En route to stable all-carbon-substituted silylenes: Synthesis and reactivity of a bis(α-spirocyclopropyl)silylene

Article abstract of DOI:10.1021/om500485m

The synthesis of a bis(α-spirocyclopropyl)silylene is reported and its reactivity revealed. Liberation of the silylene was accomplished by UV-light-mediated photolysis of a trisilane precursor. Insertion and addition reactions prove the existence and versatility of this new family of bis(α-spirocyclopropyl)-substituted silylenes. Substitution on the flanking cyclopropyls for improved steric shielding of the reactive center remains challenging.

Full text of DOI:10.1021/om500485m

Communication
pubs.acs.org/Organometallics
En Route to Stable All-Carbon-Substituted Silylenes: Synthesis and
Reactivity of a Bis(α-spirocyclopropyl)silylene
Kai M. Redies,† Thomas Fallon,† and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
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* Supporting Information
ABSTRACT: The synthesis of a bis(α-spirocyclopropyl)silylene is reported and its
reactivity revealed. Liberation of the silylene was accomplished by UV-light-mediated
photolysis of a trisilane precursor. Insertion and addition reactions prove the
existence and versatility of this new family of bis(α-spirocyclopropyl)-substituted
silylenes. Substitution on the flanking cyclopropyls for improved steric shielding of
the reactive center remains challenging.
ilylenes were for many years considered only species of
heterocyclic silylene 3 was reported by Lappert.8 In 1999, Kira
S
fleeting lifetime and enormous reactivity.1 While the
prepared the first isolable dialkyl-substituted silylene 49 and
explored its reactivity.10 Over the next decade the chemistry of
N-heterocyclic silylenes was greatly developed. Representatives
such as amidinate-ligated 511 and β-diketiminate-ligated 612
were synthesized by Roesky and Driess, respectively. The
transition-metal complexes of these silylenes have been studied
in great detail, and catalysis has been demonstrated.13 Driess
also reported a cyclic diylid-stabilized silylene14 (not shown).
Recently, stable acyclic silylenes having heteroatom substitution
have been prepared. Power and Tuononen reported the
synthesis of dithiolate-substituted silylene 7,15 whereas Jones,
Kaltsoyannis, Mountford, and Aldridge prepared the amino-
borane-substituted silylene 8;16 the latter system was shown to
activate dihydrogen.
chemistry of transient silylenes has been extensively explored,2,3
the preparation of stable silylenes has, in recent decades, rapidly
advanced (Figure 1). However, the search for new strategies to
In 2011, Shakib and co-workers introduced the design and
theoretical evaluation of a novel class of potentially stable all-
carbon-substituted silylenes (Figure 2, left).17a Their study
proposed that cyclopropyl substituents stabilize a divalent
silicon center through Walsh orbital donation, analogous to the
nonbonding orbital donation crucial in the stability of N-
heterocyclic silylenes. This led to the computational analysis of
a family of silylenes based upon the parent structure 9. Small
improvements in the projected stability were achieved by
including flanking substituents on the cyclopropyls to avoid
dimerization and, to a lesser extent, unsaturation and electron-
withdrawing substituents on the central ring. Moreover,
Kassaee and co-workers examined mixed amino/cyclopropyl-
substituted silylenes (not shown) where the orbital donation
from the nitrogen atom attached to the silicon atom is
predicted to substantially compete with orbital donation from
Figure 1. Representative isolable silylenes (Np = neopentyl, Dipp =
2,6-diisopropylphenyl, Ar = 2,6-dimesitylphenyl).
stabilize silylenes continues to be an important challenge. The
first isolable silylene, silicocene 1, was reported by Jutzi in
1986.4 Inspired by Arduengo’s synthesis of the first stable N-
heterocyclic carbene5 (not shown), Denk and West6 prepared
2, the first N-heterocyclic silylene, and studied a family of
substituted analogues.7 In a related vein, the benzo-fused N-
Received: May 7, 2014
© XXXX American Chemical Society
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Scheme 2. Synthesis of Silylene Precursor 19 for UV-Light-
Mediated Photolysis
Figure 2. Novel bis(α-spirocyclopropyl)-substituted silylenes: (left)
DFT calculations by Shakib (R1 = H, Me; R2 = H, CN, CF3, NH2, and
NO2); (right) our synthetic targets.
the α-cyclopropyl substituent.17b In addition, an orbital-energy
comparison between structure 9 and Kira’s stable silylene 4
estimated a slightly lower index of nucleophilicity and slightly
higher index of global electrophilicity.17a
Given this intriguing computational analysis,17a the utility of
bis(α-spirocyclopropyl)-substituted silylenes in achieving a
stable silylene presented to us a fascinating open question.
We began efforts to prepare and study members of this new
family of silylenes, and the parent silolane core structures 9 and
10 seemed to be suitable synthetic targets (Figure 2, right). We
reasoned that the high reactivity of the divalent silicon atom
would likely be incompatible with any functionality or
unsaturation at or in the central ring, whereas densely
substituted cyclopropyl groups were desirable. Herein, we
report the synthetic approaches toward silylene precursors for
target silylenes 9 and 10. In addition, UV-light-mediated
precursor photolysis and subsequent insertion as well as
addition reactions are demonstrated.
the dibromodiene 1318 from hexa-1,5-diene (12) on a
multigram scale in 8% overall yield. 13 was then reacted with
tert-butyllithium, and the intermediate was treated with a
