Complexes of donor-acceptor cyclopropanes with tin, titanium, and gallium chlorides - Mechanism studies

Article abstract of DOI:10.1021/om301072v

Hitherto unknown complexes of dimethyl cyclopropane-1,1-dicarboxylate with Lewis acids (Sn, Ti, and Ga chlorides) have been isolated and characterized. Their structures, which were previously assumed from theoretical considerations, have been confirmed, a

Full text of DOI:10.1021/om301072v

Article
pubs.acs.org/Organometallics
Complexes of Donor−Acceptor Cyclopropanes with Tin, Titanium,
and Gallium Chlorides  Mechanism Studies
Roman A. Novikov,†,‡ Dmitry O. Balakirev,† Vladimir P. Timofeev,‡ and Yury V. Tomilov*,†
†N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian
Federation
‡V. A. Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 32 Vavilov st., 119991 Moscow, Russian Federation
S
* Supporting Information
ABSTRACT: Hitherto unknown complexes of dimethyl
cyclopropane-1,1-dicarboxylate with Lewis acids (Sn, Ti, and
Ga chlorides) have been isolated and characterized. Their
structures, which were previously assumed from theoretical
considerations, have been confirmed, and the activating effect
of Lewis acids on the cyclopropane ring has been shown
experimentally. A single crystal of the complex with SnCl4 has
been obtained, and an X-ray diffraction study has been
performed. Hitherto unknown complexes of donor−acceptor
cyclopropanes with GaCl3 belonging to a new type and having
1
a 1,2-dipole (ylide) structure have been obtained and characterized. 1D and 2D NMR spectroscopy on H, 13C, 35Cl, 71Ga, and
119Sn nuclei were used to study the structures of the complexes. Furthermore, DOSY diffusion NMR spectroscopy was used to
determine the diffusion coefficient in solution and the molecular masses of the complexes. The data obtained enrich the picture
of transformations of donor−acceptor cyclopropanes widely used in organic synthesis. The practical use of the gallium
phenylcyclopropanedicarboxylate complex in a synthesis of polysubstituted cyclic structures in one synthetic stage has also been
demonstrated.
in the cyclopropane ring occurs, resulting in its opening with
generation of a 1,3-dipolar intermediate, which then undergoes
subsequent reactions.
However, complexes of Lewis acids with donor−acceptor
cyclopropanes and their 1,3-zwitterionic intermediates are
hypothetical structures, none of which was ever actually
detected in a reaction mixture.1
INTRODUCTION
■
Donor−acceptor cyclopropanes (cyclopropanes with donor
and acceptor substituents in vicinal positions) are now popular
in contemporary organic synthesis as sources of 1,3-dipoles that
are generated from them on treatment with Lewis acids.1 The
capability of donor−acceptor cyclopropanes to undergo [2 +
3]-, [3 + 3]-, and [3 + 4]-dipolar cycloaddition with various
substrates2 is now used to build five-, six-, and seven-membered
heterocycles (Scheme 1); these reactions can be performed
enantioselectively,3 which makes them very attractive for
applications in organic synthesis. More than 20 full syntheses
of natural compounds have been carried out to date on the
basis of donor−acceptor cyclopropanes.4
Aryl and sometimes alkyl, alkoxy, or amino groups are used
as electron-donating substituents in cyclopropanes, whereas
alkoxycarbonyls are used mainly as electron-withdrawing
substituents. Tin(II) triflates and rare-earth-element triflates
as well as chloroalanes are the most popular Lewis acids;
gallium and indium compounds are less common.1,2
In view of the wide application of donor−acceptor
cyclopropanes in organic synthesis, a study on the mechanisms
of their opening and cycloaddition was carried out by
computation methods, isotope labeling and changes in the
chirality of optically active centers during the process.1
Presumably, a complex of the Lewis acid with the donor−
acceptor cyclopropane is first formed due to coordination to an
electron-accepting group. As a result, polarization of a σ bond
RESULTS AND DISCUSSION
■
Complexes of Dimethyl Cyclopropane-1,1-dicarbox-
ylate with SnCl4, TiCl4, and GaCl3. Since no physicochemical
data are available in the literature for complexes of donor−
acceptor cyclopropanes with Lewis acids, we performed a series
of experiments in order to detect and isolate intermediate
complexes. The study was started from nonsubstituted
dimethyl cyclopropane-1,1-dicarboxylate (1). This cyclopro-
pane is among the least reactive species in the series of donor−
acceptor cyclopropanes. It was therefore expected that its
complexes with Lewis acids would be most stable for
observation by physicochemical methods. Tin, titanium, and
gallium chlorides that have a considerable Lewis acidity were
chosen as the Lewis acids.
It was found that mixing of cyclopropanedicarboxylate 1 with
the above metal chlorides in dichloromethane at room
Received: November 8, 2012
© XXXX American Chemical Society
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Scheme 1. Donor−Acceptor Cyclopropanes in Modern Organic Synthesis
temperature gave the stable complexes 2a−c (Scheme 2) in a
few minutes. These complexes separated from the solutions due
Scheme 2. Reactions of Cyclopropane-1,1-dicarboxylate 1
with Sn, Ti, and Ga Chlorides
to low solubility in CH2Cl2. The Sn and Ti complexes 2a,b
precipitated as colorless crystals, whereas the complex with
GaCl3 (2c) was a colorless, heavy, thick oil resembling ionic
liquids.
Complexes 2a−c were found to be sufficiently stable
Figure 1. ORTEP diagram of 2a, with thermal ellipsoids given at the
compounds that were almost not hydrolyzed in the air.
Retention of the cyclopropane ring was observed in all
complexes 2a−c. This ring was found to be incapable of
opening, even in the presence of rather strong Lewis acids. The
complexes obtained are insoluble in low-polarity organic
solvents, such as alkanes, benzene, and chloroform, whereas
in polar solvents (ether, ethyl acetate, acetone, methanol, etc.)
they undergo ligand exchange, releasing the original molecule
of cyclopropane 1. Dichloromethane is among the acceptable
solvents in which complexes 2a−c are at least slightly soluble.
We succeeded in performing an X-ray diffraction analysis of
tin complex 2a. Recrystallization of this complex was found to
be impossible due to its low solubility; thus, we were forced to
use the crystals obtained during the synthesis upon fast mixing
of solutions of cyclopropane 1 and tin tetrachloride in CH2Cl2.
As a result, complex 2a was obtained as large monoclinic
crystals up to 10 mm long.
50% probability level.
X-ray diffraction analysis of complex 2a showed that the
cyclopropane ring is retained, while the tin atom is coordinated
to the sp2 oxygen atoms of the two ester groups. The
coordination sphere of tin is nearly octahedral. The crystal
structure of 2a involves the intermolecular contact
Cl(1)···Cl(4) = 3.480 Å and hydrogen bond Cl(2)···H(4b) =
2.86 Å, which apparently strengthen the crystal lattice and
govern the low solubility of the complex (Figures 1 and 2).
In comparison to the cyclopropane-1,1-dicarboxylic acid
molecule (3),5 all the bonds in the cyclopropane ring in
complex 2a are elongated (Table 1). The maximum elongation
is observed in C−CH2 bonds, just as was assumed theoretically
for complexes of this type.1
Figure 2. Unit cell of 2a showing the intermolecular contact
Cl(1)···Cl(4) = 3.480 Å and hydrogen bond Cl(2)···H(4b) = 2.86 Å.
