Electronic properties and reactivity of an isolable phosphagermaheterocyclic carbene

Article abstract of DOI:10.1002/anie.201102500

A highly reactive phosphagermacarbene (PGeHC) was synthesized by [3+2] cycloaddition between phosphagermaallene Tip(tBu)GeiCiPMes* (Tip= 2,4,6-triisopropylphenyl; Mes=2,4,6-tris-tert-butylphenyl) and diphenylketene. It forms complexes with [{Rh(CO)₂Cl}₂] and Me₃P, and undergoes a Wittig-type reaction with diphenylketene (see scheme). Copyright

Full text of DOI:10.1002/anie.201102500

DOI: 10.1002/anie.201102500
Carbenes
Electronic Properties and Reactivity of an Isolable
Phosphagermaheterocyclic Carbene**
Dumitru Ghereg, Sonia Ladeira, Nathalie Saffon, Jean Escudiꢀ,* and Heinz Gornitzka*
In memory of Dumitru Ghereg
Carbenes have been considered for a long time as unstable
intermediates until the pioneering work of Bertrand and co-
workers[1,2] with the isolation of a stable noncyclic push–pull
carbene and of Arduengo and co-workers[3] with the isolation
of an N-heterocyclic push–push carbene (NHC). Today, stable
carbenes, particularly NHCs, govern a large part of modern
chemistry,[4] especially in coordination chemistry with many
applications: first of all in the field of catalysis, where NHCs
successfully replace phosphines,[5] but also in medical chemis-
try,[6] in nanoscience,[7] and in the development of new
luminescent compounds,[8] where their role is also dramati-
cally increasing.
Recently we published the unprecedented 1,3-dipole
= =
behavior of the phosphagermaallene Tip(tBu)Ge C PMes*
(1; Tip = 2,4,6-triisopropylphenyl; Mes* = 2,4,6-tris-tert-
butylphenyl) towards dimethyl acetylenedicarboxylate, lead-
ing surprisingly to a new type of nitrogen-free carbene, the
transient phosphagermaheterocyclic carbene (PGeHC) 2; the
latter undergoes, even at low temperature, a rearrangement
by insertion of the carbenic carbon atom into a CH bond of an
ortho-iPr of the Tip group to lead to the heterocyclic
compound 3 (Scheme 1).[12a]
To develop and enhance the applications of NHCs, one of
the main objectives has been to modulate their electronic
properties (acceptor and mainly donor), which are one of the
key characteristics of such carbenes. In the case of a typical
NHC system, the easiest variation concerns the substituents
on the nitrogen atoms bonded to the carbene center, which
affects the steric and electronic properties of the ligands;[9]
recently, interesting works on the backbone of NHCs have
been reported.[10] However, a greater impact on the electronic
properties of such types of heterocyclic carbenes could be
obtained by the replacement of the nitrogen atoms on the
carbenic centre by other heteroelements: the only reported
example of such an isolated nitrogen-free NHC-type carbene
comes from Bertrand and co-workers, with the synthesis of a
highly basic, means strongly s-donor, diphosphinocarbene.[11]
Scheme 1. 1,3-Dipole behavior of the phosphagermaallene 1 towards
dimethyl acetylenedicarboxylate.
The synthesis of an isolable PGeHC compound is a major
objective to determine its electronic properties and to study
its reactivity. Herein we report the first example of an isolable
and spectroscopically evidenced PGeHC, which constitutes
the second nitrogen-free NHC-type carbene, its electronic
properties, its chemical behavior, and the structure of the
corresponding Rh complex.
Stable cyclic PGeHC 4 was obtained according to a similar
procedure as that for 2, by addition of one equivalent of
[*] Dr. D. Ghereg, Dr. J. Escudiꢀ
Universitꢀ de Toulouse, UPS, LHFA
118 Route de Narbonne, 31062 Toulouse (France)
and
CNRS, LHFA, UMR 5069, 31062 Toulouse cedex 09( France)
E-mail: escudie@chimie.ups-tlse.fr
= =
diphenylketene (Ph2C C O) to phosphagermaallene 1 at
À808C in diethyl ether (Scheme 2). As in the case presented
in Scheme 1, the formation of carbene 4 can only be explained
by a 1,3-dipole behavior of the phosphagermaallene 1 giving a
=
[3+2] cycloaddition with the C O bond of the diphenylke-
S. Ladeira, Dr. N. Saffon
Institut de Chimie de Toulouse, FR 2599, Universitꢀ Paul Sabatier
118 Route de Narbonne, 31062 Toulouse cedex 09 (France)
tene.
