Versatile reactivity of phosphagermaallene tip(t -Bu)Ge=C=PMes* with α-ethylenic esters

Article abstract of DOI:10.1021/om301224t

The phosphagermaallene Tip(t-Bu)Ge=C=PMes* (1) (Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) reacts with methyl crotonate, methyl cinnamate, and methyl fumarate according to a [2+2] cycloaddition between the Ge=C and C=O double bonds to afford oxagermacyclobutanes 2-4. With methyl maleate, a [2+4] cycloaddition is observed between the Ge=C double bond and the O=C-C=C moiety, leading to oxagermacyclohexene 5. With methyl acrylate, phosphagermaallene 1 behaves as a 1,3-dipole, giving a transient five-membered-ring germa(phosphino)carbene, which rearranges by insertion of the carbenic carbon atom into a CH bond of a tert-butyl group of the Mes* group to afford the tricyclic derivative 8. These results have been supported by a theoretical approach in order to describe the reaction pathways and explain the different experimental results.

Full text of DOI:10.1021/om301224t

Article
pubs.acs.org/Organometallics
Versatile Reactivity of Phosphagermaallene Tip(t‑Bu)GeCPMes*
with α‑Ethylenic Esters
Dumitru Ghereg,† Fatima Ouhsaine,† Jean Escudie,*,† Stephane Labat,‡ Erwan Andre,‡ Karinne Miqueu,‡
́
́
́
Jean-Marc Sotiropoulos,‡ Heinz Gornitzka,*,§,⊥ and Nathalie Saffon∥
†Universite
Toulouse Cedex 09, France
‡Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mater
Pays de l’Adour, Helioparc 2 Avenue du President Angot, 64053 Pau Cedex 09, France
§CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
⊥Universite
de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
∥Structure Fed
erative Toulousaine en Chimie Moleculaire, FR 2599, Universite
́
de Toulouse, UPS, LHFA, 118 Route de Narbonne, F-31062 Toulouse, France, and CNRS, LHFA, UMR 5069, F-31062
́
́
iaux, UMR 5254, Universite de Pau et des
́
́
́
́
́
́
́
Paul Sabatier, 118 Route de Narbonne, 31062
Toulouse Cedex 09, France
S
* Supporting Information
ABSTRACT: The phosphagermaallene Tip(t-Bu)GeCPMes* (1)
(Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) reacts
with methyl crotonate, methyl cinnamate, and methyl fumarate
according to a [2+2] cycloaddition between the GeC and CO
double bonds to afford oxagermacyclobutanes 2−4. With methyl
maleate, a [2+4] cycloaddition is observed between the GeC double
bond and the OC−CC moiety, leading to oxagermacyclohexene 5.
With methyl acrylate, phosphagermaallene 1 behaves as a 1,3-dipole,
giving a transient five-membered-ring germa(phosphino)carbene, which
rearranges by insertion of the carbenic carbon atom into a CH bond of a
tert-butyl group of the Mes* group to afford the tricyclic derivative 8. These results have been supported by a theoretical
approach in order to describe the reaction pathways and explain the different experimental results.
We present in this paper the reactivity of phosphagermaal-
lene 1 toward α-ethylenic esters R−CHCH−COOMe such
as methyl cinnamate (R = Ph), crotonate (R = Me), acrylate (R
= H), and maleate and fumarate (R = COOMe), which present
R groups with different steric hindrance and electronic effects
and a Z or E stereochemistry around the CC double bond.
INTRODUCTION
■
Whereas alkenes and allenes have been very well studied and
are important organic functions, it has not been the same for
their heavier counterparts in which one or two carbon atoms
have been replaced by heavier elements of group 14 or 15: this
is due to the difficulty in stabilizing and isolating such
derivatives because of their great tendency to oligomerize. If
many phosphaallenes −PCX have been obtained,1 it is not
the case of E14CX compounds (E14 = Si, Ge), which are
still relatively rare. With one heavy element, only a few
silaallenes >SiCC<2,3 and two germaallenes4 have been
obtained. With two heavy elements, besides the transient
phosphasilaallene Tip(Ph)SiCPMes*5 and phosphager-
maallene Mes2GeCPMes*,6 only the phosphagermaallene
Tip(t-Bu)GeCPMes* (1) (Tip = 2,4,6-triisopropylphenyl,
Mes* = 2,4,6-tri-tert-butylphenyl) has been isolated.7 The first
studies show a very versatile reactivity of 1 toward compounds
that present rather similar structures: for example [2+2]
cycloadditions are observed between 1 and α-ethylenic
aldehydes, whereas [2+4] cycloadditions and ene-reactions
occur with α-ethylenic ketones.8 In order to learn more about
the chemical behavior of 1, we have continued to study its
reactivity toward new classes of unsaturated derivatives.
RESULTS AND DISCUSSION
■
Various types of reactions could occur, the most likely being the
[2+2] or [2+4] cycloaddition between the GeC and the C
O or CC−CO moieties and less probably the CC
double bond of the esters. However, this one could not be
excluded as well as reactions involving the PC double bond
or the whole GeCP system.
a. Methyl Cinnamate, Crotonate, and Fumarate.
Phosphagermaallene 1 reacts with these three ethylenic esters,
which all present the E configuration, to afford only one adduct,
according to the 31P NMR analysis of the crude reaction
mixture; signals between 258.6 and 272.1 ppm prove that the
PC double bond has not been involved in the reaction. A
CHCH moiety can be evidenced, for every derivative, from
Received: December 19, 2012
Published: February 13, 2013
© 2013 American Chemical Society
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1
the H NMR spectrum by two doublets for ethylenic protons
PC distances. The Ge−O bond (1.846(2) Å) is slightly
elongated in relation to the standard one (1.73−1.79 Å).10
Compound 3 displays an E stereochemistry at the PC
double bond (Mes* and C2 in a cis disposition), and the Tip
and CHCHPh groups are also in a cis disposition. As
compounds 2 and 4 present rather similar 1H and 13C chemical
shifts, we suppose for them the same stereochemistry.
between 3.71 and 6.67 ppm with a large 3JHH coupling constant
(15.0 to 15.9 Hz) in agreement with a E structure and from the
13C NMR spectrum by two signals between 122.11 and 148.00
ppm. For the carbon atom bonded to the methoxy group, very
similar signals are observed between 107.53 and 108.98 ppm
with a two bond C−P coupling constants of 16.8 and 16.9 Hz.
