The synthesis and conformation of sterically congested seven-membered rings containing tetracoordinate germanium(IV): Determination of the δG* for ring inversion

Article abstract of DOI:10.1021/ic010180t

The conformation of dibenzol[d,f][1,3,3,]dioxagermepin 5 was investigated by VT ¹NMR spectroscopy, utilizing a diastereotopic proton probe.The free energy of activation for ring inversion (astropisomerism about the sp²-sp² C-C single bond connecting the two aryl rings) is 13.8 kcal mol^-1.

Full text of DOI:10.1021/ic010180t

3830
Inorg. Chem. 2001, 40, 3830-3832
The Synthesis and Conformation of Sterically
Congested Seven-membered Rings Containing
Tetracoordinate Germanium(IV): Determination
of the ∆G* For Ring Inversion1
Stephen D. Pastor,* Adam Carinci,
Nancy Khoury, and David N. Rahni
Figure 1. Chemical Abstracts numbering system for the dibenzo[d,f]-
[1,3,2]dioxagermepin ring system.
Pace University, Department of Chemistry,
Pleasantville, New York 10570
rich,13 nothing is known about the corresponding dibenzo[d,f]-
[1,3,2]dioxagermepin ring system (Figure 1). We report herein
the synthesis and characterization of sterically congested mono
and spirocyclic dibenzo[d,f][1,3,2]dioxagermepin derivatives, as
well as the first determination of the free energy of activation
(∆G*) for the dioxagermepin ring system.
ReceiVed February 12, 2001
Introduction
Recently, a significant amount of research has focused on
the synthesis and conformational analysis of hetero-substituted
medium-sized rings.2 In particular, studies on the spectral
characteristics3,4 and ligand properties5 of molecules containing
the dibenzo[d,f][1,3,2]dioxaphosphepin ring system have been
reported, as well as the apical-equatorial aptitude of the seven-
membered ring where the phosphorus atom is pentacoordinate.6
A crystallographic study reported the effect of aryl substitution
upon the conformation of the seven-membered ring.7 The
synthesis of the corresponding dibenzo[d,f][1,3,2]dioxasilepin
ring system has been reported,8-10 and Holmes et al. prepared
anionic five-coordinate silicates containing a dioxasilepin ring.11
Except for the pioneering studies of Zuckerman8c,12 and Hein-
Experimental Section
All melting points were determined in open capillary tubes with a
Thomas-Hoover melting point apparatus and are uncorrected. 1H NMR
(300.08 and 499.84 MHz) spectra were obtained on a Varian Model
Gemini-300 or Unity-500 spectrometer, respectively. 13C NMR (125.6998
MHz) spectra were taken on a Varian Model Unity-500 spectrometer
1
and are proton decoupled. All H and 13C chemical shift values are
reported in ppm relative to tetramethylsilane, where a positive sign is
downfield from the standard. Merck silica gel 60 (200-400 mesh) was
used for column chromatography. Merck precoated (0.25 mm) silica
gel F-254 plates were used for TLC. Reagents were purchased from
commercial laboratory supply houses. Solvents were dried prior to use,
when necessary, with appropriate drying agents. Reactions were carried
out in a flame-dried apparatus under a dry inert atmosphere of nitrogen.
Elemental analyses were performed by Robertson Microlit Laboratories,
Madison, New Jersey.
2,4,8,10-Tetra-tert-butyl-6,6-dimethyl-dibenzo[d,f][1,3,2]-
dioxagermepin (3a). To a stirred solution of 2a (1.00 g, 5.80 mmol)