The elongation of the C−CH2 bond of the cyclopropane ring
upon complex formation is due to the fact that, as a strong
Lewis acid, the metal chloride withdraws electron density from
the cyclopropane ring, which results in polarization of the σ
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Lewis acid, the C−C bond that has vicinal donor and acceptor
substituents is polarized noticeably and cleaved by suitable
substrates.1
Table 1. Comparison of Bond Lengths in Molecule 2a and
Cyclopropane-1,1-dicarboxylic Acid (3)5 According to the
Data of the X-ray Analysis
Polarization of the σ bond in the cyclopropane ring and
charge distribution in the molecules of complexes 2a−c can be
judged by the chemical shifts in the 1H and 13C NMR spectra in
comparison with those of the original cyclopropanedicarbox-
ylate 1. Cumulated data on the chemical shifts are presented in
Table 2.
bond length, Å
a
a
bond
2a
3
3′
1.467
H2C−CH2
H2C−C
C−CO
CO
1.478
1.456
1.545, 1.564
1.531, 1.535
1.483, 1.485
1.214, 1.218
1.531, 1.540
1.483, 1.485
1.217, 1.218
1.469, 1.491
1
As expected, the H and 13C NMR spectra are characterized
1.219, 1.245
Sn−O
2.165, 2.172
by a downfield shift of all ester group signals, which normally
takes place in the case of coordination of the metal atom to the
oxygen atoms of these groups.6 However, the strongest
downfield shift is observed for signals of the CH2 groups of
Sn−Cl
2.349, 2.351, 2.367, 2.393
a
A cell contains two crystallographically independent molecules.
bond in the three-membered ring and hence causes its
weakening and elongation (Figure 3). In this case, the C−
1
the cyclopropane ring (Δδ 0.8−2.3 ppm in the H NMR
spectra and 12−19 ppm in the 13C NMR spectra), which is due
to a considerable increase in the partial positive charge6 on the
CH2 groups of the three-membered ring due to involvement of
the latter in conjugation with the ester groups and the metal
atom (Figure 3). In this case, the 13C NMR signal of the
quaternary carbon atom in the cyclopropane ring remains
almost unchanged.
As one can see from Table 2, the chemical shifts of
complexes 2a−c, in both the 1H and 13C NMR spectra, change
synchronously in accordance with the nature of the metal atom
in the complex. In fact, it is possible to build a qualitative
dependence of the chemical shift (and hence the δ+ on the CH2
group) on the type the metal atom. The observed effect
increases in the series Sn < Ti < Ga: i.e., of the Lewis acids
studied, GaCl3 should activate the cyclopropane ring most
strongly, in agreement with the experimental observations.1,7,8
A similar trend is observed for the signal width: it is the largest
for gallium complexes (evidently due to dynamic processes).
The liquid gallium complex 2c in pure form shows much larger
Figure 3. Structure of the complexes 2a−c.
CH2 bond undergoes the strongest elongation. As a result, the
cyclopropane ring becomes involved in a conjugated system of
a kind, which can be represented by limiting resonance
structures, one with an intact cyclopropane ring and one with
an open one (see Figure 3).
In reality, there is no σ-bond cleavage in the three-membered
ring if there are no additional electron-donating substituents on
the cyclopropane ring. However, owing to coordination with a
1
downfield shifts and line widths in the H NMR spectrum in
comparison with its solution, which may be explained by
additional intermolecular interactions.
Table 2. Multinuclear NMR Spectra (CD2Cl2) of Cyclopropane 1 and Its Complexes 2a−c
1H NMR
13C NMR
multinuclear NMR
71Ga
compd
data
CH2
OCH3
CH2
C
OCH3
CO
119Sn
35Cl
1
δ, ppm
W1/2, Hz
δ, ppm
1.45
3.74
16.4
27.8
52.4
170.0
0.8
0.8
SnCl4
GaCl3
−151.6
220
W1/2, Hz
δ, ppm
380
6900
220
a
228
W1/2, Hz
12600
5600
2a (M = Sn)
2b (M = Ti)
δ, ppm
Δδ, ppm
2.24
+0.79
2.2
4.02
+0.28
1.6
28.5
28.5
58.3
178.6
−591
−439
1200
+12.1
+0.7
+5.9
+8.6
W1/2, Hz
δ, ppm
2.30
+0.85
12.8
2.65
+1.2
75
4.06
+0.32
1.9
28.3
+11.9
br
28.6
+0.8
br
58.4
+6.0
br
178.8
+8.8
br
Δδ, ppm
W1/2, Hz
2c (M = Ga)
δ, ppm
4.1
252
+24
2300
252
+24
2300
(CD2Cl2)
Δδ, ppm
+0.4
60
W1/2, Hz
2c (M = Ga, neat)
δ, ppm
3.7
5.1
35.7
+19.3
br
30.5
+2.7
br
62.4
+10.0
br
183.4
+13.4
br
220
0
Δδ, ppm
+2.3
400
+1.4
400
W1/2, Hz
6100
a
Gallium chloride in solution exists as the dimer Ga2Cl6.
C
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Scheme 3. Reactions of 2-Phenylcyclopropane-1,1-dicarboxylate 4 with SnCl4 and TiCl4
Multinuclear NMR spectra of complexes 2a,c also show
strong shifts of the signals of the metal atoms in comparison
with the pure metal chlorides. In fact, the 119Sn NMR spectra of
complex 2a show an upfield shift by 439 ppm, whereas the 71Ga
NMR spectra of complex 2c show a downfield shift by 24 ppm.
These shifts are in good agreement with the literature data
concerning coordination of tin and gallium chlorides with
oxygen-containing ligands.9,10 On the other hand, no changes
occur in the 35Cl NMR spectra.
Thus, we isolated and characterized hitherto unknown
complexes of dimethyl cyclopropane-1,1-dicarboxylate with
Lewis acids, confirmed their structures previously assumed
theoretically, and experimentally showed the activating effect of
Lewis acids on the cyclopropane ring.
Complexes of 2-Phenylcyclopropane-1,1-dicarboxy-
late with GaCl3. After the studies with dimethyl cyclopropane-
1,1-dicarboxylate, we studied the complexation of 2-phenyl-
cyclopropane-1,1-dicarboxylate 4 with the same Lewis acids
(tin, titanium, and gallium chlorides). However, cyclopropane 4
was found to be much more reactive than its nonsubstituted
analogue,1 and its complexes with Lewis acids similar to 2a−c
were extremely unstable and very quickly underwent further
transformations with cyclopropane ring opening.
In fact, the reaction of cyclopropanedicarboxylate 4 with
SnCl4 or TiCl4 resulted in fast cyclopropane ring opening in the
complex formed initially upon exposure to the chloride anion
eliminated from the Lewis acid. The reaction gave enolates 5,
which on exposure to methanol or water were converted to
substituted malonates 6 (Scheme 3).7
Gallium trichloride behaves differently. It exists as a dimer in
dichloromethane solutions, and it is not a chloride anion donor
in this form,7 unlike titanium and tin chlorides. However, even
in this case, we failed to detect a primary complex of GaCl3 with
cyclopropane 4 with retention of the three-membered ring. On
the other hand, the complex formed with cyclopropane ring
opening was found to be sufficiently stable. In fact, the reaction
of phenylcyclopropanedicarboxylate 4 with an equimolar
amount of gallium trichloride in dichloromethane (c < 0.15
mol/L) at room temperature occurred almost instantaneously
in quantitative yield to give gallium complex 7 (Scheme 4) as
an orange solution (it is fairly well soluble in CH2Cl2, unlike
gallium complex 2c). The complex obtained in this way
decomposed almost completely in 6 h at 20 °C in solution.
When the solution was cooled below 0 °C, the complex
abruptly lost solubility and formed a deposit of a heavy thick
oil. It is interesting to note that this deposit no longer dissolved
in CH2Cl2 upon warming, which is apparently indicative of
irreversible polymerization. Owing to this, it did not appear
possible to work with gallium complex 7 at low temperatures.
Therefore, all necessary experiments for detection of
intermediates by NMR spectroscopy and studies of their
subsequent transformations were carried out at room temper-
ature under dry argon and with dehydrated solvents.
The processes occurring during the formation of gallium
complexes with cyclopropanedicarboxylate 4 were studied by
1D and 2D NMR spectroscopy, including the DOSY
technique,12 allowing the diffusion of components in a solution
to be analyzed. It was found that monomeric complex 7 with a
pentacoordinate gallium atom mostly exists in CD2Cl2 solution
at concentrations below 0.15 M, whereas dimeric complex 8,
which is formed owing to two additional Ga−Cl bridging
bonds11 by analogy with the Ga2Cl6 dimer (Scheme 5),
predominates at concentrations in the range of 0.15−0.25 M.
