNP pincer-ligated Rh(I) fragmentw
Yanjun Zhu,a Dan A. Smith,a David E. Herbert,a Sylvain Gatardb and Oleg V. Ozerov*a
Received 21st September 2011, Accepted 18th October 2011
DOI: 10.1039/c1cc15845g
Reactions of a series of phenyl esters with a (PNP)Rh fragment
have been studied. PhO2CPh only underwent C–H oxidative
addition (OA). PhO2CCF3 chiefly underwent acyl–oxygen OA.
PhO2CBut and PhO2CNEt2 initially underwent OA of an ortho-
C–H bond of the phenyl group but continued thermolysis led to
the phenyl–oxygen OA products.
CAryl–O2CR OA and here we disclose our results showing that
it is indeed possible with (PNP)Rh.
The key fragment 6 cannot be isolated or observed because
of its high reactivity.15 We have devised two ways to access it
in solution: (a) either by irreversible C–C reductive elimination
(RE) or (b) by reversible dissociation from a placeholder
ligand.15 For (a), RE from 3,15c 4 or 5 can be used to access
6 (Scheme 1), with 5 being conveniently isolable.
Catalytic transformations of aryl halides that rely on the oxidative
addition (OA) of a CAryl–halogen bond to a Ni(0) or Pd(0) center
are among the most explored, useful, and celebrated reactions in
chemistry.1–3 The ability to perform similar reactions using
ArO2CR (O2CR = carboxylate, carbamate, carbonate) instead
of Ar–Hal is an attractive expansion because Ar–O2CR substrates
are easily prepared from phenols (Ar–OH). In addition, Ar–O2CR
groups may provide avenues for directing functionalization of the
arene ring that are not accessible with aryl halides.4
Coupling reactions of Ar–O2CR and even other Ar–OR0
electrophiles have been successfully explored with Ni catalysts.5–7
The interest in the use of Ar–O2CR has increased significantly in
the last few years, with reports on Suzuki–Miyaura coupling by
Garg et al.,8 Nakamura et al.,9 Shi et al.,10 and Snieckus et al.11
and on aromatic amination by Chatani et al.12 using aryl
pivalates and carbamates. There is sound evidence indicating
that CAryl–O2CR OA is part of the catalytic cycle in these
reactions,8–13 however, authentic, well defined CAryl–O2CR OA
as an individual reaction has not been studied experimentally.14
Our group has been exploring15 oxidative addition reactions
of aryl halides with the T-shaped d8 (PNP)Rh species 6
(Scheme 1) wherein the Rh center is locked into a rigid PNP
pincer ligand framework (PNP = (4-Me-2-iPr2P-C6H3)2N).16
The (PNP)Rh fragment 6 participates in reactions similar to
those of LnM0 (M = Pd, Ni), despite possessing a different
geometry and a different d-electron configuration. Recently,
we have also explored catalysis of the arylation of aryl halides
with a closely related (POCOP)Rh fragment.17 We became
interested in whether such an Rh fragment can also undergo
For (b), the weakest practical adduct of (PNP)Rh would be
the one with a typical solvent. Thermolysis of 3 in fluorobenzene
resulted in the formation of a product with an empirical formula
of ‘‘(PNP)Rh(PhF)’’ (7, Scheme 1). In solution at ambient
temperature, 7 is only stable in PhF as a solvent. It gave rise
to a single 31P NMR resonance at 20 1C at 52.4 ppm (d, JP–Rh
=
113 Hz) while no identifiable 19F NMR or upfield hydride
1H NMR resonance could be detected for 7. However, upon
cooling to ꢀ34 1C, a few 19F NMR resonances in the ꢀ50
1
to ꢀ95 ppm range and a few broad H NMR resonances in
the ꢀ25 to ꢀ41 ppm range were detected.18 It seems most likely
that a rapid equilibrium between various isomers (including
rotamers) of (PNP)Rh(H)(C6H4F) takes place, perhaps also
involving a p complex of PhF. Caulton et al. reported that the
ground state of (SiPNP)Rh (where SiPNP = (tBu2PCH2SiMe2)2N)
is an intramolecular C–H OA product that exists in equili-
brium with a phenyl/hydrido ‘‘adduct’’ in benzene solution.19
The X-ray structural determination on a crystal of 7 (see ESIw)
revealed a fluorophenyl/hydride structure. No C–F OA was
observed after thermolysis at 95 1C for 18 h in neat PhF.
