Synthesis of a digermane-containing tricylic nonadecadienedione incorporating an equivalent of ring-opened THF

Article abstract of DOI:10.1021/ic0491027

The combination of 2 equiv of bis[bis(trimethylsilyl)amide]germylene (5) with 2 equiv of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in tetrahydrofuran (THF) results in the ring-opening of 1 equiv of THF to form 2,2,8,8-tetrakis-(1, 1,1,3,3,3-hexamethyl-disilazan-2-yl)-5,16-diphenyl-7,9,14-trioxa-1,3,5,16,18, 19-hexaaza-2,8-digerma-tricyclo[13.2.1.13,6]nonadeca-6(19),15(18)-diene-4, 17-dione (6). This fast and nearly quantitative reaction builds a 15-membered ring from five different molecules. The new ring, structurally assigned by X-ray crystallography, contains a flexible methylene chain that moves rapidly on the NMR time scale.

Full text of DOI:10.1021/ic0491027

Inorg. Chem. 2004, 43, 7665−7670
Synthesis of a Digermane-Containing Tricylic Nonadecadienedione
Incorporating an Equivalent of Ring-Opened THF
Amy C. Gottfried, Jia Wang, Erin E. Wilson, Larry W. Beck, Mark M. Banaszak Holl,* and
Jeff W. Kampf
Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue,
Ann Arbor, Michigan 48109-1055
Received July 8, 2004
The combination of 2 equiv of bis[bis(trimethylsilyl)amide]germylene (5) with 2 equiv of 4-phenyl-1,2,4-triazoline-
3,5-dione (PTAD) in tetrahydrofuran (THF) results in the ring-opening of 1 equiv of THF to form 2,2,8,8-tetrakis-
(1,1,1,3,3,3-hexamethyl-disilazan-2-yl)-5,16-diphenyl-7,9,14-trioxa-1,3,5,16,18,19-hexaaza-2,8-di-
germa-tricyclo[13.2.1.13,6]nonadeca-6(19),15(18)-diene-4,17-dione (6). This fast and nearly quantitative reaction
builds a 15-membered ring from five different molecules. The new ring, structurally assigned by X-ray crystallography,
contains a flexible methylene chain that moves rapidly on the NMR time scale.
Introduction
Germylenes (the germanium analogues of carbenes and
silylenes) are of increasing interest for their roles in synthetic
chemistry. Like their group 14 analogues, germylenes can
be synthesized with bulky substituents to prevent self-
polymerization and some variants may even be isolated and
stored in an air- and water-free environment.1 Germylenes
have been shown to insert into carbon-heteroatom σ-bonds
and to add to unsaturated systems such as alkenes, alkynes,
allenes, and many of their hetero analogues.1-3 More
recently, our group has shown the use of stable germylenes
for insertion into the R-C-H bond of nitriles4 and ketones5
and for C-H activation of ethers and alkanes.6
hydrogenated8 (eq 1). This reactivity prompted the hypothesis
that trienes such as 2 contain reactive double bonds amenable
to other transformations. The Diels-Alder reaction was
chosen on the basis of the pair of double bonds locked in
the s-cis conformation in triene 2 and appeared promising
on the basis of Ando’s successful Diels-Alder [4 + 2]
cycloaddition of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD)
and the germane 4 (the product of the addition of Ge[CH-
(SiMe3)2]2 (1) to 2,5-dimethyl-2,3,4-hexatriene) (eq 2).9
However, if the Diels-Alder reaction is not substantially
faster than the back reaction to regenerate 1 and benzophe-
none, the germylene may react with PTAD. Ando demon-
strated that germylenes such as 1 undergo [2 + 1] cycload-
ditions with double bonds in N-phenylmalimide (Figure 1a).9
We have also shown that germylenes activate phenones
through conversion to conjugated trienes.7 This equilibrium
1
lies far to the right such that only 2 is observed in both H
NMR and UV-vis spectroscopy.7 The conjugated triene
product of benzophenone activation (2) can be catalytically
* Author to whom correspondence should be addressed. E-mail:
mbanasza@umich.edu.
