The Cyclohexadienyl-Leaving-Group Approach toward Donor-Stabilized Silylium Ions

Article abstract of DOI:10.1021/acs.organomet.5b00609

The cyclohexa-2,5-dien-1-yl group is established as a leaving group at silicon as an alternative to the Bartlett-Condon-Schneider silicon-to-carbon hydride transfer and the allyl-leaving-group approach to generate silylium ions. Hydride abstraction from t

Full text of DOI:10.1021/acs.organomet.5b00609

This is an open access article published under an ACS AuthorChoice License, which permits
copying and redistribution of the article or any adaptations for non-commercial purposes.
Communication
pubs.acs.org/Organometallics
The Cyclohexadienyl-Leaving-Group Approach toward Donor-
Stabilized Silylium Ions
Antoine Simonneau,*,† Tobias Biberger, and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The cyclohexa-2,5-dien-1-yl group is established as a
leaving group at silicon as an alternative to the Bartlett−Condon−
Schneider silicon-to-carbon hydride transfer and the allyl-leaving-
group approach to generate silylium ions. Hydride abstraction from
the skipped diene unit employing trityl tetrakis(pentafluoro-
phenyl)borate (1, [Ph3C]+[B(C6F5)4]−) yields the silicon cation
along with benzene. Our investigations reveal that the presence of
an internal or external donor group is mandatory to allow for the
formation of intra- or intermolecularly stabilized silylium ions. If not, degradation of the precursor is observed as a result of the
reaction of the allylic silane units with the released silylium ion. It is also shown that such allylic silanes do form remarkably stable
alkyl-substituted carbenium ions when reacted stoichiometrically with benzene-stabilized silylium ions.
characterization of a free silylium ion.4 This achievement had
he preparation and characterization of positively charged
become possible thanks to the development of weakly
Tsilicon compounds have fueled intense research work over
coordinating carborate counteranions1c and the invention of
the allyl-leaving-group approach.5,6 That release mechanism
overcomes the limitation of the Bartlett−Condon−Schneider
hydride transfer to heterolytically cleave sterically inaccessible
Si−H bonds (Scheme 1b).
the last three decades.1 Since the seminal work of Corey
demonstrated the viability of the Bartlett−Condon−Schneider
silicon-to-carbon hydride transfer to efficiently generate such
entities (Scheme 1a),2,3 several groups have been involved in
this fascinating chemistry that eventually culminated in the
For the past few years, we have been involved in the use of
strong main-group Lewis acids in catalysis, e.g., silylium ions as
catalysts for challenging Diels−Alder reactions7 as well as C6F5-
containing boranes in catalytic Si−H bond activation.8 In the
arena of these electron-deficient boron Lewis acids, we recently
devised the ionic degradation of cyclohexa-2,5-dien-1-yl-
substituted silanes to hydrosilanes and an arene molecule
catalyzed by tris(pentafluorophenyl)borane, B(C6F5)3.9 This
process was quantum chemically investigated by Sakata and
Fujimoto shortly thereafter10 and is proposed to pass through
carbon-to-boron hydride transfer from the methylene group
“opposite” the silicon-bearing carbon atom, i.e., from the less
hindered face, of the cyclohexa-1,4-diene core (3 → 2′·C6H6,
Scheme 1c, gray pathway). The thus-generated Wheland
complex,11 i.e., the arene-stabilized silylium ion, 2′·C6H6, then
collapses to afford hydrosilane 4 by reaction with the
borohydride counteranion (2′·C6H6 → 4). Given our back-
ground in silylium chemistry,7,12 we asked ourselves whether
the borane in this approach could be replaced with trityl
tetrakis(pentafluorophenyl)borate (1, [Ph3C]+[B(C6F5)4]−)13
as the hydride acceptor. The plan was to intercept the
hydrosilane release at the stage of the arene-stabilized silylium
ion 2 (Scheme 1c, black pathway), hence offering a
complementary approach to that developed by Lambert.5
Scheme 1. Bartlett−Condon−Schneider Silicon-to-Carbon
Hydride Transfer (a); Generation of the Trimesitylsilylium
