The photolysis of Si,Si-di-tert-butyltetramesitylsiladigermirane in the presence of methylmagnesium iodide

Article abstract of DOI:10.1139/v00-058

The photolysis of Si,Si-di-tert-butyltetramesitylsiladigermirane in the presence of methylmagnesium iodide at 15°C has been investigated. Five products were isolated and identified from the complex product mixture: mesityldimethylgermane, dimesitylmethylg

Full text of DOI:10.1139/v00-058

1474
The photolysis of Si,Si-di-tert-
butyltetramesitylsiladigermirane in the presence
of methylmagnesium iodide
Mini S. Samuel, Kim M. Baines, and Donald W. Hughes
Abstract: The photolysis of Si,Si-di-tert-butyltetramesitylsiladigermirane in the presence of methylmagnesium iodide at
15°C has been investigated. Five products were isolated and identified from the complex product mixture:
mesityldimethylgermane, dimesitylmethylgermane, (di-tert-butylmethylsilyl)mesitylmethylgermane, meta-(di-tert-
butylmethylsilyl)toluene, and para-(di-tert-butylmethylsilyl)toluene. The characterization of these compounds and pro-
posed mechanisms for their formation will be discussed.
Key words: germasilene, Grignard reagents, germylmagnesium compounds.
Résumé : Opérant à 15°C, on a étudié la photolyse du Si,Si-di-tert-butyltétramésitylsiladigerminane en présence
d’iodure de méthylmagnésium. À partir du mélange complexe de produits, on a isolé et identifié cinq produits:
mésityldiméthylgermane, dimésitylméthylgermane, (di-tert-butylméthylsilyl)mésitylméthylgermane, méta-(di-tert-
butylméthylsilyl)toluène et para-(di-tert-butylméthylsilyl)toluène. On discute de la caractérisation de ces produits et des
mécanismes proposés pour leur formation.
Mots clés : germasilène, réactifs de Grignard, composés germylmagnésium.
[Traduit par la Rédaction] Samuel et al. 1478
Introduction
In a recent publication, we reported on a novel route to
germyl Grignard reagents by the addition of methylmag-
nesium halide to Mes2Ge=GeMes2 and Mes2Si=GeMes2 at
low temperature (Mes = 2,4,6-trimethylphenyl) (1). The ad-
dition to the germasilene is completely regioselective with
the alkyl group adding to the silicon atom (eq. [1]).
[2]
involving the α-elimination of MesMgX followed by the ad-
dition of MeMgX to the intermediate germylene has been
proposed to explain the observed ligand exchange
(Scheme 1). Evidence for the formation of the germylene
was provided by trapping the intermediate with 2,3-
dimethylbutadiene.
[1]
Scheme 1.
Furthermore, we discovered that thermolysis of the
germyl Grignard reagents in the presence of excess MeMgX
lead to a facile ligand exchange reaction in which the
mesityl groups on the germanium bonded to the magnesium
were exchanged for methyl groups (eq. [2]). A mechanism
Received July 1, 1999. Published on the NRC Research Press
website on November 2, 2000.
Dedicated to Professor Adrian G. Brook in recognition of his
seminal contributions to the field of organosilicon chemistry.
M.S. Samuel and K.M. Baines.1 Department of Chemistry,
University of Western Ontario, London, ON N6A 5B7,
Canada.
D.W. Hughes. Department of Chemistry, McMaster
University, Hamilton, ON L8S 4M1, Canada.
To our knowledge, this novel ligand exchange in germyl
Grignard reagents has no precedent in the literature. Con-
sidering the recent interest in the generation and chemistry
of both germyl (2) and silyl (3) Grignard reagents and the
importance of germylenes as intermediates in organoger-
manium chemistry, it was desirable to investigate not only
1Author to whom correspondence should be addressed.
Telephone: (519) 661-2111 ext. 86302. Fax: (519) 661-3022.
e-mail: kbaines2@julian.uwo.ca
Can. J. Chem. 78: 1474–1478 (2000)
© 2000 NRC Canada

Samuel et al.
