Radical transfer hydrosilylation/cyclization using silylated cyclohexadienes

Article abstract of DOI:10.1021/ol016160c

(matrix presented) A new method for mild metal-free hydrosilylation is described. Silylated cyclohexadienes are used as radical transfer hydrosilylating reagents for various double and triple bonds. A trialkylsilane is transferred from a cyclohexadiene mo

Full text of DOI:10.1021/ol016160c

ORGANIC
LETTERS
2001
Vol. 3, No. 15
2357-2360
Radical Transfer Hydrosilylation/
Cyclization Using Silylated
Cyclohexadienes
Stephan Amrein, Andreas Timmermann, and Armido Studer*
Fachbereich Chemie der UniVersita¨t Marburg, Hans-Meerwein-Strasse,
D-35032 Marburg
studer@mailer.uni-marburg.de
Received May 23, 2001
ABSTRACT
A new method for mild metal-free hydrosilylation is described. Silylated cyclohexadienes are used as radical transfer hydrosilylating reagents
for various double and triple bonds. A trialkylsilane is transferred from a cyclohexadiene moiety to an alkene. The hydrosilylation can be
combined with a C−C bond formation as shown for the preparation of silylated cycloalkanes from the corresponding dienes.
The hydrosilylation/cyclization reaction of dienes is a well-
known process. These reactions are generally conducted
using transition metals as catalysts. Cationic palladium
complexes have been successfully used to catalyze these
transformations.1 Furthermore, neodynium2 and more im-
portantly yttrium3 metallocene complexes catalyze these
hydrosilylation/cyclization reactions.
From an ecological point of view, it would be highly
desirable to conduct these reactions without using transition
metals. However, there are only a few reports on metal-free
hydrosilylation reactions. In general, radical hydrosilylation
of alkenes cannot be conducted using trialkylsilanes. This
is due to the rather strong Si-H bond in the trialkylsilanes.4
The alkyl radical formed after initial silyl radical addition
onto the alkene is not reduced with a trialkylsilane. Modified
silanes, such as tris(trimethylsilyl)silane, have been success-
fully used in radical hydrosilylations.5 In addition, Roberts
presented some examples using the concept of polarity
reversal catalysis.6
Recently, we introduced silylated cyclohexadienes as new
tin-free radical reducing reagents.7 Various typical radical
reactions, such as dehalogenations, deselanations, deoxy-
genations, and intermolecular additions, were performed
using these new reagents. The cyclohexadiene CH2 moiety
acts as the H-donor in these radical chain reactions. Reduc-
tion of radical R2 with 1 affords cyclohexadienyl radical 2.
Rearomatization of 2 then provides the corresponding silyl
radical, which is able to propagate the chain by reaction with
the starting halide, xanthate, or phenylselanide R2-X
(Scheme 1). As byproduct, methylated resorcin diether 3 is
formed. We conceived that the silylated cylohexadienes can
also be used as radical hydrosilylating reagents: If the silyl
radical formed in the rearomatization of 2 is allowed to react
with an alkene, â-silyl radical 4 will be generated. Reduction
of 4 with the cyclohexadiene 1 would then afford 2 and the
(1) Widenhoefer, R. A.; DeCarli, M. A. J. Am. Chem. Soc. 1998, 120,
3805. Stengone, C. N.; Widenhoefer, R. A. Tetrahedron Lett. 1999, 40,
1451. Perch, N. S.; Widenhoefer, R. A. J. Am. Chem. Soc. 1999, 121, 6960.
Pei, T.; Widehoefer, R. A. Org. Lett. 2000, 2, 1469. Perch, N. S.; Pei, T.;
Widenhoefer, R. A. J. Org. Chem. 2000, 65, 3836. Wang, X.; Chakrapani,
H.; Stengone, C. N.; Widenhoefer, R. A. J. Org. Chem. 2001, 66, 1755.
(2) Onozawa, S.; Sakakura, T.; Tanaka, M. Tetrahedron Lett. 1994, 35,
8177.
(3) Molander, G. A. Chemtracts 1998, 11, 237 and references therein.
Molander, G. A.; Dowdy, E. D. In Topics in Organometallic Chemistry;
Kobayashi, S., Ed.; Springer-Verlag: New York, 1999; Vol. 2, pp 120-
154.
(5) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188.
(6) Roberts, B. P. Chem. Soc. ReV. 1999, 28, 25.
(7) Studer, A.; Amrein, S. Angew. Chem., Int. Ed. 2000, 39, 3080.
(4) Chatgilialoglu, C. Chem. ReV. 1995, 95, 1229.
10.1021/ol016160c CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/19/2001

