Isolation of a Cyclopentasilane from Magnesium Reduction of a Linear Hexasilane

Article abstract of DOI:10.1002/ejoc.202100768

The reductive cyclization of two linear hexasilanes is contrasted, where a methylated precursor yielded a cyclohexasilane while the tert-butyl-functionalized analog unexpectedly yielded a cyclopentasilane. A comprehensive analysis using X-ray diffraction, IR, HR, HRMS, and multinuclear (¹H, ¹³C and ²⁹Si) 1D and 2D NMR spectroscopy identified the 1,2-dihydrodisilane ^tBuPhHSi-SiHPh^tBu as an additional product, which was interpreted as supportive of a mechanism involving silylene elimination. The results of this study may prove informative about substituent effects in the practice of complex organosilicon molecular synthesis.

Full text of DOI:10.1002/ejoc.202100768

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Isolation of a Cyclopentasilane from Magnesium Reduction of a
Linear Hexasilane
Authors: Ernesto Ballestero-Martínez, John T. Ferguson, Maxime A.
Siegler, and Rebekka S. Klausen
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100768
Link to VoR: https://doi.org/10.1002/ejoc.202100768
01/2020

10.1002/ejoc.202100768
European Journal of Organic Chemistry
COMMUNICATION
Isolation of a Cyclopentasilane from Magnesium Reduction of a
Linear Hexasilane
Ernesto Ballestero-Martínez,[a]‡ John T. Ferguson,[a] Maxime A. Siegler,[a] and Rebekka S. Klausen*[a]
[a]
Dr. E. Ballestero-Martínez, M. Sc. J. T. Ferguson, Dr. M. A. Siegler, Prof. Dr. R. S. Klausen
Department of Chemistry
Johns Hopkins University
3400 N. Charles Street, Baltimore, MD 21218 (USA)
E-mail: klausen@jhu.edu
www.jhu.edu/chem/klausen
Current address:
‡
Escuela de Química and Centro de Investigación en Ciencia e Ingeniería de Materiales
Universidad de Costa Rica
San José 11501-2060, Costa Rica
Supporting information for this article is given via a link at the end of the document.
Abstract: The reductive cyclization of two linear hexasilanes is
contrasted, where a methylated precursor yielded a cyclohexasilane
while the tert-butyl-functionalized analog unexpectedly yielded a
cyclopentasilane. A comprehensive analysis using X-ray diffraction,
IR, HR, HRMS, and multinuclear (1H, 13C and 29Si) 1D and 2D NMR
conductance[27] and visible light emission,[28,29] as well as utility in
ring-opening polymerization.[30,31] The [2+2] cyclodimerization of
an endocyclic disilene has been observed as an undesired
pathway during the attempted synthesis of
silylene.[32,33]
a homocyclic
t
spectroscopy identified the 1,2-dihydrodisilane BuPhHSi−SiHPhtBu
However, attempts to dimerize 1a using several conditions
and reducing agents did not cleanly afford 3a. Product
identification was complicated by the methylated structure which
resulted in similar polarity, difficult chromatographic separation
and complex, overlapping spectra. Despite the uncertain fate of
1a, we hypothesized two possible outcomes: (i) decomposition of
tricyclic 3a, as strained cyclosilanes are known to rapidly ring-
expand upon exposure to adventitious oxygen[34] or (ii)
competitive trimerization to a larger central ring system.
as an additional product, which was interpreted as supportive of a
mechanism involving silylene elimination. The results of this study
may prove informative about substituent effects in the practice of
complex organosilicon molecular synthesis.
Herein we describe the unexpected formation of a 5-
membered cyclopentasilane from
a
6-membered linear
hexasilane precursor. The influence of steric effects on the
product outcome are examined by comparison of a tert-butyl-
functionalized linear precursor to a previously described methyl-
functionalized oligosilane. This work was conducted in the context
of the attempted synthesis of a tricyclic silane building block for
complex polysilane synthesis.