(1) Neumann, W. P. Chem. ReV. 1991, 91, 311-334.
(2) Barrau, J.; Rima, G. Coord. Chem. ReV. 1998, 178-180, 593-622.
(3) Satge, J. Pure Appl. Chem. 1984, 56, 137-150.
(4) Miller, K. A.; Watson, T. W.; Bender, J. E. I.; Banaszak Holl, M. M.;
Kampf, J. W. J. Am. Chem. Soc. 2001, 123, 982-983.
(5) Sweeder, R. D.; Miller, K. A.; Edwards, F. A.; Wang, J.; Banaszak
Holl, M. M.; Kampf, J. W. Organometallics 2003, 22, 5054-5062.
(6) Miller, K. A.; Bartolin, J. M.; O’Neill, R. M.; Sweeder, R. D.; Owens,
T. M.; Kampf, J. W.; Banaszak Holl, M. M.; Wells, N. J. Am. Chem.
Soc. 2003, 125, 8986-8987.
(8) Sweeder, R. D.; Cygan, Z. T.; Banaszak Holl, M. M.; Kampf, J. W.
Organometallics 2003, 22, 4613-4615.
(9) Ando, W.; Ohgaki, H.; Kabe, Y. Angew. Chem., Int. Ed. Engl. 1994,
33, 659-661.
(7) Sweeder, R. D.; Edwards, F. A.; Miller, K. A.; Banaszak Holl, M.
M.; Kampf, J. W. Organometallics 2002, 21, 457-459.
10.1021/ic0491027 CCC: $27.50
Published on Web 11/02/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 24, 2004 7665

Gottfried et al.
solution, causing its red color to lighten. The reaction solution turned
colorless upon completion of addition. This colorless solution was
stirred for 10 min, and then volatiles were removed in vacuo,
leaving a slightly yellow film. Two washings (5 mL each) with
acetonitrile led to 0.342 g of a fine white powder of analytically
pure product (77% yield). 1H NMR (C6D6, conformer A, δ): 4.67
(2 H, t, OCH2CH2), 4.58 (2 H, t, OCH2CH2), 0.49 (36 H, s, Si-
1
(CH3)3), 0.35 (36H, s, Si(CH3)3). H NMR (C6D6, conformer B,
3
δ): 4.42 (1.6 H, t, JHH ) 5.6 Hz, OCH2CH2), 4.28 (1.6 H, t,
3JHH ) 6.4 Hz, OCH2CH2), 0.60 (28.8 H, s, Si(CH3)3), 0.23 (28.8
1
H, s, Si(CH3)3), 1.65 (1.6 H, m, OCH2CH2, by COSY). H NMR
Figure 1. Known reactions of germylenes with functional groups
analogous to PTAD.
3
(C6D6, conformers A and B, δ): 7.64 (3.6 H, m, JHH ) 7.9 Hz),
3
3
7.52 (7.2 H, m, JHH ) 8.6, Hz), 7.46 (3.6 H, m, JHH ) 7.8 Hz),
7.15-6.90 (12 H, m), 1.79 (5.6 H, m OCH2CH2). 13C NMR of
powder 6 (C6D6, conformer A by HSQC, δ): 68.70, 63.67, (OCH2)
28.47, 27.48 (OCH2CH2). 13C NMR of powder 6 (C6D6, conformer
B by HSQC, δ): 68.24, 62.13 (OCH2); 25.55, 25.31 (OCH2CH2).
