The addition of terminal alkynes to dimesitylfluorenylidenegermane

Article abstract of DOI:10.1139/cjc-2013-0532

A variety of terminal alkynes were added to dimesitylfluorenylidenegermane, Mes₂Ge=CR₂ (where CR₂ = fluorenylidene).The addition of phenylacetylene and 1-hexyne to Mes₂Ge=CR₂ gave a germacyclohexene v

Full text of DOI:10.1139/cjc-2013-0532

462
ARTICLE
The addition of terminal alkynes to
dimesitylfluorenylidenegermane
Nada Y. Tashkandi, Laura C. Pavelka, Margaret A. Hanson, and Kim M. Baines
Abstract: A variety of terminal alkynes were added to dimesitylfluorenylidenegermane, Mes2Ge=CR2 (where CR2 = fluorenylidene).
The addition of phenylacetylene and 1-hexyne to Mes2Ge=CR2 gave a germacyclohexene via a cycloaddition where the germene acts
as the 4␲ component and the alkyne as the 2␲ component. Through the use of a mechanistic probe, trans-(2-phenylcyclopropyl)-
acetylene, the reaction was postulated to proceed through a concerted [2+4] cycloaddition. The addition of ethoxyacetylene to the
germene produced both a [2+2] cycloadduct, a germacyclobutene, and a [2+4] cycloadduct, a germacyclohexene. The results of this
study are compared to the results of the addition of alkynes to Mes2Ge=CHCH2-t-Bu.
Key words: germene, alkyne, cycloaddition, mechanism.
Résumé : Divers alcynes terminaux ont été ajoutés au dimesitylfluorenylidènegermane, Mes2Ge=CR2 (où CR2 = fluorenylidène).
L’addition du phénylacetylène et du hex-1-yne au Mes2Ge=CR2 a produit un germacyclohexène par une cycloaddition lors de
laquelle le germène agit comme le composant 4␲ et l’alcyne comme le composant 2␲. En utilisant une sonde mécanistique,
trans-(2-phénylcyclopropyl)acétylène, on suppose que la réaction s’effectue par l’intermédiaire d’une cycloaddition concertée [2+4].
L’addition d’éthoxyacetylène au germène a produit a` la fois un cycloadduit [2+2], le germacyclobutène, et un cycloadduit [2+4],
le germacyclohexène. Les résultats de la présente étude sont comparés a` ceux de l’addition d’alcynes au Mes2Ge=CHCH2t-Bu.
[Traduit par la Rédaction]
Mots-clés : germène, alcyne, cycloaddition, mécanisme.
an acetylide anion, and the cycle can continue until it is quenched,
Introduction
presumably by residual Mes2GeF(CH=CH2) (7; Scheme 4).6
Cycloaddition reactions of group 14 metallenes have been attracting
the interest of chemists for more than 40 years due to the high levels of
regio- and stereospecificity.1 The addition of alkynes to Brook silenes
(Me3Si)2Si=C(R)(OSiMe3) is a classic example; the reaction produces
silacyclobutenes rapidly and quantitatively (Scheme 1).2 Through the
use of a phenyl cyclopropyl-substituted alkyne mechanistic probe,3
the cycloaddition of alkynes to Brook silenes was determined to
proceed via a biradical intermediate.4
In contrast, when a variety of terminal alkynes were added to
the neopentylsilene 1, a preference for the formation of a C–H
insertion product over the formation of formal ene and/or [2+2]
cycloadduct(s) was observed (Scheme 2).5
A comparison between the reactivity of neopentylsilene 1 with
that of neopentylgermene 2 towards alkynes revealed similar ac-
tivity. Like silene 1, three different modes of reactivity were ob-
served for germene 2: insertion of the acetylenic C–H bond to give
germylacetylenes 3, ene addition to give vinylgermanes 4, and
cycloaddition to give germacyclobutenes 5 (Scheme 3).6
A biradical pathway was proposed to account for the formation
of the cycloadducts with aromatic alkynes based on the effect of
the substituents on the rate of the reaction.6 A chain reaction
mechanism was proposed to explain the rapid formation of ger-
mylacetylenes (3). The initiator of the reaction is believed to be a
trace amount of a strong base, creating the acetylide anion, which
then adds to germene 2 producing the ␣-germyl carbanion 6. The
carbanion can deprotonate another equivalent of alkyne, reforming
The presence of the ␣-hydrogen in germene 2 facilitates the
formation of the ene adduct 4, while the basicity of the anion
generated from 2 (6) leads to the CH-insertion product 3. In con-
trast, dimesitylfluorenylidenegermane, 8, lacks an ␣-hydrogen,
which should eliminate the formation of ene-type adducts. More-
over, the amount of C–H insertion product may be reduced since
the corresponding anion is less basic than anion 6. Thus, we ex-
amined the addition of terminal alkynes to dimesitylfluorenyli-
denegermane, 8, with the expectation that the reactions will not
be as complex as those of germene 2.
Mechanistic studies of metallenes are challenging: the reactiv-
ity of metallenes towards air and moisture as well as many com-
mon functional groups limits the variability of substituents and
solvents that can be utilized. Our group has employed cyclopropyl-
based hypersensitive mechanistic probes as a means to understand
the reactivity of metallenes and their reaction pathways. Trans-(2-
phenylcyclopropyl)acetylene3 can be used as a probe to detect
the formation of a reactive vinylic intermediate. During the
cycloaddition of the alkyne moiety, rapid and regioselective cy-
clopropyl ring opening can occur upon the formation of a vinylic
biradical or cationic intermediate. In contrast, no rearrangement
of the probe will occur during a concerted addition. Thus, the
isolation of ring-opened adducts of the alkyne probe provides strong
evidence for the formation of an intermediate. In an attempt to
elucidate the mechanism of the addition of alkynes to germenes, we
Received 15 November 2013. Accepted 19 February 2014.
N.Y. Tashkandi, L.C. Pavelka, M.A. Hanson, and K.M. Baines. Department of Chemistry, University of Western Ontario, London, ON N6A 5B7,
Canada.
Corresponding author: Kim M. Baines (e-mail: kbaines2@uwo.ca).
This article is part of a Special Issue commemorating the 14th International Conference on the Coordination and Organometallic Chemistry of Germanium, Tin, and Lead
(ICCOC-GTL 2013) held in Baddeck, NS, 2013.