We selected several Ar–O2CR substrates for our study and
monitored their reaction with 5 (Scheme 2) under mild thermolysis
(60 1C, 2–3 h) by NMR spectroscopy in situ. These thermolytic
conditions were needed to complete the C–C reductive elimination
in 5; the reaction of the presumed 6 so generated with the
substrate was much faster as no intermediates were observed.
a Department of Chemistry, Texas A&M University, College Station,
TX 77842, USA. E-mail: ozerov@chem.tamu.edu
b Department of Chemistry, Brandeis University, 415 South Street,
Waltham, MA 02454, USA
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic characterization information. CCDC
[845150–845152]. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc15845g
Scheme 1 Synthesis of various precursors to (PNP)Rh (6).
c
218 Chem. Commun., 2012, 48, 218–220
This journal is The Royal Society of Chemistry 2012

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JC–Rh = 44 Hz and JC–P = 9 Hz. IR spectroscopic analysis
revealed a carbonyl stretching band at 1701 cmꢀ1 23
These
.
data are similar to those of other examples of trifluoroacetyl
complexes of Rh as well as Pd/Pt/Ir.24–26 Extended thermolysis
of 10 led to
a mixture of products that included
(PNP)Rh(CF3)(CO)(OPh)27 and (PNP)Rh(CO),28 indicating
CO deinsertion as the major pathway of reaction for 10,
typical for late metal trifluoroacetyl compounds.24,29
We also tested phenyl acetate in the reaction with 5 (Scheme 2).
A mixture of products was obtained, one of which we tentatively
identified as 11 based on the solution NMR spectroscopic data.
The presence of (PNP)Rh(CO) also indicated that decarbonylation
was taking place, likely through intermediates analogous to those
in the reactions of PhO2CCF3.
Finally, we turned to PhO2CBut and PhO2CNEt2 as substrates,
noting that aryl pivalates and carbamates were successfully used
in Ni-catalyzed coupling reactions.8–13 Mild thermolysis of 5 in
the presence of PhO2CBut and PhO2CNEt2 resulted in the
quantitative formation of compounds 12 and 13, respectively
(Scheme 2). The diagnostic NMR spectroscopic features for 12
and 13 were similar to those of 8/9. The immediate coordination
environment about Rh is the same for 8/9 and 12/13; but the
k2-C,O chelate forms a five-membered ring in 8/9 and a
six-membered ring in 12/13.
Fig. 1 ORTEP drawings of 8 (left, 50% thermal ellipsoids), and 15
(right, 30% thermal ellipsoids) showing selected atom labeling.
Hydrogen atoms (except Rh–H) are omitted for clarity. Selected bond
distances (A) and angles (deg) for 8: Rh1–P1, 2.301(4); Rh1–P2,
2.291(4); Rh1–N1, 2.128(5); Rh1–C27, 2.017(5); Rh1–O1, 2.264(4);
Rh1–H1, 1.46 (5); P1–Rh1–P2, 160.40(7). 15: Rh1–P1, 2.3061(13);
Rh1–P2, 2.3215(12); Rh1–N1, 2.044(2); Rh1–C15, 2.016(3); Rh1–O1,
2.107(2); Rh1–O2, 2.279 (2); P1–Rh1–P2, 165.87(3).
The consumption of 5 in all reactions in Scheme 2 was complete.
Compounds 8, 10, 12, and 13 were also isolated in the pure solid
form in 42–55% unoptimized yields. We were able to use
(PNP)Rh precursors other than 5 for these syntheses (see ESIw).
The reaction of 5 with phenyl benzoate and, for comparison
also with methyl benzoate, resulted in complete conversion to
the C–H OA products 8 and 9, respectively. The C–H OA took
place exclusively at the ortho-position of the benzoate Ph ring,
presumably favored by the coordination of the carbonyl
oxygen.21,22 Harsher thermolysis (100 1C, 18 h) of 8 or 9 did not
result in any further transformations. We recently described15a
analogous ortho-C–H OA taking place in nitroarenes and
arenecarboxylic acid esters. The ortho-O chelated C–H OA
products share common diagnostic spectroscopic features. For
example, 8 gave rise to an 1H NMR hydride resonance at
d ꢀ19.9 ppm as well as a 13C{1H} resonance at 181.0 ppm for
the Rh-bound carbon, both of which manifested themselves as
We were pleased to find that thermolysis at 90 1C of 12 (2 d)
and 13 (9 d) resulted in the conversion to the CAryl–O OA
products 14 and 15, respectively (Scheme 3). Isolation of
analytically pure 14 and 15 was accomplished in 46–48%
unoptimized yields. Interestingly, it was found (by NMR
spectroscopy with an internal integration standard) that the
yield of 14/15 was only ca. 50% if pure samples of 12/13 were
thermolyzed in C6D6 to 100% conversion. The yield of 14/15
increased to 82–90% when the thermolysis was carried out in the
presence of 3 equiv. of PhO2CBut or PhO2CNEt2, respectively.
