Hydrosilylation of epoxides catalyzed by a cationic η1- silane iridium(iii) complex

Article abstract of DOI:10.1039/c0cc05714b

Cationic silane complex 2, catalyzes the hydrosilylation of epoxides and cyclic ethers to give the silyl-protected alcohols, regioselectively. A mechanistic study shows that the epoxide undergoes isomerization to the ketone, followed by hydrosilylation.

Full text of DOI:10.1039/c0cc05714b

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COMMUNICATION
Hydrosilylation of epoxides catalyzed by a cationic g1-silane iridium(III)
complexw
Sehoon Park and Maurice Brookhart*
Received 21st December 2010, Accepted 25th January 2011
DOI: 10.1039/c0cc05714b
Cationic silane complex 2, catalyzes the hydrosilylation of epoxides
and cyclic ethers to give the silyl-protected alcohols, regio-
selectively. A mechanistic study shows that the epoxide undergoes
isomerization to the ketone, followed by hydrosilylation.
substrate has been employed in this catalysis to prevent
polymerization of the epoxide (See Supporting information).
Results of typical hydrosilylations are summarized in Table 1.
Table 1 Hydrosilylation of epoxides and cyclic ethers with 1a
Epoxides1 are useful precusors to a variety of materials
through ring-opening under acidic or basic conditions.2 Regio-
selectivity of the ring opening is usually observed and can be
controlled by variation in reaction conditions.3 The catalytic
hydrosilylation of epoxides by transition metal complexes,
however, has rarely been reported.3k Recently, we reported
catalytic C–O bond cleavage of alkyl ethers by silanes using
the highly electrophilic Z1-silane complex 2 generated in situ
from the conveniently prepared acetone complex 1.4 The basic
mechanism is illustrated in Scheme 1. We report here application
of this chemistry to epoxides which results in regioselective
ring-opening and one-pot conversion of epoxides to silyl-
protected alcohols.
Catalyst
Entry mol (%) Substrate t h Conv.b% Products
Yieldc%
0.25
0.25
1
1
quant.
quant.
91(78)
83d
2
3
0.25
1
1
1
quant.
quant.
quant.
68(30)
quant.
495e
4
5
6
0.5
0.5
Hydrosilylations of epoxides were conducted either in
methylene chloride or in chlorobenzene at 22 1C. Catalyst
loadings of 0.25 or 0.5 mol% were generally used together
with 1.5 equiv. of Et3SiH. Slow inverse addition of the diluted
0.25
2.5 quant.g
54
46
7
0.5
1
1
quant.
quant.
495(85)f
8h
9h
0.5
0.5
quant.
2.2 quant.i
37
63
10h
0.5
2.5 quant.i
90
10
Scheme 1
a
Reaction conditions: 0.005 mmol of 1, solvent = CD2Cl2, 1.5 equiv.
Et3SiH, 22 1C, slow addition of the substrate (1.0 or 2.0 mmol)
in CH2Cl2 (1 mL) via syringe pump over 2 h. Determined by
b
Department of Chemistry, University of North Carolina at Chapel
Hill, Chapel Hill, NC 27599-3290, USA.
E-mail: mbrookhart@unc.edu; Fax: +1 919 962 2476;
Tel: +1 919 962 0362
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic NMR data for the hydrosilylated products.
See DOI: 10.1039/c0cc05714b
1H NMR. NMR yields based on an internal standard, and isolated
c
d
yields in parentheses. 8% of Dimeric product observed. 2% of
e
f
1,2-Diphenylethane observed. trans : cis
g
59% : 41%. NMR
=
yield = 82% and isolated yield is 45%. Direct addition of the substrate
h
i
to the catalyst solution in C6D5Cl. NMR yield = quant.
c
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Styrene oxide smoothly undergoes hydrosilylation to give the
corresponding silyl-protected primary alcohol, Et3SiOCH2CH2Ph
in 91% NMR yield (entry 1). Hydrosilylation of 1,2-epoxy-2-
methyl propane at 22 1C produces the silyl-protected primary
alcohol and the dimeric product, Et3SiOCMe2CH2OCH2-
CHMe2,5 in 83% and 8% NMR yields, respectively (entry 2).
2-Methyl-2,3-epoxy butane is also selectively converted to
the silyl-protected secondary alcohol (entry 3). Unexpectedly,
hydrosilylation of 2,3-dimethyl-2,3-epoxy butane gives only
the silyl-protected secondary alcohol, not the silyl-protected
tertiary alcohol, Et3SiOCMe2CMe2, indicating that methyl
migration occurs following ring opening (entry 4). Similarly,
trans-stilbene oxide is quantitatively hydrosilylated to produce
Et3SiOCH2CHPh2, indicating a 1,2-phenyl migration (entry 5).