in 9 mL of toluene was added dropwise over 10 min a solution of 1
(1.03 g, 2.50 mmol) and triethylamine (0.51 g, 5.0 mmol) in 10 mL of
toluene. The reaction mixture was heated at 83 °C for 16 h and then
after cooling to room temperature, the precipitate of triethylamine
hydrochloride was removed by filtration. The volatiles were removed
in vacuo, and the residue was recrystallized from acetonitrile to give
1.11 g (87%) of a white solid, mp 185-187 °C. 1H NMR (CDCl3): δ
0.75 (s, 6 H), 1.33 (s, 18 H), 1.43 (s, 18 H), 7.02 (d, 4J ) 2.4 Hz, 2 H),
7.32 (d, 4J ) 2.4 Hz, 2 H). Anal. Calcd for C30H46O2Ge: C, 70.47; H,
9.07. Found: C, 70.62; H, 9.02.
2,4,8,10-Tetra-tert-butyl-6,6-diphenyl-dibenzo[d,f][1,3,2]-
dioxagermepin (3b). The procedure for the preparation of compound
3a was repeated using 2b (1.00 g, 3.1 mmol), 1 (1.03 g, 2.50 mmol),
and triethylamine (0.51 g, 5.0 mmol) in 19 mL of toluene (83 °C, 24
h). The residue was recrystallized from a mixture of toluene (10 mL)
and acetonitrile (30 mL) to give 1.21 g (76%) of a white solid, mp
221-222 °C. 1H NMR (CDCl3): δ 1.17 (s, 18 H), 1.36 (s, 18 H), 7.17
(d, 2 H), 7.26 (d, 2 H), 7.37 (m, 4 H), 7.49 (tt, 2 H), 7.59 (dd, 4 H).
13C{1H} NMR (CDCl3; 125.6998 MHz): δ 30.68, 31.55, 34.35, 35.06,
123.32, 128.10, 128.23, 130.10, 131.27, 132.81, 134.68, 139.07, 143.29,
151.71. Anal. Calcd for C40H50O2Ge: C, 75.61; H, 7.93. Found: C,
76.03; H, 7.92.
3,3′,5,5′-Tetrakis(1,1,3,3-tetramethylbutyl)-2,2′-dihydroxy-1,1′-bi-
phenyl (4). To a stirred mixture of 2,4-bis(1,1,3,3-tetramethylbutyl)phe-
nol (29.5 g, 0.1 mole) and potassium hydroxide (44.0 g, 0.8 mol) in
250 mL of distilled water at 85-90 °C was added dropwise over 1 h
a 30% aqueous solution of hydrogen peroxide (30 mL, 0.27 mol).
* To whom communication should be addressed.
(1) Presented in part at the 219th ACS National Meeting, San Francisco,
CA; March 26-30, 2000; Abstract INOR 301.
(2) For a recent monograph, see Conformational Analysis of Medium-
Sized Heterocycles; Glass, R. S., Ed.; VCH: Weinheim, 1988.
(3) Holmes, R. R.; Prakasha, T. K.; Pastor, S. D. In Phosphorus-31 NMR
Spectral Properties in Compound Characterization and Structural
Analysis; Quin, L. D., Verkade, J. G., Eds.; VCH: New York, 1994;
pp 27-39.
(4) Pastor, S. D.; Shum, S. P.; Rodebaugh, R. K.; DeBellis,; Clarke, F.
H. HelV. Chim. Acta 1993, 76, 900.
(5) (a) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. U. S. Patent 4,748,-
261; Chem. Abstr. 1987, 107, 7392. (b) Cuny, G. D.; Buchwald, S. L.
J. Am. Chem. Soc. 1993, 115, 2066. (c) Johnson, J. R.; Cuny, G. D.;
Buchwald, S. L. Angew. Chem., Int. Ed. Engl. 1995, 34, 1760. (d)
Babin, J. E.; Whiteker, G. T. U. S. Patent 5, 31994, 60, 938; Chem.
Abstr. 1995, 122, 186609. (f) Pastor, S. D.; Shum, S. P.; DeBellis, A.
D.; Burke, L. P.; Rodebaugh, R. K.; Clarke, F. H.; Rihs, G. Inorg.
Chem. 1996, 35, 949, and references therein. (f) Buisman, G. J. H.;
van der Veen, L. A.; Klootwijk, A.; de Lange, W. G. J.; Kamer, P. C.
J.; van Leeuwen P. W. N. M.; Vogt, D. Organometallics 1997, 16,
2929, and references therein. (g) Pastor, S. D.; Shum, S. P. Tetrahe-
dron: Asymmetry 1998, 9, 543, and references therein.
(6) (a) Abdou, W. M.; Denney, D. B.; Denney, D. Z.; Pastor, S. D.
Phosphorus Sulfur Relat. Elem. 1985, 22, 99. (b) Hans, J.; Day, R.
O.; Howe, L.; Holmes, R. R. Inorg. Chem. 1992, 31, 1279, and
references therein.
(7) Malen, A. H.; NabiRahni, M. A.; Pastor, S. D. Phosphorus, Sulfur
Silicon Relat. Elem. 1993, 82, 1.
(8) (a) Zuckerman, J. J. J. Chem. Soc. 1962, 873. (b) Emeleus, H. J.;
Zuckerman, J. J. J. Organomet. Chem. 1964, 1, 328. (c) Silcox, C.
