1,3-Dipole behavior of phosphagermaallene tip(tBu)Ge-C-PMes* leading to a phosphagermaheterocyclic carbene

Article abstract of DOI:10.1002/anie.201003044

A cyclic phosphagermacarbene is formed by a [3+2] cycloaddition between the 1,3-dipolar Ge-C-P unit of a phosphagermaallene and an acetylene (see scheme; Tip=2,4,6-iPr₃C₆H₃). The carbene undergoes C-H insertion or is trapped by a second equivalent of acetylene derivative, and its existence and reactivity was supported by DFT calculations.

Full text of DOI:10.1002/anie.201003044

Communications
DOI: 10.1002/anie.201003044
Heterocyclic Carbenes
= =
1,3-Dipole Behavior of Phosphagermaallene Tip(tBu)Ge C PMes*
Leading to a Phosphagermaheterocyclic Carbene**
Dumitru Ghereg, Erwan Andrꢀ, Jean-Marc Sotiropoulos, Karinne Miqueu, Heinz Gornitzka,*
and Jean Escudiꢀ*
The study of low-coordinate species of heavier Group 14 (Si,
Ge, Sn) and of Group 15 (N, P, As, Sb) elements, and
knowledge, no other example of a system containing two
heteroatoms, even a transient one, has been reported.
Addition of a solution of dimethyl acetylenedicarboxylate
in Et2O to a molar equivalent of phosphagermaallene 1
resulted in the formation of the sole derivative 2 in a nearly
quantitative yield (Scheme 1). All of the NMR spectroscopic
=
particularly that of heavy heteroalkenes E E’ and heavy
= =
heteroallenes E C E’ (E, E’ = Group 14 and 15 elements)
has attracted much attention during the last decades. Such
compounds are powerful building blocks in organometallic
and heterocyclic chemistry. Heteroallenes E C E’[1–4] behave
= =
as 1,2-dipoles, leading to addition or cycloaddition onto the
[5,6]
=
=
E C or C E’ double bonds.
Herein we present the 1,3-dipole behavior of a hetero-
allene, a new type of behavior for such a family of compounds,
= =
in the reaction of phosphagermaallene Tip(tBu)Ge C
PMes* 1 towards dimethyl acetylenedicarboxylate. This
reaction led to a transient cyclic phosphagermaheterocyclic
carbene (PGeHC). DFT calculations allowed this unprece-
dented type of reaction to be understood. By taking into
account the important role of N-heterocyclic carbenes
(NHCs) in modern coordination chemistry, the access to
higher homologues of such ligands is highly interesting. The
only example of a higher homologue of NHCs is the
diphosphinocarbene of Bertrand and co-workers.[7] To our
Scheme 1. Reactivity of 1 with dimethyl acetylenedicarboxylate.
Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl.
[*] Dr. D. Ghereg, Dr. J. Escudiꢀ
Universitꢀ de Toulouse, UPS, LHFA
118 Route de Narbonne, 31062 Toulouse (France)
and
data are in agreement with a tricyclic structure that includes a
1-phospha-3-germacyclopent-4-ene, which was unambigu-
ously confirmed by an X-ray determination (Figure 1 and
Table 1).
The formation of this tricyclic derivative can only be
understood by the intermediate formation of the PGeHC 3,
followed by insertion of the carbenic carbon atom into
CNRS, LHFA, UMR 5069
31062 Toulouse cedex 09 (France)
Fax: (+33)5-6155-8204
E-mail: escudie@chimie.ups-tlse.fr
Dr. E. Andrꢀ, Dr. J.-M. Sotiropoulos, Dr. K. Miqueu
Institut Pluridisciplinaire de Recherche sur l’Environnement
et les Matꢀriaux, UMR-CNRS 5254
ꢀ
the C H bond of an ortho isopropyl group of the Tip
(2,4,6-iPr3C6H3) group on the germanium atom. Such inser-
Universitꢀ de Pau et des Pays de l’Adour (France)
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)
Fax: (+33)5-6155-3003
E-mail: 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. For the theoretical work, we were granted access to the
HPC resources of IDRIS under allocation 2010 (i2010080045) made
by GENCI (Grand Equipement National de Calcul Intensif).
Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl.
Supporting information for this article, including full experimental
details for compounds 2, 4, 5, and 6 and details of calculations, is
available on the WWW under http://dx.doi.org/10.1002/anie.
201003044.
