A diaminocarbene-phosphonium ylide: Direct access to C,C chelating ligands

Article abstract of DOI:10.1002/anie.200701490

(Chemical Equation Presented) Captured and surrounded by C: A strongly σ-donating C,C ligand and an allyl ligand surround the palladium center in the complex shown in the scheme. The chelate, with three different types of carbon atoms, is soluble and stab

Full text of DOI:10.1002/anie.200701490

Angewandte
Chemie
DOI: 10.1002/anie.200701490
Carbon Ligands
A Diaminocarbene–Phosphonium Ylide: Direct Access to C,C
Chelating Ligands**
Yves Canac,* Carine Duhayon, and Remi Chauvin*
For a long time, the main neutral spectator ligands used in
catalysts with late transition metals (such as Ru, Rh, Ir, Ni,
and Pd) were phosphines and amines.[1] More recently,
carbon-centered ligands have appeared as a competing
category. Two kinds of such spectator carbon ligands can be
distinguished. Unsaturated ligands centered at an sp2-hybri-
dized carbon atom are represented by a wide range of
carbenes,[2] and in particular by N-heterocyclic carbenes
(NHCs).[3] Examples of saturated ligands centered at an sp3-
hybridized carbon atom have been limited until now to
specific phosphonium ylides.[4] Although these types of
neutral carbon ligands differ in their formal character
according to the Green formalism (L = two-electron ligand
for NHCs, X = one-electron ligand for ylides), they share two
noticeable common features in terms of their bonding mode:
1) They act as s donors (rather than p acceptors), and 2) they
plexes of w bisylides.[7] Owing to the prom-
ising properties of both types of dicarbon
ligand, hybrid soft–hard dicarbon ligands
are natural targets for investigation. We
describe herein the development of a
carbene–ylide ligand of a palladium com-
plex (Scheme 1).
A rigid phenylene bridge occurs in
many bidentate ligands, and in particular in hard C,C ligands.
ylides of phosphonio(phosphino)benzenes,
Scheme 1. Gene-
ric zwitterionic
formula of com-
plexes of soft–
as the bridge can not undergo undesired
deprotonation.[8] The (imidazolylphenyl)diphenylphosphine 2
was prepared in 65% yield by the double deprotonation of 1-
phenylimidazole (1) with nBuLi in THF, followed by mono-
phosphinylation of the most nucleophilic lithiated carbon
atom (at one ortho position of the phenyl substituent;
Scheme 2). A single equivalent of the hard methylating
À
+
À
+
=
À
are intrinsically a-zwitterionic ( C N , C P ), which results
À
À
in a formal b-zwitterionic form of the coordinated unit ( M
+
À
+
=
À À
C N , M C P ). The latter feature should induce electro-
static distortion in the metal coordination sphere and thus
play a special role in the control of metal-mediated processes,
for example, in catalysis. However, the effective s donation
and charge separation induced by the ligands should be less
pronounced with soft, phosphine-like, carbene ligands than
with harder ylide ligands. (All phosphonium ylides described
to date were indeed reported to act as h1 carbon-centered
2
[4]
=
ligands rather than as h C P ligands.)
A large number of homo- and heteroleptic bidentate
ligands, in particular with a diaminocarbene functionality
(NHC–X: X = N, O, P, S), have been devised to increase the
efficiency and fine-tune the stereoselectivity of complex-
ation.[5] Electrostatic distortion and electronic s donation
should also be enhanced with bidentate C,C ligands. Sym-
metrical soft–soft dicarbon ligands are exemplified by biscar-
bene ligands,[6] which have been used as alternatives to
diphosphine ligands in catalysis. Symmetrical hard–hard
dicarbon ligands can be found in doubly zwitterionic com-
Scheme 2. Straightforward synthesis of the two cationic phosphonio-
carbene–palladium complexes 4a,b (with a monodentate ligand) and
5a,b (chelated). The letters a and b refer to stereoisomers of
complexes 4 and 5.