M.; Zuckerman, J. J. J. Am. Chem. Soc. 1966, 88, 168.
(9) (a) Schwarz, R.; Kuchen, W. Z. Anorg. Allg. Chem. 1955, 84, 279.
(b) Ismail, R. M. Z. Naturforsch. B: Anorg. Chem. Biochem. Biophys.
Biol. 1964, 19B, 873. (c) Allcock, H. R.; Nugent, T. A.; Smeltz, L.
A. Synth. Inorg. Met.-Org. Chem. 1972, 2, 97. (d) Cragg, R. H.; Lane,
R. D. J. Organomet. Chem. 1985, 289, 23.
(10) (a) Mu¨ller, E.; Mayer, R.; Narr, B.; Rieker, A.; Scheffler, K. Ann.
Chem. 1961, 645, 25. (b) Kushioka, K. J. Org. Chem. 1983, 48, 4948.
(11) Kumara Swamy, K. C.; Sreelatha, C.; Day, R. O.; Holmes, J.; Holmes,
R. R. Inorg. Chem. 1991, 30, 3126.
(12) Zuckerman, J. J. J. Chem. Soc. 1963, 1322.
(13) Mu¨ller, R.; H, Heinrich, L. Chem. Ber. 1962, 95, 2276.
10.1021/ic010180t CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/19/2001

Notes
Inorganic Chemistry, Vol. 40, No. 15, 2001 3831
The reaction mixture was stirred at 85 °C for 1 h. The reaction mixture
was cooled to room temperature, and then the aqueous phase was
decanted. The organic phase was washed with water by decantation,
and then the viscous organic phase was poured into a beaker of dilute
hydrochloric acid. After standing overnight, the aqueous phase was
decanted and the organic phase was diluted with diethyl ether. The
ether solution was washed sequentially with dilute aqueous hydrochloric
acid, water, saturated sodium bicarbonate, and water. The organic phase
was dried over anhydrous sodium sulfate, and the solvent was removed
in vacuo. The product was purified by trituration with acetonitrile to
give 13.3 g (42%) of a white solid, mp 141-142 °C. Proton assignments
made by DNOE experiments. 1H NMR (CDCl3; 27 °C): δ 0.72 (s, 36
H), 1.31 (s, C(CH3)2, 12 H), 1.43 (s, 3,3′-C(CH3)A, 6 H), 1.44 (s, 3,3′-
C(CH3)B, 6 H), 1.68 (s, 4 H), 1.85 (d, 2JHCH ) 14.4 Hz, 2 H), 1.98 (d,
2JHCH ) 14.4 Hz, 2 H), 4.96 (s, OH, 1 H), 6.93 (d, 2 H), 7.32 (d, 2 H).
Anal. Calcd for C44H74O2: C, 83.22; H, 11.75. Found: C, 82.97; H,
11.36.
2,4,8,10-Tetrakis(1,1,3,3-tetramethylbutyl)-6,6-dimethyl-dibenzo-
[d,f][1,3,2]dioxa-germepin (5). The procedure for the preparation of
compound 3a was repeated using 2a (1.25 g, 7.20 mmol), 4 (1.05 g,
1.7 mmol), and triethylamine (0.58 g, 5.70 mmol) in 26.3 mL of toluene
(70 °C, 72 h). The residue was recrystallized from acetonitrile (30 mL)
to give 0.52 g (43%) of a white solid, mp 183-185 °C. 1H NMR (C6D6;
60 °C): δ 0.45 (s, 6 H), 0.86 (s, 18 H), 0.88 (s, 18 H), 1.40 (s, 12 H),
1.59 (s, 12 H), 1.78 (s, 4 H), 2.13 (s, 4 H), 7.21 (d, 2 H), 7.49 (d, 2 H).
Anal. Calcd for C46H78O2Ge: C, 75.10; H, 10.69. Found: C, 75.20;
H, 10.34.