Figure 1. Crystal structure of 2. Ellipsoids are set at 50% probability;
hydrogen atoms are omitted and Tip, tBu, and Mes* groups have
been simplified for clarity.
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Chemie
Table 1: Comparison of pertinent geometrical parameters in 2, 3, 3H,
and TS.
2[a]
2[b]
3[b]
3H[b]
TS[b]
Ge1–C7
C7–P1
P1–C2
C2–C1
C1–Ge1
Ge1-C7-P1
1.976(4)
1.849(3)
1.822(4)
1.341(5)
1.966(4)
103.3(2)
1.97
1.85
1.86
1.36
1.96
102.0
1.95
1.63
1.85
1.35
2.05
103.1
1.96
1.65
1.83
1.34
1.98
97.4
1.82
1.64
2.70
1.24
2.85
126.6
[a] X-ray data. [b] B3LYP/6-31G(d,p). Bond lengths in ꢁ, angles in 8.
ꢀ
tions into C H bonds are well known and characteristic for
carbenes.[8]
Carbene 3 would be formed by an unexpected [3+2]
Figure 2. Energy profile for the [3+2] cycloaddition reaction involved in
the formation of the carbene 3H via the transition state TS from the
= =
cycloaddition between the Ge C P moiety of the phospha-
ꢁ
ꢀ ꢁ ꢀ
parent compounds 1H and MeOOC C C COOMe. H denotes the
simplified forms of the molecules used in the calculations.
germaallene 1, which behaves as a 1,3-dipole, and the C C
triple bond. Such a 1,3-dipole behavior of a heteroallene of
= =
type E C E’ is observed for the first time herein. To gain
more insight into this unprecedented behavior, DFT calcu-
lations[9] were carried out at the B3LYP/6-31G(d,p) level of
theory.[10]
approach of the reactants has been found. Calculations
predict that this five-center transition state is located 21 kcal
molꢀ1 higher than the reactants. This TS has a planar structure
Carbene 3 is a minimum on the potential energy surface
and has a singlet ground state.[11] The [3+2] cycloaddition
with a large synchronous character: the forming Ge1 C1 and
ꢀ
= =
ꢁ
ꢀ
between the Ge C P moiety and the C C triple bond is
predicted to be exothermic (DG: ꢀ13 kcalmolꢀ1) and the
carbene 3 is less stable than 2 by about 50 kcalmolꢀ1, in
agreement with the formation of the insertion derivative as
the only product.
P1 C2 bond lengths are 2.85 and 2.70 ꢀ, respectively
(compared to 1.98 and 1.83 ꢀ in 3H) (see the Supporting
Information for all molecular orbitals involved in this study,
their shapes, and their energies). It is interesting to note the
importance of the acetylene derivative involved. Indeed, with
the bis(trifluoromethyl)acetylene, the reaction is exothermic
(ꢀ21 kcalmolꢀ1), as previously observed with the dicarbox-
ylate compound, and the barrier has the same magnitude (ca.
20 kcalmolꢀ1). In contrast, the use of dimethylacetylene gives
a weakly stabilized carbene (ꢀ9 kcalmolꢀ1; see the Support-
ing Information). Consequently, electron-deficient alkynes
seem necessary to stabilize the carbene.
Concerning the geometrical parameters of the carbene 3,
ꢀ
the Ge1 C7 bond is calculated to be about 1.95 ꢀ in length
(Table 1), which is in the range of a single bond. A short
ꢀ
P1 C7 bond (1.63 ꢀ) was calculated, which corresponds to a
=
P C double bond with a planar geometry around the
phosphorus atom (ꢀP: 3608), thus allowing a strong donation
of the phosphorus lone pair into the 2pp vacant orbital of the
carbene. Thus, this compound can be described as a
l4-phosphavinylylide, as found in the noncyclic silyl(phos-
phino)carbene of Bertrand and co-workers.[12] A very long
Despite the theoretical results (electronic stabilization of
the carbene, exothermic reaction), monitoring the reaction by
31P NMR between ꢀ808C and room temperature did not
allow the transient carbene 3 to be characterized. The reason
is that the insertion product 2 is more stable than the carbene
3; this insertion reaction is certainly facilitated by the
ꢀ
ꢀ
C1 Ge1 bond (2.05 ꢀ) was determined (standard Ge C bond
lengths: 1.95–1.98 ꢀ[13]). This lengthening is due to stabilizing
interactions between the sGe1C8/sGe1C23 and the s*Ge1C1 orbitals,
as confirmed by NBO analysis[14] (sum of stabilizing inter-
actions: about 5 kcalmolꢀ1).