1475
[3]
[4]
[5]
[6]
at 15°C, yielded a complex product mixture. Through
preparative thin layer and centripetal chromatography, five prod-
ucts, 5, 6, 7, and 8 were isolated and identified in a ratio of
approximately 5:1:2:3, respectively, and were estimated to
account for 75% of the crude product mixture. Although the
major products from the reaction mixture were identified,
there were several minor unidentified products (eq. [5]).
Compounds 5, 7, and 8 were isolated in quantities of 19,
3.5, and 4.5%, respectively, (as determined by assuming that
formation of each of these compounds occurred independ-
the generality of the Grignard addition to germasilenes as a
novel approach for the synthesis of germyl Grignard re-
agents but also the generality of the ligand exchange reac-
tion. For this reason, the addition of methylmagnesium iodide
to (t-Bu)2Si=GeMes2, generated photochemically from Si,Si-
di-tert-butyltetramesitylsiladigermirane, was investigated.
Results
The photolysis of Si,Si-di-tert-butyltetramesitylsiladiger-
mirane 1 has been shown to give di-tert-butyldimesitylger-
masilene and dimesitylgermylene, regioselectively (eq. [3])
(4). Although stable solutions of tetramesityldigermene and
tetramesitylgermasilene can be prepared by photolysis of
hexamesitylcyclotrigermane and hexamesitylsiladigermirane,
respectively, at –78°C in the presence of triethylsilane, this
methodology cannot be used to generate a solution of Si,Si-
di-tert-butyldimesitylgermasilene; germasilene 2 undergoes
photodissociation to yield dimesitylgermylene 3 and di-tert-
butylsilylene 4, both of which can be trapped by
triethylsilane (eq. [4]) (4). As a result, the study of the reac-
tivity of Si,Si-di-tert-butyldimesitylgermasilene must be car-
ried out in the presence of the trapping reagent.
1
ently of the others), and were characterized by H, 13C, and
29Si NMR, IR, and mass spectrometry. Compound 6 could
not be isolated free of compound 7, and thus, compound 6
was independently synthesized and fully characterized.
Compound 5 was isolated in yields ranging from 14 to
1
23%. The identification was based on comparison of the H
NMR spectral data to literature values (5).
The volatility of 6 largely contributes to the fact that it
could not be obtained in pure form; it was difficult to isolate
it during the work-up and purification stages. Since 6 could
not be isolated, the structural assignment is based upon data
1
obtained from a H NMR spectrum of a mixture of 6 and 7.
To confirm the structure of 6, it was independently synthe-
sized from MesGeH3 by stepwise deprotonation using tert-
butyllithium followed by methylation using methyl iodide.
We were unable to synthesize 6 in one step from MesGeH3
as has been reported in the literature (eq. [6]) (6).
Si,Si-di-tert-butyltetramesitylsiladigermirane is unreactive
toward methylmagnesium iodide in the dark. Photolysis of
Si,Si-di-tert-butyltetramesitylsiladigermirane 1 dissolved in
toluene, in the presence of excess methylmagnesium iodide
© 2000 NRC Canada

1476
Can. J. Chem. Vol. 78, 2000
Scheme 2.
1
The H NMR spectrum of compound 7 revealed the pres-
Grignard reagents to germasilenes and the subsequent be-
haviour of the newly formed germyl Grignard reagents.
Following what we have previously proposed (see
Scheme 1) (1), compound 7 is believed to arise from attack
of the Grignard reagent on the silicon atom of the germa-
silene followed by an α-elimination of mesitylmagnesium
iodide. Methylmagnesium iodide could then add to the re-
sulting silylgermylene, giving 7 after hydrolysis (Scheme 2).
The regioselectivity of the addition is the same as that of
tetramesitylgermasilene: the alkyl portion of the Grignard
reagent adds only to the silicon atom of the germasilene.