A potential solution to both challenges is partial replacement
of methyl groups with the more sterically demanding tert-butyl
group. Bulky substituents afford kinetic stabilization to
disilenes[35,36] and strained cyclotetrasilanes.[28] Reduction of
dichlorodialkylsilanes with larger alkyl groups is also known to
favor smaller rings,[37] as in the lithium reduction of Cl2SiMe(t-
Inorganic silicon nanostructures combine biocompatibility[1]
with interesting electronic and photonic properties,[2–7] which have
been key to biomedical,[8–10] energy-storage,[11,12] energy
[38,39]
Bu)[37] and Cl2Si(t-Bu)2
to cyclotetrasilanes. Therefore, we
hypothesized that 1,2-di-tert-butyl-1,2-dichloro cyclohexasilane
1b might be a reasonable precursor for the synthesis of tricyclic
3b (Scheme 1b).
production,[13]
optoelectronic,[14]
and
semiconducting
applications.[15] Towards the goal of uncovering new
nanostructured materials that combine these compelling
properties with the processability and tunability of organic
conjugated polymers, we reported the synthesis of organosilicon
polymer mimics of the crystalline silicon lattice such as
polycyclosilane poly(1,4Si6) (Scheme 1a).[16] Key to this work is
the synthesis of multifunctional cyclosilanes[17,18] amenable to
dehydrocoupling[19–23] or reductive polymerization.[24]
In our earlier siladecalin synthesis, linear hexasilane 4a was
straightforwardly reduced to cyclohexasilane 5a using Mg powder
(Scheme 2a)[25] and our prior synthesis of 4a was readily adapted
to provide the tert-butyl analog 4b (Scheme 2b). tert-
Butyl(chloro)diphenylsilane (TBDPSCl) is a common alkoxy
protecting group in organic synthesis. Lithium reduction with
Gilman-type conditions[40–42] readily provided the silyl anion
TBDPSLi as a dark red solution in THF. TBDPSLi was not isolated,
but cation exchange with isopropylmagnesium chloride[43] and salt
metathesis with dichlorooctamethyltetrasilane (Cl(SiMe2)4Cl)
afforded the linear hexasilane 6 in 69% yield over two steps.
Selective cleavage of one phenyl ring from each terminus of the
linear hexasilane was accomplished by addition of TfOH, which
formed an intermediate ditriflate. Conversion to the more
hydrolytically stable dibromo compound was accomplished by
addition of lithium bromide, yielding the target compound 4b
(50:50 d.r.) in 88% yield over two steps.
The rational design and synthesis of new building blocks
could further expand the structural and functional diversity of
polycyclosilanes and other nanostructured silanes. We previously
reported the stereocontrolled synthesis of cis- and trans-
siladecalins where a key intermediate was 1,2-dichlorocyclosilane
1a (Scheme 1a).[25] To further explore the potential applications of
1a, we envisaged reductive dimerization via a postulated
intermediate disilene 2a to yield 3a, a strained permethylated
silicon analogue of dodecahydrobiphenylene (Scheme 1b).
Embedding silicon in a four-membered ring results in compelling
properties such as enhanced Lewis acidity,[26] unusual
1
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10.1002/ejoc.202100768
European Journal of Organic Chemistry
COMMUNICATION
Scheme 1. a) Prior work: structures of molecular and polymeric polycyclic silanes synthesized in the Klausen group, including the stereocontrolled synthesis of cis-
and trans-siladecalins via 1,2-dichlorocycohexasilane intermediate 1a. b) Motivating hypothesis: dimerization of an endocyclic disilene as a route to tricyclic silanes.
c) This work: attempted synthesis of a tert-butyl functionalized cyclohexasilane yields instead a cyclopentasilane via a hypothesized silylene elimination.
quenching with 2-propanol (i-PrOH), instead of isolating the
targeted cyclohexasilane 5b as the main product, we obtained
cyclopentasilane 7 in 51% yield after column chromatography
(Scheme 3). Cyclopentasilane 7 was also the major product in
reductive cyclization with sodium naphthenalide (NaNp). The
observation of 7 from NaNp-reduction suggests that the iodine
employed to activate Mg powder was not responsible for Si–Si
bond cleavage. The structure of
7
was confirmed by
comparison to Masamune’s prior report of H and 29Si NMR
spectroscopic data for 7 (Table 1) in a report describing
29Si−29Si two-bond coupling in strained cyclosilanes.[44]
Masamune did not report a synthesis of cyclopentasilane 7.
