3-silylated cyclohexa-1,4-dienes as precursors for gaseous hydrosilanes: The B(C6F5)3-catalyzed transfer hydrosilylation of alkenes

Article abstract of DOI:10.1002/anie.201305584

Set Me₃SiH free! The strong Lewis acid B(C₆F ₅)₃ catalyzes the release of hydrosilanes from 3-silylated cyclohexa-1,4-dienes with concomitant formation of benzene. Subsequent B(C ₆F₅)₃

Full text of DOI:10.1002/anie.201305584

Angewandte
Chemie
DOI: 10.1002/anie.201305584
Transfer Hydrosilylation
3-Silylated Cyclohexa-1,4-dienes as Precursors for Gaseous
Hydrosilanes: The B(C6F5)3-Catalyzed Transfer Hydrosilylation of
Alkenes**
Antoine Simonneau and Martin Oestreich*
ꢀ
Alkene hydrosilylation is one of the prevalent methods for C
Si bond formation in academic as well as industrial settings, as
reflected by a plethora of publications and patents.[1] Usually
catalyzed by precious late-transition-metal complexes, sub-
stantial progress is currently being made in the design of
catalysts based on more abundant transition metals.[2] Various
triorganosilanes (R3SiH) but also flammable trichlorosilane
(Cl3SiH) and harmful trialkoxysilanes ((RO)3SiH with R =
Me or Et) are commonly employed in these catalytic
processes. Conversely, Me3SiH and Me2SiH2 are rarely
applied, as handling of these highly flammable and potentially
explosive gases is, aside from safety concerns, particularly
inconvenient on a laboratory scale. Practical methods avoid-
ing these issues would, therefore, be relevant to several areas
of silicon chemistry.
cyclohexa-1,4-dienes D (Scheme 1, upper). Formation of
a transient intermediate E (D!E) would be followed by
hydride abstraction (E!F+), arriving at the silicon-substi-
tuted Wheland complex F+ that could rearomatize with
release of a benzene-stabilized silicon cation[6] (F+![C(ben-
zene)]+). Aware that neither F+ nor [C(benzene)]+ are likely
to exist in the presence of HB(C6F5)3ꢀ, the sequence would
eventually produce intermediate B. The 3-silylated cyclohexa-
1,4-dienes D would thus serve as viable hydrosilane precur-
sors in B(C6F5)3 catalysis,[4] and we planned to apply this new
strategy to the in situ generation of otherwise gaseous
Me3SiH and Me2SiH2. The B(C6F5)3-activated hydrosilanes
B would then be reacted with an alkene G,[7,8] and the net
reaction corresponds to an unprecedented ionic transfer
hydrosilylation of alkenes (D!H, Scheme 1, lower).[9,10]
To assess the validity of our hypothesis, we synthesized
trimethylsilyl- and dimethylsilyl-substituted cyclohexa-1,4-
dienes 1 and 2 as well as a series of representative
triorganosilylated congeners 3–5 (Figure 1). These were all
accessed in one step by the reaction of lithiated cyclohexa-1,4-
diene with the corresponding chlorosilane (see the Support-
ing Information for details).
Our laboratory had shed light into the mechanism of the
[3,4]
ꢀ
Si H bond activation with the strong Lewis acid B(C6F5)3
where the intermediacy of a silicon cation as a result of
hydride abstraction was excluded (A!B but not C+,
Scheme 1, upper).[5] We asked ourselves whether B(C6F5)3
could be used for the deliberate generation of silicon cations
ꢀ
by the activation of a bisallylic C H bond in 3-silylated
At the outset, we decided to focus on the use of 1, as it is
an attractive trimethylsilane precursor. We investigated its
reactivity toward catalytic amounts of B(C6F5)3 in [D8]toluene
as well as CD2Cl2. Species 1 rapidly transformed into Me3SiH
and benzene (see the Supporting Information for a time-
dependent 1H NMR analysis).[11] We then conducted the same
experiments in the presence of 1-methylcyclohexene (6a;
Table 1). While conducting the reaction in n-pentane led to
complete recovery of the starting material (entry 1), we were
delighted to find that transfer hydrosilylation is effective
when switching to arene solvents, affording trimethyl(2-
methylcyclohex-1-yl)silane (7a) with excellent cis selectivity
in moderate yields (entries 2 and 3). Results were even more
satisfying in halogenated solvents. The yield slightly improved
in 1,2-dichloroethane (entry 4), and transfer hydrosilylation
occurred in even higher yield in CH2Cl2 (entry 5). With this
solvent, the amount of transfer reagent 1 was optimized. An
increase of the equivalents of 1 from 1.15 to 1.30 afforded
excellent 95% yield determined by GLC analysis, and 87%
Scheme 1. Assumed pathway for the B(C6F5)3-catalyzed release of
hydrosilanes from 3-silylated cyclohexa-1,4-dienes (upper) and planned
ionic transfer hydrosilylation of alkenes (lower).