1,2-Epoxy hexane undergoes C–O bond cleavage at both C–O
bonds of the epoxide to result in a mixture of the silyl-
protected primary and secondary alcohols in a ratio of
0.46 : 0.54 in 82% NMR yield (entry 6). Hydrosilylation of
methyl-1,2-cyclopentene oxide smoothly proceeds to give
2-methyl cyclopentyl triethyl silyl ether in excellent yield with
59 : 41 trans/cis product ratio (entry 7). Formation of two
isomers as products indicates the mechanism could be more
complex than silylation followed by ring opening with 3.
Tetrahydrofuran derivatives have been examined in the hydro-
silylation reactions (entry 8–10). Tetrahydrofuran is hydro-
silylated in 1 h to produce n-butyl triethylsilyl ether in a
quantitative yield (entry 8). 2,2-Dimethyl tetrahydrofuran
undergoes a selective cleavage of the C–O bond at the more
substituted carbon to afford the silyl-protected primary alcohol
(entry 9). The silyl alkyl ether having an internal olefin (63%)
is likely formed via silylation, ring opening to a tertiary
carbocation and elimination to the trisubstituted alkene. In
contrast, the hydrosilylation of 2-methyl tetrahydrofuran gives
the corresponding silyl-protected secondary alcohol as a main
product (entry 10).
Scheme 2
ketone, is analogous to the key intermediate in hydrosilylation of
ketones by 2 which we previously reported.6 Here, we have
observed upon hydrosilylation of 2-methyl cyclopentanone at
0 1C the same isomer ratio of the product (trans :cis = 75:25),
confirming the intermediacy of 4. Furthermore, these experiments
suggest that ketone could be intially formed via isomerization,
then, in a second step, hydrosilylated. To probe this possibility,
the reaction of 1, Et3SiH, and methyl-1,2-cyclopentene oxide in a
ratio of 1 : 53 : 27 in CD2Cl2 was monitored over ꢀ60 to ꢀ20 1C
by NMR spectroscopy (eqn (2)).
ð2Þ
Rearrangement of the epoxide to the ketone occurs at ꢀ60 1C
resulting in 79% conversion of the epoxide in 10 min. Upon
warming to ꢀ40 1C, the remaining epoxide was consumed
quantitatively over 20 min. The 13C NMR spectrum at ꢀ40 1C
clearly shows the six resonances of 2-methyl cyclopentanone, but,
at this stage, no resonances of product 2-methyl cyclopentyl
triethyl silyl ether. The 31P{1H} NMR spectrum shows 81 mol%
of several signals over d175 to 171, which are assignable to
the cationic iridium species coordinated to 2-methyl cyclo-
In the light of the unusual results for methyl-1,2-cyclopentene
oxide (both cis and trans isomers formed),3j we initiated a
mechanistic investigation of some of the hydrosilylation
reactions. Hydrosilylation of styrene oxide using Et3SiD yields
pentanone, [Ir–H][2-methyl cyclopentanone]+ (5).7
A
31P
+
signal at d190 is assigned to [Ir–H]H2 (19 mol%), is formed
due to H2O (Et3SiOSiEt3 also is observed to accompany
formation of [Ir–H]H2+). At ꢀ20 1C, 2-methyl cyclopenta-
none begins to undergo hydrosilylation to afford 2-methyl
cyclopentyl triethyl silyl ether as a main product in 78% : 22%
trans/cis ratio (quantitative conversion in 30 min). The results
of the low temperature NMR experiments confirm that the
epoxide is preferentially rearranged to the ketone, prior to
being hydrosilylated. Furthermore, the reaction of Lambert’s
b
the corresponding silyl-protected alcohol, Et3SiOCHaD–CH2 Ph,
in which the ratio of a–H to b–H is confirmed to be ca. 1:2. This
result suggests a quantitative isomerization via a hydride shift
(eqn (1)).
reagent, Et3Si+B(C6F5)4 (1 equiv.) with methyl-1,2-cyclo-
ꢀ8
ð1Þ
pentene oxide (5 equiv.) at ꢀ40 1C gives quantitatively
2-methyl cyclopentanone in 0ꢀ.5 h.9 This clearly indicates that
Et3Si+ from Et3Si+B(C6F5)4 can isomerize the epoxide to
the ketone.10
Results obtained in this study are consistent with the
proposed catalytic cycle in Scheme 3. Isomerization of the
epoxide to the ketone is rapid and followed by hydrosilylation.
Regioselectivity is observed when opening of silylated epoxides
results in selective formation of a stabilized carbocation
(tertiary (entries 2,3); benzylic (entry 1)).11 The isomerization
pathway may involve direct transfer of Et3Si+ from 4 to
epoxide (lower isomerization cycle, Scheme 3) or the transfer
In addition, the product obtained from the hydrosilylation of
methyl-1,2-cyclopentene oxide using Et3SiD at 22 1C exhibits
2
only two H signals at d3.82 and 3.46 due to a-deuterated cis
(43%) and trans (57%) isomers, respectively, indicating again a
quantitative D-incorporation into the a-carbon of the product via
hydride shift to form 4, followed by hydride transfer from 3-d2 to
the carbon a to oxygen (Scheme 2). Intermediate 4, a silylated
c
3644 Chem. Commun., 2011, 47, 3643–3645
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(g) M. B. Sassaman, G. K. S. Prakash and G. A. Olah, J. Org.