Oligomeric complexes start to predominate upon subsequent
increase in the complex concentration in solution.
DOSY diffusion spectroscopy is a well-proven NMR
approach that allows one to analyze various chemical systems
and mixtures without preliminary separation.12 Diffusion
coefficients are related to the speed of molecular motions in
solution and depend on the size of dissolved compounds. The
Scheme 5. Complexes of 4 with GaCl3 at Various
Concentrations
Scheme 4. Reaction of 2-Phenylcyclopropane-1,1-
dicarboxylate 4 with GaCl3
D
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Figure 4. 2D 1H DOSY NMR spectra processed by monoexponential fitting for cyclopropane 4 Ga complexes with calibrant (squalane). Conditions:
CD2Cl2, 0.15−0.25 mol/L, 32 °C, LEDBPP pulse sequence, Δ = 100 ms, Te = 5 ms. The figure shows several spectra recorded under the same
conditions and superimposed on each other. Due to the small variation in the viscosities of the solutions arising from differences in the solute
concentrations used, the diffusion coefficient was brought to the same value with respect to the calibrant, which is not quite correct. Each component
in the spectra is shown by a horizontal dashed line marked with different colors: 7, red; 8, blue; oligomers. green; squalane, violet.
diffusion coefficient for a molecule represented by a rigid
sphere is inversely proportional to the sixth power of
hydrodynamic radius of the sphere according to the Stokes−
Einstein equation. It may be considered in this case that in the
first approximation (in the absence of interaction of solute
molecules with the solvent), diffusion of molecules only
depends on their molecular masses; hence, one can estimate
the solute molecular masses rather precisely by measuring the
diffusion coefficient.12
estimate the concentration ranges of their existence (Figure 4).
One can also see that the monomeric and dimeric complexes
have a narrow scatter of diffusion coefficients (as expected for
individual stoichiometric compounds), whereas oligomers have
a very wide scatter of diffusion coefficients and comprise a set
of compounds with various molecular masses and degrees of
oligomerization. The use of a special mathematical algorithm
(SCORE15) for DOSY processing along with DOSYToolbox
software15 allowed us to obtain signals of separate components
in the form of 1D or 2D images even for mixtures of complexes
where the majority of signals totally overlap (see the
Supporting Information).
On the basis of the known molecular masses of the
monomeric and dimeric gallium complexes and the calibrant,
we determined the molecular masses of oligomeric complexes
and the degree of oligimerization. The calculation was carried
out under the assumption that the relationship between the
logarithms of the diffusion coefficient and the molecular mass is
nearly linear.12,14 It was found that oligomeric complexes have
molecular masses in the range of 4000−30000 D, which
corresponds to ∼10−80 molecules of the respective mono-
meric complex.
Since the diffusion coefficient strongly depends on the
solution viscosity, all DOSY experiments were carried out
under nearly the same conditions (solvent, concentration,
temperature, and pulse sequence parameters). In order to
identify the nature of gallium complexes formed, diffusion
1
coefficients were measured by 2D H DOSY NMR using the
LEDBPP pulse sequence13 in the presence of an internal
calibrant (squalane,14 C30H62).
The calibrant had to meet a number of stringent require-
ments. In particular, its molecular mass had to be close to that
of the compound being studied (in this case, complex 7), it had
to be inert toward the other solution components, and the
1
signals of the calibrant in the H spectrum should not overlap
the signals of the complexes of interest. Saturated hydrocarbons
satisfy our requirements quite well; thus, we used squalane
(C30H62, a hydrogenation product of natural squalene).14
According to DOSY data, though the chemical nature of the
calibrant and the complexes of interest differ, it has nearly the
same diffusion coefficient as monomeric complex 6, which
unambiguously confirms that the composition of the latter is
3:GaCl3 = 1:1.
Thus, DOSY NMR spectroscopy proved to be quite
promising for studying the composition and structure of
gallium complexes of donor−acceptor cyclopropanes that were
impossible to isolate in pure form, even at reduced temper-
atures.
It is interesting to note that complex 7 was formed not only
from donor−acceptor cyclopropane 4. Reactions of its isomers,
viz., unsaturated compounds 9 and 10, also gave this
compound. Both reactions occurred under the same conditions
as for cyclopropane 4 (Scheme 6). Decomposition of gallium
complex 7 with methanol gave benzylidenemalonate 9, which
Using 2D 1H DOSY NMR spectra, we were able to
distinguish unambiguously monomeric (7), dimeric (8), and
oligomeric complexes of donor−acceptor cyclopropanes and
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of the quaternary C atom in the malonyl moiety shifted upfield
by 13.0 ppm (Figure 6). Judging by the combination of the
observed signals, the methylidenemalonate moiety in gallium
complex 7 is, in fact, rather similar to the ylide structure
proposed.
Scheme 6. Different Formation Pathways of the Ga Complex
7
a
a
Reaction conditions: (i) GaCl3, CD2Cl2, 25 °C, 1 min.
additionally confirms the structure of this complex. It should
also be noted that dimethyl (2-phenylethyl)malonate, a
saturated analogue of compounds 9 and 10, almost does not
undergo complexation with GaCl3.
Thus, gallium complexes 7 and 8 have a 1,2-dipole structure
(ylide structure) that was not mentioned before for assumed
complexes of donor−acceptor cyclopropanes. The structure of
complex 7 was unambiguously confirmed by 1D and 2D NMR
1
spectroscopy on H, 13C, 35Cl, 69Ga, and 71Ga nuclei with the
use of the two-dimensional correlation techniques COSY,
1
NOESY, HSQC, and HMBC. The H NMR spectra show an
Figure 6. Chemical shifts of the signals in 1H and 13C NMR spectra of
alkene 9 and complex 7.
isolated acyclic system CHCH2 remaining from the cyclo-
propane ring, in which the CH moiety has a very strong
downfield chemical shift (δH 9.00 ppm) that approaches those
of benzyl cations,16 indicating that a considerable positive
charge is localized on it. However, the methine moiety (CH) in
the structure of this complex is directly bound to the malonyl
In the 71Ga NMR spectra, transition from gallium trichloride
to complex 7 results in a downfield shift of the signal by 54
ppm accompanied by a slight signal broadening, which is in
good agreement with literature data and agrees with gallium
coordination to two oxygen atoms (Figure 7).9 After
1
group, as suggested by the couplings in the 2D H,13C-HMBC
NMR spectrum that rule out the “classical” 1,3-dipole structure
I (Figure 5). At the same time, complex 7 was found to rather
Figure 5. Structure of Ga-complex 7.
stable. According to combined 1D, 2D, and DOSY NMR
spectroscopic data, no dynamic processes occur in solution,
except for the concentration-dependent monomer−dimer−
oligomer transformations noted above. Furthermore, the NMR
spectra do not manifest signals of the free ligand and gallium
trichloride, as they are irreversibly bound to complex 7.
Figure 7. 71Ga and 35Cl NMR spectra of complex 7 compared with
those of Ga2Cl6.
decomposition of complex 7, the gallium signal shifts back
upfield in the course of time and broadens noticeably.
Conversely, no changes occur in the 35Cl NMR spectra: all
the chlorine atoms in complex 7 are still strongly bound to the
gallium atoms and are not released to solution.
The ylide structure of complex 7 with localized charges
allows certain experimental facts to be explained in addition. In
fact, the organic ligand in complex 7 has a negative charge,
which makes it a much stronger ligand than, for example, (2-
phenylethyl)malonate. On the other hand, compounds with
separated charges and with no steric substituents, as is the case
in complex 7, show high reactivity and undergo further
1
Comparison of H and 13C NMR spectra of complex 7 and
of the corresponding alkene 9 has shown that transition of the
latter to complex 7 is accompanied by a strong downfield shift
of the CH moiety (ΔδH 1.88 and ΔδC 38.3) in both the 1H and
13C NMR spectra. We should also note a downfield shift of the
carbonyl and methoxy C atoms of the ester groups by about 10
ppm, which characterizes GaCl3 coordination to these groups.
