1,3-Diphosphorus ylide cyclopentadienylium salts: Synthesis, structures, and application in coupling reactions

Article abstract of DOI:10.1021/om201267q

The 1,3-diphosphorus ylide cyclopentadienylium salts (C₅H ₃)(PPh₃)₂I (1) and (C₅H ₃)[P(4-CH₃-Ph)₃]₂I (2) have been prepared from the reaction of 1,1′-dichloromercurioferrocene with Pd(PPh₃)₄ and with Pd[P(4-CH₃-Ph) ₃]₄, respectively. The molecular structure of 1 has been determined by X-ray diffraction analyses. The Pd(OAc)₂/1 or 2/K ^tOBu system is highly efficient for the coupling reactions of aryl chlorides at room temperature.

Full text of DOI:10.1021/om201267q

Communication
pubs.acs.org/Organometallics
1,3-Diphosphorus Ylide Cyclopentadienylium Salts: Synthesis,
Structures, and Application in Coupling Reactions
Chen Xu,*,† Zhi-Qiang Wang,† Zhen Li,‡ Wei-Zhou Wang,† Xin-Qi Hao,‡ Wei-Jun Fu,† Jun-Fang Gong,‡
Bao-Ming Ji,† and Mao-Ping Song*,‡
†College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, People's Republic of China
‡Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, People's Republic of China
S
* Supporting Information
ABSTRACT: The 1,3-diphosphorus ylide cyclopentadieny-
lium salts (C5H3)(PPh3)2I (1) and (C5H3)[P(4-CH3-Ph)3]2I
(2) have been prepared from the reaction of 1,1′-
dichloromercurioferrocene with Pd(PPh3)4 and with Pd[P(4-
CH3-Ph)3]4, respectively. The molecular structure of 1 has
been determined by X-ray diffraction analyses. The Pd(OAc)2/
1 or 2/KtOBu system is highly efficient for the coupling
reactions of aryl chlorides at room temperature.
s is known, the cyclopentadienyl anion C5H5− (Cp) is a
compounds of the first and second bands are Pd(PPh3)2I2 and
Hg2I5PPh4, respectively (Figures S1 and S2; see the Supporting
A
classic example of aromaticity and will undergo electro-
philic aromatic substitution readily. In 1957, Ramirez and Levy
first reported tripenylphosphonium cyclopentadienylide A
(Figure 1),1 which is unusually inert, unlike normal ylides.
Information). Initially, we attempted the preparation of 1
starting from A; the expected product 1 was not obtained. We
have investigated an alternative preparative scheme using
chloromercurioferrocene as reagent; however, the target
product was also not obtained. In addition, many attempts to
synthesize 1 using Pd(II) complexes such as Pd(PPh3)2Cl2 and
Ni(PPh3)4 were unsuccessful. From these results, one possible
reaction path (Scheme 1) is via electrophilic substitution of the
Cp ring with PPh3 catalyzed by a Pd(0) complex and
demetalation of ferrocene; 1,1′-dichloromercurioferrocene
plays an important role in promoting the demetalation.6 To
further extend the new method, the analogue 2 has also been
successfully synthesized using Pd[P(4-CH3-Ph)3]4 instead of
Pd(PPh3)4.
The two ligands as red solids are very soluble in chloroform,
dichloromethane, and acetone but insoluble in petroleum ether
and n-hexane. They are not very stable in the air and turn black
after several days. Therefore, they should be stored under
argon. The NMR spectra of 1 and 2 were consistent with the
proposed structures; the Cp protons are significantly shifted
downfield, and the 13C NMR chemical shifts for Cp also exhibit
changes similar to those of the phosphorus ylide cyclo-
pentadienylide.3,4 The 31P NMR spectra show signals at 14.2
and 13.3 ppm, respectively, for 1 and 2. Furthermore, in the
mass spectra of 1 and 2, the most intense peaks were found at
m/z 587.1 and 671.2, respectively, which was attributed to [M
Figure 1. Structural motifs phosphonium cyclopentadienylides.