All these data prove the formation of 1-oxa-2-germacyclobu-
tanes 2−4 consecutive to a [2+2] cycloaddition between the
GeC and CO double bonds (Scheme 1). In the case of 4,
a second addition of phosphagermaallene on the CO double
bond has never been observed.
b. Methyl Maleate. The reaction between phosphager-
maallene 1 and methyl maleate was performed according to the
same experimental conditions. The 31P NMR spectrum of the
crude reaction mixture displayed, like in the case of derivatives
2−4, the formation of a major compound (85%) with a low-
1
field chemical shift (δ = 285.9 ppm). By contrast, the H and
13C NMR spectra were completely different. One proton of the
former CHCH unit of the maleate gives a signal at relatively
high field (2.53 ppm), indicating there is no longer a CC
double bond. The large coupling constant with the phosphorus
atom (14.5 Hz) corresponds to a three-bond H−P coupling. In
the 13C NMR spectrum, there are no signals around 100−115
ppm corresponding to a saturated carbon atom bonded to two
oxygens. All the data prove that the [2+2] cycloaddition
previously observed with methyl cinnamate, crotonate, and
fumarate did not occur in this case but that a [2+4]
cycloaddition led to the 2-oxa-1-germacyclohex-3-ene 5. Thus,
there is a great difference of chemical behavior between methyl
fumarate and methyl maleate, which are E and Z isomers,
respectively. Moreover, the stereochemistry seems to play an
important role in the outcome of the reaction since methyl
cinnamate and crotonate, which are also E isomers, react like
methyl fumarate. The three minor derivatives (total about 15%)
present in the crude reaction mixture (δ31P = 304.0
corresponding to the hydrolysis product (see further), 306.8
and 340.2 ppm) could not be isolated.
Scheme 1. [2+2] and [2+4] Reactions of Tip(t-Bu)GeC
PMes*
The structure of 3 has been proved by an X-ray
determination (Figure 1). The four-membered-ring oxagerma-
Hydrolysis of Cycloadduct 5. Compound 5 is air and
moisture sensitive, due to the presence of the CC(OMe)O
moiety; addition of water leads to the saturated six-membered
ring 6 via the transient cyclic enol 7 (Scheme 2). This 2-oxa-1-
Scheme 2. Hydrolysis of 5
Figure 1. Structure of 3. Thermal ellipsoids are drawn at the 50%
probability level. All hydrogen atoms are omitted, and some atoms are
simplified for clarity. Selected bond lengths (Å) and bond angles
(deg): Ge1−C1 1.964(2); Ge1−O1 1.846(2); O1−C2 1.434(2); C1−
C2 1.542(3); C2−C41 1.498(3); C41−C42 1.324(3); C42−C43
1.471(3); O2−C2 1.427(2); P1−C1 1.663(2); P1−C22 1.870(2);
O1−Ge1−C1 74.55(7); Ge1−C1−C2 87.65(2); O1−C2−C1
101.74(14); C2−O1−Ge1 95.69(1); C22−P1−C1 104.19(9); P1−
C1−Ge1 133.97(2); P1−C1−C2 138.05(2).
germacyclohexane is easily evidenced by the presence of a
carbonyl group (δ13C = 171.03 ppm) and of an intracyclic CH2
unit (δ13C = 38.27 ppm, 3JCP = 6.9 Hz). In 1H NMR, a coupling
is observed between the two inequivalent H’s of the CH2 group
and the H of the CHCOOMe moiety, but only one of the H’s
of CH2 gives a coupling with the phosphorus atom (4JPH = 1.5
Hz).
cyclobutane is nearly planar (O1−Ge1−C1−C2 = 3.91°, Ge1−
C1−C2−O1 = 4.96°, C1−C2−O1−Ge1 = 5.30°, C2−O1−
Ge1−C1 = 4.23°) like in other rings of this type,8 although
higher folding angles from 8° to 23° are sometimes found.9
Despite the steric hindrance, the intracyclic Ge−C bond length
(1.964(2) Å) lies in the normal range10 as well as P−C and
Derivative 6 was unambiguously characterized by an X-ray
structure determination (Figure 2), which shows that a
(H2)C−C(O) single bond (1.504(12) Å) has replaced the
(H)CC(OMe) double bond. Like in the case of 2−4, an E
configuration is found for the exocyclic PC double bond.
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Figure 3. Structure of 8. Thermal ellipsoids are drawn at the 50%
probability level. All hydrogen atoms are omitted, and some atoms are
simplified for clarity. Selected bond lengths (Å) and bond angles
(deg): Ge1−C28 1.987(3); Ge1−C24 1.999(3); Ge1−C2 1.988(3);
Ge1−C1 1.989(3); P1−C1 1.835(3); P1−C3 1.913(3); C2−C3
1.550(4); P1−C6 1.858(3); O1−C4; 1.201(3); C28−Ge1−C2
111.22(12); C28−Ge1−C1 115.49(11); C2−Ge1−C1 91.65(11);
C28−Ge1−C24 112.56(11); C2−Ge1−C24 113.33(13); C1−Ge1−
C24 111.00(12); C1−P1−C6 101.72(12); C1−P1−C3 94.06(12).