Ion by the Allyl-Leaving-Group Approach (b); Planned
Cyclohexadienyl-Leaving-Group Approach (c)
Received: July 14, 2015
Published: August 5, 2015
© 2015 American Chemical Society
3927
DOI: 10.1021/acs.organomet.5b00609
Organometallics 2015, 34, 3927−3929

Organometallics
Communication
Our study commenced with subjecting a C6D6 solution of
cylohexa-2,5-dien-1-yltriisopropylsilane (3a) to a suspension of
trityl salt 1 in the same solvent. After decanting, the resulting
clathrate was analyzed by the means of NMR spectroscopy and
appeared to contain only unreacted 1. Conversely, the analysis
of the supernatant showed complete decomposition/oligome-
rization of 3a to an intractable mixture (Scheme 2, top). We
precursor 3c (Scheme 3, top). Sequential treatment of tert-
butyldichloro(methyl)silane (8) with ferrocenyllithium and the
Scheme 3. Formation of a Ferrocene-Stabilized Silylium Ion
by the Cyclohexadienyl-Leaving-Group Approach (Top) and
Evidence for the Need of Donor Stabilization (Bottom)
Scheme 2. Attempted Generation of the Triisopropylsilylium
Ion by the Cyclohexadienyl-Leaving-Group Approach (Top)
and Generation of a Stable Secondary Alkyl Carbenium Ion
(Bottom)
speculated that the formation of small amounts of the desired
arenium ion 2a·C6H6 are enough to initiate the decomposition
of 3a, likely involving the allylic silane units. To further verify
this hypothesis, we had a look at the stability of
allyltriisopropylsilane (5) in the presence of [(iPr)3Si-
(C6D6)]+[B(C6F5)4]− (2a·C6D6)13 (Scheme 2, bottom). We
observed the clean formation of the thermally stable secondary
carbenium ion [HC{CH2Si(iPr)3}2]+[B(C6F5)4]− (6). We
attribute its remarkable stability at room temperature to the
double hyperconjugative β-silicon effect exerted by the two
C(sp3)−Si σ-bonds.14 Alkyl-substituted carbenium ions of this
type are rather rare in the literature and generally not observed
at ambient temperature.15,16 This result parallels a report by
Bochmann and co-workers where isolable [HC{CH(SiMe3)-
(SnMe3)}2]+[Zr2Cl9]− was generated by the reaction of
[Me3Sn]+[Zr2Cl9]− and (3-(trimethylstannyl)prop-1-ene-1,3-
diyl)bis(trimethylsilane).17 The addition of excess allylic silane
5 to 6 led to the disappearance of 6 and consumption of 5 (not
shown). It is therefore reasonable to assume that silylium ion
precursor 3a suffers the same fate when treated with trityl salt 1.
It is also worthy of mention that we were not able to generate
any stable intermediate from the reaction of 3a and an
equimolar amount of [(iPr)3Si(C6D6)]+[B(C6F5)4]− (2a·
C6D6).
We next inferred that better stabilization of the silylium ion
will prevent it from reacting with the allylic silane units of the
cyclohexa-1,4-diene core. We initially considered kinetic
stabilization and targeted cyclohexa-2,5-dien-1-yltrimesitylsilane
(3b). We were however unable to make the TMEDA complex
of cyclohexa-2,5-dien-1-yllithium react with chlorotrimesitylsi-
lane (7). Instead of 3b, we obtained trimesitylsilane (4b) as a
result of chloride substitution in 7 with in-situ-generated
lithium hydride (see Scheme S1 in the Supporting
Information).18 Hence, we decided to switch to donor-
stabilized systems, of which many are documented in the
literature.1e Our group established such stabilization through an
electron-rich transition-metal fragment.7a,12 We therefore
embarked on the preparation of ferrocenyl-substituted
TMEDA complex of cyclohexa-2,5-dien-1-yllithium furnished
3c as dark red crystals in 34% yield. Its molecular structure is
depicted in Scheme 3; its cyclohexa-2,5-dien-1-yl group is not
planar but adopts a boat-like conformation folded toward the
silicon atom.19 This time, the addition of a solution of 3c in
C6D6 to trityl salt 1 resulted in clean formation of the
ferrocene-stabilized silylium ion 2c12a (Scheme 3, top). To
confirm the pivotal role of the ferrocene backbone, we
subjected the related phenyl-substituted precursor 3d to the
same procedure, but the targeted arenium ion 2d·C6H6 did not
form (Scheme 3, bottom).