Can. J. Chem. 92: 462–470 (2014) dx.doi.org/10.1139/cjc-2013-0532
Published at www.nrcresearchpress.com/cjc on 25 February 2014.

Tashkandi et al.
463
Scheme 1.
acetylene to germene 8 yielded two diastereomers, 10c and 10d,
in a ratio of 1:1. The coupling constants extracted from the multip-
lets of the two cyclopropyl spin systems showed maintenance of
the original trans relationship between the cyclopropyl substitu-
ents. The regiochemistry of compounds 10a and 10b was con-
firmed by X-ray diffraction (Figs. 1 and 2, respectively), whereas
the regiochemistry of 10c and 10d was assigned based on the
known regiochemistry of 10a and 10b. The germacyclohexene
ring is puckered in a half chair conformation (Figs. 1 and 2). All
bond lengths and angles are within normal ranges. In contrast, no
reaction was observed upon the addition of t-butylacetylene and
trimethylsilylacetylene to germene 8, even with gentle heating
for 2 d.
The addition of ethoxyacetylene to germene 8 showed different
reactivity. In this case, both the six-membered ring germacy-
clohexene 10e and the four-membered ring germacyclobutene 14
are formed in the reaction (Scheme 7). The ratio of 10e to 14 was 1:1
when the reaction was performed in Et2O. This ratio changed to
1:3, respectively, when THF was employed as the solvent of the
reaction. Germacyclohexene 10e and germacyclobutene 14 were
the only products of the reaction and they were identified as a
mixture by 1H NMR spectroscopy due to their sensitivity to mois-
ture.
have also examined the addition of trans-(2-phenylcyclopropyl)-
acetylene to germene 8.
Results
Germene 8 was synthesized prior to each reaction and used
in situ without purification. A pale yellow ethereal solution of
fluorenylfluorodimesitylgermane, 9, was cooled in a dry ice/ace-
tone bath and then treated with t-BuLi. To avoid the formation
of reduction products, less than 1 equiv. of t-BuLi was utilized, and
thus, germene 8 was always contaminated with residual fluo-
rogermane 9. After warming the solution to room temperature,
the colour of the solution changed from pale yellow to bright
orange.7 The orange solution of germene 8 is extremely air and
moisture sensitive; the colour of the solution changes dramatically
from bright orange to olive-green upon exposure to traces of mois-
1
ture or oxygen. The presence of germene 8 was confirmed by H
Germacyclobutene 14 hydrolyzes rapidly upon exposure to
air, yielding germanol 15, which then isomerizes to 16 upon
adsorption to silica (Scheme 8). The highest mass signals in the
mass spectra of 15 and 16 were observed at m/z 564.2078 and
564.2106, respectively, corresponding to a 1:1 adduct between
ethoxyacetylene and germene 8 and one molecule of H2O for each
isomer. The 1H NMR spectrum of 15 revealed a singlet at 4.54 ppm,
which was assigned to the sp3 C–H in the fluorenyl moiety; the
absence of coupling is consistent with the regiochemistry as-
signed. A strong absorption at 3444 cm−1 in the IR spectrum of 16
was assigned to the hydroxyl group. The key to assigning the
regiochemistry in 16 lies in the identification of the linker be-
tween the Mes2GeOH and the fluorenyl moiety. Two signals were
observed in the 13C NMR spectrum of 16 at 159.30 and 119.73 ppm,
characteristic of a vinyl ether moiety. Neither of these signals
correlated to a signal in the 1H NMR spectrum of 16 as was evident
in the 13C–1H gHSQC spectrum of 16. Furthermore, there were no
signals in the 1H NMR spectrum of 16 between 4 and 6 ppm indi-
cating the absence of a vinylic-type hydrogen. A correlation was
nuclear magnetic resonance (NMR) spectroscopy before subse-
quent reactions. The germene was then dissolved in C6D6, diethyl
ether, or tetrahydrofuran (THF), and excess alkyne was added.
The reaction was kept at room temperature and monitored by
1H NMR spectroscopy for up to 3 d. Germacyclohexene 10 was
the only product formed from the reaction of germene 8 with
phenylacetylene, 1-hexyne, or trans-(2-phenylcyclopropyl)acetylene
(Scheme 5). The reactions of germene 8 with phenylacetylene or
trans-(2-phenylcyclopropyl)acetylene were faster than that with 1-
hexyne; the addition of phenylacetylene or trans-(2-phenylcyclopropyl)
acetylene to 8 was complete within minutes, whereas the addition
of 1-hexyne required approximately 24–72 h to go to completion.
The products were isolated as yellow oils by preparative silica gel
chromatography and were identified by 1H, 13C, 1H–1H–COSY (cor-
relation spectroscopy), 13C–1H gHSQC (gradient heteronuclear
single quantum coherence), and 13C–1H gHMBC (gradient hetero-
nuclear multiple bond coherence) NMR and Fourier transform
infrared (FT-IR) spectroscopy, electron impact mass spectrome-
try (EI-MS), and, in some cases, X-ray crystallography. Due to
similarities between the spectroscopic data of 10a–10d, only
the data of 10a will be discussed.
observed between the signals at 159.30 and 119.73 ppm in the 13
C
NMR spectrum and the signal at 3.32 ppm in the 1H NMR spectrum
of 16, which was assigned to the CH2Ge moiety. Moreover, the
signal at 3.32 ppm is a singlet and integrates for two hydrogens. If
ethoxyacetylene had added to germene 8 with the opposite regio-
chemistry followed by hydrolysis (that is, to produce 17), the cor-
responding signal would integrate for a single hydrogen and
exhibit coupling to the vinylic hydrogen. Furthermore, if 17 were
indeed the structure, the characteristic signals for the vinyl ether
moiety would be absent. All data observed are completely consistent
with the regiochemistry elucidated for 16.
