A simple building-block route to (phosphanyl-carbene)palladiuni complexes via intermolecular addition of functionalised phosphanes to isocyanides

Article abstract of DOI:10.1002/ejic.200801067

We present a straightforward protocol for making (phosphanyl-carbene) Pd^II complexes. These complexes have bidentate ligands containing an acyclic diamino- or aminooxy-carbene and a phosphane. The synthesis gives good yields (typically 70-90%) for a variety of complexes (22 compounds). Moreover, it does not require the synthesis of imid azolium salts nor the a priori generation of free carbenes. Three of the new complexes were tested as catalysts for Son-ogashira and Hay coupling reactions, with good yields and selectivities. Wiley-VCH Verlag GmbH & Co, KGaA.

Full text of DOI:10.1002/ejic.200801067

SHORT COMMUNICATION
DOI: 10.1002/ejic.200801067
A Simple Building-Block Route to (Phosphanyl-carbene)palladium Complexes
via Intermolecular Addition of Functionalised Phosphanes to Isocyanides
Michael R. Eberhard,[a] Bart van Vliet,[a] Laura Durán Páchon,[a] Gadi Rothenberg,[a]
Graham Eastham,[a,b] Huub Kooijman,[a,c] Anthony L. Spek,[a,c] and Cornelis J. Elsevier*[a]
Keywords: Carbenes / Carbene complexes / Palladium / Intermolecular addition
We present a straightforward protocol for making (phos-
phanyl-carbene)PdII complexes. These complexes have bi-
dentate ligands containing an acyclic diamino- or aminooxy-
carbene and a phosphane. The synthesis gives good yields
(typically 70–90%) for a variety of complexes (22 com-
pounds). Moreover, it does not require the synthesis of imid-
azolium salts nor the a priori generation of free carbenes.
Three of the new complexes were tested as catalysts for Son-
ogashira and Hay coupling reactions, with good yields and
selectivities.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
teen different diamino-carbene-phosphane and aminooxy-
carbene-phosphane Pd complexes. Three of these were
found to be good precatalysts for Sonogashira and Hay
cross-coupling reactions.
Introduction
The interest in and the importance of carbene-based li-
gands have grown enormously[1,2] since Arduengo’s seminal
discovery in the early 1990s.[3,4] Carbenes have unique prop-
erties as ligands,[5] with catalytic applications including
hydrosilylation,[6] hydrogenation,[7,8] and various carbon–el-
ement coupling reactions.[9] Bidentate carbene ligands with
a second, other functionality are especially interesting as
they combine the strongly coordinating carbene donor with
another (more labile) potentially co-ordinating group. The
problem is that practical applications of such ligands re-
quire simple and straightforward synthesis protocols.[10]
The addition of nucleophiles to coordinated isocyanides, for
example, is a facile route to heteroatom-substituted carb-
enes.[11–17] It typically involves intramolecular addition of
a protic polar function onto a carbon–heteroatom moiety,
CϵX. Normally, this moiety must be synthesised a priori.
Here, we report an alternative protocol that follows inter-
molecular addition of readily available bidentate P–O (or
P–N) ligands to simple isocyanides. This intermolecular
route enables a building-block synthesis approach, ac-
cessing a variety of heteroatom-carbene-Pd complexes.
(Scheme 1). We demonstrate this facile synthesis with six-
Scheme 1. Modular synthetic route towards palladium(II) com-
plexes of diamino- and aminooxycarbene-phosphane ligands.
This novel class of modular, tethered open-chain carb-
enes is based on the well known idea that isocyanide coordi-
nation to a metal center activates the isocyanide carbon,