However, conversely, two signals in the 13C NMR spectrum of
complex 7 shifted upfield. In fact, the signal of the ipso C atom
of the phenyl substituent shifted by 3 ppm, whereas the signal
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Scheme 7. Mechanism of the Formation of Ga Complex 7
Scheme 8. Addition of Different Nucleophiles to Ga Complex 7
1
Figure 8. 2D H DOSY NMR spectra processed by monoexponential fitting for Ga complexes 7, 8, 11, and 13 with the calibrant (squalane).
Conditions: CD2Cl2, 0.15−0.20 mol/L, 32 °C, LEDBPP pulse sequence, Δ = 100 ms, Te = 5 ms (all spectra were recorded under the same
conditions). The spectrum of complex 13 was recorded in a ∼1/1 mixture of CDCl3 and CD2Cl2. Due to small variations in the viscosities of the
solutions, the diffusion coefficient was brought to the same value with respect to the calibrant, which is not quite correct. Each component in the
spectra is shown by a horizontal dashed line marked with different colors: 7, red; 8, blue; 11, green; 13, yellow; squalane, violet. The 1D projections
of the components were separated from the DOSY spectra of appropriate mixtures using the SCORE algorithm.
transformations as a 1,2-dipole (see Scheme 9). As already
shift of the signal by 54 ppm in comparison with Ga2Cl6,
whereas a similar shift in related gallium complex 2c is as small
noted, the 71Ga NMR spectra of complex 7 show a downfield
G
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as 24 ppm. This effect can be explained by the existence of
additional electron density on the malonyl moiety in complex 7,
which, according to literature data,9 additionally shifts the 71Ga
signal downfield.
allows us to state that they all retain their ylide structure. It is
interesting to note that, of all the gallium complexes observed,
complex 7 has the largest downfield shift of the CH moiety
carrying a positive charge. This agrees with the fact that the
stronger the positive charge localized on the CH moiety, the
larger its downfield shift in the spectra. Since, in comparison
with complex 7, each of the complexes 8 and 11−14 contains
additional ligands that donate electron density to the organic
moiety of the complex, the positive charge on the CH moiety
decreases accordingly, thus causing a reverse upfield shift of the
signal (Table 3).
According to the data obtained, the following mechanism of
formation of gallium complex 7 can be assumed. First, the
reaction of cyclopropanedicarboxylate 4 with GaCl3 results in
coordination of the latter to ester groups to give the primary
complex II, which is analogous to gallium complex 2c. The
presence of a phenyl substituent activates the cyclopropane
ring. As a result, this complex is quickly transformed to 1,3-
dipolar complex I, which in turn also quickly undergoes
migration of a hydride anion to give the detectable ylide
complex 7 (Scheme 7). Unfortunately, attempts at low-
temperature detection of complexes II and I failed due to the
reasons considered above. However, their involvement in
various transformations seems rather reasonable.
Table 3. Chemical Shifts for the CH Fragment in NMR
Spectra of Ga Complexes
1H NMR (CH)
complex
δ, ppm (t)
3J, Hz
tdecomp, h (estimated)
As noted above, the gallium atom in complex 7 is
pentacoordinate and can add more ligands:11 in particular, to
give dimeric complex 8. It was found that not only another
molecule of complex 7 but also gallium trichloride itself can act
as a nucleophile. In fact, the reaction of cyclopropanedicarbox-
ylate 4 with excess GaCl3 results in addition of the latter as a
ligand to give complex 11 with two new Ga−Cl bridging bonds
(Scheme 8). However, the resulting binuclear complex was
found to be even less stable than complex 8; even with excess
GaCl3, complex 11 is in equilibrium with mononuclear complex
7 and its content in solution does not exceed 50% according to
NMR data. What is more, complex 11 was completely
decomposed in 30 min, whereas complex 7 existed for another
several hours.
9·GaCl3 (7)
[9·GaCl3]2 (8)
[9·GaCl3]n
9·Ga2Cl6 (11)
9·GaCl3·THF (12)
9·GaCl3·S8 (13)
9·GaCl3·Pyr (14a)
9·Pyr·GaCl3 (14b)
9.00
6.9
6.8
6
8.84
3−4
<3
0.5
0.1
3
8.80 (br m)
8.97 (br)
8.52 (br)
8.88
∼6.9
∼7.1
6.8
8.69
6.7
0.3
0.2
5.93
7.6
As a result, we succeeded in synthesizing hitherto unknown
complexes of a new type with a 1,2-dipole structure (ylide
structure) formed by donor−acceptor cyclopropanes and Lewis
acids and thoroughly studied their structure and reactivity.
These data certainly enrich the understanding of the
mechanism of reactions involving donor−acceptor cyclo-
propanes, at least those that occur in the presence of gallium
trichloride, which were previously explained via the “classical”
1,3-dipole.1,7,8
For example, we recently implemented a dimerization of
donor−acceptor cyclopropanes to polysubstituted cyclopen-
tanes 15 in the presence of catalytic amounts of gallium
trichloride.7 At that time, the mechanism of the reaction was
unknown. Taking the new data into consideration, it becomes
clear that this process occurs via ylide complex 7. We have also
shown that heating of gallium complex 7 partially gives
cyclobutane structure 16, in which one molecule of complex 7
directly participates as a 1,2-dipole (Scheme 9).
The composition of complex 11 was confirmed by 1H DOSY
NMR spectroscopy. Though its signals overlapped considerably
with the signals of original complex 7, it is clear that it fits the
ratio ligand:GaCl3 = 1:2 according to the molecular mass.
Furthermore, its molecular mass is larger than that of
mononuclear complex 7 and smaller than that of dimeric
complex 8 (Figure 8). Considering that this complex (DOSY
signal) appears only if excess GaCl3 is present and that,
according to 1H and 13C NMR data, the frame and structure of
the organic ligand in complex 11 are completely the same as
those in complex 7, it becomes obvious that it is a product of
addition of a second GaCl3 molecule to complex 7.
Complex 7 also readily reacted with other nucleophilic
ligands, such as tetrahydrofuran, elementary sulfur, and
pyridine. In all the cases, the ligand was coordinated to the
gallium atom to create a six-coordinate environment of the
latter and to give complexes 12−14, respectively (Scheme 8).
The structures of complexes 12−14 were also confirmed by a
Thus, the use of complex 7 enables a one-stage synthesis of
complex polysubstituted cyclic structures from readily available
Scheme 9. Possible Transformations of Ga Complex 7
1
combination of 1D and 2D H and 13C NMR techniques, as
well as by DOSY diffusion spectroscopy. In fact, 2D 1H DOSY
NMR spectra recorded using squalane as the internal calibrant
showed unambiguously that the molecular masses of complexes
12−14 were higher than that of the original complex but lower
than that of the dimeric complex 8. The structures of the
organic ligands in the complexes remain unchanged, as
indicated by only small changes in the chemical shifts.
Hydrolysis of complexes 11−14 with methanol gave the
same product, viz., benzylidenemalonate 9, which also confirms
the structures of the complexes.
The insignificance of changes in the chemical shifts of the
1
corresponding signals in the H and 13C NMR spectra of
complexes 11−14 in comparison with those for complex 7
H
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Article
concentrated under vacuum to 0.5 mL volume. The precipitate that
formed was filtered on a Schott filter and was dried under vacuum to
afford complex 2a (515 mg, 97%) as a colorless powder: mp 138−140
°C dec.
donor−acceptor cyclopropanes. Moreover, the use of additional
ligands will expand its applicability in organic synthesis.