13C NMR of powder 6 (C6D6, conformers A and B, δ): 155.84,
155.80, 149.58, 149.29 (NdC(O)sN) and (NC(O)N), 133.57,
133.38, 132.84, 129.34, 129.24, 129.16, 129.06, 128.86, 125.72,
125.58 (aromatic), 6.096, 5.595, 5.595, 5.231 (Si(CH3)3). Solid-
state 13C NMR of powder 6 (δ): 155.8, 151.0, 149.1, 133.5, 128.5,
65.5, 61.5, 27.0, 24.9, 5.7. Solid-state 13C NMR of crystal 6 (δ):
155.9, 151.0, 149.0, 132.8, 130.7, 129.5, 128.4, 126.5, 65.7, 61.7,
27.1, 24.8, 7.4, 6.4, 5.7. IR (cm-1): ν 1713 (CdO), 1614, 1597
(CdN), 1252 (CsO). Anal. Calcd for C44H90Ge2N10O5Si8: C,
43.71; H, 7.50; N, 11.58. Found: C, 43.39; H, 7.33; N, 11.24. EI/
MS: [M/Z]+ ) 1208.2 amu.
Shoda demonstrated that Ge[N(SiMe3)2]2 (5) copolymerizes
with cyclic R,â-unsaturated ketones (Figure 1b).10
Attempts to carry out the Diels-Alder reaction of 2 with
PTAD did not yield the desired 4 + 2 cycloaddition product
but rather led to the discovery and characterization of a new
15-membered ring described in this paper.
Experimental Section
Standard air-free techniques were employed when using air-
sensitive materials.11 Tetrahydrofuran (THF), benzene-d6, tetrahy-
drofuran-d8, and toluene-d8 were degassed and dried over sodium
benzophenone ketyl. Acetonitrile was dried over P2O5 and stored
over 4-Å sieves. Ge[CH(SiMe3)2]2 (1) and Ge[N(SiMe3)2]2 (5) were
prepared according to the literature.12 4-Phenyl-1,2,4-triazoline-3,5-
dione (PTAD) was purchased from Aldrich and used as received.
All glassware was oven-dried for at least 3 h before use. 1H NMR
spectra were taken at 400 MHz on a Varian Inova 400 spectrometer
and at 300 MHz on a Varian Mercury 300 spectrometer. Spectra
were referenced to the residual protons of C6D6 at 7.150 ppm,
toluene-d8 at 2.09 ppm, and THF-d8 at 3.58 ppm. 13C NMR spectra
were taken at 100 MHz on a Varian Inova 400 spectrometer and
referenced to the natural abundance of 13C in C6D6 at 128.0 ppm.
Standard two-dimensional (2D) NMR techniques, correlation
spectroscopy (COSY) (on a Varian Inova 400), and heteronuclear
single quantum correlation (HSQC) (on a Varian Inova 500) were
used in assigning 1H and 13C spectra. Solid-state 13C cross-polarized
(CP)13 NMR spectra were taken at 75 MHz on a Bruker DSX 300
spectrometer. High-resolution mass spectra were collected on a VG-
70-250-S mass spectrometer using electron impact (70 eV) for
ionization. IR spectra were obtained as thin films of the products
formed by evaporation from solution on NaCl plates or as pressed
KBr plates on a Perkin-Elmer Spectrum BX.
All other reactions varying the germylene and the solvent were
carried out using the procedure described above.
Structure Determination of 6. Colorless needles of 6 were
grown from a benzene/acetonitrile (1:5) solution at 22 °C. A crystal
of dimensions 0.32 × 0.17 × 0.16 mm3 was mounted on a standard
Bruker SMART CCD-based X-ray diffractometer equipped with
an LT-2 low-temperature device and normal focus Mo-target X-ray
tube (λ ) 0.71073 Å) operated at 2000 W power (50 kV, 40 mA).
The X-ray intensities were measured at 150(2) K; the detector was
placed at a distance of 4.980 cm from the crystal. A total of 4615
frames were collected with a scan width of 0.2° in ω and æ with
an exposure time of 30 s/frame. The frames were integrated with
the Bruker SAINT14 software package with a narrow frame
algorithm. The integration of the data yielded a total of 83 696
reflections to a maximum 2Θ value of 56.75° of which 15 887 were
independent and 12 776 were greater than 2σ(I). The final cell
constants (Table 1) were based on the xyz centroids of 7118
reflections above 10σ(I). Analysis of the data showed negligible
decay during data collection; the data were processed with
SADABS15 and corrected for absorption. The structure was solved
and refined with the Bruker SHELXTL15 (version 5.10) software
package, using the space group P21/n with Z ) 4 for the formula
C44H90N10O5Si8Ge2. All non-hydrogen atoms were refined aniso-
tropically with the hydrogen atoms placed in idealized positions.