Monitoring the reaction by dynamic 1H, 13C, and 31P NMR
spectroscopy between À808C and room temperature showed
Prof. Dr. H. Gornitzka
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse cedex 4 (France)
and
Universitꢀ de Toulouse, UPS, INP, LCC, 31077 Toulouse (France)
E-mail: heinz.gornitzka@lcc-toulouse.fr
[**] We are grateful to the Agence Nationale pour la Recherche (contract
ANR-08-BLAN-0105-01) and PhoSciNet (CM0802) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102500.
= =
= =
Scheme 2. Reaction of Tip(tBu)Ge C PMes* 1 with Ph2C C O.
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Communications
that carbene 4 was nearly quantitatively obtained at À308C.
The product was physicochemically evidenced by a singlet at
d = 119.1 ppm in the 31P NMR spectrum. Unfortunately, the
13C signal of the carbenic carbon could not be attributed
because of the presence of many aromatic carbon atoms in the
expected field. At À808C carbene 4 is nearly insoluble in
diethyl ether and can be isolated as a white stable solid. It
showed no degradation during three weeks in ether solution
at À308C but at higher temperatures (08C) it slowly under-
À
goes in solution a C H insertion into an o-tBu group of the
Mes* group leading to the sole compound 5 (d31P =
2
À22.9 ppm, JPH = 17.6 Hz)[13] (Scheme 2); such an insertion
À
into a C H bond has been observed recently for other
unstable PGeHC compounds.[12] The difference of reactivity
with carbene 2 (insertion into an o-iPr of the Tip group) is due
to a different geometry of the heterocyclic carbene 4, the
Figure 1. Structure of 6. Ellipsoids are set at the 50% probability level.
Hydrogen atoms are omitted, and Tip, tBu, and Mes* groups have
been simplified for clarity. Selected bond lengths [ꢁ] and angles [8]:
Ge1–O1 1.862(4), O1–C40 1.362(6), C40–P1 1.804(6), P1–C1 1.639(6),
Ge1–C1 1.978(6), Ge1–C6 1.996(6), Ge1–C2 1.988(6), P1–C21
1.798(6), C40–C41 1.351(8), C1–Rh1 2.113(6), Rh1–Cl1 2.310(8), Rh1–
C54 1.869(8), Rh1–C55 1.832(12), P1-C40-O1 107.1(4); C40-O1-Ge1
121.3(4); O1-Ge1-C1 94.9(2); Ge1-C1-P1 104.4(3); C1-P1-C40 111.9(3);
Ge1-C1-Rh1 134.1(3); P1-C1-Rh1 120.4(3).
À
insertion occurring into the closest C H bond. Tricyclic
compound 5 was characterized by an X-ray structural study
(see the Supporting Information). Bond lengths are in the
normal range with the exception of the Ge–O bond, which is
slightly elongated (1.839(2) ꢀ; standard Ge–O: 1.75–
1.80 ꢀ).[14] In the tricyclic unit, the aromatic ring and the
central six-membered ring are in a same plane, except the
former carbenic carbon atom, which is 0.599 ꢀ out of this
plane.
Tetracarbonyl-di-m-chloridodirhodium(I) was added to a
solution of carbene 4 in Et2O cooled to À608C to evaluate its
s-donor properties;[15] this reaction leads nearly quantita-
tively to rhodium complex 6 (Scheme 3).[13]
Structural analysis (Figure 1) of rhodium complex 6 shows
À
a germanium carbon distance (Ge1 C1 1.978(6) ꢀ) corre-
sponding to a standard single bond,[14] illustrating the lack of
electronic stabilization by the germanium atom. By contrast,
Complex 6 was evidenced in the 13C NMR spectrum by a
signal doublet of doublet at low field for the carbenic carbon
the short phosphorus carbon distance (P1 C1 1.639(6) ꢀ) is
À
[16]
=
characteristic of a standard P C double bond.
This
1
1
atom (d = 196.68 ppm, JCRh = 34.9 Hz, JCP = 3.1 Hz). One of
the most interesting features for this compound was obtained
from the IR spectrum, which displays vibrations for the
carbonyl groups at similar wave numbers (1985 and
2061 cmÀ1) to that of Bertrandꢁs P-heterocyclic carbene
(1985 and 2059 cmÀ1).[11] Thus, from these low values, strong
basic character for carbene 4 can be deduced, which explains
its high reactivity.
situation can be best described by form 4B in Scheme 3.