The high resolution mass spectral data of 10a and the isotopic
pattern of the signal assigned to the molecular ion were consis-
1
tent with the molecular formula GeC39H36. The H NMR spectral
data of germacyclohexene 10a were similar to those reported for
the germaisoquinoline 11 formed upon the addition of benzoni-
trile to germene 88 and oxagermin 12 produced in the addition of
aldehyde 13 to germene 8 (Scheme 6).9
1
Broad singlets were observed at 1.64 and 2.32 ppm in the H
NMR spectrum of 10a and were assigned to two ortho-methyl
groups of nonequivalent mesityl substituents. A singlet at
4.65 ppm was observed and assigned to the Ge–C(saturated)H in the
germacyclohexene ring. The chemical shift of this signal was in
reasonable agreement with the chemical shifts of the analogous
signals assigned to the Ge–CH in germaisoquinoline 11 (4.55 ppm)8
and oxagermin 12 (4.62 ppm).9 Integration of the signals assigned
to the fluorenyl moiety did not correspond to the eight hydrogens
present in the starting material; there was one hydrogen less than
expected. The signal at 7.03 ppm in the 1H NMR spectrum of 10a
was assigned to the vinylic hydrogen of the germacyclohexene
ring. This signal correlated to the signal assigned to the ipso car-
bon of the phenyl substituent in the 13C–1H gHMBC spectrum
of 10a. All data are in agreement with the proposed structure of
germacyclohexene 10a. The addition of trans-(2-phenylcyclopropyl)
Germacyclohexene 10e also hydrolyzes, presumably through
the hemiacetal, to yield germacyclohexanone 18 at a slower rate
(Scheme 9). The signal at the highest mass in the mass spectrum of
18 was observed at m/z 518.1668, which is consistent with the mass
of a 1:1 adduct between ethoxyacetylene and germene 8, with the
Published by NRC Research Press

464
Can. J. Chem. Vol. 92, 2014
Scheme 2.
Scheme 3.
Scheme 4.
terminal alkyne across the Ge=C of 8 was also not observed in any
of the reactions. The absence of a C–H insertion product is attrib-
uted to the low basicity of the ␣-germyl anion derived from
germene 8 (Scheme 10). The lack of a C–H insertion product sup-
ports the proposed chain mechanism for the formation of the C–H
insertion adduct in the addition of terminal alkynes to germene 2.5
The addition of trans-(2-phenylcyclopropyl)acetylene to germene 8
resulted in the formation of germacyclohexenes 10c and 10d with
the cyclopropyl ring still intact, which rules out the presence of a
cyclopropyl vinyl radical or cationic intermediate along the reac-
tion pathway. Thus, the mechanism of the addition of alkynes to
germene 8 is proposed to proceed via a concerted [2+4] cy-
cloaddition where germene 8 acts as the diene (4␲ component)
and the alkyne acts as the dienophile (2␲ component), followed
by hydrogen transfer to give the corresponding germacy-
clohexene (Scheme 11). In all cases investigated, the proposed
intermediate 1-germa-2,4-cyclohexadiene (Scheme 11) was not
observed directly. The stereochemistry of the cyclopropyl ring in
10c and 10d remains trans; it does not scramble, which rules out
the cyclopropyl ring opening and closing during the reaction.
Analogous products were also observed in the addition of cy-
clopropylaldehyde 13 to germene 8, which also resulted in the
formation of oxagermin 12 with the cyclopropyl ring still intact.9
t-Butylacetylene and trimethylsilylacetylene did not react with
germene 8, even after gentle heating for 2 d. Similarly, no cy-
cloadduct was reported upon the addition of the same alkynes to
neopentylsilene 15 or neopentylgermene 2.6 The lack of reaction
is attributed to the steric bulk of the substituents.
For a [2+4] cycloaddition reaction to take place, the diene
must be in a cisoid conformation; the fluorenylidene substitu-
ent facilitates the coplanar cisoid conformation needed for the
cycloaddition. A similar cycloaddition was observed in the ad-
dition of alkynes to P-mesityldiphenylmethylenephosphine (19;
Scheme 12) even with the phenyl group not being held in a planar
conformation.12
The regiochemistry of the addition of phenylacetylene and
1-hexyne to germene 8 was determined unambiguously from
the X-ray structures of 10a and 10b, respectively, whereas the
regiochemistry of the products derived from the addition of
ethoxyacetylene to germene 8 was inferred from the structures
elucidated for 16 and 18. The regiochemistry of germacy-
clohexenes 10a–10e is the same. Germacyclobutene 14, formed by
the addition of ethoxyacetylene to germene 8, has the same rela-
tive regiochemistry as germacyclohexenes 10 and, in addition, the
regiochemistry of 14 is consistent with that observed in the addi-
tion of alkynes to Brook silenes (Scheme 1)2 and in the addition of
aromatic alkynes to germene 2 to give germacyclobutenes 5
(Scheme 3).6 In contrast, previous work in our group showed that
the addition of ethoxyacetylene to germene 2 results in the for-
addition of H2O and the loss of an ethoxy group. The presence of
a carbonyl group was evident from the signal at 196.30 ppm in
the 13C NMR spectrum of 18. The chemical shift of this signal is
consistent with that reported for a typical ␤-germyl ketone
(194.2 ppm).10 The IR spectrum of 18 exhibits a strong absorption
at 1664 cm−1. The C=O moieties of related ␤-keto germanes absorb
at approximately 1660 cm−1 (for example, Ph3GeCH2COPh at
1661 cm−1), whereas the corresponding carbonyl moiety in ␣-keto
germanes absorb at lower wavenumbers (for example, 1629 cm−1
for Ph3GeCOPh).11 The position of the IR absorption assigned to
the carbonyl group in 18 supports the carbonyl moiety being in
the ␤-position to Ge. The signal(s) assigned to the inequivalent
CH2 hydrogens, at 3.11 and 3.39 ppm, correlate to the signal at
196.30 ppm in the 13C NMR spectrum of 18 assigned to the carbon-
yl group, to the signal assigned to one of the ipso-mesityl carbons,
to the signal assigned to the GeCH at 45.43 ppm, and to the unsat-
urated carbon on the opposite side of the carbonyl at 133.07 ppm.
Given that the 13C–1H gHMBC spectroscopy experiment is opti-
mized for three-bond correlations (8 Hz), the regiochemistry as-
signed is most consistent with the observed correlations. All these
data unambiguously confirm the regiochemistry of germacy-
clohexanone 18. Germacyclohexanone 18 was also synthesized by
treatment of the product mixture containing 10e with aqueous
NH4Cl in THF. Germacyclohexanone 18 is unstable and decom-
poses to unidentified products upon prolonged exposure to the
ambient atmosphere.