The other regioisomer was not detected.
ence of two nonequivalent tert-butyl groups, one mesityl
group, a doublet at 0.63 ppm (3J = 4.4 Hz, 3H), a quartet at
4.70 ppm (3J = 4.4 Hz, 1H) and a methyl singlet at
0.23 ppm. The 29Si/1H heteronuclear multiple bond correla-
tion (HMBC) spectrum showed correlations between the sig-
nal at 10.9 ppm in the 29Si dimension to the singlets at 1.08,
0.93, and 0.23 ppm in the 1H dimension. These data together
with the molecular ion at 366 amu, support our assignment
of the structure of compound 7.
From the H, 13C NMR, and mass spectra, compound 8
was determined to be an isomeric mixture of (di-tert-
1
butylmethylsilyl)toluenes in a ratio of 3:1. The multiplet at
7.54 ppm in the H NMR spectrum of 8 appears to be one
The mechanism for the formation of compound 8 is in-
deed intriguing. There is ample precedence for the addition
of silyl radicals to aromatic compounds (7, 8). Thus, the
most reasonable explanation for the formation of 8 is from
the addition of the di-tert-butylmethylsilyl radical to toluene
followed by loss of a hydrogen atom. The source of the silyl
radical is unknown. It is possible that the germyl Grignard
reagent, t-Bu2MeSiGeR2MgI, undergoes homolysis of the
Si-Ge bond. Alternatively, the silyl radical may be derived
from t-Bu2MeSiMgBr. The germasilene 2 is known to
photodissociate to give t-Bu2Si: and Mes2Ge: (4). Addition
of MeMgI to the silylene may give t-Bu2MeSiMgBr which
may decompose to give the silyl radical. Further work is re-
quired to learn more about the nature of this reaction.
Compound 5 has previously been observed in the
photolysis of both hexamesitylcyclotrigermane and -siladi-
germirane in the presence of excess methylmagnesium io-
dide followed by hydrolysis. The formation of compound 5
is most likely the result of addition of methylmagnesium io-
dide to dimesitylgermylene followed by hydrolysis to the
germane (eq. [7]).
1
half of a typical AA′XX′ pattern associated with 1,4-
disubstituted benzenes, and thus, the minor component of
the mixture was assigned as the para isomer. Examination of
the multiplicity of the remaining four signals in the aromatic
1
region of the H NMR spectrum and the correlations ob-
served in the 2-D COSY spectrum indicated that the major
component of the mixture was the meta-substituted isomer.
This was based on the observation that all the signals in the
aromatic region, with the exception of the signal at
7.51 ppm, were split by a coupling of 7.5–8.0 Hz. Confirma-
tion of the assignment of the meta- and para isomers of 8
was made using COSY, 13C-1H, and 29Si-1H HSQC and
HMBC experiments, TOCSY and standard spin decoupling
experiments. Full details can be found in the supplementary
material.2
Discussion
Despite the low isolated yields of the compounds reported
herein and the complexity of the reaction mixture, we be-
lieve we can add to our understanding of the addition of
Compound 6, on the other hand, is believed to arise from
dimesitylmethylgermylmagnesium iodide by elimination of
mesitylmagnesium iodide to yield mesitylmethylgermylene
[7]
2 Copies of material on deposit may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Re-
search Council Canada, Ottawa, ON, Canada K1A 0S2 (http://www.nrc.ca/cisti/irm/unpub_e.shtml).
© 2000 NRC Canada

Samuel et al.
1477
[8]
and subsequent addition of methylmagnesium iodide to give
MesMe2GeMgI, which upon hydrolysis gives 6 (eq. [8]).
Thermolysis of hexamesitylcyclotrigermane in the pres-
ence of excess methylmagnesium bromide leads to the for-
mation not only of Mes2MeGeGeMe2H, but also mesitylene
and trimethylgermane (1). Trimethylgermane is believed to
be derived from two successive α-eliminations of mesityl-
magnesium halide from Mes2MeGeMgX followed, each
time, by addition of methylmagnesium iodide. Mesityl-
methylgermylene, the first formed intermediate germylene,
was trapped with 2,3-dimethylbutadiene to give the
germacyclopentene, providing convincing evidence for the
proposed mechanism (1). The isolation of compound 6 pro-
vides further evidence for the formation of this intermediate
germylene.
ether for the synthesis of Me2MesGeH) and the extracts
were combined with the organic phase. The organic phase
was dried over anhydrous magnesium sulfate and then fil-
tered to remove the solid.