They attributed this unusual stability to the charge delocaliza-
tion implied by resonance structure Ab.2 Because of difficulties
in characterizing and synthesizing many phosphonium cyclo-
pentadienylides, their reaction behaviors and donor properties
have not been extensively explored.3 Recently Baird and co-
workers reported mixed alkyl−aryl cyclopentadienylidene
derivatives which would be more amenable to characterize.4
To the best of our knowledge, there is presently no report
concerning the synthesis and structural characterization of
diphosphorus ylide cyclopentadienylium salts B. In view of
these findings and our continuous interest in the applications of
chloromercurioferrocene,5 we report a new method for the first
direct synthesis of 1,3-diphosphorus ylide cyclopentadienylium
salts (Scheme 1) and examined their catalytic activity in
coupling reactions.
Scheme 1 illustrates our synthetic route to target compounds
1 and 2. Heating a mixture of 1,1′-dichloromercurifoerrocene,
Pd(PPh3)4, and NaI in DMF at 150 °C produced 1, which was
obtained by flash chromatography as the third band. The
Received: December 21, 2011
Published: January 23, 2012
© 2012 American Chemical Society
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Organometallics
Communication
Scheme 1. Synthesis of 1 and 2
Figure 2. Molecular structure (a) of 1 and selected bond lengths (b).
− I]+. Finally, the structure of 1 has been confirmed by single-
compounds 1 and 2 would be efficient ligands for coupling
reactions. As a preliminary study, the catalytic activities of 1 and
2 were tested in the Buchwald−Hartwig reaction and Suzuki
reaction (Table 1). Our initial exploration of the reactions
focused on the coupling of chlorobenzene with aniline. The
reaction was performed under a nitrogen atmosphere in
dioxane in the presence of 2 equiv of KtOBu as base at room
temperature for 20 h. We are pleased to find that the
Pd(OAc)2/L(1 or 2) (1 mol %/2 mol %) system provided
coupled products in excellent yields (entries 1 and 2). Under
the same reaction conditions, similar to the results for
chlorobenzene, good yields were also obtained in the case of
4-chlorotoluene and p-toluidine (entries 3 and 4). Reactions of
electron-deficient aryl chlorides such as 4-chloronitrobenzene
gave higher yields (entries 5 and 6). Moreover, their application
to the Suzuki reaction was also investigated. However, under
the above conditions, chlorobenzene with phenylboronic acid
gave the coupled product in only a 43% yield (entry 7). When
toluene was used as solvent, the yields increased significantly to
85% and 90% (entries 8 and 9). Similar to the results of
chlorobenzene, good yields were also obtained in the case of 4-
chlorotoluene and 4-methylphenylboronic acid (entries 10 and
crystal X-ray diffraction.
Figure 2 shows clearly that 1 is a cyclopentadienium salt
containing a new triphenylphosphonium cyclopentadinylide
cation and a I− anion and the Cp ring is planar. The C−C−C
bond angles (105.9(6)−110.0(6)°) of the Cp ring are very
close to the 108° of a regular pentagon. The structure of the Cp
ring exhibits alternating patterns of bond lengths, which are
much shorter than a typical C−C single-bond length and are
similar to those of benzene (1.3894 Å). More to the point, the
P−Cp bond lengths (1.729(7) and 1.732(6) Å) are obviously
shorter than the C−P single bonds in phenyl phosphonium
ylides, while they are longer than the PCH2 bond in Ph3P
CH2 (1.66 Å).7 These results are consistent with very
significant contributions from both the uncharged and
zwitterionic resonance structures.4
Recently, great progress has been obtained in coupling
reactions by using bulky electron-rich phosphines8 and N-
heterocyclic carbene (NHCs) ligands.9 However, palladium-
catalyzed coupling reactions of aryl chlorides at room
temperature have been less reported.10 As the structures of
1,3-diphosphorus ylide cyclopentadienylium salts are similar to
those of imidazolium salts, we were interested to see whether
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Communication
a
Table 1. Coupling Reactions of Aryl Chlorides Catalyzed by Pd(OAc)2/1 or 2
b
entry
R1
R2
ligand
yield (%)
1
2
3
4
5
6
7
H
H
Ph
Ph
1
2
1
2
1
2
1
1
2
1
2
1
2
A
A
86
93
83
91
90
97
43
85
90
81
87
94
99
0
4-CH3
4-CH3
4-NO2
4-NO2
H
4-CH3-Ph
4-CH3-Ph
4-CH3-Ph
4-CH3-Ph
Ph
c
8
H
Ph
c
9
H
Ph
c
10
4-CH3
4-CH3
4-NO2
4-NO2
4-CH3
H
4-CH3-Ph
4-CH3-Ph
Ph
c
11
c
12
c
13
Ph
c
14
4-CH3-Ph
H
15
0
a
Reaction conditions: Pd(OAc)2 (1 mol %), ligand (2 mol %), aryl chloride (1.0 mmol), amine (1.2 mmol) or arylboronic acid (1.5 mmol), KtOBu
b
c
(2 mmol), dioxane (5 mL), room temperature, 20 h. Isolated yields (average of two runs). Toluene was used.
