Low-coordinate germanium(II) centers within distorted axially chiral seven-membered chelates: Stereo- and enantioselective cycloadditions

Article abstract of DOI:10.1002/anie.201201566

Distorted germylene: A distorted seven-membered N-heterocyclic germylene, which has a germanium center of relatively high Lewis acidity, reacts with benzaldehyde to give (2+1+1) or (2+2+1) cycloadducts. The type of cycloadduct produced in the reaction is dependent on the ratio of substrates used. It is considered that the reactions involve stereoselective 1,3-dipolar cycloadditions of an in situ generated carbonyl germaylide. Copyright

Full text of DOI:10.1002/anie.201201566

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DOI: 10.1002/anie.201201566
Main Group Chemistry
Low-Coordinate Germanium(II) Centers Within Distorted Axially
Chiral Seven-Membered Chelates: Stereo- and Enantioselective
Cycloadditions**
Hidekazu Arii, Taiyo Amari, Junji Kobayashi, Kunio Mochida,* and Takayuki Kawashima
The heavier group 14 analogues of carbenes (R2ED) have
attractive structures and interesting reactivity toward unsa-
turated compounds.[1] R2ED compounds have two covalent
bonds with high p character, a single electron lone pair with
high s character, and a single vacant p orbital in the singlet
ground state. The intrinsic ambiphilic nature of R2ED com-
pounds allows their participation in a variety of cycloaddition
reactions with compounds containing unsaturated bonds.[2,3]
(2 + 2 + 1) Cycloaddition reactions of divalent group 14 ele-
ment species provide one of the most interesting ways for
synthesizing a heterocycle in one step but have been limited
to a number of transient silylenes and [Si(Cp*)2] (Cp* =
Scheme 1. Two-coordinate N-heterocyclic germylenes.
pentamethylcyclopentadienyl).[4,5] The reactions of these
silylenes with a ketone or a aldehyde afforded the corre-
sponding 1:2 cycloadducts, although different reaction inter-
mediates were proposed for the used silylenes. In both cases,
the high Lewis acidity of the group 14 element atom was
believed to be an essential factor in promoting the formation
of these intermediates.
however, their reactivity was not explored. On the other hand,
seven-membered N-heterocyclic carbenes, such as 1,3-disub-
stituted 1,3-diazacycloheptane-2-ylidenes and dibenzo-1,3-
diazaazepine-2-ylidenes, have been reported.[12] Palladium
complexes containing these carbenes were reported to be
very useful catalysts for Mizoroki–Heck coupling reac-
tions.[12d] However, there has been no discussion on the
Lewis acidity of the divalent carbon centers, although there
has been discussion about the influence of ring size and steric
hindrance of the carbenes on catalytic performance. Herein,
we describe the synthesis and characterization of [GeL] (1)
and a stereoselective cycloaddition reaction of an in situ
generated carbonyl germaylide derived from the reaction of
1 with benzaldehyde.
Since the appearance of a report on diaminogermylene by
Harris and Lappert,[6] most germylenes have been synthesized
by taking advantage of the thermodynamic stabilization
achieved through their binding with p-donor atoms
(Scheme 1); in this stabilized form, their Lewis acidity is
presumed to be relatively low.[7–9] We envisioned that a seven-
membered N-heterocyclic germylene containing an
N,N’-bis(trimethylsilyl)-2,2’-diamino-6,6’-dimethylbiphenyl
(LH2) unit would engender a germanium center with rela-
tively high Lewis acidity. We expected that the seven-
membered chelate would exhibit significantly lower p dona-
tion from the nitrogen atoms to the vacant p orbitals of the
germanium center than planar five- or six-membered chelates
because of the twisted conformation of the seven-membered
chelate. Indeed, a number of N- and O-heterocyclic germy-
lenes of larger ring size form adducts with Lewis donors,[10,11]
The reaction of dilithio derivative [LLi2] and
GeCl2·dioxane in THF at À808C under Ar afforded the
corresponding germylene 1 as yellow crystals in a 19% yield
upon isolation. The crystal structure of 1 (Figure 1)[13]
revealed that a twofold rotation axis exists through the
À
germanium center and the midpoint of the C(1) C(1)’ bond
and that the distance between germanium atoms on neigh-
boring molecules is approximately 6.6 ꢀ, thus indicating that
there are no intermolecular Ge···Ge interactions (Figure 1).