Figure 2. Structure of 6. Thermal ellipsoids are drawn at the 30%
probability level. All hydrogen atoms are omitted, and some atoms are
simplified for clarity. Selected bond lengths (Å) and bond angles
(deg): Ge1−C1 1.978(4); C1−C2 1.523(12); C2−C3 1.523(11);
C3−C4 1.504(12); C4−O1 1.368(11); Ge1−O1 1.827(3); C4−O2
1.217(11); P1−C1 1.677(4); P1−C26 1.866(4); C5−O3 1.184(12);
C5−O4 1.330(13); O1−Ge1−C1 95.99(15); C4−O1−Ge1 131.4(7);
O1−C4−C3 118.1(12); C4−C3−C2 113.1(12); C3−C2−C1
109.5(12); C2−C1−Ge1 111.2(8).
by an insertion of the carbenic carbon atom into a C−H bond
of a tert-butyl group in ortho position on the supermesityl
group. This type of insertion of a carbene into a C−H bond is
well known.11,12 Thus, the phosphagermaallene behaves as a
1,3-dipole in such a reaction, giving a [3+2] cycloaddition with
the CHCH2 moiety. It is worth noting that the more positive
part of the CHCH2 unit (CH2) is bonded to the more
positive part of the GeCP (Ge) unit.
c. Methyl Acrylate. Surprisingly, the reaction between the
phosphagermaallene 1 and methyl acrylate, under the same
experimental conditions as with other ethylenic esters, was
completely different. A 31P NMR analysis of the crude reaction
mixture showed the presence of only one signal, at −19.9 ppm.
This high-field chemical shift proves that the PC double
bond has been involved in the reaction. The insensitivity of this
compound toward oxygen and moisture suggests the absence of
the extremely reactive GeC double bond. The 13C NMR
spectrum displays a signal at 177.54 ppm corresponding to the
carbonyl group of the COOMe moiety; the 1H NMR spectrum
shows the disappearance of the vinylic protons and the
presence of only two tert-butyl groups on the supermesityl
and of a proton on the carbon between the germanium and
phosphorus atoms (δ = 2.35 ppm). A 1/1 adduct was proved
by mass spectrometry. All these spectral data are in agreement
with the structure 8, which has been unambiguously proved by
an X-ray determination (Scheme 3 and Figure 3).
Monitoring the reaction between −80 °C and room
temperature in the 31P NMR did not allow evidencing the
formation of the carbene 9, which probably has a very short
lifetime. Trapping experiments with excess methyl acrylate were
also unsuccessful.
CALCULATIONS
■
In order to get more insight into these different experimental
results, DFT calculations have been performed at the B3LYP/
6-31G** level of theory.13 The energy profiles of the [2+2] and
[2+4] cycloaddition reactions between the real molecule 1 and
the three ethylenic esters, respectively the fumarate and the
crotonate (Figure 4) and the maleate (Figure 5), have been
computed. Taking into account the frontier orbitals of the
phosphagermaallene 1 and for instance those of the fumarate
(see SI, Figure S1), it is noteworthy that the [2+2] and [2+4]
cycloaddition reactions can compete since they involve the
same orbitals (interaction between πGeC − nP of 1 and π*CO
+ π*CC of the ethylenic ester).
The formation of 8 can be explained only by the intermediate
formation of the cyclic germa(phosphino)carbene 9 followed
Scheme 3. [3+2] Reaction of Tip(t-Bu)GeCPMes*
Calculations have shown that the reactions involving the
fumarate are exergonic (−19.4 kcal·mol−1 for the [2+2]
cycloaddition and −13.5 kcal·mol−1 for the [2+4] cyclo-
addition) and that the [2+2] cycloadduct is the thermodynamic
product of the reaction. The activation barriers are calculated at
21.7 and 25.7 kcal·mol−1 for the [2+2] and [2+4] cycloaddition
reactions, respectively. These results are in agreement with the
experimental data since the [2+2] adduct has been formed,
showing that this reaction seems kinetically and thermodynami-
cally controlled. In this [2+2] mechanism, the heteroallene
approach is supra−supra facial (GeOC1C dihedral angle ∼10°)
with the Ge−O and Ge−C bonds not yet formed (respectively
2.374 and 1.836 Å). The [2+2] adduct is obtained according to
an asynchronous mechanism with a difference of the Ge−O
and Ge−C bond lengths in the TS[F2u+m2a]rate compared to the final
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Figure 4. Energy profiles (ΔG in kcal·mol−1) for the [2+2] and [2+4] cycloaddition reactions involving the phosphagermaallene 1 and the fumarate
or the crotonate. Selected bond lengths (Å) and bond angles (deg).
[2+2]
product of respectively about 0.5 and 0.12 Å. In this TS
,
kinetic control; [2+4]: thermodynamic control). The compar-
ison between calculations and the experimental data allows us
to conclude that the reaction involving the methyl crotonate is
kinetically controlled since the [2+2] adduct has been
Fumarate
the GeCP bond angle (130.87°) decreases drastically (about
33°) compared to the phosphagermaallene 1 (164.25°). The
HOMO corresponds to the πGeC − nP orbital with a strong
weight on the central carbon atom (see SI, Figure S2). In fact, it
looks like a σ-carbon lone pair localized on the central carbon
in antibonding interaction with the phosphorus lone pair. The
LUMO is the bonding combination between π*CO and
π*CC orbitals and the LUMO+1 is the π*PC orbital. This TS
can be described as a carbenoid species. It is noteworthy that
we have previously observed such a TS in the reaction between
1 and methyl vinyl ketone.8 By looking in detail at the
electronic and repulsion energies (see SI, Table S3) in the
experimentally characterized. Consequently, the transition
[2+2]
state TS
TS
, which is less sterically hindered than the
Crotonate
[2+4]
(smallest repulsion energy, see SI Table S3), seems
Crotonate
to direct the reaction.
The energy profile for the reaction between the phospha-
germaallene 1 and the maleate has also been calculated. It is
very similar to the crotonate case. The [2+2] and [2+4]
cycloaddition reactions are exergonic. The thermodynamic
product is the [2+4] adduct. It presents an energy difference of
about 12 kcal·mol−1 from the [2+2] adduct. It is the highest
calculated energetic difference (−5.9 kcal·mol−1 for the
[2+2]
[2+4]
Fumarate
transition states TS
and TS
as well as in the final
Fumarate
products of the two cycloaddition reactions, the privileged
[2+2] attack presents the smallest repulsion energies, which
means that the steric hindrance of the substituents seems to
control this reaction.
fumarate and +4.2 kcal·mol−1 for the crotonate). The transition
[2+2]
state TS
is more accessible (21.1 kcal·mol−1) than
Maleate
[2+4]
TS
(27.8 kcal·mol−1), with a difference of 6.7 kcal·mol−1.