Hydrosilylation of allyl acetate could easily be conducted
with reagent 6 in hexane at 80-85 °C (sealed tube) using
R,R′-azoisobutyronitrile (AIBN) as initiator. Best results were
obtained with 1.5 equiv of the silylated cyclohexadiene and
0.3 equiv of AIBN. Under these conditions, the hydrosilyl-
ation product 10 was isolated in 54% yield (Scheme 3).
Scheme 1. Silylated Cylohexadienes as Radical Reducing
Reagents
Scheme 3. Hydosilylation of Various Double Bonds with
Reagent 6
hydrosilylation product 5. Radical 2 will eventually fragment
a silyl radical, thereby propagating the chain (Scheme 2).
Scheme 2. Silylated Cylohexadienes as Radical
Hydrosilylating Reagents
This hydrosilylation of the alkene can formally be regarded
as a transfer-hydrosilylation, since reagent 1 is transformed
to the corresponding arene in a reverse hydrosilylation
process. The driving force of this transfer-hydrosilylation is
the resonance energy of the arene. Herein, we report the first
applications of the silylated cyclohexadienes in metal-free
hydrosilylation reactions.
The synthesis of cyclohexadiene 6 has previously been
described.7 The dimethylphenyl silyl derivative 7, the tri-
isopropyl compound 8, and cyclohexadiene 9 were prepared
in analogy (Figure 1).
Hydrosilylation of 4-phenyl-1-butene worked equally well
and 11 was isolated in 55% yield. Terminal substituted
double bonds can also be hydrosilylated, as shown for the
reaction of cyclohexene with 6 to form tetraalkylsilane 12
(60%). Hydrosilylation of â-pinene afforded the ring-opened
cyclohexene derivative 13 in 70% yield. The initially formed
â-silyl radical undergoes fast ring opening to form a tertiary
radical, which is subsequently reduced with reagent 6 to
provide compound 13.
We next studied the hydrosilylation of various dienes
which are easily prepared as described in the Supporting
Informations. In these reactions the initially formed â-silyl
radical undergoes a 5-exo radical cyclization to afford the
corresponding primary radical, which after reduction provides
a silylated cycloalkane. Treatment of diene 14 with reagent
6 provided the hydrosilylation/cyclization product 19 in 80%
yield as a 4.3:1 (cis:trans) mixture of diastereoisomers (Table
1). The diastereoselectivity was determined by GC analysis.
The relative configuration of the major isomer was assigned
by comparison of the NMR data with literature values.8 All
other new compounds were assigned in analogy. The
predominant formation of the cis-isomer is in agreement with
the Beckwith-Houk model for 5-exo cyclization reactions.9
Figure 1. Silylated cyclohexadienes 6-9.
(8) Miura, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1993, 66,
2348.
2358
Org. Lett., Vol. 3, No. 15, 2001