The sum of angles around the phosphorus atom (ꢀ = 3608)
À
and the P1 C21 bond situated in the nearly planar five-
membered ring Ge1-C1-P1-C40-O1 support this interpreta-
À
tion. The very short P1 C1 distance observed in 6 is also in
À
good agreement with calculations that predicted a P C bond
length of 1.63 ꢀ for the closely related phosphagermacarbene
2.[12a]
From these special geometrical characteristics, we could
=
expect high reactivity of this formal P C double bond: this is
= =
the case for the addition of one equivalent of Ph2C C O to
the carbene 4 to afford the new allenic derivative 7[13]
(Scheme 3 and Figure 2). The first step was probably a
Wittig-type reaction, leading to transient bicyclic compound
8, followed by rearrangement. The same result was obtained
by addition of two equivalents of diphenylketene to the
starting phosphagermaallene 1 at À808C. Derivative 7 was
evidenced in the 31P NMR spectrum by a signal at d =
30.6 ppm, convenient for a RR’R’’P(O) derivative and in
the 13C NMR spectrum by a characteristic low-field shifted
signal for the sp carbon atom of the allenic unit (d =
2
207.77 ppm, JPC = 1.0 Hz). The structure of 7 was unambig-
uously proved by X-ray analysis. The allenic unit displays a
typical bond angle of 177.9(3)8 for C20-C21-C22, which is
close to 1808, with short C20–C21 and C21–C22 bond lengths;
as expected, the P1–C20 distance (1.832(2) ꢀ) lies in the
normal range, which is in marked contrast with the short
corresponding bond in the rhodium complex 6 (1.639(6) ꢀ).
Scheme 3. Reaction of carbene 4 with a rhodium complex and with
= =
Ph2C C O.
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Angew. Chem. Int. Ed. 2011, 50, 7607 –7610

equivalent of diphenylketene dissolved in Et2O (10 mL). The reaction
was monitored by dynamic 1H, 13C, and 31P NMR spectroscopy
between À808C and room temperature. The NMR spectroscopic
analysis showed the nearly quantitative formation of the carbene 4 at
À308C. Owing to the slow rotation of the Tip and Mes* groups, their
signals were generally extremely broad. By cooling to À808C, 4 is
insoluble in Et2O and can be isolated as a white powder (0.71 g, 87%).
By warming the Et2O solution of 4 to room temperature, the signals of
carbene 4 slowly disappeared and were replaced by those of the
insertion product 5. The solvent was removed under reduced pressure
and replaced by 30 mL of pentane. LiF was eliminated by filtration.
After concentration, white crystals of 5 (0.64 g, 78%) were obtained
by cooling to À208C from pentane. 4: 31P NMR (CDCl3): d =
119.1 ppm. 5: 1H NMR (CDCl3): d = GeCHP = 2.08–2.27 (m),
CH2P = 1.73–1.89 (m) and 2.28–2.48 ppm (m); 31P NMR (CDCl3):
d = À22.9 ppm, 2JPH = 17.6 Hz.
Synthesis of 6, 7, and 9: To a solution of carbene 4, prepared
in situ as previously described or by starting from the isolated
carbene, cooled to À608C, was added half an equivalent of
Figure 2. Structure of 7. Ellipsoids are set at the 50% probability level.
Hydrogen atoms are omitted, and Tip, tBu, Ph, and Mes* groups have
been simplified for clarity. Selected bond lengths [ꢁ] and angles [8]:
Ge1–O1 1.833(2), O1–C53 1.379(3), C53–P1 1.849(2), P1–C20
1.832(2), C20–Ge1 1.979(2), Ge1–C1 1.967(2), Ge1–C16 1.995(2),
P1–O2 1.485(2), C20–C21 1.291(3), C21–C22 1.322(3); P1-C53-O1
114.6(2); C53-O1-Ge1 117.5(2); O1-Ge1-C20 91.1(1); Ge1-C20-P1
107.2(1); C20-P1-C53 97.5(1); C20-C21-C22 177.9(3).