Discussion
The addition of alkyl and aromatic terminal alkynes to germene
8 is clean and quantitative and produces germacyclohexenes 10a–
10d. As anticipated, there was no evidence for the formation of an
ene adduct, which was attributed to the lack of an ␣-hydrogen in
germene 8. A product derived from insertion of the C–H of the
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Tashkandi et al.
465
Scheme 5.
Scheme 6.
Fig. 1. Thermal ellipsoid plot of 10a (30% probability surface).
Selected bond lengths (Å) and angles (°): Ge–C1 = 1.994(3), Ge–C5 =
1.963(3), Ge–C31 = 1.971(3), Ge–C22 = 1.984(3), C1–C2 = 1.490(5),
C2–C3 = 1.380(5), C3–C4 = 1.499(5), C4–C5 = 1.336(5); C5–Ge–C1 =
92.85(15), C2–C1–Ge = 107.5(2), C3–C2–C1 = 127.2(3), C2–C3–C4 =
120.5(3), C5–C4–C3 = 122.2(3), C4–C5–Ge = 122.0(3).
Fig. 2. Thermal ellipsoid plot of 10b (30% probability surface).
Selected bond lengths (Å) and angles (°): Ge–C19 = 1.924(2), Ge–C32 =
1.961(2), Ge–C1 = 1.961(2), Ge–C10 = 1.964(2), C32–C33 = 1.490(3),
C33–C21 = 1.378(3), C20–C21 = 1.467(3), C19–C20 = 1.334(3);
C20–C19–Ge = 123.75(18), C19–C20–C21 = 122.8(2), C33–C21–C20 =
121.3(2), C19–Ge–C32 = 94.40(10), C33–C32–Ge = 109.67(15),
C21–C33–C32 = 127.6(2).
mation of germacyclobutene 20 (and vinylgermane 21), which has
the opposite regiochemistry compared to germacyclobutene 14.
Complexation between the alkyne and germene 2 was suggested
to account for the unusual regiochemistry (Scheme 13).6
To gain more insight into the addition of alkynes to germenes
and the regiochemistry of the reactions, density functional theory
(DFT) calculations were performed at the TPSSTPSS/6-31G(d) level
of theory. TPSSTPSS is a nonempirical meta-generalized gradient
approximation (GGA) functional.13 Meta-GGA functionals mini-
mize the number of approximations used in the density func-
tional. TPSSTPSS has been shown to give accurate results for a
wide range of systems. In particular, we have found that it gives
results for germanium systems, which are comparable to the
popular B3LYP functional in a shorter amount of time.14 Dime-
sitylfluorenylidenegermane 8 was modeled by 1-germabutadiene
or 1,1-dimethylgermastyrene and acetylene was used to model
1-hexyne. All geometries were optimized at the TPSSTPSS/6-31G(d)
level of theory.
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466
Can. J. Chem. Vol. 92, 2014
Scheme 7.
Scheme 8.
Scheme 9.
Scheme 10.
dimethylgermastyrene, interacting with the LUMO of the alkyne,
known as a normal demand Diels–Alder reaction, or the LUMO of
the conjugated germene interacting with the HOMO of the alkyne,
known as an inverse demand Diels–Alder reaction. In the case of
phenylacetylene, the calculated differences between the energy
levels for both the HOMO of the germastyrene and the LUMO of
the alkyne and the HOMO of the alkyne and the LUMO of the
germastyrene are the same at 328 kJ/mol (3.40 eV; Table 2). How-
ever, when ethoxyacetylene or acetylene is the alkyne, the calcu-
lated energy differences between the LUMO of germastyrene and
the HOMO of the alkyne are much smaller than the calculated
differences for the opposite combinations (Table 2). Thus, we be-
lieve these reactions are best classified as inverse demand Diels–
Alder reactions. In general, inverse demand Diels–Alder reactions
are favoured when a dienophile with electron-donating substituents
reacts with a diene. The electron-donating groups on the dienophile
raise the energy of the HOMO, which decreases the energy difference
between the HOMO of the dienophile and the LUMO of the diene,
and thus, accelerates the rate of the reaction. Phenylacetylene, trans-
(2-phenylcyclopropyl)acetylene, and ethoxyacetylene are conjugated
alkynes with electron-donating groups and the addition of these
alkynes to germene 8 was complete within a few minutes, whereas
the addition of 1-hexyne to germene 8 took several days to go to
completion. Correspondingly, the energy gap between the LUMO of
1,1-dimethylgermastyrene and the HOMO of acetylene (440 kJ/mol,
4.54 eV) decreases significantly to 303 kJ/mol (3.13 eV) when the
alkyne is changed to ethoxyacetylene and to 328 kJ/mol (3.40 eV)
when it is changed to phenylacetylene (Table 2), which is in agree-
ment with the qualitative assessment of the reaction kinetics.
Thus, the formation of germacyclohexenes 10a–10d follows the
trends established for carbon chemistry. However, even though
As was observed for R2C=Ge(SiMe3)(SiMe2-t-Bu) (R2C = adamantyli-
dene),15 the highest occupied molecular orbital and lowest unoccu-
pied molecular orbital (HOMO–LUMO) energy gap of 1-germabutadiene
(257 kJ/mol, 2.67 eV) was much less than in butadiene (399 kJ/mol,
4.14 eV; Table 1).
The HOMO and LUMO surfaces of butadiene, 1-germabutadiene,
phenylacetylene, and ethoxyacetylene are show in Fig. 3. The larg-
est orbital coefficient in the LUMO of 1-germabutadiene is located
on the germanium atom (0.7633) and contributes 58% to the mo-
lecular orbital. In ethoxyacetylene, the largest orbital coefficient
in the HOMO is located on the carbon bearing the hydrogen
(0.7206, 52%) rather than the carbon bearing the ethoxy group
(0.6933, 48%), which is consistent with the observed regiochemis-
try. In acetylene and phenylacetylene, the orbital coefficients on
each carbon are equal. Since no difference in the frontier molec-
ular orbital coefficients was observed in these cases, the observed
regiochemistry is likely governed by minimization of steric inter-
actions.
Two possibilities for the reaction pathway were considered:
the HOMO of the conjugated germene, as modelled by 1,1-
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Tashkandi et al.
467
Scheme 11.
Scheme 12.