C2 symmetry that renders the methyl group protons isochronous
or a ring that is rapidly inverting (atropisomerization about the
C-C bond connecting the two aryl rings) on the NMR time
scale.14
In an analogous manner, the diphenyl derivative 3b was
prepared by the reaction of 1 with 2b. In the 1H NMR spectrum
of 3b, a doublet of doublets, a triplet of triplets, and a doublet
of doublets were observed at δ 7.37, 7.49, and 7.59, respectively,
which were assigned to the aromatic protons that are meta, para,
and ortho to germanium in the 6,6-diphenyl substituents. The
aromatic protons of the two phenyl substituents bonded to
germanium are expected to be isochronous if the dioxagermepin
is either rapidly or slowly inverting on the NMR time scale.15
However, the observation of only one signal for the two aromatic
protons ortho to germanium in the phenyl substituent is only
consistent with a dioxagermepin ring rapidly inverting on the
NMR time scale. This must be the case because slow (or non)
ring inversion on the NMR time scale would render the ortho
aromatic protons anisochronous due to the presence of a
stereoaxis about the single bond connecting the aryl groups in
the dioxagermepin ring. In the 13C{1H} NMR spectrum of 3b,
all of the carbon atoms expected to be isochronous in a rapidly
inverting dioxagermepin ring were found to be isochronous.
The conformation of the dibenzo[d,f][1,3,2]dioxagermepin
ring was further delineated utilizing a diastereotopic proton
probe. The substituted dioxagermepin 5 with diastereotopic
methylene and methyl group protons was synthesized for a VT
1H NMR study. The prerequisite bisphenol 4 was prepared by
the oxidative coupling of 2,4-bis(1,1,3,3-tetramethylbutyl)phenol
with hydrogen peroxide under basic conditions. In the 1H NMR
spectrum (C2D2Cl4) of 4 at 27 °C, two doublets were observed
at δ 1.85 and 1.98 that were assigned to two equivalent pairs
of anisochronous methylene protons bonded to the C-2 carbon
atom of the two 1,1,3,3-tetramethylbutyl substituents ortho to
the oxygen atoms. The protons of the adjacent gem-dimethyl
groups were also observed to be anisochronous. These observa-
tions are consistent with slow rotation about the stereoaxis (C-C
bond connecting the two aryl groups) on the NMR time scale.16
6-[(2,4,8,10-Tetra-tert-butyl-6-ethyl-dibenzo[d,f][1,3,2]-
dioxagermepin-6-yl)oxy]- 2,4,8,10-Tetra-tert-butyl-6-ethyl-dibenzo-
[d,f][1,3,2]dioxagermepin (6). The procedure for the preparation of
compound 3a was repeated using ethylgermanium(IV) trichloride (1.00
g, 4.80 mmol), 1 (3.28 g, 8.00 mmol), and triethylamine (1.46 g, 14.40
mmol) in 20 mL of toluene (74 °C, 21 h). The residue was recrystallized
from a mixture of acetonitrile (40 mL) and toluene (7 mL) to give
1
1.89 g (46%) of a white solid, mp 236-238 °C. H NMR (C2D2Cl4;
110 °C): δ 0.96 (t, 6 H), 1.18 (q, 4 H), 1.31 (s, C(CH3)3, 36 H), 1.41
(s, C(CH3)3, 36 H), 6.95 (d, 4 H), 7.29 (d, ArH, 4 H). MS: The desired
molecular ions (M+•) were observed in the ratio calculated for the
76
naturally abundant isotope mixture of 70Ge, 72Ge, 73Ge, 74Ge, and
-
Ge. Anal. Calcd for C60H90O5Ge2: C, 69.52; H, 8.75. Found: C, 71.44;
H, 8.70.
Bis(3,3′,5,5′-tetra-tert-butyl-1,1′-biphenylen-2,2′-dioxy)germanium-
(IV) (7). The procedure for the preparation of compound 3a was
repeated using germanium(IV) chloride (1.50 g, 7.0 mmol), 1 (5.80 g,
14.0 mmol), and triethylamine (2.90 g, 29.0 mmol) in 37 mL of toluene
(60 °C, 2 h). The residue was recrystallized from a mixture of
acetonitrile (20 mL) and toluene (3 mL) to give 3.92 g (62%) of a
white crystalline solid, mp > 250 °C. Proton assignments were made
by NOE experiments. 1H NMR (CDCl3; 27 °C): δ 1.26 (s, 4,8-C(CH3)3,
36 H), 1.35 (s, 2,10-C(CH3)3, 36 H), 7.10 (d, 1,11-ArH, 4 H), 7.38 (d,
1
3,9-ArH, 4 H) [for atom numbering, see Figure 1; assignments for /2
of the S2 symmetric molecule]. Anal. Calcd for C56H80O4Ge: C, 75.59;
H, 9.06. Found: C, 76.39; H, 8.96.
