Synthesis, Characterization, and Thermolysis of Dibenzo-7-dimethylgermanorbornadiene

Article abstract of DOI:10.1021/acs.organomet.5b00529

The dibenzo-7-dimethylgermanorbornadiene Me₂GeA (A = C₁₄H₁₀) has been synthesized in one step by treatment of MgA·3THF with Me₂GeCl₂ in tetrahydrofuran (-35 °C) and isolated in 69% yield. The thermolysis of Me₂GeA in toluene leads to the effective expansion of the bicyclic framework to the dibenzo-7,8-tetramethyldigermabicyclo[2.2.2]octadiene (Me₂Ge)₂A, isolated in 71% yield (based on germanium). The bicyclic compounds Me₂GeA and (Me₂Ge)₂A have been characterized by single-crystal X-ray diffraction studies and their structures discussed.

Full text of DOI:10.1021/acs.organomet.5b00529

Communication
pubs.acs.org/Organometallics
Synthesis, Characterization, and Thermolysis of Dibenzo-7-
dimethylgermanorbornadiene
Alexandra Velian, Wesley J. Transue, and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
ABSTRACT: The dibenzo-7-dimethylgermanorbornadiene
Me2GeA (A = C14H10) has been synthesized in one step by
treatment of MgA·3THF with Me2GeCl2 in tetrahydrofuran (−35
°C) and isolated in 69% yield. The thermolysis of Me2GeA in
toluene leads to the effective expansion of the bicyclic framework
to the dibenzo-7,8-tetramethyldigermabicyclo[2.2.2]octadiene
(Me2Ge)2A, isolated in 71% yield (based on germanium). The bicyclic compounds Me2GeA and (Me2Ge)2A have been
characterized by single-crystal X-ray diffraction studies and their structures discussed.
nteresting primarily as sources of germylenes (R2Ge), 7-
germanorbornadienes are traditionally prepared in a
the case for other heavy alkenes,9 tetramethyldigermene could
be trapped by anthracene, giving rise to (Me2Ge)2A in a [4+2]
Diels−Alder cycloaddition process.7,8
I
cumbersome multistep synthesis culminating with the [4 + 2]
Diels−Alder reaction of a germole with a benzyne.1 Herein we
show that dibenzo-7-dimethylgermanorbornadiene Me2GeA (A
= C14H10) can be assembled directly in good yield in a one-step
synthesis from MgA·3THF and Me2GeCl2 (see Scheme 1).
Our recent synthesis and isolation of unprotected 7-
phosphanorbornadienes obtained directly from the reaction of
select dichlorophosphines with MgA·3THF6 made us wonder if
7-germanorbornadienes could be accessed similarly. Addition of
solid MgA·3THF (1 equiv) to a chilled (−35 °C)
tetrahydrofuran (THF) solution of Me2GeCl2 (1 equiv)
Scheme 1. Synthesis of Me2GeA (Isolated in 69% Yield) by
Treatment of MgA·3THF with Me2GeCl2 at −35 °C and
Formation of (Me2Ge)2A (Isolated in 71% Yield)
1
effected an immediate color change to pale yellow. H NMR
spectroscopic analysis of the crude reaction mixture, after
removal in vacuo of all volatile materials, revealed the formation
of one major product, Me2GeA, as well as the presence of small
amounts of (Me2Ge)2A and anthracene. Isolated pure in 69%
yield by selective crystallization from a chilled saturated Et2O
solution, Me2GeA is an air-stable, crystalline, colorless
compound.
We wondered if under similar reaction conditions other
germanium dichlorides could be used to form the 7-
germanorbornadiene architecture; treatment of diphenylgerma-
nium dichloride (Ph2GeCl2), germanium tetrachloride (GeCl4),
or the germanium(II) dichloride (dioxane)GeCl2 with MgA·
3THF in thawing THF, however, only led to the formation of
anthracene, magnesium chloride, and insoluble reduced
germanium species, likely (Ph2Ge)n and Ge(0).11 These results
mirror our previous observation that PhPCl2 and PCl3 do not
give rise to the corresponding 7-phosphanorbornadiene
derivatives on treatment with MgA·3THF.6,12
Assembling 7-heteroatom-norbornadiene architectures via the
salt metathesis reaction of an anthracenide salt and a main-
group halide reaction partner is precedented for silicon,2,3 tin,4,5
and more recently phosphorus,6 yet Me2GeA has not been
previously reported in the literature. In 1980 Neumann was first
to report the synthesis of a 7-germanorbornadiene and
established that such species are thermal precursors to
germylene (Me2Ge), the heavier analogue of a carbene.
Proposing this process occurs in two steps through the
intermediacy of a biradical,1 he showed that Me2Ge inserts
into the Ge−C bond of a 7-germanorbornadiene, giving rise to
7,8-digermabicyclo[2.2.2]octadienes.7 Exploring the behavior of
the latter species upon thermolysis, the tetramethyldigermene
Me2GeGeMe2 was generated for the first time;8 similar to
Probing the thermal stability and chemical behavior toward
retro Diels−Alder chemistry, a solution of Me2GeA in toluene