Full-matrix least-squares refinement based on F2 converged at
R1 ) 0.0301 and wR2 ) 0.0685 (based on I > 2σ(I)), R1 ) 0.0457
and wR2 ) 0.0745 for all data. Additional details are presented in
Table 1 and are given as Supporting Information in a CIF file
2,2,8,8-Tetrakis(1,1,1,3,3,3-hexamethyl-disilazan-2-yl)-5,16-di-
phenyl-7,9,14-trioxa-1,3,5,16,18,19-hexaaza-2,8-digerma-tricyclo-
[13.2.1.13,6]nonadeca-6(19),15(18)-diene-4,17-dione (6). Bis[bis-
(trimethylsilyl)amide]germylene (5) (0.290 g, 0.737 mmol) was
dissolved in 10 mL of THF, forming a pale yellow solution.
4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) (0.129 g, 0.737 mmol)
was dissolved in 10 mL of THF, forming a bright red solution.
Both solutions were stirred for 2-3 min to ensure complete
solvation. The solution of 5 was added via cannula to the PTAD
(10) Shoda, S.; Iwata, S.; Yajima, K.; Yagi, K.; Ohnishi, Y.; Kobayashi,
S. Tetrahedron 1997, 53, 15281-15295.
(11) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-SensitiVe
Compounds; Wiley: New York, 1986.
(12) Fjeldberg, T. H. A.; Schilling, B. E. R.; Lappert, M. F.; Thorne, A. J.
J. Chem. Soc., Dalton Trans. 1986, 1551-1556.
(13) Fyfe, C. A. Solid State NMR for Chemists; C.F.C. Press: Guelph,
Ontario, Canada, 1983.
(14) Saint Plus, v. 7.01; Bruker Analytical X-ray: Madison, WI, 2003.
(15) Sheldrick, G. M. SADABS: Program for Empirical Absorption
Correction and Scaling of Area Detector Data, v. 2.05; University of
Gottingen: Gottingen, Germany, 2002.
7666 Inorganic Chemistry, Vol. 43, No. 24, 2004

Digermane-Containing Tricyclic Nonadecadienedione
Table 1. Crystal Data and Structure Refinement for 6
empirical formula
fw
C44H90Ge2N10O5Si8
1209.16
temperature/K
wavelength/Å
cryst syst, space group
unit cell dimensions
a/Å
150(2)
0.71073
monoclinic, P21/n
11.496(3)
R/deg
b/Å
90
28.227(7)
â/deg
c/Å
94.416(5)
19.713(5)
ø/deg
90
6378(3)
4, 1.259
1.139
volume/Å3
Z, calcd density/Mg/m3
abs coeff/mm-1
F(000)
2560
crystal size/mm
Θ range/deg
limiting indices
0.32 × 0.17 × 0.16
2.80 to 28.37
-15 e h e 15
-37 e k e 37
-26 e l e 26
83 696/15 887
[R(int) ) 0.0480]
99.4%
semiempirical from equivalents
0.8388 and 0.7120
full-matrix least-squares on F2
15 887/0/646
1.009
no. of reflns collect/unique
completeness to Θ ) 28.37
abs corr
max and min transmission
refinement method
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
Figure 2. ORTEP diagram of 5 with CH3 groups omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ge2-O3 ) 1.74190(12); Ge2-
O4 ) 1.8200(12); Ge1-N5 ) 1.8647(15); Ge1-N9 ) 1.8730(14); N8-
C37 ) 1.288(2); N8-N9 ) 1.4225(19); O3-Ge2-O4 ) 112.85(5); C36-
O3-Ge2 ) 124.71(11); O3-C36-C-35 ) 108.31(15); C36-C35-C34
) 112.78(15); O2-C33-C34 ) 109.24(15); C32-O2-C33 ) 114.66-
(14); N5-Ge1-N9 ) 100.95(6).