Fig. 3. Highest occupied and lowest unoccupied molecular orbitals
of (A) butadiene, (B) 1-germabutadiene, (C) phenylacetylene, and
(D) ethoxyacetylene. All molecular orbitals were calculated at the
TPSSTPSS/6-31G(d) level.
Scheme 13.
Table 1. HOMO and LUMO energies (hartree) and energy differences
between the HOMO and the LUMO (kJ/mol) for various germenes and
alkynes.
Compound
HOMO
LUMO
HOMO−LUMO gap
1-Germabutadiene
Butadiene
Acetylene
Ethoxyacetylene
Phenylacetylene
1,1-Dimethylgermastyrene
−0.176
−0.199
−0.245
−0.193
−0.203
−0.154
−0.078
−0.047
0.024
0.038
−0.051
−0.056
257
399
706
606
399
257
the reaction of 1-hexyne with germene 8 took longer to go to
completion compared to the conjugated alkynes, the reaction pro-
ceeds much faster than the analogous all-carbon systems. In com-
parison, the addition of nonactivated alkynes to dienes requires
high temperature and (or) the presence of a catalyst to proceed at
a reasonable rate.16
The substituent in ethoxyacetylene is strongly electron donat-
ing. In this case, both the dienophile (ethoxyacetylene) and the
diene (germene 8) are electron rich species. The mismatch in
electron demand presumably slows the rate of the Diels–Alder
reaction and allows for the competitive formation of another cy-
cloadduct. The mechanism for the formation of germacyclobutene
14 is likely stepwise, where the nucleophile (ethoxyacetylene) at-
tacks germene 8 to give a zwitterionic intermediate, followed by
cyclization (Scheme 14). This is in contrast to the formation of
germacyclobutene 20, from the addition of ethoxyacetylene to
germene 2, which has the opposite regiochemistry and was pro-
posed to proceed via complexation (Scheme 13). Furthermore, the
zwitterionic intermediate proposed from the nucleophilic addi-
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468
Can. J. Chem. Vol. 92, 2014
Table 2. Magnitude of the HOMO−LUMO gaps (kJ/mol)
General experimental details
for 1,1-dimethylgermastyrene and various alkynes.
All manipulations were performed in flame-dried Schlenk
tubes, or NMR tubes sealed with a septum, under an inert atmo-
sphere of argon. Benzene-d6 was distilled from LiAlH4, stored over
4 Å molecular sieves, and degassed prior to use. t-BuLi was pur-
chased from Sigma-Aldrich. The NMR standards used were resid-
ual C6D5H (7.15 ppm) for 1H NMR spectra, C6D6 central transition
(128.00 ppm) for 13C NMR spectra, residual CHCl3 (7.26 ppm) for 1H
NMR spectra, and CDCl3 central transition (77.00 ppm) for 13C
NMR spectra. IR spectra were recorded (cm−1) from thin films on a
Bruker Tensor 27 FT-IR spectrometer. Electron impact mass spec-
tra were obtained using a MAT model 8400 mass spectrometer
using an ionizing voltage of 70 eV. Mass spectral data are reported
in mass-to-charge units, m/z. Trans-(2-phenylcyclopropyl)acetylene
was synthesized as follows: reduction of trans-(2-phenylcyclopropyl)-
carboxylic acid using excess LiAlH4 produced the corresponding
alcohol, which was oxidized under Swern conditions. The result-
ing aldehyde was converted to the dibromoolefin and then to
trans-(2-phenylcyclopropyl)acetylene using Corey–Fuchs condi-
Germene
Alkyne
HOMO−alkyne
LUMO gap
HOMO−germene
LUMO gap
Alkyne
Phenylacetylene
Acetylene
Ethoxyacetylene
328
525
562
328
440
303
Scheme 14.
1
tions. The product was identified by comparison of the H NMR
data to those in the literature.6 A modified route to synthesize
fluorenylfluorodimesitylgermane (9) was developed due to the com-
plexity of the purification step in the reported procedure.9 Hydroly-
sis using an NaOH solution of chlorofluorenyldimesitylgermane
to give the hydroxy analog followed by fluorination using HF
gave 9. The overall yield (40%) is comparable to the yield from the
original route (41%).9
tion of ethoxyacetylene to germene 8 (Scheme 14) would be stabi-
lized due to the delocalization of the negative charge over the
fluorenyl group17 and the positive charge can be stabilized by the
ethoxy group. The observed increase in the ratio of the germacy-
clobutene 14 over germacyclohexene 10e when the reaction is
carried out in THF rather than in diethyl ether also supports our
proposed stepwise, zwitterionic cycloaddition mechanism, as the
solvent (THF) with the greater dielectric constant can stabilize the
zwitterionic intermediate and lead to an increase in the rate of
germacyclobutene 14 formation. Thus, the difference in regio-
chemistry between the addition of ethoxyacetylene to germenes 2
and 8 may be a consequence of the relative ability of the germenic
carbon to accommodate a negative charge.
In summary, our hypothesis that the reaction of germene 8
with alkynes would indeed be simpler than the reactions of
germene 2 because of the lack of an ␣-hydrogen and the reduced
basicity of the conjugate base of 8 proved to be correct. The addi-
tion reactions of alkynes to germene 8 are clean; they undergo
cycloaddition exclusively. As in carbon chemistry, a [4+2] cy-
cloaddition pathway is favoured over a [2+2] cycloaddition
pathway unless an extremely electron-rich alkyne is utilized.