Chem., 1990, 55, 2016; (h) D. Mitchell and T. M. Koenig,
Tetrahedron Lett., 1992, 33, 3281; (i) P. V. de Weghe and
J. Collin, Tetrahedron Lett., 1995, 36, 1649; (j) H. C. Brown and
N. M. Yoon, J. Am. Chem. Soc., 1968, 90, 2686; (k) Nagashima
and co-workers have tested two epoxides as substrates in catalytic
hydrosilylation by triruthenium clusters bearing acenaphthylene.
The expected alcohols are accompanied by formation of considerable
amounts of oligomers. H. Nagashima, A. Suzuki, T. Lura, K. Ryu
and K. Matsubara, Organometallics, 2000, 19, 3579.
4 J. Yang, P. S. White and M. Brookhart, J. Am. Chem. Soc., 2008,
130, 17509.
5 The addition of the epoxide in CD2Cl2 to the catalyst solution
(0.5 mol%) containing Et3SiH (1.5 equiv.) over less than 10 min
produces the dimeric isomer in 40% NMR yield. This isomer has
been fully characterized by NMR spectroscopy (see Supporting
informationw). This indicates that a slower addition of the epoxide
allows higher yield of the desired product, and inhibits dimerization
of the epoxide.
6 S. Park and M. Brookhart, Organometallics, 2010, 29, 6057.
7 The complex 5 generated in situ from the reaction of 1, Et3SiH, and
2-methyl cyclopentanone in a ratio of 1 : 4 : 16 at ꢀ40 1C shows
four sharp 31P signals at d174.2, 172.6, 171.6, and 169.9 with a ratio
of 1 : 4 : 4 : 1, which are quite similar to those observed at d175.1,
173.6, 172.4, and 170.9 with the same ratio during the catalytic
reaction at ꢀ40 1C. Warming the solution to 22 1C causes the four
peaks to coalesce into two main peaks at d174.0 and 173.2 with a
ratio of 1 : 1. These results indicate that complex 5 exists as four
diastereomers with rapid interconversion of two pairs occurring at
22 1C.
Scheme 3
could occur via the intermediacy of 2 (upper isomerization cycle).
The dominant resting state in the dual cycle is [Ir–H][ketone]+, 5.
The hydrosilylation cycle using 2 was previously described6 and
is similar to an earlier report by Nikonov12 who established a
similar mechanism of hydrosilylation of carbonyls and nitriles
employing the cationic ruthenium silane complex [Cp(Pri3P)-
Ru(NCMe)(Z2–HSiMe2Ph)]+.
8 (a) J. B. Lambert, Y. Zhao and H. J. Wu, J.Org. Chem., 1999, 64,
2729; (b) J. B. Lambert, S. Zhang and S. M. Ciro, Organometallics,
1994, 13, 2430.
We gratefully acknowledge funding by the National Institutes
of Health (Grant No. GM-28939).
9 There have been a number of reports of catalytic rearrangement of
epoxides to ketones or aldehydes by Lewis acids: (a) J. N. Coxon,
M. P. Hartshorn and W. J. Rae, Tetrahedron, 1970, 26, 1091;
(b) S. Gilbert and J. E. Michael, J. Am. Chem. Soc., 1974, 96, 3682;
(c) M. David and B. Ouri, Tetrahedron Lett., 1974, 26, 2257;
(d) M. Suzuki, Y. Oda and R. Noyori, J. Am. Chem. Soc., 1979,
101, 1623; (e) J. K. Whitesell, R. S. Mattews, M. A. Minton and
A. M. Helbling, J. Am. Chem. Soc., 1981, 103, 3468;
(f) M. Keisuke, S. Masato and T. Gen-ichi, J. Am. Chem. Soc.,
1986, 108, 3827; (g) M. Keiji, N. Shigeru, O. Takashi and
Y. Hisashi, Tetrahedron Lett., 1989, 30, 5607.
Notes and references
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2 Reviews on reactions of epoxides: (a) S. Winstein and
R. B. Henderson, in Heterocyclic Compounds, ed. R. C. Elderfield,
+
10 [Ir–H]H2 is also found to promote the isomerization of methyl-
1,2-cyclopentene oxide (40 equiv.) at ꢀ40 1C to afford the ketone
(69% in 2.5 h), although the conversion to the ketone is sharply
retarded with reaction time probably due to the displacement of H2
by the excess ketone formed during reaction. Thus, we can not rule
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+
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lytic reaction might partially catalyze the isomerization of the
epoxide to the ketone, 2-methyl cyclopentanone.
11 The observation that phenyl migration is favoured over hydride
migration in entry 5 is unusual. The reason for this preference
awaits further experiments.
12 (a) D. V. Gutsulyak, S. F. Vyboishchikov and G. I. Nikonov,
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c
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Chem. Commun., 2011, 47, 3643–3645 3645