Results and Discussion
The reaction of the tetra-tert-butyl substituted biphenyldiol
110 with dimethylgermanium(IV) dichloride, 2a, using triethy-
lamine as an acid acceptor gave the dibenzo[d,f][1,3,2]-
dioxagermepin 3a as a white crystalline solid (87% recrystal-
lized) (Figure 1; Structures 1-5). The diphenyl derivative 3b
was prepared in an analogous reaction of 1 with 2b.
In the 1H NMR spectrum of 3a at probe temperature (27 °C),
a singlet was observed at δ 0.75, which was assigned to the
protons of two equivalent methyl groups bonded to germanium.
The protons of two equivalent pairs of tert-butyl substituents
(singlets at δ 1.33 and 1.43, respectively) and aromatic protons
(doublets at δ 7.02 and 7.32, respectively) were observed in
the 1H NMR spectrum. These observations are consistent either
with a slow or noninverting staggered ring conformation with
1
In the H NMR spectrum (C2D2Cl4) of 4 above 80 °C, the
coalescence temperature (TC), the two doublets observed at δ
(14) The examination of a Drieding molecular model suggests that the
alternate explanation of a planar ring conformation is highly unlikely.
(15) For a discussion on topicity and anisochrony in NMR, see Eliel, E.
L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Com-
pounds; Wiley-Interscience: New York, 1994; pp 465-508.

3832 Inorganic Chemistry, Vol. 40, No. 15, 2001
Notes
1.85 and 1.98 collapsed into a broad singlet, which sharpened
upon raising the temperature to 110 °C. The free energy of
activation for the process required to render these diastereotopic
of 6 at 110 °C, two sharp singlets were observed, which were
assigned to the protons of two isochronous pairs of tert-butyl
substituents.
protons equivalent is ∆G*353 ) 17.2 kcal mol-1 17,18
This
.
process can reasonably be assigned to rotation about the C-C
single bond connecting the two aryl groups in 4. Simple ortho-
substituted biphenyls are reported to have activation energies
ranging between 17 and 20 kcal mol-1 19
Small increases in
.
the ∆G* for rotational processes in hydroxyaryl molecules due
to hydrogen bonding has been reported.20
The 1,1,3,3-tetramethylbutyl-substituted dioxagermepin 5 was
prepared by the reaction of 4 with 2a as a white crystalline
solid (43% recrystallized). Interestingly, in the 1H NMR
spectrum (CDCl3) of 5 at probe temperature (27 °C), the signals
for the gem-dimethyl and methylene protons of both pairs of
1,1,3,3-tetramethylbutyl substituents were observed to be broad.
A reasonable explanation for these observations is that below
the TC, ring inversion of the seven-membered rings is slow on
the NMR time scale, which leads to observable diastereoiso-
merism because of the presence of two independent stereoaxes
(sp2-sp2 C-C single bond connecting the two aryl rings).
Below the TC, two atropisomers would be expected to be
observable with relative absolute configurations of (R*, R*) and
(R*, S*), which refers throughout this paper to the relative
configurations the two stereoaxes.15,16 The observation of two
pairs of equivalent tert-butyl substituents at 110 °C is consistent
with rapid ring inversion of the dibenzo[d,f][1,3,2]dioxagermepin
rings. Conformational averaging, because of rapid ring inversion
and rotation of exocyclic bonds at 120 °C, leads to an overall
C2V symmetry for 3, which renders the corresponding pairs of
tert-butyl protons in each ring homotopic.23
1
In the H NMR spectrum (C6D6) of 5 at 60 °C, these signals
1
sharpened considerably. In the VT H NMR spectra (CD2Cl2)
below 15 °C, the TC, two doublets were observed at δ 1.60 and
1.75 (upfield and downfield arms of an AB quartet, respectively;
JAB ) 14.6 Hz), which were assigned to the anisochronous
methylene protons of one pair of equivalent 1,1,3,3-tetrameth-
ylbutyl substituents (interconverted by a C2 symmetry operation
below TC). The free energy of activation for the process required
to render these diastereotopic protons equivalent is ∆G*288
)
13.8 kcal.mol-1. The process that interconverts these protons
can be reasonably assigned to be ring inversion (atropisomerism
about the sp2-sp2 single bond connecting the two aryl rings)
The spirocyclic derivative 7 was prepared by the reaction of
germanium(IV) tetrachloride with 2 mol of the bisphenol 1 (see
1
of the dioxagermepin ring. As expected, in the VT H NMR
1
Structure 7). In the H NMR spectrum of 7, two singlets at δ
spectrum (CD2Cl2) of 5 at -40 °C, the methylene and gem-
dimethyl group protons of both pairs of equivalent 1,1,3,3-
tetramethylbutyl substituents were observed to be diastereotopic.