reagent methyl triflate (MeOTf) reacted selectively with 2 in
CH2Cl2 to give the N-methylated monocation, the treatment
of which with a second equivalent of MeOTf afforded the
iminio–phosphonio dication 3 in 95% yield. The conjugated
diacid 3 of the targeted ligand precipitated from the reaction
mixture and was thus obtained simply in pure form. Com-
pound 3 was fully characterized, including by X-ray diffrac-
tion analysis of a single crystal (Table 1, Figure 1).[9]
Despite the similarity of the pKa values of the imidazo-
lium and methylphosphonium salts (pKa 22–24 in DMSO),
the heteroleptic nature of the targeted ligand prompted us to
investigate a sequential complexation route. We found that
standard transmetalation through an NHC–Ag intermediate
can be avoided.[3] Instead, the direct and mild treatment of 3
[*] Dr. Y. Canac, C. Duhayon, Prof. R. Chauvin
Laboratoire de Chimie de Coordination (LCC) of the CNRS
205 route de Narbonne, 31077 Toulouse Cedex 4 (France)
Fax: (+33)5-6155-3003
E-mail: yves.canac@lcc-toulouse.fr
remi.chauvin@lcc-toulouse.fr
[**] This research was supported by the Centre National de la Recherche
Scientifique and by the Minist›re de l’Enseignement SupØrieur de la
Recherche et de la Technologie.
Supporting information for this article, including synthetic proce-
dures, characterization data for all new compounds, catalytic
procedures, and crystal-structure data, is available on the WWW
under http://www.angewandte.org or from the authors.
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Table 1: Comparison of structural data (selected bond lengths [Š] and angles [8]) from X-ray diffraction
studies of dication 3 and complexes 4a,b and 5a,b.
chemical shift of the carbon atom
directly bonded to both nitrogen
atoms in the imidazolyl moiety
shifted from dC = 137.1 ppm in 3 to
dC = 181.8–182.0 ppm in 4a,b. The
existence of two stereoisomers of 4
is possibly a result of restricted
3
4a,b
5a,b[a]
+
À
P
CH2
1.783(3)
1.330(4)
1.321(4)
–2.037(2)
–
1.788(2)
1.358(2)
1.345(3)
1.73(1); 1.71(1)
1.38(2); 1.37(2)
1.36(2); 1.36(2)
À
ArN CN
CH3N CN
À
À
Pd CN2
2.01(1); 2.02(2)
A···CH2P+
A=Cl: 3.539(2)
A=Pd: 2.10(1);
2.15(1)
45.7; 55.6
À
À
rotation about the Pd CN2 or N
aryl bond. On the basis of data in
the literature,[10] the observed ste-
reochemical disorder can be attrib-
uted reasonably to the flip of the p-
allyl ligand with respect to the
P+C-C-N-CN dihedral angle 89.8
N2C-Pd···CH2 –45.6
(Ph2P+CHx)···imidazole inter- p–p stacking
103.3
99.9; 97.9
weak CH···p/p interaction? p–p stacking (x=2)
(x=3)
action
(x=3)
P+Ca···NAr
Ca =Cipso
3.153(4)
Ca =Cipso
3.145(4)
:
:
Ca =CH3: 3.056(3)
Ca =Cipso: 3.12(2);
3.08(2)
disymmetric
diaminocarbene
P+Ca···CN2
Ca =CH3: 3.290(3)
Ca =Cipso: 3.00(2);
3.10(2)
ligand, whereby the MePh2P+
group lies anti to the p-allyl ligand.
[a] The left-hand value corresponds to one of the molecules in the asymmetric unit, and the right-hand This hypothesis and the overall
value to the other molecule in the same asymmetric unit (see CCDC files).[9]
structure of the complex were con-
firmed by X-ray diffraction analysis
of colorless single crystals of 4a,b
deposited from ethanol (Table 1,
Figure 1). The air-stable complex 4a,b is stable on silica gel
and, more interestingly, soluble and stable in water.
The preliminary complexation of the carbene enabled the
preparation of a new type of phosphoniocarbene complex and
was also expected to facilitate the chelation of the palladium
center by the ylide of the appended phosphonium ion.