Maleate
As previously observed, the reaction involving the crotonate
leads to an activation barrier for the [2+2] cycloaddition
reaction (26.4 kcal·mol−1) slightly lower than in the [2+4] one
(30.7 kcal·mol−1). The energy difference between these two
TSs (4.3 kcal·mol−1) is found similar to that calculated for the
fumarate (4.0 kcal·mol−1). These reactions are also exergonic
(−7.4 versus −11.6 kcal·mol−1 respectively for the [2+2] and
[2+4] cycloadditions), with a [2+4] adduct stabilized by about
4.2 kcal·mol−1 from the [2+2] one. Such theoretical results
show that the [2+2] and [2+4] reactions can compete ([2+2]:
In fact, for this [2+2] cycloaddition reaction, the activation
barrier is similar to the fumarate one. In contrast, the activation
barrier for the [2+4] cycloaddtion reaction is found to be
intermediate between those of the two previous studied
reactions (27.8 kcal·mol−1 versus 30.7 kcal·mol−1 for the
crotonate and 25.7 kcal·mol−1 for the fumarate). Taking into
account the small energetic difference in the activation barriers
between the [2+2] and [2+4] cycloaddition reactions, the two
reactions can compete. Nevertheless, the strong stabilization of
the [2+4] adduct compared to the [2+2] one (11.8 kcal·mol−1)
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Figure 5. Energy profiles (ΔG in kcal·mol−1) for the [2+2] and [2+4] cycloaddition reactions involving the phosphagermaallene 1 and the maleate.
Selected bond lengths (Å) and bond angles (deg).
leads experimentally to the formation of only the [2+4] adduct.
This means that the reaction is thermodynamically controlled.
This [2+4] adduct is the thermodynamic product of the
reaction due to the presence of stabilizing interactions (Eelec
more important than in the [2+2] adduct: see SI Table S3),
which counterbalance the steric hindrance.
kcal·mol−1 for the acrylate and 36.0 kcal·mol−1 for the
crotonate. The energy barrier found for the acrylate is in the
same range as those for the [2+2] and [2+4] cycloaddition
reactions previously described. This can be one of the reasons
that this process can occur only with the acrylate. With the
other ethylenics, the activation barriers are too high and the
If we compare the energy profile between the two maleate
and fumarate isomers, it is noteworthy that the [2+2] adduct
involving the maleate is slightly less stable than the one
involving the fumarate by about 3.3 kcal·mol−1. In contrast, the
[2+4] adduct involving the maleate is strongly stabilized by
about 14.4 kcal·mol−1 compared to that of the fumarate. This
means that with the maleate the weak destabilization of the
[2+2] final product (higher steric hindrance) and the strong
stabilization of the [2+4] adduct (higher electronic energy)
favor the [2+4] cycloaddition process. Consequently, the
reaction becomes thermodynamically controlled.
To get more insight into the original 1,3-dipole behavior of
the phosphagermaallene 1, we have also calculated the energy
profiles for the [3+2] cycloaddition reactions (GeCP +
CC) involving all three ethylenic esters (maleate, fumarate,
and crotonate) and also the acrylate derivative (Figure 6). All
the cyclic carbenes have been found as minima on the potential
energy surface. However, the reactions are calculated to be
endergonic (∼+13 kcal·mol−1 for the maleate and acrylate and
up to +25.6 kcal·mol−1 for the crotonate), in agreement with
the fact that these carbenes are unstable intermediates (the
germa(phosphino)carbene obtained with methyl acrylate could
not be isolated nor characterized by low-temperature NMR).
The activation barriers have been calculated between 26.8
[2+2] and [2+4] cycloaddition reactions are privileged. With
[3+2]
the acrylate, the transition state TS
presents a quasi planar
Acrylate
five-membered ring (GePC2C3 = −13°) with a Ge−C bond
length of 1.851 Å and a P−C bond of 1.629 Å, characteristic of
a double bond with a planar geometry around the phosphorus
atom (see Figure 6). These geometrical features show a strong
donation from the phosphorus lone pair into the carbene 2pπ
vacant orbital. The GeCP bond angle is acute (around 124.06°
versus 164.25° in 1), and the Ge−C3 and P−C2 bond lengths
are respectively 2.414 and 2.798 Å. This TS has an
asynchronous character since the Ge−C3 and P−C2 bond
lengths in the carbene adduct are respectively 2.014 and 1.975
Å. However this carbene is not the thermodynamic product of
σ
the reaction. In fact, the insertion of the nC lone pair of the
central carbon (between Ge and P) into the σ*CH orbital of the
tert-butyl group leads to a product that is strongly stabilized, by
about 52.7 kcal·mol−1, from the starting products (see SI,
Figure S3). This means that the insertion product remains
indisputably the driving force of this reaction. Taking into
account (i) that the activation barrier obtained for the [3+2]
cycloaddition reaction between the phosphagermaallene 1 and
the acrylate is the smallest calculated and (ii) the strong
stabilization of the insertion product, which is the thermody-
namic compound of the reaction, we can understand why the
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Figure 6. Energy profiles (ΔG in kcal·mol−1) for the [3+2] cycloaddition reactions involving the phosphagermaallene 1 and the fumarate, crotonate,
maleate, or acrylate. Selected bond lengths (Å) and bond angles (deg).
phosphagermaallene 1 experimentally behaves as a 1−3 dipole
feasible. Unfortunately, the latter has not been experimentally
characterized, in agreement with an endothermic reaction. The
driving force of the reaction remains the formation of the
insertion product (insertion of the carbenic carbon atom into a
CH bond of a t-Bu group of the Mes*), showing that the
reaction is thermodynamically controlled.