3. CONCLUSION
Method B (Growth of Crystals for X-ray Analysis). A solution of
SnCl4 (329 mg, 1.26 mmol) in CH2Cl2 (0.5 mL) was added in one
portion under an argon atmosphere at room temperature without
stirring to a solution of cyclopropane 1 (200 mg, 1.26 mmol) in dry
CH2Cl2 (2.5 mL). After 1 min complex 2a began to precipitate from
the solution as colorless crystals. The reaction mixture was kept at
room temperature for 10 min without stirring until the end of the
precipitation of crystals. The precipitate that formed was filtered on a
Schott filter, washed with CH2Cl2 (2 × 2 mL), and dried under
vacuum. Complex 2a (450 mg, 85%) was obtained as large colorless
monoclinic crystals.
We have isolated and characterized for the first time complexes
of dimethyl cyclopropane-1,1-dicarboxylate with Lewis acids,
confirmed their structures that were previously assumed
theoretically, and experimentally showed the activating effect
of Lewis acids on the cyclopropane ring. A single crystal of the
complex 1·SnCl4 has been grown, and an X-ray diffraction
study has been carried out. We have synthesized hitherto
unknown complexes of a new type with 1,2-dipole structure
(ylide structure) formed by donor−acceptor cyclopropanes and
gallium trichloride and thoroughly studied their structures and
reactivities. The data obtained provide a new understanding of
the mechanisms of reactions of donor−acceptor cyclopropanes
on treatment with Lewis acids, in particular GaCl3. A practical
use of the gallium phenylcyclopropanedicarboxylate complex in
organic synthesis to obtain polysubstituted cyclic structures in
one synthetic stage has been demonstrated as an example.
Anal. Calcd for C7H10Cl4O4Sn: C, 20.08; H, 2.41. Found: C, 20.01;
H, 2.27. IR (KBr): ν 3116, 3029, 2958, 2853, 1728 br (CO), 1639,
1
1561, 1442, 1357, 1340, 1222, 1199, 1139 cm−1. H NMR (400.1
MHz, CD2Cl2): δ 2.24 (s, 4H, 2CH2, W1/2 = 2.2 Hz), 4.02 (s, 6H,
2OCH3, W1/2 = 1.6 Hz). 13C NMR (100.6 MHz, CD2Cl2): δ 28.5 (C
and 2CH2), 58.3 (2OCH3), 178.6 (2COO). 119Sn NMR (149.2 MHz,
CD2Cl2): δ −591 (s, W1/2 = 1200 Hz). MS (m/z, %): 260 (8,
+
+
+
120Sn35Cl4 ), 225 (54, 120Sn35Cl3 ), 190 (2, 120Sn35Cl2 ), 158 (8,
+
C7H10O4 ), 155 (8, 120Sn35Cl+), 127 (100, C7H10O4 − OMe+), 120 (9,
4. EXPERIMENTAL SECTION
120Sn+), 100 (28, C7H10O4 − CO2Me + H+), 98 (30), 95 (33), 68
(33), 59 (80), 39 (39).
General Experimental Details. All reagents and solvents used
were commercial grade chemicals (>99.5%) without additional
purification. All operations with metal chlorides and their complexes
were carried out under a dry argon atmosphere. TLC analysis was
performed on Silufol chromatographic plates. For preparative
Selected geometric parameters for 2a (from X-ray analysis data):
monoclinic crystals, a = 7.3546(11) Å, b = 12.4212(19) Å, c =
14.965(2) Å, V = 1330.5(3) Å3, dcalc = 2.090 g cm−3, space group P21/
c. Bond lengths (Å): Sn(1)−O(1), 2.165; Sn(1)−O(3), 2.172; Sn(1)−
Cl(1), 2.349; Sn(1)−Cl(4), 2.351; Sn(1)−Cl(2), 2.367; Sn(1)−Cl(3),
2.393; O(1)−C(1), 1.219; O(3)−C(3), 1.245; C(1)−C(2), 1.491;
C(2)−C(3), 1.469; C(2)−C(5), 1.545; C(2)−C(4), 1.564; C(4)−
C(5), 1.478. Angles (deg): O(1)−Sn(1)−O(3), 78.5; O(1)−Sn(1)−
Cl(1), 88.6; O(3)−Sn(1)−Cl(1), 166.8; Cl(1)−Sn(1)−Cl(4), 101.5;
Cl(1)−Sn(1)−Cl(2), 95.7. Intermolecular contacts (Å): Cl(1)···Cl(4),
3.480; Cl(2)···H(4b), 2.86.
Complex (CH2)2C(CO2Me)2·TiCl4 (2b). A solution of TiCl4 (239
mg, 1.26 mmol) in CH2Cl2 (0.5 mL) was added dropwise over 1 min
under an argon atmosphere to a solution of cyclopropane 1 (200 mg,
1.26 mmol) in dry CH2Cl2 (2.5 mL), and the mixture was stirred at a
room temperature for 15 min. Then the reaction mixture was
concentrated under vacuum to 0.5 mL volume. The precipitate formed
was filtered on a Schott filter and was dried under vacuum to afford
complex 2b (403 mg, 92%) as light yellow crystals: mp 100−102 °C
dec.
1
chromatography, silica gel 60 (0.040−0.063 mm) was used. H and
13C NMR spectra were recorded on 400 MHz (400.1 and 100.6 MHz,
respectively) and 300 MHz (300.1 and 75.5 MHz, respectively)
spectrometers in CD2Cl2 and CDCl3 containing 0.05% Me4Si as the
1
internal standard. Assignments of H and 13C signals were made with
the aid of 2D COSY, TOCSY, NOESY, HSQC, and HMBC spectra
1
where necessary. Some chemical shifts in H and 13C NMR spectra,
due to the impossibility of direct detection, were taken from the
corresponding two-dimensional spectra. 35Cl, 71Ga, and 119Sn NMR
spectra were recorded on a 400 MHz spectrometer (39.2, 122.0, and
149.2 MHz, respectively) in CD2Cl2; standards were NaCl and
Ga(NO3)3 solutions in water and Me4Sn with the addition of C6D6,
respectively. Monitoring of the reactions in NMR tubes was carried
out in CD2Cl2 solution containing 0.05% Me4Si as the internal
standard. Measurements of the diffusion coefficients were performed
1
using 2D H DOSY NMR spectroscopy (diffusion ordered spectros-
copy)12 in CD2Cl2 and CDCl3 solutions on a 300 MHz spectrometer
(300.1 MHz for 1H). A BPP-LED pulse sequence13 was used with Δ =
100 ms and Te = 5 ms. The DOSY spectra were processed by
monoexponential fitting and the SCORE algorithm using Bruker
TopSpin and DOSYToolbox software.15 Squalane (C30H62, mol wt
422) was used as the internal calibrant of the molecular weight in
DOSY spectra in ∼1:1 molar ratio to the studied complexes. IR spectra
were obtained on a FT-IR spectrometer in CHCl3 solution (1%). Mass
spectra were recorded using electron impact ionization (EI, 70 eV,
direct inlet probe). High-resolution mass spectra were obtained using
simultaneous electospray (ESI). The elemental compositions were
determined on a Perkin-Elmer Series II 2400 CHN analyzer. X-ray
crystallographic data for complex 2a were obtained on a “Bruker 1K
SMART CCD” diffractometer (Mo Kα radiation) at 100 K. The
starting arrays of measured intensities were processed using the
APEX2 program.17 The structure was solved by direct methods and
refined by full-matrix least squares in the anisotropic approximation for
non-hydrogen atoms on F2hkl. Hydrogen atoms were placed in
geometrically calculated positions.
Anal. Calcd for C7H10Cl4O4Ti: C, 24.17; H, 2.90. Found: C, 23.51;
H, 3.07. IR (KBr): ν 2959, 1737 br (CO), 1629, 1439, 1321, 1308,
1246, 1219, 1200, 1150 cm−1. 1H NMR (400.1 MHz, CD2Cl2): δ 2.30
(s, 4H, 2CH2, W1/2 = 12.8 Hz), 4.06 (s, 6H, 2OCH3, W1/2 = 1.9 Hz).