The divalent germanium species has a bent geometry, thus
indicating the presence of a vacant p orbital and a lone pair of
[*] Dr. H. Arii, T. Amari, Prof. Dr. K. Mochida, Prof. Dr. T. Kawashima
Department of Chemistry, Gakushuin University
1-5-1 Mejiro, Toshima-ku, 171-8588 Tokyo (Japan)
E-mail: kunio.mochida@gakushuin.ac.jp
À
electrons on the germanium center. The Ge N bonds
(1.8651(13) ꢀ) of 1 are slightly longer than those of the
previously reported planar five-membered N-heterocyclic
germylenes; this difference can be attributed to the twisted
geometry of the seven-membered chelate, wherein there is
weak p donation from the nitrogen atoms to the germanium
center. The large N-Ge-N bond angle (98.13(8)8) indicates
Prof. Dr. J. Kobayashi
Department of Material Science, International Christian University
3-10-2 Osawa, Mitaka-shi, 181-8585 Tokyo (Japan)
[**] This work was financially supported by a Grant-in-Aid for Scientific
Research on Priority Areas from the Ministry of Education, Culture,
Sports, Science and Technology (Japan).
À
that the the Ge N bond has higher s character and thus a lone
pair of higher p character than that of typical non-distorted
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201566.
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Figure 2. Crystal structure of 2. Thermal ellipsoids are shown at 50%
probability and hydrogen atoms are omitted for clarity.
Figure 1. Crystal structure of 1. Thermal ellipsoids are shown at 50%
probability and hydrogen atoms are omitted for clarity.
1H NMR spectrum showed that 2 had completely transformed
into 1 and free benzaldehyde in a molar ratio of 2:1.
Interestingly, this temperature-dependent spectral change
was reversible between À40 and 208C, thus indicating that 2
is in equilibrium with two mole equivalents of 1 and one
mole equivalent of benzaldehyde. The thermodynamic
parameters of the equilibrium were DH = À114(5) kJmolÀ1
and DS = À302(17) JKÀ1 molÀ1, as estimated using the
vanꢁt Hoff equation (see the Supporting Information,
Figure S5).[15] At high temperatures, the large negative
entropy of formation governs the equilibrium and 1 and
benzaldehyde predominate. The report on an N-heterocyclic
silylene by Lappert and co-workers suggests that it is unlikely
that a digermene, which could form by a dimerization of 1, is
an intermediate in the reaction pathway toward 2.[16] We
believe that 1 reacts with benzaldehyde to give an adduct, that
is, carbonyl germaylide I-1 or 1,2-oxagermirane I-2; either of
these potential intermediates could then react rapidly with
a second equivalent of 1 to afford 2. These results prompted
us to investigate the reaction in the presence of excess
benzaldehyde to determine whether benzaldehyde could
compete with 1 as a reactant in the further transformation
of the potential intermediates I-1 and I-2. The reaction of
1 with three equivalents of benzaldehyde at 608C in benzene
gave the expected adduct derived from a (2 + 2 + 1) cyclo-
addition, 1,3,4-dioxagermolane 3, in a yield of 88%, as
determined by NMR experiments (Scheme 3).[17] The
1H NMR spectrum of the reaction mixture showed that
1 was completely consumed resulting in the formation of one
major product together with several minor products. The
major product 3 crystallized from hexane solution and
colorless crystals were obtained in a yield of 27% upon
isolation. Compound 3 crystallized as the racemate (M,R,S)-
3/(P,S,R)-3, in which the two phenyl groups are cis to each
other (Figure 3).[13] It is considered that 3 forms through
a regio- and stereoselective cycloaddition via a 1,3-dipolar
intermediate, that is, a carbonyl germaylide I-1. The 1H NMR
spectrum of a solution of 3 in [D6]benzene showed two
singlets at 5.16 and 6.24 ppm, which were assigned to the
aliphatic methine protons. In the 13C NMR spectrum of 3, the
aliphatic methine carbon nuclei resonated at 73.9 and
100.2 ppm. In contrast to the equilibrium associated with 2,
germylenes. The dihedral angle of the biphenyl backbone
(approximately 628) is similar to that of the binaphthyl moiety
in the previously reported O-substituted germylene, despite
the steric repulsion between the methyl groups on 6 and
6’ positions of the biphenyl moiety.[10a] In accordance with the
1
results of the X-ray diffraction (XRD) analysis, the H and
13C NMR spectra of a solution of 1 in C6D6 showed two
equivalent sets of signals that are characteristic of a C2-
symmetric molecule.