variety of silicon electrophiles. Addition of dichlorodiphenylsi-
lane furnished diphenylsilane 14 in high yield, followed by
Simmons−Smith cyclopropanation to give the bis(α-spirocy-
clopropyl)-substituted diphenylsilane 15. Unfortunately, all
attempts to convert 15 into dihalosilane 11 under standard
conditions using bromine, chlorine, or strong Brønsted acids
led to either decomposition or single dephenylation. An
alternative strategy involved treating 13 with tert-butyllithium
followed by addition of silicon tetrachloride and in situ
reduction with DIBAL-H to give dihydrosilane 16 as a
promising congener. However, the reaction exclusively
provided spirosilane 17 together with various byproducts,
regardless of stoichiometry or order of addition. Furthermore,
the same result was obtained by applying other halosilanes such
as silicon tetrabromide, diethoxydichlorosilane, and dichloro-
silane.
Since the reduction of dihalosilanes using an alkali metal
represents the most common synthetic approach to isolable
silylenes, we targeted dihalosilane 11 as a suitable precursor to
silylene 9 (Scheme 1). A two-step procedure was used to obtain
Scheme 1. Synthetic Approaches toward Dihalosilane 11 as a
Silylene Precursor for Reductive Dehalogenation
Since all synthetic efforts to construct a suitable silylene
precursor for reductive dehalogenation remained unsuccessful,
we turned our attention toward the photolytic generation of
silylenes from trisilanes. In 1994, Kira and Sakurai reported the
matrix isolation of a divinyl-substituted silylene generated from
trisilane 18 under irradiation with UV light (λ 254 nm) at 77 K
in 2-MeTHF.19 Encouraged by these results, trisilane 18 was
obtained from 13 via an optimized two-step procedure by
lithiation and trapping with 2,2-dichloro-1,1,1,3,3,3-hexamethyl-
trisilane.20 Subsequent cyclopropanation afforded bis(α-spiro-
cyclopropyl)-substituted trisilane 19 in good yield (Scheme 2).
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We anticipated that our synthetic methodology would enable
access to trisilanes bearing sterically more demanding
substituted cyclopropyl groups. Our attempts to prepare the
more densely substituted trisilane 20 (anti isomer shown)
proved to be a significant challenge. Initially, we subjected
trisilane 18 to Simmons−Smith reaction conditions using 2,2-
diiodopropane.21,22 Unfortunately, all such attempts led to no
reaction and recovery of the starting material. We then
prepared the tetramethyltrisilane 23 in three steps from the
known diene 21.23 Bromination followed by elimination gave
the dibromodiene 22 in 68% overall yield which was then
transformed into 23 by a reaction analogous to the synthesis of
trisilane 18. Unfortunately, when standard Simmons−Smith
reaction conditions were employed, no reaction was observed.
Under prolonged heating, traces of the desired trisilane 20
could be detected; however, this was accompanied by severe
decomposition of the substrate.
Trisilane 19 was then irradiated in cyclohexane in the
1
absence of a trapping agent and monitored by H and 29Si
NMR spectroscopy. After 2 h, the conversion was approx-
imately 20% on the basis of the appearance of hexamethyldi-
silane. However, no new signals for either a free silylene or the
corresponding disilene were observed.24 Furthermore, no
additional resonances apart from hexamethyldisilane were
1
found in the H and 29Si NMR spectra. This implies that
silylene 9 is formed but rapidly decomposes in the absence of a
trapping reagent, presumably forming oligomeric material.
Irradiation of 19 in the presence of 1,2-diethyl-1,1,2,2-
tetramethyldisilane25 showed no silane scrambling, suggesting
that the formation of silylene 9 is irreversible.
In conclusion, we accomplished the synthesis of the novel
bis(α-spirocyclopropyl)trisilane 19 and its UV-light-mediated
photolysis. Although neither a free silylene nor a disilene was
observed, the formation of insertion as well as addition
products provides strong evidence for the generation of
transient silylene 9. However, the predicted stability of 9 was
shown to be insufficient to allow isolation of the free silylene.
The de novo preparation of silylene precursor 20 with greater
steric bulk on the cyclopropyl groups has been unsuccessful in
our hands so far and remains an ongoing challenge.
With trisilane 19 as a suitable precursor for silylene 9 in
hand, we commenced initial photochemical experiments
(Scheme 3). We first irradiated 19 in cyclohexane using an
Scheme 3. UV-Light-Mediated Silylene Generation:
Insertion and Addition Reactions
ASSOCIATED CONTENT
* Supporting Information
Text and figures giving experimental details, characterization
data, and NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail for M.O.: martin.oestreich@tu-berlin.de.
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.F. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2013−2014). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship. We are grateful to Julien Fuchs (TU Berlin) for
his experimental contributions.
array of low-pressure mercury lamps (λ 254 nm, 46 W) in the
presence of an excess of tert-butyl alcohol at 30 °C. To our
delight, the corresponding insertion product 24 was obtained in
55% isolated yield. In addition, the only byproduct was
identified as hexamethyldisilane. When 0.5 equiv of water was
used, disiloxane 25 was isolated from a complex mixture, which
might be caused by side reactions with diethyl ether as solvent.
Gratifyingly, (4 + 1) cycloaddition with 2,3-dimethylbuta-1,3-
diene gave silacyclopentene 26 in moderate yield. Irradiation of
19 in the presence of cyclohexene or bis(trimethylsilyl)-
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