Rearrangement of the cyclopropyl ring did not occur during the
addition of the alkynyl mechanistic probe, trans-(2-phenylcyclopropyl)-
acetylene, to germene 8, which rules out the presence of a radical or
cationic intermediate along the reaction pathway. Thus, the forma-
tion of germacyclohexenes 10a–10e is proposed to occur via a
concerted [2+4] cycloaddition followed by hydrogen transfer. The
addition of ethoxyacetylene to germene 8 produced germacy-
clobutene 14 and germacyclohexene 10e, where 14 is likely formed
via a stepwise [2+2] cycloaddition through a zwitterionic interme-
diate. The difference in reaction rate between aromatic and ali-
General procedure for the reaction of alkynes with
germene 8
A solution of fluorenylfluorodimesitylgermane (50 mg, 0.1 mmol)
dissolved in diethyl ether (3 mL) was cooled to −78 °C. t-BuLi (0.06 mL,
1.7 mol/L in pentane, 0.1 mmol) was added slowly. The colour of the
solution changed from pale yellow to bright orange. The reaction
mixture was allowed to stir at room temperature for 2 h. An
aliquot was analyzed by 1H NMR spectroscopy (C6D6) and was
found to contain germene 8 (ϳ88%). The ether was removed in
vacuo (if required), yielding a bright orange residue, and the res-
idue was dissolved in C6D6 (1.1 mL). Alkyne (1.2 equiv.) was added
directly to the germene solution. The progress of the reaction was
monitored by 1H NMR spectroscopy for up to 3 d. The solvent was
removed by rotary evaporation yielding a yellow residue. The ra-
tio of products in the crude reaction mixture was determined by
1H NMR spectroscopy. The product mixture was contaminated
with residual alkyne and unreacted fluorogermane 9. The prod-
ucts were separated from fluorogermane 9 and residual alkyne by
preparative thin layer chromatography on silica gel (hexanes and
dichloromethane, 50:50) followed by second preparative thin
layer chromatography on silica gel (hexanes) yielding yellow oil
(mixed with a colourless solid in the case of 10a and 10b), 10a–10d
(43%–45%).
phatic alkynes toward germene
8 reveals that conjugated
10a
germenes mimic carbon-based dienes in terms of their cyclo-
addition chemistry. We continue to investigate the cycloaddition
reactions of germenes with the goal of understanding their reac-
tivity more completely.
Experimental
DFT calculations
First principles calculations were performed using Gaussian
0918 on the Shared Hierarchical Academic Research Computing
Network (SHARCNET, www.sharcnet.ca). Calculations were per-
formed on an 8 core Xeon 2.83 GHz CPU with 16 GB memory. All
calculations were performed at the TPSSTPSS13/6-31G(d) level of
theory. Molecular orbitals were calculated using NBO version 3.
Cube files were generated using the cubegen utility and visualized
in GaussView 3.09.
IR (thin film, cm−1): 2922 (s), 2851 (s), 1731 (m), 1603 (m), 1449 (s),
1377 (m), 1264 (m), 1028 (m), 847 (m), 810 (m), 740 (m). H NMR
(CDCl3, 600 MHz) ␦: 7.89 (d, 1H, G, J = 7.8 Hz), 7.76 (d, 1H, A, J =
1
Published by NRC Research Press

Tashkandi et al.
469
7.8 Hz), 7.44–7.32 (m, 6H, Ph-H and F), 7.32–7.28 (m, 1H, D), 7.27–
7.22 (m, 2H, B and E), 7.03 (s, 1H, L), 6.93 (d, 1H, C, J = 7.2 Hz), 6.91
(s, 2H, m-Mes H), 6.55 (s, 2H, m-Mes-H), 4.65 (s, 1H, Q), 2.32 (br s, 9H,
o,p-Mes), 2.11 (s, 3H, p-Mes), 1.64 (br s, 6H, o-Mes). 13C NMR (CDCl3,
100 Hz) ␦: 150.92 (N), 145.80 (J), 143.81 (i-Ph), 143.24 (o-Mes), 142.98
(K), 141.56 (I), 141.53 (o-Mes), 140.37 (H), 138.51 (p-Mes), 138.07 (p-
Mes), 137.71 (i-Mes), 134.81 (i-Mes), 134.06 (M), 132.91 (L), 129.40 (m-
Mes), 128.67 (o-Ph), 128.26 (m-Mes), 128.10 (m-Ph), 127.33 (p-Ph),
126.60 (C), 126.37 (E), 126.12 (B), 125.76 (F), 124.35 (D), 120.16 (G),
119.27 (A), 41.88 (GeCH), 25.56 (br s, o-Mes), 22.71 (o-Mes), 21.03
(p-Mes), 20.80 (p-Mes). High-resolution EI-MS for C39H3670Ge (m/z),
calcd.: 574.206; found: 574.204.
1 Hz), 6.92 (s, 1H, L), 6.86 (m, 1H, Ph), 6.85 (s, 4H, m-Mes2), 6.41 (s,
2H, m-Mes1), 6.39 (s, 2H, m-Mes1), 4.64 (s, 1H, Q), 4.54 (s, 1H, Q), 2.46
(s, 6H, o-Mes2), 2.43 (s, 6H, o-Mes2), 2.24–2.20 (m, T=), 2.18, 2.17 (s
each, 6H total, p-Mes2), 2.15–2.10 (m, T), 2.08–2.02 (m, R=), 1.87 (s,
3H, p-Mes1), 1.85 (s, 3H, p-Mes1), 1.80 (br s, 12H, o-Mes1), 1.80–1.77
(m, R), 1.49 (ddd, 1H, S1, J = 5.4, 6.3, 9.6 Hz), 1.26 (ddd, 1H, S=, J = 4.8,
6.3, 9.0 Hz), 1.18 (ddd, 1H, S=, J = 4.8, 6.6, 9.0 Hz), 1.07 (ddd, 1H, S1, J =
4.8, 5.4, 8.4 Hz). 13C NMR (C6D6, 150 Hz) ␦: 149.43 (N), 149.34 (N),
146.42 (J), 146.56 (J), 143.62 (o-Mes2), 143.53 (o-Mes2), 143.20 (Ph),
143.16 (K), 143.07 (Ph), 142.94 (K), 142.35 (I), 142.31 (I), 141.61 (o-Mes1),
141.58 (o-Mes1), 140.93 (2H), 138.88 (p-Mes2), 138.35 (p-Mes1), 138.29
(i-Mes1), 138.09 (i-Mes1), 135.76 (i-Mes2), 135.66 (i-Mes2), 135.12 (M),
135.05 (M), 130.11 (m-Mes2), 129.09 (m-Mes1), 129.05 (m-Mes1), 128.89
(L), 127.15 (B), 127.05 (B), 126.94 (F), 126.92 (F), 126.50 (2E), 126.37
(Ph), 126.35 (Ph), 126.31 (L), 126.30 (Ph), 126.17 (Ph), 126.14 (Ph),
126.04 (Ph), 124.75 (D), 124.71 (D), 124.65 (C), 124.63 (C), 120.79 (G),
119.90 (A and G), 119.76 (A), 42.58 (Q), 42.52 (Q), 30.41 (T), 29.79 (T=),
28.19 (R=), 25.93 (o-Mes2), 25.85 (o-Mes2), 24.53 (R), 23.21 (o-Mes1),
21.23 (p-Mes2), 20.89 (p-Mes1), 20.71 (p-Mes1), 17.96 (S=), 14.64 (S).