R1 ) 0.0301
wR2 ) 0.0685
R1 ) 0.0457
wR2 ) 0.0745
0.375 and -0.276
R indices (all data)
from both the carbonyl (1713 cm-1) and imine (1614, 1597
cm-1) functional groups and elemental analysis. Analysis by
mass spectrometry confirmed that 6 had a molecular weight
of 1209 g/mol (C44H90Ge2N10O5Si8).
largest diff peak and hole/e A-3
Results
1H and 13C spectroscopy were both consistent with the
structural assignment of 6 but contained what appeared to
be two conformers (A and B) present in benzene solution.
The 1H NMR spectrum contains four trimethylsilyl singlets
at 0.49 and 0.35 ppm (A) and at 0.60 and 0.23 ppm (B) as
well as four, broad pseudotriplets from the methylenes R to
the oxygen atoms at 4.67 and 4.58 (A) and 4.42 4.28 ppm
(B). 2D NMR (COSY) was used to confirm that the multiplet
at ∼1.8 ppm derived from the four methylene carbons â to
the oxygen. All of the methylene signals in 6 originate from
THF as confirmed by running the reaction of 5 and PTAD
in THF-d8, which results in 6-d8 with an 1H NMR spectrum
identical to 6 except for the absence of all methylene protons.
The ratio of conformers changes with the solvent, appearing
in a ∼1:0.8 A/B ratio in benzene-d6, a ∼1:1 A/B ratio in
toluene-d8, and a 1:1.5 A/B ratio in THF-d8.
The appearance in the 1H NMR spectrum of the methylene
groups R to the oxygen merits further analysis. One might
naively presume that as a result of being present in a ring
all eight methylene protons in 6 would be diastereotopic. If
this were the case, the four protons R to the oxygen atoms
would be observed as four doublets of doublets of doublets
in a 1:1:1:1 ratio. For a freely moving chain, the two
methlyene groups R to oxygen would each appear as a triplet
in a 1:1 ratio. The experimental observation in benzene-d6
is four slightly broad triplets in a 1:1:0.8:0.8 ratio. This
observations leads to the conclusion that there are two
conformers in a 1:0.8 ratio, each with freely moving
methylene chains. The 13C NMR spectrum contains four
As PTAD mixed with 2 in THF, the solutions color rapidly
changed from bright red to colorless. The 1H NMR spectrum
of the reaction products clearly indicated that a Diels-Alder
[4 + 2] cycloaddition had not taken place between the triene
and PTAD. To confirm that germylene 1 (in equilibrium with
2 and benzophenone) was indeed reacting with the PTAD,
1 was added directly to PTAD in THF, resulting in multiple
products as indicated by the number of trimethylsilyl peaks
1
observed in the H NMR spectrum.
The addition of Ge[N(SiMe3)2]2 (5) to PTAD in THF was
also carried out. This reaction resulted in a clean product
with four unanticipated triplets in the 4-5 ppm region of its
1H NMR spectrum. X-ray quality crystals were grown by
slow evaporation from benzene/acetonitrile (1:5) in order to
allow an X-ray crystal structural determination of 6. A large
tricyclic ring made from five reactant molecules (2 equiv of
5, 2 equiv of PTAD, and 1 equiv of THF) was revealed
(Figure 2, eq 3). The formation of 2,2,8,8-tetrakis(1,1,1,3,3,3-
hexamethyl-disilazan-2-yl)-5,16-diphenyl-7,9,14-trioxa-1,3,5,-
16,18,19-hexaaza-2,8-digerma-tricyclo[13.2.1.13,6]nonadeca-
6(19),15(18)-diene-4,17-dione (6) is supported by IR stretches
Inorganic Chemistry, Vol. 43, No. 24, 2004 7667

Gottfried et al.