Treatment of germylene 1 with one equivalent of benzal-
dehyde in toluene at À308C gave a (2 + 1 + 1) cycloadduct,
1,2,3-oxadigermetane 2 (Scheme 2), which has one stereo-
genic carbon atom and two axes of chirality. The addition of
Scheme 2. Equilibrium reaction involving 1 (two mole equivalents),
benzaldehyde, and 2.
excess benzaldehyde was required to shift the equilibrium to
the formation of 2. Among the four racemic forms that are
possible, 2 crystalized as a single racemate (M,M,R)-2/(P,P,S)-
2, wherein the two biphenyl moieties of each molecule have
the same absolute configurations (Figure 2).[13] Each of the
germanium centers has a tetrahedral geometry, and the length
À
À
of the Ge(1) Ge(2) and C(47) O(1) bonds in the four-
membered ring were 2.4613(13) and 1.431(8) ꢀ, respectively,
both of which correspond to single bonds.[14] The H NMR
1
spectrum of a solution of 2 in [D8]toluene at À408C, indicated
that the structure of 2 in the crystal form was maintained in
solution. The NMR signal associated with the methine proton
was observed at 5.95 ppm as only one singlet, thus ruling out
the presence of other diastereomers. However, when the
solution of 2 in [D8]toluene was warmed to 408C, the resulting
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of 59% and 15%, respectively, as determined by NMR
spectroscopy (Scheme 3). The use of three equivalents of tert-
butyl isocyanide was sufficient for preventing the competing
reaction that yields 3. The diastereoselectivity for the cyclo-
addition of 4 was slightly lower than that of 3, with respect to
a three-component system . The isolated crystals of adduct 4
were composed of only the racemate (M,R)-4/(P,S)-4, as
determined by XRD (see the Supporting Information,
Figure S1)[13] and NMR spectroscopy (see the Supporting
Information, Figure S7).
Optically active (M)-1, which was prepared by using the
same method as that used to prepare racemic 1, crystalizes in
the asymmetric space group P212121.[13] The distance between
neighboring germanium centers Ge···Ge in (M)-1 is
2.6102(5) ꢀ, which is much shorter than that of racemic 1,
À
but significantly longer than that of typical Ge Ge single
bonds (2.42–2.45 ꢀ);[19] this difference probably has its origin
in the crystal-packing forces in the crystal. The 1H NMR
spectrum of (M)-1 is identical to that of racemic 1, thus
indicating that (M)-1 behaves as a monomer in solution. The
reaction of optically active germylene (M)-1 with benzalde-
hyde afforded the optically active (M,R,S)-3 in a yield of 78%
together with traces of other diastereomers as determined by
NMR spectroscopy (Scheme 4 and see the Supporting
Information, Figure S8). Thus, it was shown that (M)-1 can
be used as a chiral source in the enantiospecific cycloaddition
to give the heterocycle.
Scheme 3. Cycloaddition involving a 1,3 dipole derived from 1.
Figure 3. Crystal structure of 3. Thermal ellipsoids are shown at 50%
probability and hydrogen atoms are omitted for clarity.
Scheme 4. Enantiospecific cycloaddition of (M)-1.
the formation of 3 was exergonic. To our knowledge, a
(2+2+1) cycloaddition involving a germylene and a carbonyl
compound is unprecedented, although Banaszak Holl et al.
reported that germylene [Ge{CH(SiMe3)2}2] reacts with
benzophenone to give an adduct derived from a concerted
(4 + 1) cycloaddition.[3b] In the reaction with carbonyl com-
pounds, when a transient silylene was generated in the
presence of AgI, which is thought to act as an electrophile by
the authors, the (2 + 2 + 1) cycloadducts were formed through
a regio- and stereoselective cycloaddition via a carbonyl
silaylide. On the other hand, when isolable [Si(Cp*)2] and
silylenoides, which are thought to act as nucleophiles, were
In conclusion, we designed and synthesized a divalent N-
heterocyclic germylene 1 representing a seven-membered
chelate containing an axially chiral biphenyl backbone. In situ
generation of a 1,3 dipole, that is, a carbonyl germaylide, by
electrophilic attack of 1 on benzaldehyde, followed by its
stereoselective cycloaddition with benzaldehyde or tert-butyl
isocyanide, gave the corresponding cycloadducts 3 and 4.
(M,R,S)-3 was obtained by the diastereoselective cycloaddi-
tion involving enantiomerically pure (M)-1. The incorpora-
tion of a germylene moiety within a seven-membered chelate
was demonstrated to be very effective way of engendering
a germanium center of high Lewis acidity, thus allowing its
involvement in several types of cycloadditions.
À
used, the (2 + 2 + 1) cycloadducts involving C C bond
formation were obtained via oxasirilanes.[4d,5] Taking the
relatively high Lewis acidity of germylene 1 into consider-
ation,[18] it is most likely that an intermediate is the carbonyl
germaylide I-1, which could form through the electrophilic
attack of 1 on the carbonyl oxygen atom.
Received: February 27, 2012
Revised: April 13, 2012
Published online: June 5, 2012
When benzaldehyde and tert-butyl isocyanide were added
to a benzene solution of 1 at 608C, a (2 + 1 + 1) cycloadduct,
3-imino-1,2-oxagermetane 4, was obtained as a mixture of two
diastereomers, (M,R)-4/(P,S)-4 and (M,S)-4/(P,R)-4, in yields
Keywords: chirality · cycloaddition · germanium · heterocycles ·
ylides
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