In an attempt to prepare a hydroxyaryl-substituted dioxager-
mocin by the reaction of 2 mol of 2a with ethylgermanium(IV)
trichloride, the digermoxane 6 was obtained as a white crystal-
line solid (46% recrystallized; see Structure 6). The formation
of the digermoxane 6 is suggested to be the result of the reaction
of the intermediate 6-chloro-substituted dioxagermepin with
1.26 and 1.35 were observed, which were assigned to the protons
of two pairs of equivalent tert-butyl substituents. This observa-
tion is consistent with rapid ring inversion leading to an overall
Td averaged symmetry.24
No evidence was found for the formation of the corresponding
pentacoordinate germanium(IV) species resulting from coordi-
nation of the chloride counterion of the triethylammonium cation
during the preparation of the dibenzo[d,f][1,3,2]dioxagermepin
derivatives of this study, which has been observed for certain
spirocyclic catecholate derivatives.25 A reasonable explanation
for this observation is the steric inhibition of coordination of
chloride to germanium as a result of the tert-alkyl substitution
and the lower electron acceptor properties of germanium in
derivatives 3a,b and 5 because of the alkyl or aryl substitution
at germanium. A similar steric effect, leading to reduced
coordination at germanium, was observed by Holmes25 in
spirocyclic catecolate derivatives of germanium due to tert-butyl
substitution.
1
adventitious water.21 In the VT H NMR spectrum (C2D2Cl4)
of 6 at 27 °C, four broad singlets were observed at δ 1.26, 1.37,
1.47, and 1.56 which were assigned to the protons of four
nonequivalent tert-butyl substituents.22 Upon heating, the tert-
butyl signals underwent coalescence. In the 1H NMR spectrum
(16) (a) For the observation of atropisomers in a bisphenol, see Pastor, S.
D.; Hyun, J. L.; Odorisio, P. A.; Rodebaugh J. Am. Chem. Soc. 1988,
110, 6547, and references therein. (b) Pastor, S. D.; Richardson, C.
F.; Ali NabiRahni, M. Phosphorus, Sulfur Silicon Relat. Elem. 1994,
90, 95.
(17) (a) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic
Chemistry; VCH: Weinheim, 1985; pp 3-11. (b) Eliel, E. L.; Wilen,
S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley-
Interscience: New York, 1994; pp 502-507.
(18) For a discussion of errors associated with the calculation of rate and
∆G* from coalescence data, see (a) Rix, F. C.; Brookhart, M.; White,
P. S. J. Am. Chem. Soc. 1996, 118, 2436. (b) Eliel, E. L.; Wilen, S.
H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley-
Interscience: New York, 1994; p 504.
IC010180T
1
(22) In the VT H NMR spectrum (C2D2Cl4) of 6 at 27 °C, the aromatic
protons were observed as broad multiplets between δ 6.9-7.3. A broad
triplet and a broad multiplet were observed at δ 0.91 and 1.08, which
were assigned to the protons of the ethyl substituent.
(23) For a discussion of averaged symmetry, residual stereoisomers, and
the NMR of conformationally mobile systems, see Eliel, E. L.; Wilen,
S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley-
Interscience: New York, 1994; pp 54-58.
(24) The alternate explanation for the observation of two tert-butyl proton
signals is that inversion on the NMR time scale is slow and requires
the formation of only one diastereoisomer. Although less likely, this
explanation cannot be totally ruled out.
(19) OÄ ki, M. Applications of Dynamic NMR Spectroscopy to Organic
Chemistry; VCH: Weinheim, 1985; pp 140-154.
(20) (a) OÄ Ki, M.; Iwamura, H.; Nishida, T. Bull. Chem. Soc. Jpn. 1968,
41, 656. (b) OÄ Ki, M.; Akashi, H. K.; Yamamoto, G,; Iwamura, H.
Bull. Chem. Soc. Jpn. 1971, 44, 1683.
(21) A MS of the starting ethylgermanium(IV) trichloride suggested the
hydrolysis to the corresponding 1,1,3,3-tetrachloro-1,3-dimethyldi-
germoxane prior to use.
(25) (a) Sau, A. C.; Holmes, R. R. Inorg. Chem. 1981, 20, 4129. (b) For
related work, see Day, R. O.; Holmes, J. M.; Sau, A. C.; Holmes, R.
R. Inorg. Chem. 1982, 21, 281.