High-resolution EI-MS for C42H4070Ge m/z, calcd.: 614.2372; found:
614.2373. (Primes or numbers indicate the signals belong to the
same diastereomer.)
10b
Addition of ethoxyacetylene to germene 8
A solution of fluorogermane 9 (50 mg, 0.1 mmol) dissolved in
diethyl ether (3 mL) was cooled to −78 °C. t-BuLi (0.06 mL, 1.7 mol/L
in pentane, 0.1 mmol) was added slowly. The colour of the solution
changed from pale yellow to bright orange. The reaction mixture
was allowed to stir at room temperature for 2 h. An aliquot was
analyzed by 1H NMR spectroscopy (C6D6) and was found to contain
germene 8 (ϳ88%). The ether was removed in vacuo (if required),
yielding a bright orange residue. The residue was dissolved in
one of three solvents: C6D6, diethyl ether, or THF (1.1 mL).
Ethoxyacetylene (1.2 equiv.) was added directly to the germene
solution. The solvent was removed by rotary evaporation yield-
ing a yellow residue. The ratio of products (10e:14) in the crude
reaction mixture was determined by 1H NMR spectroscopy. The
product mixture was contaminated with residual alkyne and
unreacted fluorogermane 9. The mixture was separated by prepar-
ative thin layer chromatography on silica gel (hexanes and dichlo-
romethane, 50:50) to give contaminated samples of 10e and 14.
Further attempts at purification by chromatography resulted in
the hydrolysis of 10e and 14; compounds 15, 16, and 18 were
isolated from the plate. Compound 15 underwent isomerization
to 16 in solution. A third attempt to purify compounds 16 and 18
by preparative thin layer chromatography on silica gel (hexanes)
gave 16 in acceptable purity, however, 18 was contaminated with
Mes2GeOHCHR2.7
1H NMR (CDCl3, 600 MHz) ␦: 7.86 (d, 1H, D, J = 7.8 Hz), 7.74 (d, 1H,
A, J = 7.8 Hz), 7.37–7.31 (m, 4H, B, G, and C), 7.23–7.19 (m, 2H, F and
E), 6.91 (s, 2H, m-Mes-H), 6.79 (s, 1H, L), 6.51 (s, 1H, m-Mes-H), 4.45 (s,
1H, L), 2.87 (dt, 1H, P, J = 7.8, 14.4 Hz), 2.53 (dt, 1H, P, J = 7.8, 14.9 Hz),
2.32 (s, 9H, o,p-Mes), 2.08 (s, 3H, p-Mes), 1.63–1.57 (m, 8H, o-Mes and
Q), 1.4 (m, 2H, R), 0.93 (t, 3H, S, J = 7.2 Hz). 13C NMR (CDCl3, 150 Hz)
␦: 149.07 (N), 145.96 (K), 143.22 (bs, o-Mes), 142.73 (I), 141.57 (J), 141.38
(o-Mes), 140.36 (H), 138.38 (p-Mes), 138.21 (i-Mes), 137.96 (p-Mes),
135.27 (i-Mes), 133.80 (M), 129.34 (m-Mes), 129.00 (L), 128.17 (m-Mes),
126.25 (B), 126.23 (F), 125.61 (E), 124.23 (D), 123.30 (C), 120.04 (G),
118.87 (A), 42.00 (Ge-CH), 37.21 (P), 30.48 (Q), 25.5 (br s, o-Mes), 22.60
(o-Mes), 22.50 (R), 21.05 (p-Mes), 20.78 (p-Mes), 14.02(S). High-
resolution EI-MS for C37H4070Ge (m/z), calcd.: 554.2373; found:
554.2369. Anal. calcd. for C37H4070Ge: C 79.73, H 7.23; found: C
75.36, H 7.13.
10c and 10d
10e
Yellow oil contaminated with fluorene and other impurities
(1:0.8). 1H NMR (C6D6, 600 MHz) ␦: 7.96 (d, 1H, J = 7.2 Hz), 7.78 (d, 1H, J =
7.8 Hz), 7.71 (d, 1H, J = 7.8 Hz), 7.51 (d, 1H, J = 7.2 Hz), 7.35 (t, 1H, J =
7.2 Hz), ϳ7.23 (m; all for fluorenyl-H), 6.85 (s, 2H, m-Mes), 6.44 (s, 2H,
m-Mes), 5.58 (s, 1H, vinyl H), 4.58 (s, 1H, Ge-CH), 3.70 (dq, 1H, OCH, J =
16.8, 6 Hz), 3.52 (dq, 1H, OCH, J = 16.2, 6.6 Hz), 2.47 (br s, 6H, o-Mes),
2.18 (s, 3H, p-Mes), 1.90 (s, 3H, p-Mes), 1.80 (br s, 6H, o-Mes), 1.15 (t, 3H,
CH3, J = 6.6 Hz).
Mixture of diastereomers, a colourless solid contaminated with
[Mes2Ge(fluorenyl)]2O7 (1:1:0.13) and other minor impurities. 1H
NMR (C6D6, 600 MHz) ␦: 7.79 (d, 1H, G, J = 7.8 Hz), 7.78 (d, 1H, G, J =
7.8 Hz), 7.67 (d, 1H, A, J = 7.8 Hz), 7.62 (d, 1H, A, J = 7.8 Hz), 7.57 (d,
1H, C, J = 7.8 Hz), 7.52 (d, 1H, C, J = 7.2 Hz), 7.50 (d, 2H, 2D, J = 7.2 Hz),
7.25 (m (2× td), 2H, 2E), 7.21 (t, 1H, B, J = 7.2 Hz), 7.16–7.14 (m, B + F +
Ph), 7.12 (t, F, J = 7.8 Hz), 7.07–7.02 (m, 5H, Ph), 6.99 (d, 1H, L, J =
14
Yellow oil. 1H NMR (C6D6, 600 MHz) ␦: 7.68 (d, 2H, J = 7.2 Hz), 7.29
(d, 2H, J = 7.8 Hz), 7.16 (t, 2H, J = 7.8 Hz), 7.00 (t, 2H, J = 7.2 Hz; all for
fluorenyl-H), 6.66 (s, 4H, m-Mes), 5.48 (s, 1H, vinyl H), 3.56 (q, 2H,
OCH2 J = 7.2 Hz), 2.12 (br s, 12H, o-Mes), 2.06 (s, 6H, p Mes), 0.74 (t,
3H, CH3, J = 6.6 Hz).