Figure 3. 13C NMR of 6 in (a) solution, (b) solid state of powder, and
(c) solid state of crystal.
unique methlyene carbons R to the oxygen atoms (68.70,
68.24, 63.67, and 62.13 ppm) and four unique methylene
carbons â to oxygen atoms (28.47, 27.48, 25.55, and 25.28
ppm), supporting the conclusion that two conformers must
be present in solution (Figure 3a).
The data from the X-ray crystal structure indicates that 6
adopts a single conformation in the solid phase. A sample
of 6 prepared by dissolving single crystals from the same
batch used to obtain the X-ray structure in benzene-d6 at
room temperature produced an 1H NMR spectrum identical
to that obtained for noncrystalline 6. This suggests that the
single conformer present in the solid state can rapidly
equilibrate with a second conformer when dissolved in
benzene at ambient temperature.
1
Figure 4. Variable temperature H NMR spectra (300 MHz, toluene-d8
-60 to 90 °C) of methylene protons R to oxygen (5-4 ppm) starting with
single conformer crystals.
conformers that are interchanging rapidly. Perhaps multiple
conformers of the methylene chain are attaining the energy
necessary to quickly interconvert formally diastereotopic
protons. With an increase of temperature, this single broad
peak grows into a pair of the pseudotriplets previously
identified as conformer B in the spectra of 6 taken at 20 °C.
The second conformer (A) is initially absent. However, when
the sample of 6 was warmed to -30 °C a broad peak for
the methylene groups of A begins to appear and by 20 °C
both pairs of pseudotriplets are present in a 1:1 ratio.
To determine which, if any, of the NMR peaks observed
in solution represent the conformer obtained by slow crystal
formation, more crystals were grown using the same method.
X-ray powder diffraction was employed to confirm that the
material crystallized was in the same space group, P21/n,
with the same unit cell constants. The 13C CP solid-state
NMR spectra of crystalline 6 and powder 6, which showed
no crystallinity by X-ray powder diffraction, are shown in
Figure 3. The solid-state NMR of powder 6 (Figure 3b)
shows broad peaks in both methylene regions (20-30 and
60-70 ppm), which are attributed to the presence of multiple
conformers in the solid powder. In Figure 3c, the solid-state
NMR spectrum of crystalline 6, the two methylene carbons
R to oxygen atoms and the two methylene carbons â to
oxygen atoms appear as well-defined and distinct peaks
consistent with the presence of single conformer in the
crystalline solid.
The sequential appearance of the two conformers is not
due to a large difference in solubility. (Previous H NMR
1
experiments showed both conformers in their room-temper-
ature ratios in solution down to -30 °C.) Rather, the two
conformers appear to be in equilibrium with conformer A
growing in slowly as the temperature increases. As the
temperature is increased from 20 to 60 °C, both pairs of
pseudotriplets sharpen and begin moving toward one another
in a manner expected for coalescence due to fast intercon-
version of the two conformers at high temperatures. The
boiling point of the NMR solvent was reached before the
coalescence temperature could be attained. However, a lower
limit of coalescence (∆Gq ) 70 kJ/mol) was calculated using
the average ∆ν of the set of conformer peaks at 0 °C with
Tc ) 90 °C.16
1
A solution H NMR spectrum of a single conformer was
obtained by dissolving crystals of 6 in toluene-d8 at -78 °C
and placing this solution in an NMR spectrometer that had
been cooled to -60 °C. The sample of 6 was warmed slowly
in 10 °C intervals with 5 min of equilibration at each
temperature while obtaining NMR data. The 4-5 ppm region
Spartan ‘04 molecular mechanics calculations were used
to search for possible low-energy conformations of 6. A
search resulted in over 80 unique conformers with energies
over a 10 kcal/mol range. Density functional theory geometry
1
(the methylenes R to the oxygen) of the H NMR data is
shown in Figure 4. At low temperatures, 6 is not very soluble
in toluene. As the temperature is warmed and 6 dissolves,
its methylene peaks initially appear very broad in a manner
that may indicate the coalescence of peaks from many
(16) Gunther, H. NMR Spectroscopy: Basic Principles, Concepts, and
Applications in Chemistry, 2nd ed.; Wiley: Chichester, NY, 1995.