ꢀ
proximity between the carbenic carbon atom and the C H
bond of the iPr group (3.18 ꢀ between the two carbon atoms
and 2.35 ꢀ between carbon C7 and the H atom of the iPr
group).
NBO analysis also shows other interactions that allow an
electronic stabilization of the carbene. Indeed, interaction
between the sGe1C7 orbital and the antiperiplanar s*P1C27
orbital is large in magnitude (ca. 18 kcalmolꢀ1), probably
owing to ring constraint.[15] Moreover, the carbon lone pair
nsC7 interacts with the vicinal s*P1C2 orbital (ca. 15 kcalmolꢀ1),
To characterize the carbene 3, a trapping experiment was
performed. Due to its short lifetime, the trapping agent must
be added to the phosphagermaallene solution before addition
of dimethyl acetylenedicarboxylate, but the high reactivity of
=
the pC1
orbital with the p*C7
orbital, and the pC7
the Ge C double bond of 1 complicates and limits the choice;
=
=
=
P1
C2
P1
orbital with the p*C1 orbital (ca. 8 kcalmolꢀ1).
the best trapping agent appeared to be the acetylenic
derivative itself used in excess. Considering the frontier
molecular orbital of the dimethyl acetylenedicarboxylate
=
C2
The energy profile for the cycloaddition reaction was
calculated on model compounds (1H + dimethyl acetylene-
dicarboxylate) to lower calculation times.[16] As previously
observed for the real molecules, the formation of carbene 3H
is exothermic (DG = ꢀ20 kcalmolꢀ1; Figure 2) and non-rever-
sible. Moreover, the geometrical parameters of 3H are close
to those of 3.[17] A transition state (TS) involving a syn
(ꢀ1.50 eV/p*C C + p*C
and ꢀ7.87 eV/pC CꢀpC
for the
=
O
=
=
O
=
LUMO and HOMO respectively), and those of the carbene
(LUMO: ꢀ1.99 eV/p*P C + p*C C , HOMO: ꢀ5.65 eV/nsC7),
=
=
the latter could behave as a nucleophile, leading to a
cyclopropenation adduct.
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Communications
2JCP = 2.7 Hz, PCCOOCH3), 168.88 ppm (GeCCOOCH3); 31P NMR
(121.51 MHz, CDCl3): d = 15.35 ppm.
Thus, slow addition of phosphagermaallene 1 to two
equivalents of dimethyl acetylenedicarboxylate at low tem-
perature, led, along with about 10% of the derivative 2, to the
spiro compound 4 (Scheme 1). Compound 4 is stable in the
solid state, but slowly decomposes in solution (Et2O or
pentane) to give two diastereoisomeric compounds 5 and 6
(d 31P: ꢀ36.2 and ꢀ18.5 ppm; Scheme 2), probably via a
4: A solution of phosphagermaallene 1 (1 mmol) in Et2O (20 mL)
was added to a solution of two equivalents of dimethyl acetylenedi-
carboxylate dissolved in Et2O (10 mL) cooled to ꢀ808C. The reaction
mixture was slowly warmed to room temperature and turned brown
in color. The solvents were removed under reduced pressure and
replaced by 30 mL of pentane, and LiF was removed by filtration.
After concentration to 5 mL, light brown crystals of 4 (0.73 g, 81%,
mp 1478C) were obtained by cooling the pentane solution to ꢀ208C.
1H NMR (300.13 MHz, CDCl3): d = 2.90, 3.68, 3.78, 3.84 ppm (4 s, 4 ꢁ
3H, COOCH3); 13C NMR (75.47 MHz, CDCl3): d = 51.40, 51.66,
51.75, and 52.18 (COOCH3), 104.17 (d, 1JCP = 146.0 Hz, GeCP),
114.95 (d, JCP = 9.9 Hz), 124.29 (d, JCP = 45.8 Hz), 145.57 (d, JCP
=
=
20.1 Hz) and 152.20 (d, JCP = 34.9 Hz) (C C), 160.15 (d, JCP = 7.6 Hz),
165.41 (d, JCP = 10.0 Hz), 170.87 (d, JCP = 15.6 Hz), 173.02 ppm (d,
J
CP = 3.2 Hz) (COOCH3); 31P NMR (121.51 MHz, CDCl3): d =
ꢀ34.71 ppm (d, 4JPH = 6.2 Hz).