[*] Dr. A. Simonneau, Prof. Dr. M. Oestreich
Institut fꢀr Chemie, Technische Universitꢁt Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
Homepage: http://www.organometallics.tu-berlin.de
[**] This research was (in part) supported by the Deutsche For-
schungsgemeinschaft (Oe 249/9-1). M.O. is indebted to the
Einstein Foundation (Berlin) for an endowed professorship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305584.
Figure 1. Various 3-silylated cyclohexa-1,4-dienes tested as transfer
hydrosilylation reagents.
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Table 1: Optimization of the transfer hydrosilylation employing trime-
thylsilane precursor 1.[a]
Table 2: B(C6F5)3-catalyzed transfer hydrosilylation of various alkenes
employing reagent 1.
Entry
Equiv of 1
Solvent
Yield of 7a [%][c]
Entry Alkene
1
Alkyltrimethylsilane
7b
Yield [%][a]
84
[d]
1
2
3
4
5
6
1.15
1.15
1.15
1.15
1.15
1.30
1.50
1.30
n-pentane
toluene
benzene
1,2-dichloroethane
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
–
6b
6c
48
53
68
2
3
4
R2 =H
7c
7d
7e
78
85
94
R2 =Me 6d
R2 =Ph 6e
82
95 (87)[e]
5
6
6 f
7 f
85
81
7
93
–
8[f]
6g
7g
[a] Unless otherwise noted, all reactions were conducted using B(C6F5)3
(5 mol%) with a substrate concentration of 1.0m in the indicated solvent
at room temperature on a 0.2 mmol scale. [b] Obtained as a single
diastereomer according to NMR spectroscopic analysis. Assigned by
analogy with the related B(C6F5)3-catalyzed hydrosilylation using
PhMe2SiH.[7] [c] Determined by GLC analysis using tetracosane as
internal standard. [d] B(C6F5)3-catalyzed liberation of Me3SiH from 1 but
no consumption of the alkene. [e] Yield of isolated product on
a 0.5 mmol scale. [f] No catalyst.
7
8
X=H
X=Br
6h
6i
7h
7i
69
65
9
6j
7j
66
10
11
n=1
n=2
6k
6l
7k
7l
61
71
12
13
6m
6n
7m 69[b]
7n
73[c]
yield of isolated product was obtained at larger scale
(entry 6). This result could not be improved any further by
adding more of 1 (entry 7). No reaction was seen in the
absence of the B(C6F5)3 catalyst (entry 8). It is remarkable
that only little excess of 1 is necessary for high yield, because
Me3SiH is gaseous at room temperature. Another practical
aspect of this reaction is that the byproducts, benzene and
unreacted Me3SiH, are easily removed from the crude
reaction mixture.
With the optimized procedure in hand, we subjected
various terminal and internal alkenes to the transfer hydro-
silylation. Gratifyingly, our method emerged as widely
applicable with all examples reacting at room temperature
in acceptable reaction times (Table 2). The monosubstituted
alkene oct-1-ene (6b) was converted in good yield of isolated
product with complete anti-Markovnikov regioselectivity
(entry 1). Typical styrenes 6c–6e showed excellent reactivity,
and the yield correlated with the cation-stabilizing ability of
the a substituent in the order H < Me < Ph (entries 2–4). A
few other 1,1-disubstituted alkenes were screened, and we
were pleased to find that methylenecyclohexane (6 f) fur-
nished cyclohexylmethyl-substituted 7 f in high yield
(entry 5); a comparable result was obtained with 2-methyl-
non-1-ene (6g; entry 6). We then turned to endocyclic
alkenes. Applying our method to indenes 6h and 6i allowed
us to isolate indanes 7h and 7i with a trimethylsilyl group at
C2 (entries 7 and 8). Similarly, 1,2-dihydronaphthalene (6j)
[a] Yield of isolated product after purification by flash chromatography
on silica gel or Kugelrohr distillation. [b] exo:endo>95:5 according to
NMR spectroscopic analysis. [c] Obtained along with trace amounts of
the other regioisomer (relative configuration not assigned).