Published by NRC Research Press

470
Can. J. Chem. Vol. 92, 2014
15
Supplementary data
Yellow oil. 1H NMR (C6D6, 400 MHz) ␦: 7.58–7.54 (m, 2H), 7.2 (m,
Supplementary data (figures containing NMR spectra for all
new compounds, details of the X-ray structure determinations of
10a and 10b and tables summarizing the computational data) are
available with the article through the journal Web site at http://
nrcresearchpress.com/doi/suppl/10.1139/cjc-2013-0532. CCDC 960004
and 960005 contain the X-ray data in CIF format for this manuscript.
These data can be obtained, free of charge, via http://www.ccdc.
cam.ac.uk/products/csd/request (Or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2 1E2, UK;
fax: +44 1223 33603; or e-mail: deposit@ccdc.cam.ac.uk).
6H; all fluorenyl-H), 6.72 (s, 4H, m-Mes), 5.25 (s,1H, vinyl), 4.54 (s,1H,
CHR2), 3.09 (q, 2H, J = 7.2 Hz, OCH2), 2.56 (s, 12H, o-Mes), 2.08 (s, 6H,
p-Mes), 0.49 (t, 3H, J = 7.2 Hz, OCH2CH3). High-resolution EI-MS for
C35H38O274Ge m/z, calcd.: 564.2088; found: 564.2078.
16
Acknowledgements
Computational work was made possible by the facilities of
the Shared Hierarchical Academic Research Computing Net-
work (SHARCNET; www.sharcnet.ca) and Compute/Calcul Can-
ada. We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of Western Ontario
for financial support, King Abdul Aziz University (Saudi Arabia)
for a scholarship to N.Y.T., and Aneta Borecki for the X-ray struc-
ture determination of 10b.
Yellow oil. IR (thin film, cm−1): 3444 (br s), 2923 (m), 1612 (w),
1734 (w), 1602 (w), 1303 (s), 1025 (w), 758 (s). 1H NMR (C6D6, 400 MHz)
␦: 8.54 (dt, 1H, M, J = 7.8, 0.8 Hz), 7.86 (d, 1H, G, J = 7.6 Hz), 7.72 (dt,
1H, A, J = 7.8, 0.8 Hz), 7.68 (ddd, 1H, D, J = 6.8, 1.2, 0.8 Hz), 7.39 (td,
1H, C, J = 7.8, 1.2 Hz), 7.27 (td, 1H, B, J = 7.6, 1.2 Hz), 7.12 (td, 1H, F, J =
7.6, 1.2 Hz), 7.07 (td, 1H, E, J = 7.2, 1.6 Hz), 6.60 (s, 4H, m-Mes), 3.50
(q, 2H, O-CH2, J = 6.8 Hz), 3.32 (s, 2H, L), 2.37 (s, 12H, o-Mes), 2.03 (s,
6H, p-Mes), 0.92 (t, 3H, CH3, J = 7.2 Hz). 13C NMR (C6D6, 100 Hz) ␦:
159.30 (N), 143.01 (o-Mes), 139.97 (J), 139.38 (p-Mes), 139.01 (K), 138.22
(H), 138.11 (I), 136.09 (i-Mes), 129.83 (m-Mes), 127.50 (C), 126.67 (E),
126.40 (B), 126.21 (M), 125.89 (F), 123.30 (G), 120.27 (D), 119.73 (Q),
119.65 (A), 64.63 (OCH2), 27.83 (L), 23.85 (o-Mes), 21.16 (p-Mes), 15.02
(CH3). High-resolution EI-MS for C35H38O274Ge m/z, calcd.: 564.2088;
found: 564.2106.
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Yellow oil contaminated with Mes2GeOHCHR27 in a 1:0.5 ratio. IR
(thin film, cm−1): 2919 (m), 2849 (m), 1665 (s), 1601 (m), 1448 (s), 1415
(m), 1414 (m), 1276 (m), 1261 (m), 1025 (s), 849 (m), 795 (s), 765 (m), 737
(s). 1H NMR (C6D6, 600 MHz) ␦: 8.2 (d, 1H, C, J = 7.8 Hz), 7.71 (d, 1H, G, J =
7.2 Hz), 7.70 (d, 1H, A, J = 6.6 Hz), 7.34 (d, 1H, D, J = 7.8 Hz), 7.18 (t, 1H,
B, J = 7.2 Hz), ϳ7.22 (E, overlap with signal from the germol), 7.11 (t,
1H, F, J = 7.8 Hz), 6.74 (s, 2H, m-Mes2), 6.35 (s, 2H, m-Mes2), 4.23 (s, 1H,
Q), 3.39 (d, 1H, L, J = 12 Hz), 3.11 (d, 1H, L, J = 12 Hz), 2.20 (s, 6H, o-Mes2),
2.10 (s, 3H, p-Mes2), 1.82 (s, 3H, p-Mes1), 1.60 (s, 6H, o-Mes1). 13C NMR
(C6D6, 100 Hz) ␦: 196.30 (N), 147.98 (K), 145.59 (J), 143.04 (o-Mes1), 142.05
(o-Mes2),141.81 (H), 141.37 (I), 139.47 (p-Mes2), 138.78 (p-Mes1), 134.99
(i-Mes2), 133.48 (i-Mes1), 133.07 (M), 130.03 (m-Mes2), 129.27 (m-Mes1),
127.34 (F), 127.04 (B), 126.55 (E), 126.38 (C), 124.55 (D), 124.18 (A), 120.61
(G), 45.43 (Q), 42.74 (L), 24.70 (o-Mes2), 22.43 (o-Mes1), 20.98 (p-Mes2),
20.70 (p-Mes1). High-resolution EI-MS for C33H32O74Ge m/z, calcd.:
518.1642; found: 518.1668.
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