was heated for ca. 16 h at 100 °C, to ensure complete
1
consumption of the starting material. H NMR analysis of the
Special Issue: Gregory Hillhouse Issue
Received: June 22, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00529
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
crude mixture indicated the formation of anthracene and
(Me2Ge)2A, which after removal of anthracene using a charcoal
plug was isolated as a pure substance in 71% yield (based on
germanium). Obtained previously from Li2A and [Me2Ge-
(Cl)]2 in 59% yield,7 (Me2Ge)2A is an air-stable, colorless
compound that crystallizes readily from Et2O. When the
thermolysis of Me2GeA was performed in the presence of
excess diphenylacetylene (6 equiv), a competent trap for
dimethylgermylene,1 the only observed products were
anthracene and (Me2Ge)2A. This suggests that, at the high
temperature necessary for the thermolysis, Me2GeA is a better
trap for dimethylgermylene than the alkyne. It is likely that the
process of (Me2Ge)2A formation in the thermolysis of Me2GeA
involves the insertion of dimethylgermylene into the Ge−C
bond of Me2GeA, although dimerization of free Me2Ge
followed by reaction of Me2GeGeMe2 with anthracene
cannot be excluded. Using DFT methods,13 we estimated the
change in free energy (ΔG) associated with the thermal
conversion of Me2GeA to (Me2Ge)2A in the transformation 2
Me2GeA → (Me2Ge)2A + A to be ca. −37 kcal/mol of
(Me2Ge)2A (see DFT Calculations in the Supporting
Information).
Figure 2. Solid-state molecular structure of (Me2Ge)2A with ellipsoids
at the 50% probability level, rendered using PLATON.10 Hydrogen
atoms are omitted for clarity. Selected interatomic distances (Å) and
angles (deg): C(13)−Ge(2) 2.015(1), C(14)−Ge(1) 2.017(1),
Ge(1)−Ge(2) 2.4128(2); C(13)−Ge(2)−Ge(1) 95.53(3).
of a 7-germanorbornadiene, with Ge−Cbh bond distances
averaging 2.02 Å and Cbh−Ge−Cbh angle approximating
78.5°.16 A CCDC search reveals the X-ray structure of
(Me2Ge)2A to be the first reported for a 7,8-digerma-
bicyclo[2.2.2]octadiene.17
The preparation of (Me2Ge)2A from Me2GeA, in turn
prepared in one step from MgA·3THF and Me2GeCl2, is
significantly better than that originally reported7 and may
enable an expansion of the chemistry of Ge2Me4.
The higher thermal stability of Me2GeA in comparison to
that of benzo-7-dimethylgermanorbornadiene, with a half-life
t1/2 = 40 min in CCl4 solution at 70 °C,1 suggests a stabilizing
effect of the second benzo group on the 7-heteronorbornadiene
architecture, also observed in the case of phosphorus and
nitrogen.6,14
X-ray-quality crystals of Me2GeA and (Me2Ge)2A were
grown from saturated solutions in diethyl ether (see Figures 1
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00529.
■
S
Crystallographic data for Me2GeA (CIF)
Crystallographic data for (Me2Ge)2A (CIF)
Further characterization data of the compounds, details
of DFT studies, and crystallographic details (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.C.C.: ccummins@mit.edu.
Notes
■
Figure 1. Solid-state molecular structure of Me2GeA with ellipsoids at
the 50% probability level, rendered using PLATON.10 Hydrogen
atoms are omitted for clarity. Selected interatomic distances (Å) and
angles (deg): Ge(1)−C(13) 2.030(1), Ge(1)−C(14) 2.029(1),
Ge(1)−C(15) 1.940(2), Ge(1)−C(16) 1.947(2); C(13)−Ge(1)−
C(14) 77.72(5).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under CHE-1362118. The authors declare
no competing financial interests.
and 2 and X-ray Diffraction Studies in the Supporting
Information). Of approximate C2v symmetry, the two structures
feature a strained architecture at the germanium centers, with a
C13−Ge−C14 angle of 77.72(5)° in Me2GeA and a C13−
Ge2−Ge1 angle of 95.53(3)° in (Me2Ge)2A, respectively. The
strain imposed by the bicyclic architectures is also visible in the
DEDICATION
■
This work is dedicated in fond remembrance of Professor
Gregory L. Hillhouse.
Ge−C bond lengths, which average 1.943 Å for Ge−Cmethyl
,
typical for single, covalent Ge−C bonds,15 but are significantly
lengthened for those connecting the germanium atoms to the
bridgehead (Cbh) carbon atoms of the anthracene framework to
2.016 Å (av) in (Me2Ge)2A and to 2.030 Å (av) in the more
strained Me2GeA. These values compare well with those
reported for 1,4-dihydro-9,9-dimethyl-1,2,3,4-tetraphenyl-1,4-
germanaphthalene, the only other reported solid-state structure
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DOI: 10.1021/acs.organomet.5b00529
Organometallics XXXX, XXX, XXX−XXX