The [3+2] cycloaddition reaction does not occur with the
maleate because the activation barrier (29.4 kcal·mol−1) is
higher than that of the [2+4] one (27.8 kcal·mol−1), and
moreover the [2+4] adduct is the most thermodynamically
favored. It can be noted that the activation barrier of this [2+4]
reaction is close to that of the experimentally observed [3+2]
cycloaddition reaction of acrylate (26.8 kcal·mol−1).
only with acrylate.
CONCLUSION
■
A rich reactivity is observed between ethylenic esters R−CH
CH−COOMe and the phosphagermaallene 1 since, depending
on the R group and on the Z or E stereochemistry around the
CC double bond, various reactions occur involving the sole
GeC double bond or the whole GeCP unit. In all the
reactions involving the phosphagermaallene 1 and the ethylenic
esters (fumarate, maleate, crotonate), the activation barrier for
the [2+2] reaction is found smaller than for the [2+4] one.
These [2+2] cycloaddition reactions, which are orientated by
the steric hindrance of the substituents, are favored with the
fumarate and the crotonate, showing that these pathways are
kinetically controlled. Moreover, with the fumarate the [2+2]
cycloaddition adduct is also the thermodynamic product of the
reaction, thus involving both a kinetic and thermodynamic
control. With the maleate the reaction is thermodynamically
controlled, since the compound experimentally characterized
corresponds to the [2+4] cycloadduct, which is the
thermodynamic product of the reaction. The latter is strongly
stabilized compared to the [2+2] cycloadduct (Δ = 11.8
kcal·mol−1). This is the highest energy difference calculated in
the studied ethylenic esters. The comparison with the reaction
involving the fumarate shows that both the weak destabilization
of the [2+2] cycloadduct and the strong stabilization of the
[2+4] one seem responsible for the formation of the latter.
Considering the acrylate reagent, the calculated activation
barrier for the [3+2] cycloaddition reaction makes the
formation of an intermediate germa(phosphino)carbene
The very versatile reactivity of this phosphagermaallene
continues to be under active investigation.
EXPERIMENTAL PART
■
All experiments were carried out in flame-dried glassware under a
nitrogen or argon atmosphere using high-vacuum-line techniques,
Solvents were dried and freshly distilled using an SPS-5MB system.
NMR spectra were recorded (with CDCl3 as solvent) on a Bruker
Avance 300 spectrometer at the following frequencies: 1H, 300.13
MHz; 13C. 75.47 MHz (reference TMS); 31P (121.51 MHz)
(reference H3PO4). Melting points were determined on a Wild
Leitz-Biomed apparatus. Mass spectra were obtained on a Hewlett-
Packard 5989A spectrometer by EI at 70 eV and on a Nermag R10-10
spectrometer by CI. Elemental analyses were performed by the
“Service de Microanalyse de l’Ecole de Chimie de Toulouse”. NMR
spectra have been elucidated using COSY, HMBC, and HSQC
techniques.
Synthesis of 1. The phosphagermaallene 1 was synthesized as
previously described7,8 by addition of a solution of tert-butyllithium 1.7
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Organometallics
Article
M in pentane (0.6 mL) to a solution of fluorophosphagermapropene
Tip(t-Bu)Ge(F)-C(Cl)PMes* (0.676 g, 1 mmol) in diethyl ether
(10 mL) cooled to −78 °C. The solution turned immediately from
colorless to red, due to the formation of the lithio compound Tip(t-
Bu)Ge(F)-C(Li)PMes*. After warming to room temperature, a 31P
NMR analysis (δ 249.9 ppm) showed the nearly quantitative
formation of phosphagermaallene 1. Red-orange solutions of 1
containing LiF were used directly without further purification.
General Procedure for the Reaction between Phosphager-
mallene 1 and Methyl Crotonate, Methyl Cinnamate, and
Methyl Fumarate. To a red-orange solution of phosphagermaallene
1 (1.00 mmol) in Et2O (10 mL) cooled to −80 °C was added one
equivalent of ethylenic ester dissolved in Et2O (10 mL). The reaction
mixture was stirred for 15 min at −80 °C, then warmed to room
temperature to give a yellow solution. Solvents were distilled off under
reduced pressure and replaced by pentane. LiF was removed by
filtration. Cooling to −20 °C afforded white crystals of 2−4.
Mes*, 20); 333 (Tip(t-Bu)Ge − 1, 7); 275 (TipGe − 2, 58); 233
(TipGe − i-Pr, 25); 131 (t-BuGe, 15); 57 (t-Bu, 100). Anal. Calcd for
C48H71GeO2P (783.65): C, 73.57; H, 9.13. Found: C, 73.80; H, 9.26.