perfect exo selectivity is in agreement with that of another
Lewis acid-catalyzed hydrosilylation.[12] Finally, indene 6n
with a methyl group at C2 showed an unexpected preference
ꢀ
for selective C Si bond formation in the more hindered
position (entry 13) as opposed to the regioselective hydro-
silylation of 1-methylcyclohexene (cf. 6a!7a, Table 1). This
finding indicates that stabilization of the positive charge
developed at the (former) alkene carbon atoms during and
after the transfer of the electrophilic silicon atom onto the
trisubstituted double bond overrides steric factors. A secon-
dary but benzylic carbocation outrivals a tertiary in this
ꢀ
particular case (6n!7n). However, the C C double bond is
still sterically accessible in trisubstituted 6a and 6n whereas
congested tetramethylethylene was too hindered to react with
the B(C6F5)3-activated Me3SiH (not shown).[13]
Encouraged by this success, we addressed the release and
transfer of Me2SiH2 from precursor 2 (Scheme 2). The silicon
atom in 2 is decorated with an Si H bond that could
ꢀ
participate in another B(C6F5)3-catalyzed hydrosilylation.[7]
Indeed, its reaction with equimolar amounts of styrene (6c) in
the presence of 5 mol% B(C6F5)3 led to both single and
double hydrosilylation, as evidenced by GLC-MS analysis
(6c!8c/9c; not shown). Adjusting the stoichiometry to
0.5 equiv of 2 allowed us to form 9c arising from double
addition selectively (Scheme 2, upper). We then reasoned that
reaction with a sterically more demanding alkene would stop
at the stage of monoaddition. Transfer hydrosilylation of the
ꢀ
displayed regioselectivity in favor of C Si bond formation in
the homobenzylic position of 7j (entry 9). Cyclic alkenes,
such as cyclohexene (6k) and cycloheptene (6l), were
converted into the corresponding cycloalkyltrimethylsilanes
7k and 7l, respectively (entries 10 and 11); the rather low
yield of 7k might be explained by its volatility. The transfer
hydrosilylation of norbornene (6m) proceeded smoothly,
exclusively yielding 7m with exo selectivity (entry 12). This
2
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Keywords: boranes · hydrosilylation · Lewis acids ·
.
ꢀ
Si H bond activation · silane transfer
[1] a) I. Ojima, Z. Li, J. Zhu in The Chemistry of Organic Silicon
Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley,
Chichester, 2003, pp. 1687 – 1792; b) I. Ojima in Organic Silicon
Compounds (Eds.: S. Patai, Z. Rappoport), Wiley, New York,
1989, pp. 1479 – 1526.
[2] For a recent review, see: D. Troegel, J. Stohrer, Coord. Chem.
Rev. 2011, 255, 1440 – 1459, and references therein.
Scheme 2. Reactivity of dimethylsilane precursor 2: double (upper)
versus single hydrosilylation (lower) governed by steric factors.
[3] For reviews of the chemistry of B(C6F5)3, see: a) W. E. Piers,
A. J. V. Marwitz, L. G. Mercier, Inorg. Chem. 2011, 50, 12252 –
12262; b) G. Erker, Dalton Trans. 2005, 1883 – 1890; c) W. E.
Piers, Adv. Organomet. Chem. 2004, 52, 1 – 76; d) W. E. Piers, T.
Chivers, Chem. Soc. Rev. 1997, 26, 345 – 354.
[4] For a Highlight providing a comprehensive list of synthetic
applications of catalysis with B(C6F5)3, see: T. Robert, M.
Oestreich, Angew. Chem. 2013, 125, 5324 – 5326; Angew. Chem.
Int. Ed. 2013, 52, 5216 – 5218, and references therein.
[5] a) S. Rendler, M. Oestreich, Angew. Chem. 2008, 120, 6086 –
6089; Angew. Chem. Int. Ed. 2008, 47, 5997 – 6000; our inves-
tigation is based on the seminal work of Piers and co-workers:
b) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000,
65, 3090 – 3098.
trisubstituted alkene 6a with a slight excess of 2 yielded 8a
ꢀ
with the Si H bond still intact as the major product along with
unidentified heavier compounds (Scheme 2, lower).
To further demonstrate the generality of our method, we
examined the transfer hydrosilylation from cyclohexa-1,4-
dienes 3–5 onto alkene 6e (Scheme 3). Compared to 1 with its
trimethylsilyl group, 3 and 4 are more bulky. Transfer of the
triethylsilyl group was possible, but required prolonged
[6] J. B. Lambert, S. Zhang, C. L. Stern, J. C. Huffman, Science 1993,
260, 1917 – 1918.
[7] M. Rubin, T. Schwier, V. Gevorgyan, J. Org. Chem. 2002, 67,
1936 – 1940.
[8] For the related alkene hydrosilylation employing silicon cations
directly, see: a) J. B. Lambert, Y. Zhao, J. Org. Chem. 1999, 64,
2729 – 2736 (intermolecular); b) H.-U. Steinberger, C. Bauch, T.