that can then react with a polar function from another
[a] Van ’t Hoff Institute for Molecular Sciences, University of Am-
sterdam,
Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Nether- molecule.[10,18,19] By using a bidentate phosphane-amine or
lands
a phosphane-alcohol as the nucleophile, we create a new
Fax: +31-20-525-5653
bidentate phosphane-carbene ligand in the first coordina-
tion sphere via intermolecular attack of the polar function
on the isocyanide.
Using the modified phosphanes 1–6, the phosphorus
atom coordinates to the metal, whilst the amine or alcohol
reacts with the isocyanide, forming a carbene. In this way
one obtains directly and cleanly a bidentate phosphane-
E-mail: C.J.Elsevier@uva.nl
http://home.medewerker.uva.nl/c.j.elsevier/
[b] Lucite International UK Ltd., Wilton Centre,
Wilton, Redcar, TS10 4RF, United Kingdom
[c] Department of Crystal and Structural Chemistry, Bijvoet Cen-
tre for Biomolecular Research, Utrecht University
Utrecht, The Netherlands
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2009, 1313–1316
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. J. Elsevier et al.
SHORT COMMUNICATION
carbene ligand coordinated to the metal center. Thus, re- shorter than Pd(1)–Cl(1) (Cl trans to P) with 2.3857(10) Å.
acting compounds 1–6 with four commercially available iso- In view of the relative trans influences of the phosphane
cyanides RNC (R = tBu, Cy, 4-MeOC6H4, MeO2CCH2) in compared to the carbene, the opposite would be expected.
the presence of [PdCl2(cod)] provides the new compounds Possibly, the non-ideal angles due to the chelate conforma-
7–22 in good yields.
tion are responsible for this unusual result.
Figure 1. Displacement ellipsoid plot of 9, drawn at 30% prob-
ability level. Pertinent bond lengths [Å] and angles [°]: Pd(1)–Cl(1)
2.3857(10); Pd(1)–Cl(2) 2.3513(11); Pd(1)–P(1) 2.2193(10); Pd(1)–
C(4) 1.968(2); Cl(1)–Pd(1)–Cl(2) 91.71(3); Cl(1)–Pd(1)–C(4)
88.16(6); Cl(2)–Pd(1)–P(1) 95.41(3); P(1)–Pd(1)–C(4) 85.17(7).
We tested these new (phosphanyl-carbene)PdCl2 com-
plexes as precatalysts in the Sonogashira and Hay coupling
of various aryl and pyridyl halides. Pyridyl iodides, bro-
mides and chlorides were smoothly coupled with phenyl-
acetylene in the presence of 1 mol-% of Pd-carbene catalyst
Table 1. Pd-carbene-catalyzed Sonogashira and Hay coupling.[a]
In a typical reaction, the functional phosphane dissolved
in anhydrous dichloromethane was added to an equimolar
amount of [PdCl2(cod)] dissolved in the same solvent. The
mixture was stirred for ca. 1 h at 20 °C. Subsequently, one
equivalent of the isocyanide dissolved in dichloromethane
was added and the reaction mixture was stirred overnight.
The solvent was then removed and the product was washed
repeatedly with pentane or diethyl ether. Drying in vacuo
yielded the carbene-heteroatom-Pd complexes, which were
usually obtained in pure form; in some cases recrystalli-
zation from dichloromethane/alkanes was necessary.
Crystals of the (phosphanyl-carbene)PdCl2 compound 9
were grown from dichloromethane/pentane. The complex
crystallizes in the space group P21/c (see structure in Fig-
ure 1). Interestingly, the bond length Pd(1)–Cl(2) (Cl trans
[a] Reaction conditions: 0.25 mmol aryl or pyridyl halide substrate,
0.38 mmol phenyl acetylene, 0.4 mmol Et3N, 0.5 mmol CuI and
1 mol-% catalyst relative to 24 in 2.5 mL of DMF at 110 °C for
24 h. [b] Based on GC analysis, corrected for the presence of in-
to the carbene) is 2.3513(11) Å, which is significantly ternal standard.
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Eur. J. Inorg. Chem. 2009, 1313–1316