4: 0.63 g; 82%; mp 196 °C. 1H NMR: δ (ppm) 1.23 (s, 9H,
GeCMe3); 1.27 (d, 3JHH = 6.6 Hz, 6H, p-CHMeMe′); 1.29, 1.31, 1.39,
3
and 1.44 (4d, JHH = 6.6 Hz, 4 × 3H, o-CHMeMe′); 1.35 (s, 9H, p-
CMe3 of Mes*); 1.42 and 1.54 (2s, 2 × 9H, o-CMe3 of Mes*); 2.82 (s,
3H, OMe); 2.82−2.88 (m. 3H, o- and p-CHMeMe′); 3.65 (COOMe);
5.27 and 5.90 (2d, 3JHH = 15.3 Hz, 2 × 1H, MeOOCCHCH); 6.99
and 7.04 (2d, 4JHH = 1.5 Hz, 2 × 1H, m-CH of Tip); 7.30 and 7.35 (2s,
2 × 1H, m-CH of Mes*). 13C NMR: δ (ppm) 23.77 and 23.90 (p-
CHMeMe′); 24.62, 25.48, 26.47, and 26.95 (o-CHMeMe′); 27.90
3
(GeCMe3); 31.00 (d, JCP = 4.5 Hz, GeCMe3); 31.25 (p-CMe3 of
Mes*); 33.24 (d, 4JCP = 8.1 Hz) and 34.86 (d, 4JCP = 6.7 Hz) (o-CMe3
of Mes*); 34.13 (p-CHMeMe′); 34.59 and 37.59 (o-CHMeMe′);
34.77 (p-CMe3 of Mes*); 37.65 and 39.06 (o-CMe3 of Mes*); 48.19
2
(OMe); 51.10 (COOMe); 107.53 (d, JCP = 16.8 Hz, C-O); 120.56
2: 0.41 g; 57%; mp 178 °C. 1H NMR: δ (ppm) 1.25 (s, 9H,
and 121.64 (m-CH of Mes*); 121.19 and 122.09 (m-CH of Tip);
3
3
GeCMe3); 1.29, 1.34, 1.37, and 1.47 (4d, JHH = 6.6 Hz, 4 × 3H, o-
122.11 and 148.00 (MeOOCCHCH); 130.84 (d, JCP = 4.5 Hz,
3
ipso-C of Tip); 135.05 (d, 1JCP = 71.1 Hz, ipso-C of Mes*); 149.43 (p-
CHMeMe′); 1.30 (d, JHH = 6.6 Hz, 6H, p-CHMeMe′); 1.38 and 1.58
(2s, 2 × 9H, o-CMe3 of Mes*); 1.41 (dd, 4JHH = 1.5 Hz, 3JHH = 6.6 Hz,
3H, MeCH); 1.43 (s, 9H, p-CMe3 of Mes*); 2.86 (s, 3H, OMe);
2.92−3.00 (m, 3H, o- and p-CHMeMe′); 3.71 (dq, 4JHH = 1.5 Hz, 3JHH
2
C of Mes*); 150.58 (p-C of Tip); 152.61 (d, JCP = 2.2 Hz) and
155.43 (o-C of Mes*); 153.19 and 154.33 (o-C of Tip); 166.74
1
(COOMe); 188.64 (d, JCP = 79.3 Hz, CP). 31P NMR: δ (ppm):
3
3
= 15.0 Hz, 1H, MeCHCH); 5.50 (dq, JHH = 6.6 Hz, JHH = 15.0
Hz, 1H, MeCH=CH); 7.04 and 7.06 (2s, 2 × 1H, m-CH of Tip); 7.33
and 7.37 (2s, 2 × 1H, m-CH of Mes*). 13C NMR: δ (ppm) 17.33
(MeCHCH); 23.76 and 23.92 (p-CHMeMe′); 24.43, 25.46, 26.56,
and 27.02 (o-CHMeMe′); 28.14 (GeCMe3); 30.87 (GeCMe3); 31.43
272.1. MS (m/z %): 766 (M, 2); 736 (M − OMe + 1, 3); 710 (M − t-
Bu + 1, 4); 623 (M − MeOOCCHCHCOOMe + 1, 3); 566 (M −
MeOOCCHCHCOOMe − t-Bu + 1, 15); 510 (M −
MeOOCCHCHCOOMe − 2t-Bu + 2, 10); 377 (M −
MeOOCCHCHCOOMe − Mes*, 16); 333 (Tip(t-Bu)Ge − 1,
5); 275 (TipGe − 2, 43); 233 (TipGe − i-Pr − 1, 20); 131 (t-BuGe,
3); 57 (t-Bu, 100). Anal. Calcd for C44H69GeO4P (765.59): C, 69.03;
H, 9.08. Found: C, 68.84; H, 8.86.
4
4
(p-CMe3 of Mes*); 33.22 (d, JCP = 8.4 Hz) and 34.86 (d, JCP = 6.6
Hz) (o-CMe3 of Mes*); 33.90 (p-CHMeMe′); 34.45 and 37.44 (o-
CHMeMe′); 34.71 (p-CMe3 of Mes*); 37.66 and 39.41 (o-CMe3 of
2
Mes*); 47.54 (OMe); 108.91 (d, JCP = 16.9 Hz, C−O); 119.88 and
Reaction between Phosphagermallene 1 and Methyl
Maleate. To a red-orange solution of phosphagermaallene 1 (1.00
mmol) in Et2O (10 mL) cooled to −80 °C was added one equivalent
of methyl maleate dissolved in Et2O (10 mL). The reaction mixture
was stirred for 15 min at −80 °C, then warmed to room temperature
to give a yellow solution. A 31P NMR analysis showed the presence of
a major signal (285.9 ppm, 85%) with minor signals at 304.0
(hydrolysis product: see further) and 306.8 and 340.2 ppm (about 5%
each). The usual workup (elimination of solvents under reduced
pressure, addition of 10 mL of pentane followed by removal of LiF,
then attempted crystallization) did not allow obtaining pure
cycloadduct 5. The latter was obtained in about 85% purity, and its
structure was unambiguously proved by its hydrolysis product.
Hydrolysis of the Reaction Mixture Containing 5. After
addition of 20 mL of diethyl ether to the previously obtained crude
reaction mixture of cycloadduct 5, the solution was left in an open
Schlenk and a small amount of water was added. An NMR study
showed the formation of the new hydrolysis derivative 6, which was
obtained as white crystals from pentane at −20 °C (0.49 g; 64%; mp
177 °C). 1H NMR: δ (ppm) 1.17, 1.18, 1.32, and 1.45 (4d, 3JHH = 6.6
Hz, 4 × 3H, o-CHMeMe′); 1.24 (d, 3JHH = 6.6 Hz, 6H, p-CHMeMe′);
1.32 (s, 9H, p-CMe3 of Mes*); 1.38 (s, 9H, GeCMe3); 1.45 and 1.58
122.21 (m-CH of Mes*); 121.02 and 121.90 (m-CH of Tip); 126.31
3
(MeCHCH); 131.76 (d, JCP = 4.5 Hz, ipso-C of Tip); 132.00 (d,
1
3JCP = 2.8 Hz, MeCHCH); 135.56 (d, JCP = 72.1 Hz, ipso-C of
Mes*); 149.10 (p-C of Mes*); 150.16 (p-C of Tip); 152.72 (d, 2JCP
=
2.8 Hz) and 155.46 (o-C of Mes*); 153.28 and 154.35 (o-C of Tip);
191.26 (d, 1JCP = 75.5 Hz, CP). 31P NMR: δ (ppm): 258.6. MS (m/
z %): 723 (M + 1, 5); 692 (M − OMe + 1, 5); 666 (M − t-Bu + 1, 4);
566 (M − MeCHCHCOOMe − t-Bu + 1, 10); 509 (M −
MeCHCHCOOMe − 2t-Bu + 1, 12); 377 (M − MeCH
CHCOOMe − Mes*, 10); 275 (TipGe − 2, 20); 233 (TipGe − i-Pr,
10); 131 (t-BuGe, 2); 57 (t-Bu, 100). Anal. Calcd for C43H69GeO2P
(721.58): C, 71.57; H, 9.64. Found: C, 71.81; H, 9.48.