7668 Inorganic Chemistry, Vol. 43, No. 24, 2004

Digermane-Containing Tricyclic Nonadecadienedione
optimizations were carried out on 6 without the phenyl or
TMS groups. Both the crystal structure coordinates and an
arbitrarily drawn molecule were minimized, resulting in two
different conformers of the ring. Semiempirical geometry
minimizations (AM1) of the full-crystal structure and another
arbitrarily drawn full ring also resulted in two different
conformers. This set of calculations did not resolve the
question of specific conformations likely to be obtained by
6.
Discussion
Upon the basis of NMR data and molecular mechanics
simulations, we conclude that in a solution of 6 there are
two groups of interconverting conformers with comparable
energies. All of the conformers have flexible methylene
chains, resulting in the appearance of triplets in the1H NMR.
The presence of many conformers within a group at lower
temperatures (-30 to 10 °C in toluene-d8) results in the
broader triplets. As the temperature is raised, all conformers
within a group interconvert quickly, resulting in sharp
methylene triplets. Finally, at high temperatures, the two
groups of conformers appear to be moving toward intercon-
version. The activation energy for conversion between the
two groups (lower boundary ) 70 kJ/mol) is sufficiently high
so that both sets exist in tolene-d8 up to at least 90 °C as
shown by 1H NMR spectroscopy in Figure 4. Upon crystal-
lization, a single conformer is formed as indicated by solid-
state NMR spectroscopy and X-ray crystallography. It is
possible that the solid state 13C CP NMR spectrum of 6 and
Figure 5. Literature precedent for mechanistic proposal.
dropyran as the solvent produced mostly the nonring products
observed in the reactions of 5 and PTAD in benzene.
To propose a reaction mechanism for ring formation, the
reactivities of the individual components of the ring were
examined. 5 has been shown to form an alternate copolymer
with R,â-unsaturated ketones similar to PTAD in the
presence of salts such as lithium chloride (Figure 1b).10 The
proposed mechanism is the addition of germylene anion
(formed by complexation with the salt) to the R,â-unsaturated
ketone via a Michael-type addition. The steric bulk of the
amide ligand and the poor homopolymerizability of the R,â-
unsaturated ketone are stated as reasons for prevention of
homo unit formation.
PTAD is primarily used as a dieneophile for Diels-Alder
type reactions, but Butler and Jones have demonstrated that
it will undergo the 1,4-cycloaddition that leads to polymer-
ization (Figure 5a).18,19 Weber and co-workers have even
shown the formation of cyclododecadienes from 2 equiv of
PTAD and 2 equiv of a metal-substituted diphosphene in
diethyl ether (Figure 5b). This cycloaddition product differs
from 6 in that a symmetric molecule is formed with each
diphosphene attached to one of the PTAD fragment through
a P-N bond and to the second PTAD fragment through a
P-O bond.
1
the low-temperature H NMR spectrum in toluene-d8 arise
from the same conformer. The AM1 minimized structure and
the conformer found by crystallography do differ in detail
in terms of both the methylene chain and the PTAD plane
angles. However, the <10 kcal/mol difference is well within
the energetics range of crystal packing interactions.
The formation of 6 is a facile, complete, and unique result
of a simple mixing of the reagents needed to form the ring.
Other large, multifunctional rings such as porphyrins and
crown ethers are usually only achievable through high
dilution techniques, the use of a template, or if there exists
a small ∆S between products and reactants.17 Compound 6
was formed independent of concentration and rate of addition
and without the aid of the template. The combination of five
independent molecules to make one single compound is not
indicative of a small change in entropy.
Exploration of related reaction conditions and reactants
was carried out. The reaction of 5 with PTAD in benzene
yields a mixture of products with eight trimethylsilyl peaks.