= =
[{Rh(CO)2Cl}2] dissolved in Et2O (or one equivalent of Ph2C C O
in Et2O, or a tenfold excess of Me3P). The reaction mixture was
warmed to room temperature and treated as above. 6: 13C NMR
(CDCl3): d = 184.28 (dd, 3JCP = 9.6 Hz, 1JCRh = 74.1 Hz, CO), 185.33
1
(d, 1JCRh = 56.7 Hz, CO), 196.68 ppm (dd, 1JCP = 3.1 Hz, JCRh
=
2
34.9 Hz, GeCP); 31P NMR (CDCl3À)1: d = 140.2 ppm (d, JPRh
=
3.8 Hz); IR: n = 1985.6 and 2061.3 cm (CO). 7: 13C NMR (CDCl3):
~
d = 97.83 (d, 1JCP = 66.8 Hz, GeC C = CPh2), 207.77 ppm (d, JCP
= =
=
2
=
1.0 Hz, Ge-C C CPh2); 31P NMR (CDCl3): d = 30.6 ppm. 9:
31P NMR (CDCl3): d = 5.3 (dd, 2JPP = 181.0 Hz, 2JPH = 11.2 Hz,
The four atoms O1-C53-P1-C20 are in a plane, while the
germanium atom Ge1 is slightly out from this plane;
surprisingly, the two bulkiest groups, Tip and Mes*, are in a
cis disposition.
Addition of trimethylphosphine to the carbene 4 at
À608C led to transient complex 9 (Scheme 4). Because of
its low stability (crystallization attempts failed owing to its
decomposition back to the starting products), this complex
2
PMe3), 8.3 ppm (d, JPP = 181.0 Hz, PMes*).
CCDC 821063 (5), 821064 (6), and 821065 (7) 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.
Received: April 11, 2011
Published online: June 29, 2011
Keywords: allenes · carbenes · heterocycles · rhodium ·
.
Wittig reactions
[1] a) A. Igau, H. Grꢂtzmacher, A. Baceiredo, G. Bertrand, J. Am.
Chem. Soc. 1988, 110, 6463 – 6466; b) A. Igau, A. Baceiredo, G.
Trinquier, G. Bertrand, Angew. Chem. 1989, 101, 617 – 618;
Angew. Chem. Int. Ed. Engl. 1989, 28, 621 – 622.
[2] T. Kato, H. Gornitzka, A. Baceiredo, A. Savin, G. Bertrand,
J. Am. Chem. Soc. 2000, 122, 998 – 999.
Scheme 4. Reaction of the carbene 4 with trimethylphosphine.
could not be isolated but was evidenced in the 31P NMR
spectrum by the presence of two doublets at d = 5.3 ppm
(PMe3) and d = 8.3 ppm (PMes*) with a rather high JPP
coupling constant (181 Hz). This reaction proves that the
carbene 4 also presents an electrophilic character.
In conclusion, the surprising 1,3-dipole behavior of
phosphagermaallene 1 towards unsaturated reagents consti-
tutes a good route to a new type of carbenes such as
phosphagermacarbene 4 (PGeHC). The latter compound,
which is highly basic, can also behave as electrophile and is
particularly active in Wittig-type reactions. The study of this
compound and the generalization of this route to the synthesis
of other PGeHCs are now under active investigation.
2
[3] A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1991, 113, 361 – 363.
[4] A huge number of reviews concerning NHCs is published every
year; one of the most interesting concerning our subject is: M.
Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. 2010,
122, 8992 – 9032; Angew. Chem. Int. Ed. 2010, 49, 8810 – 8849.
[5] a) W. A. Herrmann, Angew. Chem. 2002, 114, 1342 – 1363;
Angew. Chem. Int. Ed. 2002, 41, 1290 – 1309; b) L. H. Gade, S.
Bellemin-Laponnaz, Coord. Chem. Rev. 2007, 251, 718 – 725;
c) V. Cꢃsar, S. Bellemin-Laponnaz, L. H. Gade, Chem. Soc. Rev.
2004, 33, 619 – 636; d) M. C. Perry, K. Burgess, Tetrahedron:
Asymmetry 2003, 14, 951 – 961; e) “N-heterocyclic carbenes in
transition metal catalysis”: Topics in Organometallic Chemistry,
Vol. 21 (Ed.: F. Glorius), Wiley-VCH, Weinheim, 2006; f) Spe-
cial issue (Ed.: R. H. Crabtree), Coord. Chem. Rev. 2007, 251,
595; g) Special issue (Ed.: G. Bertrand), J. Organomet. Chem.