di-tert-butylperoxide as initiator at 140 °C in hexane (sealed
tube), high yields can be obtained even with these bulky
reagents, as shown for the hydrosilylation/cyclization with
reagent 8 (82%, see Table 2). It turned out that the
trimethylsilyl derivative 9 lacking the methoxy substituents
is slightly less reactive.10 The hydrosilylation could not be
initiated using AIBN. However, with di-tert-butylperoxide
as initiator smooth reaction occurred in hexane at 140 °C
and the hydrosilylation/cyclization product 28 was isolated
in 81% yield.
Table 1. Hydrosilylation/Cyclization of Various Dienes
We also studied hydrosilylation reactions comprising a
6-exo cyclization (Scheme 4). Reaction of diene 29 with
Scheme 4. Radical Hydrosilylation Comprising a 6-Exo
Cyclization
a Reaction took 7 h for completion. b 3 equiv of 6, 0.6 equiv of AIBN.
Ether 15, tosylamide 16, and ketal 18 were readily trans-
formed to the corresponding cycloalkanes 20, 21, and 23 in
good yields with moderate selectivities. For tosylamide 16,
reaction took longer (7 vs 4 h). Under our standard
conditions, hydrosilylation of diol 17 did not go to comple-
tion. However, with 3 equiv of Si reagent 6 and 0.6 equiv
of initiator, high conversion was obtained and compound 22
was isolated in 62% yield (dr ) 2.3:1).
Since the silyl group in the Si reagents can be readily
varied (see Figure 1), basically any silyl group can be
introduced via radical hydrosilylation using our approach.
To demonstrate this fact, we studied the hydrosilylation of
diene 24 using the Si reagents 6-9. Hydrosilylation with
reagent 6 under the standard conditions provided carbocycle
25 in 84% yield (Table 2). Hydrosilylation/cyclization with
the bulkier dimethylphenylsilyl 7 and the triisopropylsilyl
reagent 8 provided the corresponding products 26 (57%) and
27 (44%, not shown) in lower yields. However, upon using
reagent 6 afforded cyclohexane derivative 30 in 61% yield
as 1:1 mixture of diastereoisomers.11 As expected, the silyl
radical added regioselectively at the unsubstituted terminal
double bond of bisalkene 29.
Hydrosilylation of bisalkyne 31 under our standard condi-
tions (Scheme 5) afforded double hydrosilylation product 32
Scheme 5. Hydrosilylation of Bisalkyne 31
Table 2. Hydrosilylation/Cyclization of Diene 24 with Si
Reagents 6-9
in 11% yield along with unreacted starting material and mono
silylation/cyclization compound 33.12 Under optimized con-
ditions (4 equiv of 6, t-BuONdNOt-Bu as initiator),13 55%
of 32 was isolated. Reduction of the allyl radical 34 formed
(9) Beckwith, A. L.; Schiesser, C. H. Tetrahedron 1985, 41, 3925.
Spellmeyer, D. C.; Houk, K. N. J. Org. Chem. 1987, 52, 959.
(10) Amrein, S. Unpublished results.
(11) About 5% unreacted starting material remained.
(12) Ojima, I.; Zhu, J.; Vidal, E. S.; Kass, D. F. J. Am. Chem. Soc. 1998,
120, 6690.
a t-BuOO-t-Bu (0.5 equiv) was used as initiator at 140 °C.
Org. Lett., Vol. 3, No. 15, 2001
2359

after silyl radical addition to diene 33 occurred regioselec-
tively at the less hindered site (secondary vs tertiary radical).
The hydrosilylation products, if appropriately substituted
at silicon, can be readily oxidized to the corresponding
alcohols using the well-known Tamao-Fleming oxidation
protocol.14 This certainly expands the utility of our new
method. For instance, bisester 26 (cis:trans ) 4:1) was
transformed to alcohol 35 in 73% yield (cis:trans ) 4:1,
Scheme 6).15
transfer-hydrosilylating reagents. Double as well as triple
bonds can readily be hydrosilylated with the new method.
1,5- and 1,6-dienes undergo the hydrosilylation/cyclization
reaction to afford silylated cycloalkanes. To the best of our
knowledge, this is the first report of a transfer-hydrosilylation
reaction.
Acknowledgment. We thank the Swiss Science National
Foundation (2100-055280.98/1) for funding our work. The
work described herein is part of the planned dissertation of
S.A.
Scheme 6. Tamao-Fleming Oxidation
Supporting Information Available: Full experimental
details and spectroscopic and analytical data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL016160C
(13) Mendenhall, G. D. Tetrahedron Lett. 1983, 24, 451.
(14) Jones, G. R.; Landais, Y. Tetrahedron 1996, 52, 7599.
(15) The same oxidation has already been reported: Widenhoefer, R.
A.; Stengone, C. N. J. Org. Chem. 1999, 64, 8681.
In conclusion, we report a new method for radical
hydrosilylation using silylated cyclohexadienes as metal-free
2360
Org. Lett., Vol. 3, No. 15, 2001