NMR spectra were recorded on a Varian Gemini 200, an
XL-300, a Varian Gemini 300 or a Bruker Avance DRX-500
using benzene-d6 as a solvent. The 2-D spectra were ac-
quired using standard techniques. IR spectra were recorded
(cm–1) as thin films on a Perkin-Elmer 2000 FT IR spec-
trometer. A Finnegan MAT model 8230 mass spectrometer,
with an ionizing voltage of 70 eV, was used to obtain elec-
tron impact mass spectra (recorded in mass-to-charge units,
m/z, with ion identity and peak intensities relative to the
base peak in parentheses).
Compounds 6 and 7, which are believed to originate from
the α-elimination of MesMgI from germyl Grignard re-
agents, have been observed in the photolysis of Si,Si-di-tert-
butyltetramesitylsiladigermirane with excess methylmag-
nesium iodide at 15°C. This is in contrast to the previously
studied systems (1), where ligand exchange products were
observed well above room temperature (~105°C). The differ-
ence in the ease of the proposed α-elimination of the Grig-
nard reagents may be the result of the need to relieve steric
congestion in the case of the bulkier di-tert-butylmethylsilyl-
substituted germylmagnesium halide. However, at 15°C the
elimination of a second equivalent of MesMgI does not ap-
pear to take place, allowing for the formation of 6 and 7
rather than the products of complete ligand exchange.
In summary, the addition of Grignard reagents to germa-
silenes to give germyl Grignard reagents and the α-elimination
of mesitylmagnesium halide from mesitylgermylmagnesium
halides to give the germylenes do appear to be general reac-
tions.
Photolysis of Si,Si-di-tert-butyltetramesitylsiladigermirane
in the presence of excess methylmagnesium iodide
A solution of (t-Bu)2SiGe2Mes4 (100 mg, 0.131 mmol)
and freshly prepared methylmagnesium iodide (3 mL,
84 mmol in Et2O) in toluene (15 mL) was photolyzed for
16 h at 15°C. Following the photolysis, the reaction mixture
was quenched with aqueous ammonium chloride, followed
by an acidic work-up. Removal of the solvent in vacuo
yielded an oily residue (106.5 mg), which contained several
compounds including dimesitylmethylgermane
5
(5),
mesityldimethylgermane 6, (di-tert-butylmethylsilyl)mesityl-
methylgermane 7 and a mixture of the meta- and para-iso-
mers of (di-tert-butylmethylsilyl)toluene 8. Separation of the
components was accomplished using preparative thin layer
chromatography (Chromatotron) with hexanes:CH2Cl2 (8:2)
as the eluent. Compounds 5, 7, and 8 were isolated in 19%
(16.25 mg), 3.5% (1.65 mg), and 4.5% (1.5 mg) yields, re-
spectively. Compound 6 could not be separated from 7.