11). As expected, 4-chloronitrobenzene could provide excellent
yields (entries 12 and 13).
To explore the role of 1 and 2 in the above coupling
reactions, we performed control coupling reactions catalyzed by
A (entries 14 and 15); the phosphorus ylide A was inactive
under the same conditions, suggesting that 1 and 2 probably
participated in the catalytic cycles as carbene precursors.
However, numerous attempts to isolate the corresponding
carbenes of 1 and 2 at low temperature failed because of their
instability. Recently Kawashima and co-workers reported a
carbene bearing a phosphorus ylide, which had the highest
electron-donating ability of a carbene at the time.11 Soon after,
Furstner and co-workers also reported the (amino)(ylide)-
̈
Figure 3. HOMO (left) and LUMO+12 (right) of the corresponding
carbene of 1.
carbenes.12 These ligands have high σ-donating ability because
the inductive effect of the phosphorus ylide moiety is smaller
than that of the amino group. From this viewpoint, the
corresponding carbenes of diphosphorus ylides cyclopentadie-
nylium salts 1 and 2 may have higher electron-donating ability.
To gain more insight into the origin of the donating property
of the corresponding carbenes of 1 and 2, theoretical
calculations13 were performed at the B3LYP/6-31G(d) level
according to the literature.11 The π-acceptor orbitals of the
corresponding carbenes of 1 and 2 (LUMO+12, 1.70 eV;
LUMO+12, 1.84 eV), which mainly consist of the p(π) orbital
of the carbene center, are located at a higher energy level
relative to that of a carbene bearing a phosphorus ylide (0.60
eV).11 At the same time, the σ-donating HOMO orbital of the
corresponding carbenes of 1 and 2 (HOMO, −3.58 eV;
HOMO, −3.42 eV) are also higher than that of carbene bearing
a phosphorus ylide (−4.4 eV)11 (Figures 3 and 4 and Figures
S3−S6 (Supporting Information)). The results suggest that the
enhanced electron-donating ability of the corresponding
carbenes of 1 and 2 is derived from both the high σ-donating
ability and the low π-accepting ability. To the best of our
knowledge, the corresponding carbene of 2 has the highest
electron-donating ability among carbenes known to date, which
Figure 4. HOMO (left) and LUMO+12 (right) of the corresponding
carbene of 2.
is consistent with its high catalytic activity in the above coupling
reactions.
In summary, we found a new method for the first direct
synthesis of 1,3-diphosphorus ylide cyclopentadienylium salts.
The Pd(OAc)2/1 or 2/KtOBu system is highly efficient for the
coupling reactions of aryl chlorides at room temperature. The
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Organometallics
Communication
Navarro, O.; Mei, J. G.; Stevens, E. D.; Scott, N. M.; Nolan, S. P. J. Am.
Chem. Soc. 2006, 128, 4101.
(11) Nakafuji, S. Y.; Kobayashi, J. J.; Kawashima, T. Angew. Chem.,
synthesis and application of these carbene complexes are
currently under investigation in our laboratory.
Int. Ed. 2008, 47, 1141.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and a CIF file giving experimental details,
characterization data, details of the calculations, and X-ray
crystal structure data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
(12) Furstner, A.; Alcarazo, M.; Radkowski, K.; Lehmann, C. W.
̈
S
Angew. Chem., Int. Ed. 2008, 47, 8302.
(13) Frisch, M. J.; et al. Gaussian 09, Revision B.01 (see the
Supporting Information).
AUTHOR INFORMATION
Corresponding Author
*E-mail: xubohan@163.com (C.X.); maopingsong@zzu.edu.cn
(M.-P.S.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (No. 20902043) and the Natural Science Foundation of
Henan Province and Henan Education Department, People's
Republic of China (Nos. 102300410220 and 2009B150019) for
financial support of this work.
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