After the demonstration of the feasibility of the cyclo-
hexadienyl-leaving-group approach, we probed its generality
with the aid of an external donor. We had recently been
successful with stabilizing silylium ions with sulfides, resulting
in chemically rather robust Lewis pairs.7c,d,20 Revisiting the
generation of 2a from cylohexa-2,5-dien-1-yltriisopropylsilane
(3a) in the presence of diphenylsulfide was met with success
(Scheme 4). Analysis of the clathrate confirmed the formation
of [(iPr)3Si(SPh2)]+[B(C6F5)4]− (2a·SPh2) with a diagnostic
29Si NMR chemical shift of δ 58.8 ppm.7c,d We repeated the
same experiment with tert-butyl(cyclohexa-2,5-dien-1-yl)-
(methyl)(phenyl)silane (3d), and silylium ion 2d readily
formed as its adduct 2d·SPh2 with a chemical shift of δ 41.6
ppm. The difference in deshielding of the silicon atoms in 2a·
SPh2 and 2d·SPh2 probably stems from a stronger silicon−
sulfur interaction in Lewis adduct 2d·SPh2 of sterically less
hindered 2d.
We showed here that cyclohexa-2,5-dien-1-ylsilanes are viable
precursors for the generation of silylium ions through carbon-
3928
DOI: 10.1021/acs.organomet.5b00609
Organometallics 2015, 34, 3927−3929

Organometallics
Communication
In Science of Synthesis: Knowledge Updates 2013/3; Oestreich, M., Ed.;
Thieme: Stuttgart, 2013; pp 1−42. (c) Reed, C. A. Acc. Chem. Res.
2010, 43, 121−128. (d) Lambert, J. B.; Zhao, Y.; Zhang, S. M. J. Phys.
Org. Chem. 2001, 14, 370−379. (e) Lambert, J. B.; Kania, L.; Zhang, S.
Chem. Rev. 1995, 95, 1191−1201.
Scheme 4. Generation of Intermolecularly Sulfur-Stabilized
Silylium Ions by the Cyclohexadienyl-Leaving-Group
Approach in the Presence of a Sulfide Donor
(2) (a) Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237−3238.
(b) Corey, J. Y.; Gust, D.; Mislow, K. J. Organomet. Chem. 1975, 101,
C7−C8.
(3) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science
1993, 260, 1917−1918.
(4) Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.;
Lin, L.; Lambert, J. B. Science 2002, 297, 825−827.
(5) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J. Am.
Chem. Soc. 1999, 121, 5001−5008.
(6) The technique itself had been reported earlier by Schade and
Mayr: Schade, C.; Mayr, H. Makromol. Chem., Rapid Commun. 1988, 9,
477−482.
(7) (a) Klare, H. F. T.; Bergander, K.; Oestreich, M. Angew. Chem.,
Int. Ed. 2009, 48, 9077−9079. (b) Schmidt, R. K.; Muther, K.; Muck-
̈
̈
to-carbon hydride transfer upon treatment with equimolar
amounts of trityl salt 1. However, sufficient stabilization of the
released silylium ion by an internal or external donor group
must be available to prevent its reaction with the allylic silane
units of the unreacted precursor, i.e., the cyclohexa-1,4-diene
moiety. Intramolecular stabilization was realized in a ferrocenyl-
substituted system, and intermolecular Lewis pair formation
was achieved with diphenylsulfide. As part of this work, we also
demonstrated that the stoichiometric reaction of an allylic
silane and a benzene-stabilized silylium ion yields a remarkably
stable alkyl-substituted secondary carbenium ion.
Lichtenfeld, C.; Grimme, S.; Oestreich, M. J. Am. Chem. Soc. 2012,
134, 4421−4428. (c) Rohde, V. H. G.; Pommerening, P.; Klare, H. F.
T.; Oestreich, M. Organometallics 2014, 33, 3618−3628. (d) Rohde, V.
H. G.; Muller, M. F.; Oestreich, M. Organometallics 2015, 34, 3358−
̈
3373.
(8) For a recent review, see: (a) Oestreich, M.; Hermeke, J.; Mohr, J.
Chem. Soc. Rev. 2015, 44, 2202−2220. For mechanistic work, see:
(b) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47, 5997−
6000. (c) Hermeke, J.; Mewald, M.; Oestreich, M. J. Am. Chem. Soc.