(Di-tert-butylmethylsilyl)mesitylmethylgermane (7): Colour-
less, waxy solid. IR (thin film, cm–1): 2022 (m) (Ge-H); 1H
NMR (ppm): 6.78 (s, 2H, Mes H), 4.70 (q, 1 H, J = 4.4 Hz,
Ge-H), 2.45 (s, 6 H, Mes oCH3), 2.11 (s, 3 H, Mes pCH3),
1.08 (s, 9 H, tert-butyl CH3), 0.93 (s, 9 H, tert-butyl CH3),
0.63 (d, 3 H, J = 4.4 Hz, Ge-CH3), 0.23 (s, 3 H, Si-CH3);
13C NMR (ppm): 143.05, 137.58, 136.21 (Mes C), 128.93
(Mes CH), 29.18, 29.00 (C(CH3)3), 25.45 (Mes oCH3),
21.35, 21.18 (C(CH3)3), 21.03 (Mes pCH3), –3.01 (Ge-
CH3), –7.23 (Si-CH3); 29Si NMR (ppm): 10.9; MS (m/z):
366 (M+, 3), 208 (MesGeMe, 34), 193 (MesGe, 39), 119
(Mes, 19), 84 (42), 73 (SiMe3, 100), 49 (69); High resolu-
tion MS: calc. for C19H36SiGe: 366.1798, found: 366. 1801.3
Experimental
All experiments were carried out in flame-dried glassware
under an inert argon atmosphere. Toluene, diethyl ether and
THF were freshly distilled from sodium–benzophenone.
Methylmagnesium iodide was prepared from commercially
available methyl iodide (BDH) and magnesium (Lancaster
Chemical Co.), stored under argon and the concentration
was determined by titration, prior to use. Mesitylgermane
was prepared following the published procedure (9). Chro-
matography was carried out using a Chromatotron (Harrison
Research) or on conventional silica gel preparative plates.
Photolyses were carried out at 350 nm using a Rayonet Pho-
tochemical Reactor.
(Di-tert-butylmethylsilyl)toluene (mixture of meta-isomer
and para-isomer) (8): Colourless, waxy solid. IR (thin
film, cm–1): 3020 (w), 2962 (s), 2930 (s), 2892 (m), 2857
(s), 1471 (m), 1388 (m), 1364 (m), 1252 (m), 1119 (m),
An acidic work-up entails the separation of the aqueous
and organic phases following quenching of the reaction mix-
ture. The aqueous phase was extracted with four portions of
an organic solvent (hexanes for the photolyses and diethyl
1
1098 (w), 1012 (w), 935 (w), 853 (w), 821 (m); H NMR
3 Due to the small quantities of samples, it was very difficult to separate the compounds from grease and, therefore, the compounds were
unsuitable for elemental analysis.
© 2000 NRC Canada

1478
Can. J. Chem. Vol. 78, 2000
(ppm)4: 7.54 (half of an AA′XX′ pattern, Ar H-3/5 of p-iso-
then quenched with 1 M HCl, followed by an acidic work-
up. Removal of the solvent in vacuo yielded a yellow oil.
Isolation and purification of 6 was accomplished by
Kugelrohr distillation (85°C, 2 mm Hg) which yielded
42.4 mg (30% yield) of 6 as a clear, colourless oil.
5
4
mer), 7.51 (bs, J = 0.6 Hz, J = 1.3, 1.9 Hz, Ar H-2 of m-
isomer), 7.46 (pseudo d, 3J = 7.5 Hz, 4J = 1.3, 1.3 Hz, Ar H-
4 of m-isomer), 7.17 (pseudo t, 3J = 7.5, 7.6 Hz, 5J = 0.6 Hz,
Ar H-5 of m-isomer), 7.06 (m, Ar H-2/6 of p-isomer), 7.05
3
4
(m, J = 7.6 Hz, J = 1.3, 1.9 Hz, Ar H-6 of m-isomer), 2.17
(s, Ar CH3 of m-isomer), 2.14 (s, Ar CH3 of p-isomer),
1.050 (s, tert-butyl CH3 of m-isomer), 1.047 (s, tert-butyl
CH3 of p-isomer), 0.29 (s, Si-CH3 of m-isomer), 0.27 (s, Si-
CH3 of p-isomer); 13C NMR (ppm): 136.8, 136.76 (Ar C-
1/C-3 of m-isomer), 136.10 (Ar C-2 of m-isomer), 132.73
(Ar C-4 of m-isomer) 129.77 (Ar C-6 of m-isomer), 127.8
(Ar C-5 of m-isomer), 138.5 (Ar C-1/C-4 of p-isomer),
135.59 (Ar C-3 and C-5 of p-isomer), 128.61 (Ar C-2 and
C-6 of p-isomer), 29.10 (C(CH3)3 of both isomers), 21.66
(Ar CH3 of m-isomer), 21.34 (Ar CH3 of p-isomer), 19.51
(C(CH3)3 of both isomers), –8.59 (Si-CH3 of both isomers);
29Si NMR (ppm): 4.5 (m-isomer), –10.9 (p-isomer); MS
(m/z): 248 (M+, 2), 233 (M+-CH3, 3), 191 (M+-tert-butyl,
100), 149 (63), 73 (SiMe3, 93); High resolution MS: calcd.