1
3: 0.55 g; 70%; mp 185 °C. H NMR: δ (ppm): 1.26 (s, 9H,
GeCMe3); 1.27 (d, 3JHH = 6.6 Hz, 6H, p-CHMeMe′); 1.28, 1.32, 1.39,
and 1.45 (4d, 3JHH = 6.6 Hz, 4 × 3H, o-CHMeMe′); 1.36 and 1.57 (2s,
2 × 9H, o-CMe3 of Mes*); 1.41 (s, 9H, p-CMe3 of Mes*); 2.87 (s, 3H,
3
OMe); 2.88 and 3.07 (2sept, JHH = 6.6 Hz, 2 × 1H, o-CHMeMe′);
2.91 (sept, 3JHH = 6.6 Hz, 1H, p-CHMeMe′); 4.54 (d, 3JHH = 15.9 Hz,
3
1H, PhCHCH); 6.67 (d, JHH = 15.9 Hz, 1H, PhCHCH); 6.98
4
and 7.06 (2d, JHH = 1.5 Hz, 2 × 1H, m-CH of Tip); 7.10−7.27 (m
4
5H, Ph); 7.37 and 7.42 (2d, JHH = 1.2 Hz, 2 × 1H, m-CH of Mes*).
3
2
13C NMR: δ (ppm) 23.78 and 23.90 (p-CHMeMe′); 24.72, 25.54,
26.57, and 26.98 (o-CHMeMe′); 28.10 (GeCMe3); 31.06 (d, 3JCP = 4.2
Hz, GeCMe3); 31.43 (p-CMe3 of Mes*); 33.15 (d, 4JCP = 8.2 Hz) and
(2s, 2 × 9H, o-CMe3 of Mes*); 2.29 (dd, JHH = 5.1 Hz, JHH = 17.1
Hz, 1H, CHH′); 2.67 (ddd, 3JHH = 5.1 Hz, 2JHH = 17.1 Hz, 4JPH = 1.5
3
Hz, 1H, CHH′); 2.77 (s, 3H, OMe); 2.82 (d, JHH = 5.1 Hz, 1H,
CHCOOMe); 2.88 (sept, 3JHH = 6.6 Hz, 1H, p-CHMeMe′); 3.23 and
3.61 (2sept, 3JHH = 6.6 Hz, 2 × 1H, o-CHMeMe′); 7.03 and 7.10 (2d,
4
34.97 (d, JCP = 6.5 Hz) (o-CMe3 of Mes*); 34.10 (p-CHMeMe′);
34.40 and 37.52 (o-CHMeMe′); 34.83 (p-CMe3 of Mes*); 37.71 and
39.32 (o-CMe3 of Mes*); 47.81 (OMe); 108.98 (d, 2JCP = 16.8 Hz, C-
O); 119.81 and 122.79 (m-CH of Mes*); 121.15 and 122.08 (m-CH
of Tip); 126.79, 127.03, and 128.23 (CH of Ph); 130.31 (d, 3JCP = 3.1
4
4JHH = 1.5 Hz, 2 × 1H, m-CH of Tip); 7.32 and 7.39 (2d, JHH = 1.8
Hz, 2 × 1H, m-CH of Mes*). 13C NMR: δ (ppm): 23.79 (p-
CHMeMe′); 24.73, 25.34, 25.75, and 27.55 (o-CHMeMe′); 28.19 (d,
4JCP = 5.3 Hz, GeCMe3); 31.17 (p-CMe3 of Mes*); 32.27 and 36.91 (o-
CHMeMe′); 32.55 (GeCMe3); 33.60 (d, 4JCP = 6.6 Hz) and 34.10 (d,
4JCP = 8.2 Hz) (o-CMe3 of Mes*); 33.99 (p-CHMeMe′); 34.77 (p-
3
Hz, PhCHCH); 130.71 (PhCHCH); 131.34 (d, JCP = 3.0 Hz,
1
ipso-C of Tip); 136.08 (d, JCP = 71.8 Hz, ipso-C of Mes*); 136.98
(ipso-C of Ph); 149.10 (p-C of Mes*); 150.36 (p-C of Tip); 152.70 (d,
2JCP = 2.6 Hz) and 155.85 (o-C of Mes*); 153.29 and 154.33 (o-C of
Tip); 190.37 (d, 1JCP = 75.5 Hz, CP). 31P NMR: δ (ppm) 262.6. MS
(m/z %): 785 (M + 1, 4); 753 (M − OMe, 5); 728 (M − t-Bu + 1, 5);
696 (M − t-Bu − OMe, 5); 623 (M − PhCHCHCOOMe + 1, 4);
566 (M − PhCHCHCOOMe − t-Bu + 1, 20); 510 (M − PhCH
CHCOOMe − 2t-Bu + 2, 30); 377 (M − PhCHCHCOOMe −
3
CMe3 of Mes*); 38.13 (o-CMe3 of Mes*); 38.27 (d, JCP = 6.9 Hz,
CHH′); 46.83 (d, 2JCP = 14.3 Hz, CHCOOMe); 50.94 (OMe); 121.15
and 123.33 (m-CH of Tip); 121.50 and 122.38 (m-CH of Mes*);
3
1
130.20 (d, JCP = 8.5 Hz, ipso-C of Tip); 134.60 (d, JCP = 77.0 Hz,
ipso-C of Mes*); 150.25 (p-C of Tip); 150.62 (p-C of Mes*); 153.43
and 155.94 (o-C of Tip); 154.12 (d, 2JCP = 2.5 Hz) and 155.22 (o-C of
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Article
Mes*); 171.03 (CO); 171.15 (d, JCP = 9.4 Hz, COOMe). 31P
NMR: δ (ppm) 304.0. MS (m/z %): 753 (M + 1, 5); 695 (M − t-Bu,
15); 663 (M − t-Bu − OMe − 1, 5); 275 (TipGe − 2, 30); 233
(TipGe − i-Pr, 10); 57 (t-Bu, 100); 43 (i-Pr, 25). Anal. Calcd for
C43H67GeO4P (751.56): C, 68.72; H, 8.99. Found: C, 68.80; H, 9.20.