13C NMR (100.6 MHz, CD2Cl2): δ 28.3 (br.s, 2CH2), 28.6 (br s, C),
58.4 (br s, 2OCH3), 178.8 (br.s, 2COO). MS (m/z, %): 190 (1,
+
48Ti35Cl337Cl+), 158 (1, C7H10O4 ), 155 (2, 48Ti35Cl237Cl+), 127 (12,
+
C7H10O4 − OMe+), 118 (1, 48Ti35Cl2 ), 100 (15, C7H10O4 − CO2Me
+ H+), 69 (13), 59 (23), 36 (100).
Complex (CH2)2C(CO2Me)2·GaCl3 (2c). Solid GaCl3 (222 mg,
1.26 mmol) was added in one portion under an argon atmosphere
with stirring to a solution of cyclopropane 1 (200 mg, 1.26 mmol) in
dry CH2Cl2 (2.5 mL), and the mixture was kept at room temperature
for 15 min without stirring. Complex 2c drops out as a thick, heavy,
oily precipitate, which was separated by decantation. Complex 2c
obtained by this method (420 mg, 99%) is nearly pure by NMR data
but contains traces of CH2Cl2 and is a colorless, thick, heavy oil.
Complex 2c can be additionally dried under vacuum to give a white
semisolid powder: mp 54−59 °C.
Complex (CH2)2C(CO2Me)2·SnCl4 (2a). Method A. A solution of
SnCl4 (329 mg, 1.26 mmol) in CH2Cl2 (0.5 mL) was added dropwise
over 5 min under an argon atmosphere to a solution of cyclopropane 1
(200 mg, 1.26 mmol) in dry CH2Cl2 (2.5 mL), and the mixture was
stirred at room temperature for 10 min. Then the reaction mixture was
Anal. Calcd for C7H10Cl3GaO4: C, 25.16; H, 3.02. Found: C, 24.59;
H, 3.07. IR (KBr): ν 3009, 2957, 2855, 1739 br (CO), 1615, 1444,
1364, 1229, 1197, 1132 cm−1. 1H NMR (400.1 MHz, CD2Cl2): δ 2.65
(s, 4H, 2CH2, W1/2 = 75 Hz), 4.1 (s, 6H, 2OCH3, W1/2 = 60 Hz). 71Ga
I
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1
NMR (122.0 MHz, CD2Cl2): δ 252 (s, W1/2 = 2300 Hz). H NMR
(400.1 MHz, neat): δ 3.7 (s, 4H, 2CH2, W1/2 = 400 Hz), 5.1 (s, 6H,
2OCH3, W1/2 = 400 Hz). 13C NMR (100.6 MHz, neat): δ 30.5 (br.s,
C), 35.7 (br.s, 2CH2), 62.4 (br.s, 2OCH3), 183.4 (br.s, 2COO). 71Ga
NMR (122.0 MHz, neat): δ 252 (s, W1/2 = 2300 Hz). MS (m/z, %):
was added in one portion at 32 °C to a solution of styrylmalonate 10
(15 mg, 0.064 mmol) in dry CD2Cl2 (0.5 mL), and the mixture was
shaken for 0.5 min. Complex 7 (∼99%) was obtained as a solution in
CH2Cl2, which was nearly pure by NMR data.
Complex [PhCH2CH+C−(CO2Me)2·GaCl3]2 (8). The reaction was
carried out in an NMR tube under an argon atmosphere. The solution
was purged with argon flow before the reaction was started. Solid
GaCl3 (28 mg, 0.16 mmol, 1.2 equiv) was added in one portion at 32
°C to a solution of cyclopropane 4 (30 mg, 0.13 mmol) in dry CD2Cl2
(0.5 mL) and the mixture was shaken for 0.5 min. The necessary NMR
experiments were made immediately after synthesis of the gallium
176 (17, 69Ga35Cl237Cl+ and 71Ga35Cl3 ), 158 (3, C7H10O4 ), 141 (76,
+
+
+
69Ga35Cl37Cl+ and 71Ga35Cl2 ), 127 (41, C7H10O4 − OMe+), 106 (11,
69Ga37Cl+ and 71Ga35Cl+), 100 (21, C7H10O4 − CO2Me + H+), 59
(50), 59 (51), 50 (100), 44 (79), 36 (55), 29 (48).
Dimethyl 2-(2-Chloro-2-phenylethyl)malonate (6) and Its 2-
Deuterium Analogue. Method A. A solution of SnCl4 (221 mg, 0.85
mmol) in dry CH2Cl2 (0.5 mL) was added under an argon atmosphere
to a solution of cyclopropane 4 (200 mg, 0.85 mmol) in CH2Cl2 (5
mL), and the mixture was stirred at room temperature for 5 min to
give a pure solution of the Sn-enolate 5. Then an aqueous solution of
HCl (5%) (or DCl (5%) in D2O) was added until pH 3 was achieved
and the reaction mixture was extracted with CH2Cl2 (3 × 10 mL). The
organic layer was dried over MgSO4, and the solvent was removed in
vacuo to afford compound 6 (230 mg, 99%) as a colorless oil. 1H and
13C NMR spectra correspond to literature data.18
Method B. A solution of TiCl4 (161 mg, 0.85 mmol) in dry CH2Cl2
(0.5 mL) was added under an argon atmosphere to a solution of
cyclopropane 4 (200 mg, 0.85 mmol) in CH2Cl2 (5 mL), and the
mixture was stirred at room temperature for 5 min to give a pure
solution of the Ti-enolate 5. Then an aqueous solution of HCl (5%)
(or DCl (5%) in D2O) was added until pH 3 was achieved and the
reaction mixture was extracted with CH2Cl2 (3 × 10 mL). The organic
layer was dried over MgSO4, and the solvent was removed in vacuo to
afford compound 6 (227 mg, 98%) as a colorless oil. 1H and 13C NMR
spectra correspond to literature data.18
1
complex. For recording of 2D H DOSY NMR spectra with calibrant
for diffusion coefficient measurements squalane C30H62 (∼15 mg,
1
0.036 mmol) was added to the reaction mixture. According to the H
DOSY data the resulting solution contains mainly binuclear complex 8
with a small amounts of monomeric complex 7 and oligomeric
complexes. Complex 8 significantly decomposed in solution after
several hours at room temperature. When the solution was cooled, 8
rapidly oligomerized and decomposed; attempts to isolate this
complex failed.
1H NMR (400.1 MHz, CD2Cl2): δ 4.26 and 4.45 (both s, 2 × 3H,
2OCH3), 4.39 (d, 2H, CH2, 3J = 6.8 Hz), 7.24−7.45 (m, 5H, Ph), 8.84
3
(t, 1H, CH+, J = 6.8 Hz). 13C NMR (100.6 MHz, CD2Cl2): δ 42.2
(CH2), 61.2 and 61.6 (2OCH3), 116.8 (C−), 129.5 (p-CH), 130.5 and
130.8 (2 o-CH and 2 m-CH), 136.2 (i-C), 174.7 and 174.9 (2COO),
184.0 (CH+).
Oligomeric Complexes [PhCH2CH+C−(CO2Me)2·GaCl3]n. The
reaction was carried out in an NMR tube under an argon atmosphere.
The solution was purged with an argon flow before the reaction was
started. Solid GaCl3 (59 mg, 0.34 mmol, 1.2 equiv) was added in one
portion at 32 °C to a solution of cyclopropane 4 (65 mg, 0.28 mmol)
in dry CD2Cl2 (0.5 mL), and the mixture was shaken for 0.5 min. The
necessary NMR experiments were made immediately after synthesis of
Complex PhCH2CH+C−(CO2Me)2·GaCl3 (7). Method A (Prepa-
rative Synthesis). Solid GaCl3 (165 mg, 0.94 mmol, 1.1 equiv) was
added in one portion under an argon atmosphere at 5 °C with stirring
to a solution of cyclopropane 4 (200 mg, 0.85 mmol) in CH2Cl2 (6
mL), and the mixture was stirred at the same temperature for 1 min.
Complex 7 (∼99%) was obtained as a solution in CH2Cl2, which is
nearly pure by NMR data. The prepared solution of complex 7 was
immediately used for further syntheses.