For crystal data for 2, 5, and 6, see the Supporting Information.
CCDC 777357 (2), 777358 (5), and 777359 (6) contain the supple-
mentary 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.
Scheme 2. Decomposition reaction of 4.
transient seven-membered ring phosphinocarbene (see the
Supporting Information). Single crystals suitable for an X-ray
determination have not been obtained; however, the struc-
ture of 4 was unambiguously shown by its physicochemical
data: mass spectrometry (m/z = 906) and elemental analyses
showed the presence of two molecules of acetylenic reagent
Received: May 19, 2010
Revised: July 2, 2010
Published online: September 10, 2010
Keywords: carbenes · 1,3-dipoles · germanium · heteroallenes ·
for one of 1, 13C NMR spectra showed a quaternary carbon
.
phosphorus
1
atom bonded to phosphorus (d = 104.17 ppm, JPC
=
=
146.0 Hz), two C C double bonds, and four different
1
COOMe groups, and H NMR spectra showed three isopro-
pyl groups for the Tip group, thus excluding a similar
cyclization observed in 2. Although not yet observed with a
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ꢁ
C C triple bond, rather similar trapping of silyl(phosphino)-
=
carbene with C C double bonds of various electron-poor
alkenes has already been reported.[18]
In conclusion, this first example of a 1,3-dipole behavior
= =
of a heteroallene E C E’ is an important milestone in the
1
2
3
4
chemistry of these cumulative doubly-bonded species and
opens new perspectives in their use as building blocks in
heterocyclic chemistry. Furthermore, a highly interesting
access to heavy heteroatom containing cyclic carbenes has
been opened. DFT calculations points out that this PGeHC is
more stable than the starting compounds. Our research is now
focused on the goal to find the right combination between
heteroallenes and unsaturated systems for the isolation of
stable heterocyclic carbenes of this type.
= =
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Experimental Section
1
2
3
= =
2: Dimethyl acetylenedicarboxylate (1 mmol) dissolved in Et2O
(10 mL) was slowly added to a solution of phosphagermaallene (1,
1 mmol)[4c] in Et2O (20 mL) cooled to ꢀ808C. A brown coloration
appeared after warming to room temperature. The solvents were
removed under reduced pressure and replaced by pentane (30 mL),
and LiF was removed by filtration. After concentration of the filtrate,
white crystals of 2 (0.55 g, 72%, m.p. 1538C) were obtained by cooling
[4] a) Transient R R Si C PR : L. Rigon, H. Ranaivonjatovo, J.
Escudiꢂ, A. Dubourg, J.-P. Declercq, Chem. Eur. J. 1999, 5, 774 –
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2
3
= =
781; b) Transient R R Ge C PR : H. Ramdane, H. Ranaivon-
jatovo, J. Escudiꢂ, S. Mathieu, N. Knouzi, Organometallics 1996,
1
2
3
= =
15, 3070 – 3075; c) stable R R Ge C PR : Y. El Harouch, H.
Gornitzka, H. Ranaivonjatovo, J. Escudiꢂ, J. Organomet. Chem.
2002, 643–644, 202 – 208.
1
= =
the pentane solution to ꢀ208C. H NMR (300.13 MHz, CDCl3): d =
[5] The only exceptions are the phosphaallenes RP C X, which
2.63 (d, 2JPH = 6.0 Hz, 1H, CH P), 3.61 and 3.75 ppm (2s, 2 ꢁ 3H,
react sometimes by the phosphorus lone pair (1acde), and SiCN
(6ab) derivatives, which can behave as silylenes > Si and
ꢀ
1
COOCH3); 13C NMR (75.47 MHz, CDCl3): d = 49.96 (d, JCP
=
2
ꢀ
=
33.2 Hz, CH P), 51.68 and 51.91 (COOCH3), 134.66 (d, JCP
=
isocyanides C NR owing to their intermediary structure
31.5 Hz, GeC C), 166.61 (d, 1JCP = 44.8 Hz, PC C); 168.65 (d,
between azasilaallenes R N C SiR R and silylene–isonitrile
1
2
3
=
=
= =
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Chemie
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2
3
=
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[16] This mechanism does not take into account steric effects. It
should be noted that the reintroduction of the real substituents
into the TS structure calculated for the parent compound shows
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approach between 1 and the acetylene derivative.
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