The remainder of the fully consumed 12/13 gave rise to a mixture
of unidentified products. We hypothesize that the reaction path
from 12/13 to 14/15 involves intermediate liberation of PhO2CBut
or PhO2CNEt2 by C–H RE and an attack of 6 on the CAryl–O
bond in the free substrate. In the absence of added extra substrate,
the substrate concentration in solution is low and 6 may
undergo unselective and irreversible attack on itself. The
notion of intermediate liberation of PhO2CBut or PhO2CNEt2
is consistent with the observation of a mixture of 14 and 15
when 12 is thermolyzed in the presence of PhO2CNEt2 or 13 in
the presence of PhO2CBut.30
doublets of triplets (JH–Rh = 35 Hz, JH–P = 14 Hz; JC–Rh
=
28 Hz, JC–P = 10 Hz). A single crystal X-ray diffraction study
confirmed the structure of 8 (Fig. 1).20 As expected, the
carbonyl oxygen is coordinated to Rh, completing the d6
Rh(III) octahedral coordination sphere.
We wondered if phenyl trifluoroacetate might be the most
‘‘aryl halide-like’’ in OA reactivity. However, the reaction of
in situ generated 7 with it resulted in the acyl–oxygen OA main
product 10,14 with no evidence for either aryl–oxygen OA or
C–H OA (Scheme 2). The diagnostic 13C NMR resonance for
the Rh-bound acyl carbon at 191.9 ppm was observed as a
doublet of triplets in the 13C{19F,1H} NMR spectrum with
Concerted RE from a six-coordinate d6 metal center typically
requires prior dissociation of one of the ligands.22,31 In the case
of 12/13, this would necessitate dissociation of the O-donor as
the first step. The longer time needed for the conversion of
13 is likely a reflection of the stronger donor power of the
carbonyl oxygen in carbamate.
The solid-state structure of 15 was established in a single-
crystal X-ray diffraction study (Fig. 1). The environment
about Rh is distorted octahedral, with the acute bite angle
of the k2-carbamate being the major source of deviation.
In summary, the outcome of the reaction of the (PNP)Rh
fragment with Ph–O2CR substrates is strongly dependent on the
nature of R. It appears that a bulky and/or more electron-
donating R group helps block the undesirable Cacyl–O OA and
Scheme 2 Reactions of 5 with various esters.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 218–220 219

View Online
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Scheme 3 Formation of the aryl–oxygen OA products upon thermolysis
at 90 1C. The proposed mechanistic path is shown in green.
R. C¸ elenligil-C¸ etin, C. Guo, B. M. Foxman and O. V. Ozerov,
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18 See ESIw for figures containing these NMR spectra.
19 A. Y. Verat, M. Pink, H. Fan, J. Tomaszewski and K. G. Caulton,
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channel the reaction towards CAryl–O OA. The competition
between Cacyl–O and CAryl–O OA has been recognized before
in the chemistry of Ni/Rh.8a,13,24b Our findings here also highlight
C–H OA as another potential side reaction and illustrate that
pivalate and carbamate may be particularly suitable for pursuing
CAryl–O OA reactions regardless of the metal involved. We hope
20 Persistence of Vision Ray Tracer (POV-Ray) and Ortep-3 for
Windows (L. Farugia, J. Appl. Crystallogr., 1997, 30, 565).
21 (a) D. H. Wang, T. S. Mei and J. Q. Yu, J. Am. Chem. Soc., 2008,
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that the insight from this study with a well-defined (PNP)Rh
fragment will help design and understand other systems as well.
Support of this research by the US National Science Foundation
(grants CHE-0944634), the Welch Foundation (grant A-1717), and
the Dreyfus Foundation (Camille Dreyfus Teacher-Scholar Award
to O. V. O) is gratefully acknowledged. We express our gratitude to
Mr Steven K. Silber for assistance with 13C{19F,1H} experiments
and to Ms Linda Redd for assistance with manuscript preparation.
22 X. Zhang, M. Kanzelberger, T. J. Emge and A. S. Goldman,
J. Am. Chem. Soc., 2004, 126, 13192.
23 We have also prepared (PNP)Rh(COCF3)(I) by OPh/I metathesis
of 10 with Me3Si–I in situ; (PNP)Rh(COCF3)(I) gave rise to a
13C{1H} multiplet at 192 ppm and a nCO = 1693 cmꢀ1
.
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c
220 Chem. Commun., 2012, 48, 218–220
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