Mꢁller, N. Auner, Can. J. Chem. 2003, 81, 1223 – 1227 (intra-
molecular).
[9] A radical transfer hydrosilylation also using (functionalized) 3-
silylated cyclohexa-1,4-dienes had already been disclosed about
a decade ago. However, only one example using a trimethylsilane
precursor was reported: a) S. Amrein, A. Timmermann, A.
Studer, Org. Lett. 2001, 3, 2357 – 2360; b) S. Amrein, A. Studer,
Helv. Chim. Acta 2002, 85, 3559 – 3574.
[10] There is one report by Nikonov and co-workers where the idea
of transfer hydrosilylation from an N-silylated 1,4-dihydropyr-
idine onto various acceptors is briefly addressed. Catalyzed by
a ruthenium catalyst, reversible transfer of the hydrosilane onto
another pyridine molecule, and hydrosilylation of a nitrile at low
reaction rate and conversion were possible. Stoichiometric
amounts of ZnCl2 slowly mediated the transfer hydrosilylation
of an aldimine with moderate conversion: D. V. Gutsulyak, A.
van der Est, G. I. Nikonov, Angew. Chem. 2011, 123, 1420 – 1423;
Angew. Chem. Int. Ed. 2011, 50, 1384 – 1387.
[11] There was no obvious indication that one of the allyl fragments is
activated by B(C6F5)3. For B(C6F5)3-catalyzed allylation employ-
ing allylic silanes or stannanes, see: a) M. Rubin, V. Gevorgyan,
Org. Lett. 2001, 3, 2705 – 2707; b) J. M. Blackwell, W. E. Piers, R.
McDonald, J. Am. Chem. Soc. 2002, 124, 1295 – 1306.
[12] AlCl3 was used as catalyst: a) K. Yamamoto, M. Takemae,
Synlett 1990, 259 – 260; b) Y.-S. Song, B. R. Yoo, G.-H. Lee, I. N.
Jung, Organometallics 1999, 18, 3109 – 3115.
Scheme 3. Transfer hydrosilylation with triorganosilylated cyclohexa-
1,4-dienes 3–5.
reaction time (6e!10e using 3); the triisopropyl moiety did
not transfer at all[7] (6e!11e using 4). Complete conversion
of 4 into iPr3SiH and benzene by B(C6F5)3 was however
observed. As the trimethylsilyl group is not easily function-
alized,[14] we included precursor 5 with a phenyl-substituted
silicon atom into our survey. That would later allow for
ꢀ
oxidative degradation of the C Si bond. The reaction with 5
afforded the target compound in near quantitative yield
(6e!12e using 5).
To summarize, we disclosed herein an unprecendented
ionic transfer hydrosilylation of alkenes. The ability of the
commercially available Lewis acid B(C6F5)3 to catalyze the
release of hydrosilanes from 3-silylated cyclohexa-1,4-dienes
was instrumental for this. The hydrosilylation of alkenes is
promoted by the same catalyst.[7] With the easy-to-handle 3-
trimethylsilyl- or 3-dimethylsilylcyclohexa-1,4-dienes 1 and 2,
we applied this method to the in situ generation of gaseous
Me3SiH and Me2SiH2, silanes that are often prohibited from
laboratories for safety considerations.
[13] For rare examples of hydrosilylation (catalyzed by AlCl3) of
tetrasubstituted double bonds, see: K. Oertle, H. Wetter,
Tetrahedron Lett. 1985, 26, 5511 – 5514.
ꢀ
[14] For an iridium(I)-catalyzed C H bond borylation of methyl
Received: June 28, 2013
groups attached to a silicon atom, see: T. Ohmura, T. Torigoe, M.
Published online: && &&, &&&&
Publication was delayed at the authorsꢀ request.
Suginome, J. Am. Chem. Soc. 2012, 134, 17416 – 17419.
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Communications
Transfer Hydrosilylation
Set Me3SiH free! The strong Lewis acid
B(C6F5)3 catalyzes the release of hydro-
silanes from 3-silylated cyclohexa-1,4-
dienes with concomitant formation of
benzene. Subsequent B(C6F5)3-catalyzed
A. Simonneau,
M. Oestreich*
&&&& — &&&&
ꢀ
3-Silylated Cyclohexa-1,4-dienes as
Precursors for Gaseous Hydrosilanes:
The B(C6F5)3-Catalyzed Transfer
Hydrosilylation of Alkenes
Si H bond activation allows for alkene
hydrosilylation (see scheme). The net
reaction is an ionic transfer hydrosilyla-
tion. The new technique is particularly
attractive in the case of otherwise gas-
eous, highly flammable hydrosilanes.
4
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