If THF is added to these products, no formation of 6 is
observed. The reaction of 5 and PTAD in benzene with 2
equiv of THF yields a mixture of 6 and the products from
the reaction of 5 and PTAD in benzene. Clean conversion
to the large ring of 6 only took place with the Ge[N(SiMe3)2]2
(5) and not with Ge[CH(SiMe3)2]2 (1). Reactions employing
other ethers including dioxane, diethyl ether, and tetrahy-
THF has been shown to ring open through nucleophilic
attack when coordinated to a Lewis acid (such as germylene)
often leading to polymerization (Figure 5c).20,21
These precedents suggest formation of 6 begins with
addition of germylene 5 to the R,â-unsaturated ketone of
PTAD (Scheme 1). We suggest that the germylene attacks a
nitrogen in the NdN double bond, rather than at a carbonyl
carbon, based on the Michael-type addition cited by Shoda9
and the resulting connectivity observed in the final product
6. The resulting anion then carries out a nucleophilic attack
on THF coordinated to the Lewis acidic germylene. With
four of the five components put together, completion of the
mechanism with steps based solely on 2 e- acid-base
reactions no longer appears feasible. It is proposed that an
(18) Butler, G. B.; Guilbault, L. J.; Turner, S. R. Polym. Lett. 1971, 9,
115-124.
(19) Hall, J. H.; Jones, M. L. J. Org. Chem. 1983, 48, 822-826.
(20) Penczek, S.; Kubisa, P.; Matyjaszewski, K. AdV. Polym. Sci. 1980,
37, 1-144.
(17) Stang, P. J.; Mangum, M. G. J. Am. Chem. Soc. 1975, 97, 3856-
(21) Dreyfuss, P.; Dreyfull, M. P. ComprehensiVe Chemical Kinetics;
Elsevier: Amsterdam, 1976; Vol. 15, pp 259-330.
3859.
Inorganic Chemistry, Vol. 43, No. 24, 2004 7669

Gottfried et al.
Germyl anions have been shown to be donors22,23 and PTAD
also has been shown to be an acceptor in single electron-
transfer reactions.24,25
Scheme 1. Proposed Mechanism for Formation of 6
In conclusion, a unique tricyclononadecadiene ring has
been formed by simply mixing Ge[N(SiMe3)2]2 (5) with
PTAD in THF. The reaction proceeds quickly and in high
yield. The combination of five independent molecules to form
a single ring without use of a template or dilution techniques
is unexpected. Analysis of the ring reveals a single conformer
in the crystalline solid and two groups of conformers, each
with a flexible methylene chain, in solution.
Acknowledgment. Support of this work from the Petro-
leum Research Fund and from the Research Corporation is
gratefully acknowledged. We also thank B. Coppola and the
Chemical Sciences at the Interface of Education program at
the University of Michigan. Nate Ockwig is gratefully
acknowledged for his assistance with X-ray powder diffrac-
tion.
electron transfer from the germyl anion to PTAD generates
a PTAD anion (7) that can couple with the resulting germyl
radical (8) to form the second Ge-O bond. The resulting
species can complete the intramolecular coupling by attack-
ing the initially formed germyl cation to close the ring.
Clearly, this is just one of many possible mechanistic
schemes and an electron-transfer step may well occur in the
first germylene-PTAD reaction. On the basis of numerous
literature reports over the past 20 years,22 the presence of
electron-transfer steps in reactions with germylenes should
always be considered as a likely mechanistic possibility.
Supporting Information Available: 1H NMR spectrum
and the full 13C NMR spectrum (solid-state crystals, solid-state
powder, and solution). X-ray crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0491027
(23) Riviere, P.; Castel, A.; Desor, D.; Abdennadher, C. J. Organomet.
Chem. 1993, 443, 51-60.
(24) Alstanei, A. M.; Hornoiu, C.; Aycard, J. P.; Carles, M.; Volanschi, E.
J. Electroanal. Chem. 2003, 542, 13-21.
(22) Riviere, P.; Riviere-Baudet, M.; Castel, A. Main Group Met. Chem.
1994, 17, 679-705.
(25) Bausch, M. J.; David, B. J. Org. Chem. 1992, 57, 1118-1124.
7670 Inorganic Chemistry, Vol. 43, No. 24, 2004