2005, 690, 5397; h) H. Clavier, S. P. Nolan, Chem. Commun.
2010, 46, 841 – 861.
Experimental Section
Syntheses of 4 and 5: To a solution of phosphagermaallene 1[17]
(1 mmol) in Et2O (20 mL) cooled to À808C was slowly added one
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Communications
[6] a) K. M. Hindi, M. J. Panzner, C. A. Tessier, C. L. Cannon, W. J.
doza-Espinosa, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc.
2010, 132, 7264 – 7265.
[11] D. Martin, A. Baceiredo, H. Gornitzka, W. W. Schoeller, G.
Bertrand, Angew. Chem. 2005, 117, 1728 – 1731; Angew. Chem.
Int. Ed. 2005, 44, 1700 – 1703.
Youngs, Chem. Rev. 2009, 109, 3859 – 3884; b) M.-L. Teyssot,
A.-S. Jarrousse, M. Manin, A. Chevry, S. Roche, F. Norre, C.
Beaudoin, L. Morel, D. Boyer, R. Mahioue, A. Gautier, Dalton
Trans. 2009, 6894 – 6902.
[7] a) J. Vignolle, T. D. Tilley, Chem. Commun. 2009, 7230 – 7232;
b) C. K. Lee, C. S. Vasam, T. W. Huang, H. M. J. Wang, R. Y.
Yang, C. S. Lee, I. J. B. Lin, Organometallics 2006, 25, 3768 –
3775.
[8] a) A. K. Ghosh, V. J. Catalano, Eur. J. Inorg. Chem. 2009, 1832 –
1843; b) V. J. Catalano, A. L. Moore, J. Shearer, J. Kim, Inorg.
Chem. 2009, 48, 11362 – 11375; c) C. E. Strasser, V. J. Catalano,
J. Am. Chem. Soc. 2010, 132, 10009 – 10011; d) V. Ka-Man Au,
K. Man-Chung Wong, N. Zhu, V. Wing-Wah Yam, J. Am. Chem.
Soc. 2009, 131, 9076 – 9085; e) F. Jean-Baptiste dit Dominique,
H. Gornitzka, A. Sournia-Saquet, C. Hemmert, Dalton Trans.
2009, 340 – 352.
[9] a) N. M. Scott, S. P. Nolan, Eur. J. Inorg. Chem. 2005, 1815 –
1828; b) R. Dorta, E. D. Stevens, N. M. Scott, C. Costabile, L.
Cavallo, C. D. Hoff, S. P. Nolan, J. Am. Chem. Soc. 2005, 127,
2485 – 2495; c) S. Dꢄez-Gonzꢅlez, S. P. Nolan, Coord. Chem. Rev.
2007, 251, 874 – 883.
[12] a) D. Ghereg, E. Andrꢃ, J.-M. Sotiropoulos, K. Miqueu, H.
Gornitzka, J. Escudiꢃ, Angew. Chem. 2010, 122, 8886 – 8889;
Angew. Chem. Int. Ed. 2010, 49, 8704 – 8707; b) D. Ghereg, H.
Gornitzka, J. Escudiꢃ, S. Ladeira, Inorg. Chem. 2010, 49, 10497 –
10505.
[13] See the Supporting Information for complete physicochemical
data.
[14] a) K. M. Baines, W. G. Stibbs, Coord. Chem. Rev. 1995, 145, 157 –
200; b) C. E. Holloway, M. Melnik, Main Group Met. Chem.
2002, 25, 185 – 266.
[15] For comparative infrared spectroscopic studies, see: a) M. Mayr,
K. Wurst, K.-H. Ongania, M. Buchmeiser, Chem. Eur. J. 2004, 10,
1256 – 1266; b) W. A. Herrmann, K. ꢆfele, D. von Preysing, E.
Herdweck, J. Organomet. Chem. 2003, 684, 235 – 248.
[16] R. C. Fischer, P. P. Power, Chem. Rev. 2010, 110, 3877 – 3923.
[17] Y. El Harouch, H. Gornitzka, H. Ranaivonjatovo, J. Escudiꢃ,
J. Organomet. Chem. 2002, 643–644, 202 – 208.
[10] a) L. Benhamou, N. Vujkovic, V. Cꢃsar, H. Gornitzka, N. Lugan,
G. Lavigne, Organometallics 2010, 29, 2616 – 2630; b) D. Men-
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