2013, 135, 17537−17546.
(9) (a) Simonneau, A.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52,
11905−11907. (b) Simonneau, A.; Friebel, J.; Oestreich, M. Eur. J.
Org. Chem. 2014, 2077−2083. (c) Keess, S.; Simonneau, A.; Oestreich,
M. Organometallics 2015, 34, 790−799.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00609.
■
(10) Sakata, K.; Fujimoto, H. Organometallics 2015, 34, 236−241.
(11) Arenium ion intermediates are named after Wheland, but their
existence had already been noted by Pfeiffer and Wizinger before
Wheland’s quantum-mechanical analysis: (a) Wheland, G. W. J. Am.
Chem. Soc. 1942, 64, 900−908. (b) Pfeiffer, P.; Wizinger, R. Liebigs
Ann. Chem. 1928, 461, 132−154.
S
Synthetic procedures and figures giving NMR spectra of
the previously unknown compounds synthesized in this
paper (PDF)
(12) (a) Muther, K.; Frohlich, R.; Muck-Lichtenfeld, C.; Grimme, S.;
̈
̈
̈
Oestreich, M. J. Am. Chem. Soc. 2011, 133, 12442−12444. (b) Muther,
̈
K.; Hrobar
́ ́ ́
ik, P.; Hrobarikova, V.; Kaupp, M.; Oestreich, M. Chem. -
CIF file (CIF)
Eur. J. 2013, 19, 16579−16594.
(13) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993,
383−384.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: antoine.simonneau@chem.tu-berlin.de.
*E-mail: martin.oestreich@tu-berlin.de.
■
(14) Lambert, J. B.; Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu,
X.; So, J.-H.; Chelius, E. C. Acc. Chem. Res. 1999, 32, 183−190.
(15) For seminal reports of carbenium ions/carbocations stabilized
through the β-silicon effect, see: (a) Lambert, J. B.; Zhao, Y.; Wu, H. J.
Present Address
Org. Chem. 1999, 64, 2729−2736. (b) Meyer, R.; Werner, K.; Muller,
̈
†Laboratoire de Chimie de Coordination (LCC), CNRS, 205
Route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4,
France.
T. Chem. - Eur. J. 2002, 8, 1163−1172.
(16) (a) Olah, G. A.; Prakash, G. K. S., Eds. Carbocation Chemistry;
Wiley: Hoboken, 2004. (b) Vogel, P. Carbocation Chemistry; Elsevier:
Amsterdam, 1985. (c) Bethell, D.; Gold, V. Carbonium Ions; Academic
Press: London, 1967. (d) Olah, G. A.; Schleyer, P. v. R., Eds.;
Carbonium Ions; Wiley: New York, 1968−1976; Vols I−V.
(17) Schormann, M.; Garratt, S.; Hughes, D. L.; Green, J. C.;
Bochmann, M. J. Am. Chem. Soc. 2002, 124, 11266−11267.
(18) For the formation of lithium hydride from cyclohexa-2,5-dien-1-
yllithium, see: Bates, R. B.; Gosselink, D. W.; Kaczynski, J. A.
Tetrahedron Lett. 1967, 199−204.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was (in part) supported by the Deutsche
Forschungsgemeinschaft (Oe 249/9-1). A.S. gratefully acknowl-
edges the Alexander von Humboldt Foundation for a
postdoctoral fellowship (2014−2015). M.O. is indebted to
the Einstein Foundation (Berlin) for an endowed professorship.
We thank Dr. Elisabeth Irran (TU Berlin) for the X-ray
analysis.
(19) For conformational analyses of cyclohexa-1,4-dienes, see:
Bennett, M. J.; Purdham, J. T. Can. J. Chem. 1978, 56, 1358−1363
and cited references .
(20) Prakash, G. K. S.; Bae, C.; Wang, Q.; Rasul, G.; Olah, G. A. J.
Org. Chem. 2000, 65, 7646−7649.
REFERENCES
■
(1) (a) Muller, T. In Structure and Bonding (Berlin); Scheschkewitz,
̈
D., Ed.; Springer: Berlin, 2014; Vol. 155, pp 107−162. (b) Muller, T.
̈
3929
DOI: 10.1021/acs.organomet.5b00609
Organometallics 2015, 34, 3927−3929