1
Me2
Si
Me2Si 3
Me2Si 3’
Ph
2
Si
Me
Me
2’
1
Si
Me2
Me
Table 1. Tabulated 29Si NMR chemical shifts for compound 7.
Scheme 2. a) Synthesis of cyclohexasilane 1a from linear hexasilane 4a.[25]
b) Synthesis of linear hexasilane 4b. i) Li, THF, 24 h; ii) i-PrMgCl (2.1 equiv.),
30 min; then Cl(SiMe2)4Cl, 15 h, 69% (two steps); iii) TfOH (2.0 equiv.), DCM,
0 °C, 2 h; then LiBr (2.1 equiv.), Et2O, RT, 15 h, 88% (two steps). RT = room
temperature.
Atom
δ (this work) a,b
-19.00
δ (ref. 35) a,c
-18.5
Si-1
Si-2, Si-2’
-42.25
-41.73
Si-3, Si-3’
-43.74
-43.22
With linear hexasilane 4b in hand, we investigated
reductive cyclization using similar conditions as had proved
successful in the synthesis of methylated 5a[25] (powdered Mg0
(4.0 equiv.), THF) (compare Scheme 2a and Scheme 3). Upon
a Relative to SiMe4. b 1-D 29Si {1H} DEPT (distortionless enhancement by
polarization transfer) spectrum determined in benzene- d6 (25 °C) at 79 MHz.
c Determined in benzene-d6 (25 °C) at 53.7 MHz.
2
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10.1002/ejoc.202100768
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COMMUNICATION
Scheme 3. Reduction of linear hexasilane 4b yields predominantly cyclopentasilane 7, as well as trace quantities of disilane 8 and target cyclohexasilane 5b.
Figure 1. Displacement ellipsoid plot (50% probability level) of 8 (CCDC
2091373) at 110(2) K. H atoms and disorder are omitted for clarity. Selected
bond length [Å]: Si1-Si1’ 2.3483(7).
In addition to cyclopentasilane 7, a second more polar
fraction was isolated, which was observed to be a complex
mixture of compounds by 1H NMR spectroscopy (Figure S14).
Fortuitously, after dissolution in hexanes (25 °C, 7 d), a mixture
of a glassy solid and colorless crystals was observed. Single
crystal X-ray crystallographic analysis suggested that the
crystalline solid could be assigned to 1,2-dihydrodisilane 8
(Figure 1). The crystal structure of 8 only shows the meso
diastereomer. The Si−Si bond distance (dSi−Si) of 2.3483(7) Å
was slightly shorter than the analogous dSi−Si observed for the
related compounds iPr2PhSi−SiPhiPr2 (2.3898(4) Å) and
t
tBuPh2Si−SiPh2 Bu (2.4002(6) Å).[45]
The structural assignment to 1,2-dihydrodisilane 8 was
supported by extensive spectroscopic analysis. FTIR
spectroscopy showed the presence of a sharp stretching
frequency at ṽ = 2093 cm-1 consistent with a Si–H bond, but a
strong, broad feature at ca. 1000 cm-1 consistent with a Si–OR
bond was not observed (Figure S30). 1H and 29Si NMR
spectroscopy (Figures S15 and S20) further supported the
presence of a Si–H functional group, as in the observation of a
Figure 2. a) Cropped 1H−29Si HSQC spectrum showing one-bond 1H-29Si
one-bond correlation between H (δ 4.55) and 29Si (δ -18.03)
in the 1H-29Si HSQC spectrum (Figure 2a). High resolution
mass spectrometry (HRMS) also revealed a molecular ion
consistent with 8 (calcd. M = 326.18861, found = 326.18929,
Figure S27). No evidence of the other diastereomer of 8 was
found, for example, the 29Si NMR data in Figure 2 is consistent
with only one Si–H containing compound.
