Carbocyclic carbene ligands derived from aromatic nitrones: Formation and catalytic activity of their Pd(II) complexes

Article abstract of DOI:10.1021/ja0701659

Nitrones derived from aromatic aldehydes were shown to be convenient precursors to a novel class of carbocyclic carbene ligands. Air- and moisture-stable palladium complexes derived from the carbocyclic carbene ligands exhibit unprecedented catalytic efficiency in the Heck reaction. Deactivated aryl bromides, such as 4-bromoanisole, undergo coupling with styrene in the presence of 8 × 10^-5 mol % of the catalysts with TON exceeding 0.9 million. Copyright

Full text of DOI:10.1021/ja0701659

Published on Web 02/21/2007
Carbocyclic Carbene Ligands Derived from Aromatic Nitrones: Formation
and Catalytic Activity of Their Pd(II) Complexes
Qingwei Yao,* Mark Zabawa, Joyce Woo, and Chong Zheng
Department of Chemistry and Biochemistry, The Michael Faraday Laboratories, Northern Illinois UniVersity,
DeKalb, Illinois 60115
Received January 9, 2007; E-mail: yaogroup@hotmail.com
First introduced in 1995 by Herrmann1 into the field of
Pd-catalyzed cross-coupling reactions, N-heterocyclic carbenes
2
(
NHCs) have become a paradigmatically new generation of strong
σ donor ligands. NHCs are widely used in transition metal
catalysis,2 as exemplified by the development of highly active
,3
4
catalysts for olefin metathesis and for room-temperature Suzuki
5
coupling involving aryl chlorides. Electronic and skeletal variation
2a,b
on the original Arduengo carbene A has led to the discovery of
a number of diaminocarbene ligands such as B-G6 (Chart 1).
More recent additions to the list include cyclic diaminocarbenes
H-J which are built on inorganic frameworks.8
,7
Chart 1. Structural Motifs of NHCs
Figure 1. Carbocyclic carbenes and carbene-containing metal complexes.
Scheme 1
Compared to the aforementioned cyclic diaminocarbenes and
6
other heterocyclic carbenes, nucleophilic carbocyclic carbenes are
much less well-known.9 Analogous to the synthesis of NHCs,
which can be obtained by deprotonation of azolium salts, carbocy-
clic carbenes could in principle be approached from their conjugate
acids. Recently, the first isolated three-membered carbocyclic
carbene of the type K, the bis(dialkylamino)cyclopropenylidene 1,
was synthesized by deprotonation of a cyclopropenium salt (Figure
,10
and the anionic oxygen serve as a directing group for selective C-H
activation via orthometalation.
When nitrone 4a was allowed to react with Pd(OAc)
2
in acetic
11
12
13
acid, Pd complex 5a was isolated in 82% yield as an air-stable
1). Stable transition metal complexes such as 2 and 3,
1
13
pale yellow solid (mp 213 °C) (Scheme 1). Both H NMR and
C
containing ligand L based on the aromatic tropylium system, have
previously been synthesized and characterized. We were intrigued
by the possibility of a six-membered congener of L, the anionic
carbene M, as a new donor ligand. As depicted in Figure 1, carbene
M represents an otherwise rarely considered resonance of the phenyl
anion. As such, a benzene derivative could in principle serve as its
precursor. We report here on our efforts toward the realization of
this concept.
NMR data revealed 5a to exist as a mixture of two diastereoisomers
in a ratio of ca. 10:1. The major isomer was assigned as the C-2
symmetric dinuclear transoid form, 5a-I (Figure 2). The bridging
acetate groups in 5a-I are diastereotopically equivalent (singlet at
1
13
σ 2.12 ppm for H and σ 180.7 ppm for C). The minor isomer,
the symmetric cisoid form, 5a-II, has two diastereotopically
1
different acetate groups (singlet at σ 2.01 and 2.15 ppm for H,
In contemplating a carbocyclic carbene based on the benzene
skeleton, we took notice the formulistic distinction between a
and σ 179.8 and 182.6 ppm for 13C). In a similar manner, the
“phenylide” ligand and an anionic carbene ligand; the latter requires
that the coordinating carbon atom serve as a neutral donor. We
further reasoned that a stable cationic group such as an iminium
group appended to the benzene ring would provide the required
charge balance. Along this line of reasoning, we designed a
bidentate ligand N featuring an anionic carbene attached to a nitrone
group. The choice of this group is based on the anticipation that
the iminium nitrogen would stabilize the charge on the phenyl ring
Figure 2. Stereoisomers of 5a.
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J. AM. CHEM. SOC. 2007, 129, 3088-3089
10.1021/ja0701659 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
alternation on the rest of the benzene ring point to the significance
of structure 5b-B as a major contributor.
A preliminary test of their catalytic activity revealed that these
Pd complexes are extremely efficient precatalysts for the Heck
reaction (Table 1). The coupling of unactivated and deactivated
aryl bromides with styrene could be achieved with TON greater
than 1 000 0000. The catalysts were active at extremely low loading
and remained active over a long period of reaction time, attesting
to their excellent stability under the conditions employed. The
unprecedented efficiency of 5a and 5b may be attributed to the
anionic nature of the carbene ligand. The electron-donating methoxy
groups in 5b make it significantly more reactive than 5a, pointing
to the possibility of electronic and steric tuning on the carbene
ligand to further increase the activity of the resulting metal catalysts.
In conclusion, we have shown that highly active and catalytically
useful Pd complexes can be conveniently assembled from readily
available ligand precursors. The concept of using an anionic
carbocyclic carbene as ancillary ligand for palladium as demon-
strated here opens a new opportunity for the development of
structurally novel carbene ligands for transition metal catalysis.
Figure 3. X-ray crystal structure and selected bond lengths (Å) of 5b.
Table 1. The Heck Reaction of Aryl Halides Catalyzed by Pd(II)
Complexes 5a and 5b
Supporting Information Available: Experimental details and
spectroscopic and X-ray data of compounds 5a and 5b. This material
is available free of charge via the Internet at http://pubs.acs.org.
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1
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6 6
Crystallization of 5b from C D yielded yellow crystals suitable
for X-ray structure determination. As shown in Figure 3, 5b
crystallizes as a single diastereomer, the transoid acetato-bridged
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(
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1
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