for C16H28Si: 248.1960; found: 248.1959.
Mesityldimethylgermane (6): IR (thin film, cm–1): 2050 (s,
Ge-H); 1H NMR (ppm): 6.75 (s, 2 H, Mes H), 4.85 (septet, 1
H, J = 3.7 Hz, Ge-H), 2.34 (s, 6 H, Mes oCH3), 2.13 (s, 3 H,
Mes pCH3), 0.42 (d, 6 H, J = 3.7 Hz); 13C NMR (ppm):
143.27, 138.19, 134.01 (Mes C), 128.79 (Mes CH), 24.11
(Mes oCH3), 21.04 (Mes pCH3), –2.18 (Ge-CH3); MS (m/z):
224 (M+, 26), 209 (M+ -CH3, 100), 191 (MesGe, 12), 119
(Mes, 32), 105 (M+ – Mes, 19), 89 (GeCH3, 29); High reso-
lution MS: calcd. for C11H18Ge: 224.0620; found: 224.0608.
Acknowledgements
The financial support of the Natural Sciences and Engi-
neering Council of Canada is gratefully acknowledged.
Preparation of mesitylmethylgermane
References
Tert-butyllithium (1.7 M in hexanes, 5 mmol) was added
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in THF (10 mL) at –50°C. The orange-yellow mixture was
allowed to stir at –50°C for 10 min before the addition of
methyl iodide (1.77 g, 13 mmol). After 20 min at –50°C, the
reaction mixture was warmed to room temperature and al-
lowed to stir for 1 hour. The reaction mixture was then
quenched with 1 M HCl, followed by an acidic work-up. Re-
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and purification of 9 was accomplished by Kugelrohr distil-
lation (60°C, 2 mm Hg) which yielded 131.85 mg (24%
yield) of 9 as a clear, colourless oil.
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Mesitylmethylgermane (9): IR (thin film, cm–1): 2064 (s,
1
Ge-H); H NMR (ppm): 6.74 (s, 2 H, Mes H), 4.53 (q, 2 H,
J = 3.9 Hz, Ge-H), 2.32 (s, 6 H, Mes oCH3), 2.11 (s, 3 H,
Mes pCH3), 0.34 (t, 3 H, J = 3.9 Hz, Ge-CH3); 13C NMR
(ppm): 143.30, 138.44, 131.6 (Mes C), 128.5 (Mes CH),
24.00 (Mes oCH3), 21.10 (Mes pCH3), –6.78 (Ge-CH3); MS
(m/z): 210 (M+, 21), 193 (MesGe, 64), 121 (MesH2, 100),
105 (MeGeH, 43), 91 (M+ _ Mes, 26); High resolution MS:
calcd. for C10H16Ge: 210.0464; found: 210.0451.
Preparation of mesityldimethylgermane
Tert-butyllithium (1.7 M in hexanes, 0.85 mmol) was
added to a solution of mesitylmethylgermane (0.131 g,
0.63 mmol) in THF (5 mL) at –50°C. The orange-yellow
mixture was allowed to stir at –50°C for 10 min before the
addition of methyl iodide (0.45 g, 3.2 mmol). After 20 min
at –50°C the reaction mixture was warmed to room tempera-
ture and allowed to stir for 1 h. The reaction mixture was
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1
4 Coupling constants were measured from the homonuclear decoupled H NMR spectrum (irradiated at 2.17 ppm; see Supplementary
Material).
© 2000 NRC Canada