À
Although examples of the C H activation or deprotonation
of N-alkyl substituents of NHC ligands have been reported,[11]
the treatment of 4a,b with potassium bis(trimethylsilyl)amide
(KHMDS) in THF afforded cleanly a dehydrochlorinated
complex (electrospray MS: m/z 503) as a 7:3 mixture of
stereoisomers in 79% yield. As the signal in the 31P NMR
spectrum was shifted significantly to dP = 34.2–34.1 ppm
(from dP = 21.70–21.73 ppm for 4a,b), the complex was
believed to be the expected carbene–ylide complex in the
form of a mixture of the stereoisomers 5a and 5b. The
À
presence of a Pd CH2 methylene group was confirmed by
multinuclear NMR spectroscopy, in particular on the basis of
the high-field 13C NMR signals dC = À11.2 and À10.9 ppm.
The geometrically constrained chelate configuration was
deduced from the coupling constants observed in the
13C NMR spectrum for the coupling of the carbene carbon
center (dC = 180.7 and 181.2 ppm) with the positively charged
phophorus center through the shortest pathway between the
two nuclei (C–Pd–CH2–P+; 3JP, C = 7.8 and 8.6 Hz).
The structure of complex 5a,b was confirmed by X-ray
diffraction analysis of colorless single crystals deposited from
a CH3CN/Et2O mixture (Table 1, Figure 1). As commonly
observed,[10] the p-allyl ligand was found to be disordered
over two orientations with respect to the frozen N2C–Pd–
CH2P+ chelated core. The seven-membered palladacycle is
boat-shaped, the prow occupied by the phosphorus center and
Figure 1. ORTEP views of the X-ray crystal structures of compounds
a) 3, b) 4a,b, and c) 5a,b with thermal ellipsoids drawn at the 30%
probability level (for representative bond lengths and angles, see
Table 1).
with half an equivalent of allylchloropalladium dimer [{PdCl-
(p-allyl)}2] and triethylamine in acetonitrile afforded cleanly a
phosphoniocarbene mononuclear complex as a 1:1 mixture of
the stereoisomers 4a and 4b in 85% yield. The chemical shift
observed for the phosphorus atom in 4a,b in the 31P NMR
spectrum (dP = 21.70–21.73 ppm) is indeed very similar to that
for 3 (dP = 20.50 ppm), whereas in the 13C NMR spectrum the
À
the stern by the C NAr bond. Although this conformation is
dictated mainly by steric constraints, an auxiliary driving force
is a p stacking of the imidazolyl ring with a phenyl ring of the
+PPh2 unit (Table 1). A similar p-stacking interaction was also
observed in the free dication 3. In the nonchelated phospho-
niocarbene complex 4a,b, this interaction is replaced by short
6314
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Chemie
contacts between the methyl group on the phosphorus atom
and the CPNAr bond, and between the same methyl group
and the chloride ligand. The formation of these contacts is
[1] E. W. Abel, F. G. A. Stone, G. Wilkinson, Comprehensive
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probably directed by weak CH···X interactions (X = N, C, Cl).
+
À
In complex 5a,b, the P CH2 bond (ca. 1.73 Š) is markedly
+
À
shorter than that in 4a,b (ca. 1.79 Š); however, the P CH2
Angew.Chem.Int.Ed.
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Pd bond length (ca. 2.10 Š) is comparable to classical
+
[8b]
À
P CH(R) Pd distances. The chelating substitution of the
chloride ligand in 4a,b by the ylide ligand in 5a,b results in a
À
slight shortening of the N2C Pd bond (ca. 2.04 Š in 4a,b
À
versus 2.01 Š in 5a,b) and a slight lengthening of the ArN C
[3] a) N. M. Scott, S. P. Nolan, Eur.J.Inorg.Chem. 2005, 1815; b) N.
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bond (ca. 1.36 Š in 4a,b versus 1.38 Š in 5a,b). These data
suggest that the strong s donation of the hard ylide is
absorbed by the residual p acceptance of the softer diamino-
carbene ligand.
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The catalytic properties of the air-stable complex 5a,b
were tested in the Tsuji–Trost allylation of the dimethyl
malonate anion by 3-acetoxy-1,3-diphenylpropene. The reac-
tion in the presence of 5a,b (5 mol%) in THF as the solvent
was complete within 12 h at 608C to give the desired product
with 100% conversion and 100% selectivity according to
1H NMR spectroscopic analysis of the crude material
(Scheme 3).