Method B (NMR Studies). The reaction was carried out in NMR
tube under an argon atmosphere. The solution was purged with an
argon flow before the reaction was started. Solid GaCl3 (14 mg, 0.077
mmol, 1.2 equiv) was added in one portion at 32 °C to a solution of
cyclopropane 4 (15 mg, 0.064 mmol) in dry CD2Cl2 (0.5 mL), and the
mixture was shaken for 0.5 min. The nearly pure complex 7 (∼99%)
was obtained as a solution in CD2Cl2. The necessary NMR
experiments were made immediately after synthesis of gallium
1
the gallium complex. For recording of 2D H DOSY NMR spectra
with calibrant for diffusion coefficient measurements squalane C30H62
(∼15 mg, 0.036 mmol) was added to the reaction mixture. According
to the 1H DOSY data the resulting solution contains mainly oligomeric
gallium complexes with small amounts of binuclear complex 8. The
oligomeric complex significantly decomposed in solution after several
hours at room temperature. When the solution was cooled, the
oligomer rapidly decomposed; attempts to isolate this complex failed.
1H NMR (400.1 MHz, CD2Cl2): δ 4.35 (br s, 6H, 2OCH3), 4.35
(br m, 2H, CH2), 7.10−7.40 (br m, 5H, Ph), 8.80 (br m, 1H, CH+).
Complex PhCH2CH+C−(CO2Me)2·Ga2Cl6 (11). The reaction was
carried out in an NMR tube under an argon atmosphere. The solution
was purged with an argon flow before the reaction was started. Solid
GaCl3 (75 mg, 0.44 mmol, 4 equiv) was added in one portion at 32 °C
to a solution of cyclopropane 4 (25 mg, 0.11 mmol) in dry CD2Cl2
(0.5 mL), and the mixture was shaken for 0.5 min. The necessary
NMR experiments were carried out immediately after synthesis of the
gallium complex. According to the 1H DOSY data the resulting
solution contains a mixture of binuclear complex 11 with mononuclear
complex 7 in about an equimolar ratio. The binuclear complex 11 is
much less stable than complex 7 and completely decomposes in
solution in less than 0.5 h, while complex 7 remains in solution.
1H NMR (400.1 MHz, CD2Cl2): δ 4.38 and 4.53 (both s, 2 × 3H,
1
complex. For recording of 2D H DOSY NMR spectra with calibrant
for diffusion coefficient measurements squalane C30H62 (∼15 mg,
0.036 mmol) was added to the reaction mixture. Complex 7
significantly decomposes in solution after several hours at room
temperature. When the solution was cooled, 7 rapidly oligomerized
and decomposed; attempts to isolate this complex failed.
1H NMR (400.1 MHz, CD2Cl2): δ 4.42 and 4.58 (both s, 2 × 3H,
2OCH3), 4.43 (d, 2H, CH2, 3J = 6.9 Hz), 7.30 (m, 2H, 2 o-CH), 7.42
(m, 1H, p-CH), 7.46 (m, 2H, 2 m-CH), 9.00 (t, 1H, CH+, 3J = 6.9 Hz).
13C NMR (100.6 MHz, CD2Cl2): δ 41.9 (CH2), 61.7 and 61.9
(2OCH3), 115.1 (C−), 129.0 (p-CH), 129.5 and 130.2 (2 o-CH and 2
m-CH), 134.1 (i-C), 174.1 and 174.3 (2COO), 186.1 (CH+). 35Cl
NMR (39.2 MHz, CD2Cl2): δ −221 (s, W1/2 = 6500 Hz). 71Ga NMR
(122.0 MHz, CD2Cl2): δ 282 (s, W1/2 = 2300 Hz).
3
2OCH3), 4.37 (br d, 2H, CH2, J ≈ 6.9 Hz), 7.10−7.50 (br m, 5H,
3
Ph), 8.97 (br t, 1H, CH+, J ≈ 6.9 Hz). 13C NMR (100.6 MHz,
CD2Cl2): δ 41.5 (CH2), 63.4 and 63.6 (2OCH3), 110.8 (C−), 132.5
and 133.5 (5CH, Ph), 142.4 (i-C), 174.5 (2COO), 182.4 (CH+).
Complex PhCH2CH+C−(CO2Me)2·GaCl3·THF (12). The reaction
was carried out in an NMR tube under an argon atmosphere. The
solution was purged with an argon flow before the reaction was
started. Solid GaCl3 (30 mg, 0.17 mmol, 1.5 equiv) was added in one
portion at 32 °C to a solution of cyclopropane 4 (25 mg, 0.11 mmol)
in dry CD2Cl2 (0.4 mL); after that a solution of THF (8 mg, 0.11
mmol) in CD2Cl2 (0.1 mL) was added and the mixture was shaken.
The necessary NMR experiments were carried out immediately after
synthesis of the gallium complex. According to the NMR data the
resulting solution contains complex 12 with products of destruction.
Method C. The reaction was carried out in an NMR tube under an
argon atmosphere. The solution was purged with argon flow before the
reaction was started. Solid GaCl3 (14 mg, 0.077 mmol, 1.2 equiv) was
added in one portion at 32 °C to a solution of benzylidenemalonate 9
(15 mg, 0.064 mmol) in dry CD2Cl2 (0.5 mL), and the mixture was
shaken for 0.5 min. Complex 7 (∼99%) was obtained as a solution in
CH2Cl2, which was nearly pure by NMR data.
Method D. The reaction was carried out in an NMR tube under an
argon atmosphere. The solution was purged with an argon flow before
the reaction was started. Solid GaCl3 (14 mg, 0.077 mmol, 1.2 equiv)
J
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Complex 12 is highly unstable and completely decomposes in solution
in less than 10 min.
20:1) to afford compound 15 (165 mg, 83%) as a colorless oil (∼2:1
mixture of two diastereomers). The product obtained can be
additionally separated on a Silufol chromatographic plate (20 × 20
cm) with benzene−EtOAc (10:1) as eluent to afford the pure isomers.
1H and 13C NMR spectra correspond to the literature data.7
1H NMR (400.1 MHz, CD2Cl2): δ 2.17 (m, 4H, CH2, THF), 4.27
3
(br d, 2H, CH2, J ≈ 7.1 Hz), 4.36 (m, 4H, CH2O, THF), 8.52 (br t,
1H, CH+, 3J ≈ 7.1 Hz), other signals overlap with the signals of
products of destruction. 13C NMR (100.6 MHz, CD2Cl2): δ 26.7
(CH2, THF), 41.3 (CH2), 75.1 (CH2O, THF), 178.5 (CH+), other
signals overlap with the signals of products of destruction.
(2R*,3R*,4R*)-Dimethyl 2-Benzyl-4-(1,3-dimethoxy-1,3-di-
oxopropan-2-yl)-3-phenylcyclobutane-1,1-dicarboxylate (16).
Solid GaCl3 (83 mg, 0.47 mmol, 1.1 equiv) was added in one portion
under an argon atmosphere at 0−5 °C with stirring to a solution of
cyclopropane 4 (100 mg, 0.43 mmol) in dry CH2Cl2 (3 mL), and the
mixture was stirred at the same temperature for 1 min. The nearly pure
complex 7 (∼99%) was obtained as a solution in CH2Cl2. A prepared
solution of complex 7 was heated at 25−30 °C for 3 h. After that, an
aqueous solution of HCl (5%) was added until pH 3 was achieved and
the reaction mixture was extracted with dichloromethane (3 × 10 mL).