Reaction of Phosphagermaallene 1 with Methyl Acrylate. To
a red-orange solution of phosphagermaallene 1 (1.47 mmol) in Et2O
cooled to −80 °C was added 0.12 g (1.44 mmol) of methyl acrylate. A
brown coloration appeared immediately. After 15 min stirring at −80
°C, the reaction mixture was allowed to warm to room temperature;
the 31P NMR analysis of the crude light red reaction mixture showed
only one signal. Solvents were eliminated under vacuum and replaced
by pentane (20 mL), and the lithium salts were removed by filtration.
Crystallization at −20 °C from pentane afforded 0.79 g (76%) of white
crystals of 8 (mp 205 °C). 1H NMR: δ (ppm) 1.15 and 1.41 (2s, 2 ×
3H, CMeMe′); 1.18, 1.22, 1.49, and 1.52 (4d, 4 × 3H, 3JHH = 6.6 Hz,
o-CHMeMe′); 1.29 (d, 6H, 3JHH = 6.6 Hz, p-CHMeMe′); 1.29 (s, 9H,
t-BuGe); 1.31 and 1.52 (2s, 2 × 9H, t-Bu on Mes*); 1.65−1.91 (m,
3H, CHH′CMeMe′ and GeCHH′); 2.02−2.15 (m, 1H, GeCHH′);
2.35 (ddd, J = 4.1 Hz, J = 13.9 Hz, J = 22.7 Hz, 1H, PCHGe); 2.52,
2.81, and 3.12 (3 sept, 3 × 1H, 3JHH = 6.7 Hz, CHMeMe′); 2.95 (t, J =
15 Hz, 1H, PCHCOOMe); 6.87−6.95 (m, 2H, arom H of Tip); 7.16−
3
ACKNOWLEDGMENTS
■
We are grateful to the “Agence Nationale pour la Recherche”
(contract ANR-08-BLAN-0105-01) and PhoSciNet (CM0802)
for financial support. The theoretical work was granted access
to the HPC resources of Idris under allocation 2012
(i2012080045) made by GENCI (“Grand Equipement Na-
tional de Calcul Intensif”).
DEDICATION
■
Dedicated to Dumitru Ghereg, a young brilliant chemist who
died prematurely.
REFERENCES
■
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16.5 Hz, CH2Ge); 21.45 (d, 1JCP = 26.1 Hz, PCHGe); 23.73 and 24.01
(p-CHMeMe′); 24.40 (Me3CGe); 26.37, 25.49, 26.46, and 27.73 (o-
4
CHMeMe′); 30.42 (d, JCP = 6.4 Hz, Me3CGe); 31.36 (p-Me3C on
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37.22 (d, JCP = 1.6 Hz) and 38.07 (CMe3 on Mes* and CH2CMe2);
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́
2
121.00, 121.24, and 122.25 (arom CH of Tip and Mes*); 128.04 (d,
1JCP = 37.4 Hz, ipso-C of Mes*); 131.54 (d, JCP = 8.3 Hz, ipso-C of
3
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Tip); 149.32, 150.60, 154.08, 154.82, 155.10, and 155.39 (arom C of
Tip and Mes*); 177.54 (d, 2JCP = 16.2 Hz, CO). 31P NMR: δ (ppm):
−19.9. MS (m/z %): 708 (M+, 1); 665 (M+ − i-Pr, 1); 651 (M+ − t-
Bu, 20); 619 (M+ − t-Bu − OMe − 1, 2); 595 (M+ − 2 t-Bu + 1, 2);
566 (M+ − 2 t-Bu − CO, 8); 523 (M+ − 2 t-Bu − CO − i-Pr, 1); 493
(M+ − 2 t-Bu − CO − i-Pr − 2Me, 1); 364 (M+ − t-Bu − Ar − i-Pr +
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(707.55): C, 71.30; H, 9.54. Found: C, 71.18; H, 9.57.
X-ray Structure Determinations. The data of the structures for
compounds 3, 6, and 8 were collected on a Bruker-AXS APEX II
diffractometer using a 30 W air-cooled microfocus source (ImS) with
focusing multilayer optics at low temperature (173(2) and 193(2) K),
with graphite-monochromated Mo Kα radiation (wavelength =
0.71073 Å) by using phi- and omega-scans. The data were integrated
with SAINT,14 and an empirical absorption correction with SADABS
was applied.15 All structures were solved by direct methods, using
SHELXS-97,16 and refined using the least-squares method on F2.17 All
non-H atoms were treated anisotropically. The H atoms were fixed
geometrically and refined with a riding model. Crystallographic data of
3, 6, and 8 are summarized in Table 1 (see Supporting Information).
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́
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ASSOCIATED CONTENT
■
́
́
J.;
S
* Supporting Information
Ranaivonjatovo, H.; Gornitzka, H.; Saffon, N.; Miqueu, K.; Lazraq,
CIF files for 3, 6, and 8. Theoretical calculation details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
M. Organometallics 2010, 29, 2566.
(9) Lazraq, M.; Couret, C.; Escudie,
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jean.escudie@numericable.fr; gornitzka@lcc-toulouse.
fr.
Notes
The authors declare no competing financial interest.
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