[4] a) L. R. Falvello, J. C. GinØs, J. J. Carbó, A. Lledós, R. Navarro,
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Chauvin, Eur.J.Inorg.Chem.
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1999, 193–195, 1143; d) O. I.
Kolodiazhnyi, Tetrahedron 1996, 52, 1855; e) W. C. Kaska,
K. A. Ostoja Starzewski in Ylides and Imines of Phosphorus
(Ed.: A. W. Johnson), Wiley, New York, 1993, p. 14; f) H.
Schmidbaur, Angew.Chem. 1983, 95, 980; Angew.Chem.Int.Ed.
Engl. 1983, 22, 907.
[5] For recent examples, see: S. J. Roseblade, A. Ros, D. Monge, M.
Alcarazo, E. lvarez, J. M. Lassaletta, R. Fernµndez, Organo-
metallics 2007, 26, 2570, and references therein.
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Angew.Chem.Int.Ed. 2005, 44, 444.
Scheme 3. 1,3-Diphenylallylation of dimethyl malonate under the catal-
ysis of complex 5a,b.
In summary, we have developed an efficient short
sequential synthesis of two novel p-allyl–palladium com-
plexes, monodentate 4a,b and bidentate 5a,b, from the
readily accessible dication 3. The diaminocarbene–phospho-
nium-ylide complex 5a,b contains a novel type of strongly s-
donating C,C chelating ligand. This PdC5-type complex is
“homoleptic” in the first coordination sphere: The palladium
atom is bonded to five carbon atoms of three different types:
allylic (sp2/sp3), carbenic (sp2), and phosphonioalkyl (sp3).
Preliminary experiments illustrated the catalytic properties of
complex 5a,b. The solubility and stability in water of 4a,b and
5a,b, which are readily accessible on a multigram scale,
suggest the possibility of catalytic applications in aqueous
media. Finally, the ligands might be made intrinsically
[7] a) L. R. Falvello, M. E. Margalejo, R. Navarro, E. P. Urriolabei-
tia, Inorg.Chim.Acta 2003, 347, 75; b) A. Spannenberg, W.
Baumann, U. Rosenthal, Organometallics 2000, 19, 3991.
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2004, 61, 298; b) R. Zurawinski, B. Donnadieu, M. Mikolajczyk,
R. Chauvin, J.Organomet.Chem. 2004, 689, 380; c) R. Zur-
awinski, B. Donnadieu, M. Mikolajczyk, R. Chauvin, Organo-
metallics 2003, 22, 4810; d) L. Viau, C. Lepetit, G. Commenges,
R. Chauvin, Organometallics 2001, 20, 808.
[9] CCDC 642733 (3), 642732 (4a–b), and 642731 (5a–b) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[10] a) J. Vignolle, B. Donnadieu, D. Bourissou, M. Soleilhavoup, G.
Bertrand, J.Am.Chem.Soc.
2006, 128, 14810; b) Y. Ding, R.
À
atropoisomeric with respect to the central C(Ar) N
Goddard, K.-R. Pꢀrschke, Organometallics 2005, 24, 439.
[11] a) N. M. Scott, R. Dorta, E. D. Stevens, A. Correa, L. Cavallo,
S. P. Nolan, J.Am.Chem.Soc. 2005, 127, 3516; b) F. E. Hahn, D.
Le Van, M. C. Moyes, T. v. Fehren, R. Frꢀhlich, E. U.
Würthwein, Angew.Chem. 2001, 113, 3241; Angew.Chem.Int.
Ed. 2001, 40, 3144.
[12] For related attempts by our research group, see: M. Soleil-
havoup, L. Viau, G. Commenges, C. Lepetit, R. Chauvin, Eur.J.
Inorg.Chem. 2003, 207, and references [8a,d].
bond,[12] and applications in asymmetric catalysis can be
envisaged. Efforts toward the development of such chiral
ligands are in progress.
Received: April 5, 2007
Revised: May 31, 2007
Published online: July 16, 2007
Keywords: carbene ligands · chelates · homogeneous catalysis ·
.
palladium · phosphonium ylides
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