Building-Block Route to (Phosphanyl-carbene)Pd Complexes
may form. Next,
a solution of the respective isocyanide
(0.25 mmol) in anhydrous CH2Cl2 (5 mL) was added dropwise. The
colour of the reaction mixture may change to yellow or become
colourless. In some cases, the intermediate precipitate redissolves,
and a precipitate may form again. In other cases, the product pre-
cipitates directly from the homogeneous solution. After stirring
overnight, the solvent is removed under vacuum (in case of a pre-
cipitate, this may also be filtered off and washed with CH2Cl2 and
diethyl ether and dried in vacuo). The residue is then suspended in
pentane or diethyl ether, washed three times, and dried in vacuo.
Example: Compound
9
(beige solid) yield 72%. 1H NMR
(CD2Cl2): δ = 10.68 (s, 1 H), 8.00–6.60 (m, 14 H), 4.07 (s, 2 H),
3.63 (s, 3 H) ppm. 31P{1H} NMR (CD2Cl2): δ = 21.5 ppm. FAB-
MS: m/z = 540 [M+ – CH3], 517 [M+ – Cl – 2 H], 482 [M+ – 2 Cl –
2 H]. C22H20Cl2NO3PPd (554.7): calcd. C 47.63, H 3.63, N 2.52;
found C 46.25, H 3.45, N 2.38. Data of all other new compounds
11, 15 or 21 [Equation (1)]. High selectivity for the Hay have been compiled in the Supporting Information.
product 26 was found for the three catalysts, specifically in
the cases of catalysts 15 and 21 with 4-iodoacetophenone,
Procedure for Sonogashira and Hay Cross-coupling: The reactions
were performed in sets of 16 reactions using a Chemspeed
Smartstart 16-reactor block, modified in-house for efficient reflux
and stirring. GC analysis was performed on an Interscience GC-
8000 gas chromatograph with a 100% dimethylpolysiloxane capil-
lary column (DB-1, 30 mϫ0.325 mm). GC conditions: isotherm at
105 °C (2 min); ramp at 30 °C min–1 to 280 °C; isotherm at 280 °C
(5 min). Pentadecane was used as internal standard. All other
chemicals were purchased from commercial sources (Ͼ 98% pure).
3-amino-6-iodopyridine
and
2-chloro-4-nitropyridine
(Table 1).
To discard any possibility of a stoichiometric Stephens–
Castro[20] reaction with copper that would lead to 26, we
ran control experiments without catalyst, both in the pres-
ence and absence of halide substrate. No conversion was
observed in either case. A selection of bases and additives
was also evaluated. For aryl iodides as substrates, the best
Hay coupling results were obtained using Et3N with CuI.
Using TBAF with KF, lower conversions were obtained for
the bromo and chloro pyridines. In these cases, the selectiv-
ity to the Hay product 26 decreased and the Sonogashira
product 25 appeared. The Hay product 25 was generally
formed with selectivities between 20% and 89% (Table 1)
and with TOFs of 60–200 h–1, depending on the catalyst
and substrate. Unfortunately, catalytic methoxycarbon-
ylation of ethylene using 7, 8, 11, 12, 14, 15 or 21, failed.
Example: 1-(4-Methoxyphenyl)-2-phenylacetylene (25f), catalyst 7.
A Schlenk-type glass vessel equipped with a septum and a magnetic
stirrer was charged with phenylacetylene (23) (0.38 mmol,
0.042 mL), 4-bromoanisole (24f) (0.25 mmol, 0.031 mL), Et3N
(1.6 equiv., 0.4 mmol, 0.056 mL), CuI (2 equiv., 0.5 mmol, 95 mg)
and catalyst 7 (1 mol-%, 0.0025 mmol, 1.33 mg) in 2.5 mL of DMF.
The reaction mixture was stirred at 110 °C for 24 h under slight N2
overpressure. Reaction progress was monitored by GC (pentade-
cane internal standard). Samples for GC were added in equivalent
amount of water, extracted with hexane and filtered through an
alumina plug prior to injections.
X-ray Crystalographic Study of 9: CCDC-671362 contains the sup-
plementary 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.
Conclusion
In summary, we have introduced a simple and straight-
forward route towards PdII complexes with bidentate li-
gands containing an acyclic diamino- or aminooxy-carbene
and a phosphane. This route allows efficient synthesis of a
series of potential catalysts. It does not require the synthesis
of imidazolium salts nor the a priori generation of sensitive
free carbenes. An intrinsic limitation of this route is that
one of substituents on the nitrogen is restricted to be hydro-
gen. We believe that this modular synthesis of carbene com-
plexes will find many applications in homogeneous cataly-
sis.
Supporting Information (see also the footnote on the first page of
this article): Product yields and analytical data of new compounds
and details of the catalytic Sonogashira reactions.
Acknowledgments
We thank the European network “Atom-economic syntheses using
palladium, the chameleon catalyst”, part of the Fifth Framework
Program (contract number HPRN-CT-2002-00196), the Nether-
lands Council for the Chemical Sciences and the Vernieuwingsim-
puls program of the Netherlands Organization for Scientific Re-
search (NWO) for funding.
Experimental Section
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SHORT COMMUNICATION
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Received: October 31, 2008
Published Online: February 17, 2009
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Eur. J. Inorg. Chem. 2009, 1313–1316