The organic layer was dried over MgSO4, and the solvent was removed
in vacuo. The residue was purified by column chromatography on
silica gel (benzene−EtOAc, 20:1) to afford a mixture (45 mg, 45%) of
compound 15 (∼30% by NMR data, about 2:1 mixture of two
diastereomers) and compound 16 (∼15% by NMR data, single
diastereomer) as a colorless oil; attempts to separate this mixture by
chromatography failed. IR (CHCl3): ν 3055, 2988, 2955, 1734 br
(CO), 1601, 1550, 1496, 1436, 1423 cm−1. MS (m/z, %): 468 (3,
M+), 437 (1, M+−OCH3), 376 (1), 336 (28), 276 (35), 245 (14), 217
(28), 203 (21), 171 (21), 145 (24), 115 (908), 91 (60), 77 (33), 59
(100), 51 (24), 39 (14). HRMS calcd for C26H28O8: M + H, 469.1857;
M + Na, 491.1676; M + K, 507.1416. Found: m/z 469.1649, 491.1672,
Complex PhCH2CH+C−(CO2Me)2·GaCl3·S8 (13). The reaction
was carried out in an NMR tube under an argon atmosphere. The
solution was purged with an argon flow before the reaction was
started. Solid GaCl3 (29 mg, 0.17 mmol, 1.5 equiv) was added in one
portion at 32 °C to a solution of cyclopropane 4 (25 mg, 0.11 mmol)
and molecular sulfur (28 mg, 0.11 mmol) in dry CDCl3 (0.5 mL), and
the mixture was shaken for 0.5 min. The necessary NMR experiments
were carried out immediately after synthesis of the gallium complex.
According to the NMR data the resulting solution contains a mixture
of complex 13 with complex 7 in about an equimolar ratio. Conversion
is not complete because of the poor solubility of sulfur in chloroform.
Complex 13 significantly decomposes in solution after several hours at
room temperature.
1H NMR (300.1 MHz, CD2Cl2): δ 4.62 (d, 2H, CH2, 3J = 6.8 Hz),
8.88 (t, 1H, CH+, 3J = 6.8 Hz), other signals overlap with the signals of
products of destruction and signals of complex 7.
Complexes PhCH2CH+C−(CO2Me)2·GaCl3·Py (14a) and
PhCH2CH(Py+)C−(CO2Me)2·GaCl3 (14b). The reaction was carried
out in an NMR tube under an argon atmosphere. The solution was
purged with an argon flow before the reaction was started. Solid GaCl3
(30 mg, 0.17 mmol, 1.5 equiv) was added in one portion at 32 °C to a
solution of cyclopropane 4 (25 mg, 0.11 mmol) in dry CD2Cl2 (0.4
mL); after that a solution of pyridine (9 mg, 0.11 mmol) in CD2Cl2
(0.1 mL) was added and the mixture was shaken. The necessary NMR
experiments were carried out immediately after synthesis of the
gallium complex. According to the NMR data the resulting solution
contains complexes 14a and 14b in about a 3:1 molar ratio with
products of destruction. Complexes 14 are highly unstable and
completely decompose in solution in less than 15 min.
1
507.1422. Data for compound 16 are as follows. H NMR (400.1
3
MHz, CDCl3): δ 2.56 (ddd, 1H, CH(2), J = 11.0, 10.0, and 4.5 Hz),
2.80 (dd, 1H, CH2(a), 2J = 16.5, 3J = 4.5 Hz), 3.09 (dd, 1H, CH(3), 3J
2
3
= 11.0 and 7.0 Hz), 3.10 (dd, 1H, CH2(b), J = 16.5, J = 10.0 Hz),
3
3.33 (dd, 1H, CH(4), J = 7.0 and 3.0 Hz), 3.51, 3.79, 3.84, and 3.86
(all s, 4 × 3H, 4OCH3), 3.92 (d, 1H, CH(2′), 3J = 3.0 Hz), 7.05−7.23
(m, 10H, 2 Ph). 13C NMR (100.6 MHz, CDCl3): δ 29.0 (CH2), 43.9
(CH(4)), 44.3 (CH(2′)), 52.1, 52.2, 52.6, and 53.1 (4OCH3), 51.9
(CH(2)), 55.2 (CH(3)), 58.5 (C(1)), 126.4 and 127.3 (2 p-C), 127.4,
127.8, 128.7, and 128.9 (2 × 2 o-C and 2 × 2 m-C), 141.5 and 141.6 (2
i-C), 168.7, 168.8, 169.0, and 172.2 (4COO).
Data for complex 14a are as follows. 1H NMR (400.1 MHz,
CD2Cl2): δ 4.15 and 4.34 (both s, 2 × 3H, 2OCH3), 4.36 (d, 2H, CH2,
3
3J = 6.7 Hz), 7.05−7.45 (m, 5H, Ph), 8.13 (br dd, 2H, H(3), Py, J =
ASSOCIATED CONTENT
* Supporting Information
■
7.9 and 5.3 Hz), 8.66 (tt, 1H, H(4), Py, 3J = 7.9 Hz, 3J = 1.4 Hz), 8.69
S
(t, 1H, CH+, 3J = 6.7 Hz), 8.78 (br d, 2H, H(2), Py, 3J = 5.3 Hz). 13
C
Figures, tables, and a CIF file giving NMR spectra for all new
compounds and crystallographic data and refinement parame-
ters of complex 2a. This material is available free of charge via
the Internet at http://pubs.acs.org.
(100.6 MHz, CD2Cl2): δ 39.6 (CH2), 58.7 and 59.0 (2OCH3), 124.9−
131.6 (5CH, Ph), 127.9 (2 CH(3), Py), 140.5 (2 CH(2), Py), 148.1
(CH(4), Py), 171.8 and 172.5 (2COO), 178.3 (CH+), other signals
could not be detected due to the instability of the complex.
Data for complex 14b are as follows. 1H NMR (400.1 MHz,
CD2Cl2): δ 3.69 (d, 2H, CH2, 3J = 7.6 Hz), 5.93 (t, 1H, CHN, 3J = 7.6
AUTHOR INFORMATION
Corresponding Author
*Tel/fax: +7 499 135 6390. E-mail: tom@ioc.ac.ru.
■
3
Hz), 7.05−7.45 (m, 5H, Ph), 7.98 (br dd, 2H, H(3), Py, J = 7.0 and
6.4 Hz), 8.39 (br t, 1H, H(4), Py, 3J = 7.0 Hz), 8.62 (br d, 2H, H(2),
Py, 3J = 6.4 Hz), other signals overlap with signals of complex 14a and
products of destruction. 13C (100.6 MHz, CD2Cl2): δ 39.4 (CH2),
71.8 (CHN), 128.4 (2 CH(3), Py), 142.5 (2 CH(2), Py), 148.2
(CH(4), Py), other signals could not be detected due to the instability
of the complex.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(2R*,3R*)-Dimethyl 2-(1,3-Dimethoxy-1,3-dioxopropan-2-
yl)-3,4-diphenylcyclopentane-1,1-dicarboxylate (15). Solid
GaCl3 (83 mg, 0.47 mmol, 1.1 equiv) was added in one portion
under an argon atmosphere at 0−5 °C with stirring to a solution of
cyclopropane 4 (100 mg, 0.43 mmol) in dry CH2Cl2 (3 mL), and the
mixture was stirred at the same temperature for 1 min. The nearly pure
complex 7 (∼99%) was obtained as a solution in CH2Cl2. A solution
of cyclopropane 4 (100 mg, 0.43 mmol, 1 equiv) in dry CH2Cl2 (3
mL) was immediately added to a prepared solution of complex 7 at 0−
5 °C, and the reaction mixture was stirred at the same temperature for
30 min. After that an aqueous solution of HCl (5%) was added until
pH 3 was achieved and the reaction mixture was extracted with
dichloromethane (3 × 10 mL). The organic layer was dried over
MgSO4, and the solvent was removed in vacuo. The residue was
purified by column chromatography on silica gel (benzene−EtOAc,
This study was financially supported by the Russian Federation
President Council for Grants (Program for the State Support of
Leading Scientific Schools of RF, Grant NSh-604.2012.3) and
by the Division of Chemistry, Materials Science of the Russian
Academy of Sciences (Programme for Basic Research
“Theoretical and Experimental Study of the Nature of Chemical
Bond and Mechanisms of the Most Important Reactions and
Processes”) and the Program “Molecular and cell Biology” of
the Presidium of the Russian Academy of Sciences.
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dx.doi.org/10.1021/om301072v | Organometallics XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/om301072v | Organometallics XXXX, XXX, XXX−XXX