1
correlation assigned to disilane 8. b) Cropped 1H−29Si HMBC spectrum
showing three-bond 1H-29Si correlations assigned to 5b and 8.
correlation (0.95, -18.03, Figure 2b) that allowed assignment
of the δ 0.95 resonance to the CMe3 group of 8. The δ 1.20 and
1.26 1H resonances could be assigned to the CMe3
resonances of the cis and trans diastereomers of 5b based on
similar three-bond correlations to δ -18.47 and -18.72 29Si
resonances, respectively (Figure 2b). Additional NMR
experiments allowed complete assignment of Si-Me
resonances to the cis and trans stereoisomers of 5b (Figures
S17, S18, S23, S24).
We suggest that all four identified products,
cyclopentasilane 7, disilane 8, and cis and trans
cyclohexasilane 5b, could arise from a common mechanism
(Scheme 4a). Magnesium insertion into one Si–Br bond of 4b
HRMS also suggested the presence of trace quantities of
the originally desired cyclohexasilane 5b (calcd.
M =
556.26847, found = 556.26905; calcd. for M−tBu = 499.19805,
found = 499.19819, Figures S28 and S29). A series of NMR
relationships made possible the identification and
differentiation of cyclohexasilane 5b and 1,2-dihydrodisilane 8
in the complex mixture isolated after column chromatography.
The same δ -18.03 29Si resonance with a one-bond correlation
to 1H in Figure 2a (assigned to 8) showed a three-bond 1H-29Si
3
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10.1002/ejoc.202100768
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COMMUNICATION
is proposed to give intermediate silanide 9. Silanide 9 could
directly cyclize to give cyclohexasilane 5b. The known rapid
stereochemical inversion of silanides[46,47] may rationalize the
low diastereoselectivity in this cyclization and the cyclization of
methylated 4a (55:45 trans:cis).[25]
Slower rates of cyclization might be observed with 4b
relative to 4a due to replacement of Si-Me for the bulkier Si-
tBu group. As a result, silylene elimination (9®10+11) could
become competitive with direct cyclization (9®5b). Silylene
elimination would provide the less hindered silanide 11 which
could directly cyclize to yield cyclopentasilane 7. We also note
the alternative possibility of nucleophilic attack on an internal
Si–Si bond[48–50] to directly afford cyclopentasilane 7 and a
silylenoid equivalent of 10 (Pht-BuSi(MgBr)Br, Figure S31).
via one of several possible mechanisms. A 2-propanol quench
was used herein and diorganosilylenes typically undergo high-
yielding O–H insertion products upon alcoholysis, which would
suggest tert-butyl(isopropoxy)(phenyl)silane as
a
major
product. For example, West reported yields between 74-90%
for the ethanol trapping reaction of the closely structurally
related tert-butylmesitylsilylene in a variety of solvents (Mes(t-
Bu)Si
+ HOEt ®
Mes(t-Bu)SiH(OEt)).[52] However, tert-
butyl(isopropoxy)(phenyl)silane was not detected by HRMS.
Another common reaction of silylenes is intramolecular C-
insertion[53] and a 1,3-CH insertion reaction with the tert-butyl
group would afford a silirane. Although the silirane was not
detected, this does not preclude its formation, as siliranes
thermally decompose.[54–56] Finally, we note the possibility that
1,2-dihydrodisilane 8 could arise from an Si–H bond insertion
reaction, such as R2Si + R2SiH2 ® R2HSi-SiHR2. Silane traps
have frequently been used to study silylene intermediates.[55,57]
However, such a reaction would be expected to produce a
diastereomeric mixture. A source of Ph(t-Bu)SiH2 is not
immediately obvious. The difference in isolated yield between
7 and 8 (51% vs. 6%) may reflect that multiple reaction
pathways exist (e.g. the reactive intermediate giving rise to 8
also decomposes to volatile, unidentified products).
We considered that an equilibrium between silylene 10
and disilene 12 might instead be the origin of disilane 8
(Scheme 4b). Silylene dimerization to the corresponding
disilene is well-known[52] and we have identified two different
reports of 1,2-dihydrodisilanes arising from disilenes. In
seminal work on tetramesityldisilene (Mes2Si=SiMes2), West
and Michl reported that photochemical activation (λ = 254 nm)
converted tetramesityldisilene to the 1,2-dihydrodisilane.[58]
The authors speculated radical character to the excited state
would result in C-H abstraction. Matsumoto and Nagai
reported that tetra(iso-propyl)disilene ((i-Pr)2Si=Si(i-Pr)2)
(generated via photolysis of an anthracene-bridged masked
disilene) yielded a 1,2-dihydrodisilane (5% yield) via postulated
C–H abstraction from solvent.[59]
While both reactions above were photochemical, our
work was performed without high-energy irradiation. A more
closely related example is a study by Belzner et al. in which
Mg0 reduction of
silacyclobutane (19%) and
a
dichlorodiarylsilane yielded
a
a
1,2-dihydrodisilane (5%,
diastereomer not specified) via a putative silylene-disilene
equilibrium (Scheme 4c).[51] That different starting materials
under similar reaction conditions yielded similar products is
indicative of a common reactive intermediate. We note the use
of excess Mg0 in both reactions and suggest that the two-
electron reducing agent might provide an intermediate dianion
that affords a 1,2-dihydrodisilane upon protic workup (Scheme
4b). Sekiguchi reported one-electron reduction of a disilene
with tert-butyllithium to a disilane radical anion.[60] In neither our
work nor Belzner’s was the typical disilene alcoholysis product
(R2(H)Si-Si(OR)R2) detected which may indicate
concentration of the disilene under the reaction conditions.
a low
Scheme 4. a) A silanide intermediate could account for the four isolated
products cis-5b, trans-5b, cyclosilane 7 and disilane 8 via direct cyclization
or competitive silylene elimination. b)
A silylene-disilene equilibrium
In summary, we report that the reduction of the linear
hexasilane 4b with Mg0 resulted in the unexpected formation
of the ring contraction product cyclopentasilane 7 and 1,2-
dihydrodisilane 8, as well as trace quantities of the desired
cyclohexasilane 5b as a mixture of diastereomers. The product
distribution was characterized by single crystal X-ray
crystallography, high resolution mass spectrometry, and 1H
followed by two-electron reduction and protonation could account for
disilane 8. c) Prior work: reduction of a dichlorodiorganosilane yields unusual
disilene derivatives.[51]
We expect that 1,2-dihydrodisilane 8 arises from silylene
10 (Ph(t-Bu)Si) or silylenoid equivalent (Ph(tBu)Si(Br)MgBr))
4
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10.1002/ejoc.202100768
European Journal of Organic Chemistry
COMMUNICATION
and 29Si NMR spectroscopy. Ring contraction was
hypothesized to arise from steric effects that reduced the rate
of cyclization relative to silylene elimination. Two mechanisms
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permission granted to perform a postdoctoral stay in the
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U.S. Department of Energy (DOE), Office of Science, Basic
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10.1002/ejoc.202100768
European Journal of Organic Chemistry
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The Mg0 promoted reduction of a sterically hindered 1,6-dibromohexasilane yielded a cyclopentasilane as the main product. The
disilane t-BuPhHSi−SiHPht-Bu was identified as a minor product, which suggests a possible mechanism involving elimination of a
transient silylene SiPht-Bu.
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