Iridium-catalyzed regioselective silylation of secondary alkyl C-H bonds for the synthesis of 1,3-diols

Article abstract of DOI:10.1021/ja5026479

We report Ir-catalyzed intramolecular silylation of secondary alkyl C-H bonds. (Hydrido)silyl ethers, generated in situ by dehydrogenative coupling of a tertiary or conformationally restricted secondary alcohol with diethylsilane, undergo regioselective silylation at a secondary C-H bond Υ to the hydroxyl group. Oxidation of the resulting oxasilolanes in the same vessel generates 1,3-diols. This method provides a strategy to synthesize 1,3-diols through a hydroxyl-directed, functionalization of secondary alkyl C-H bonds. Mechanistic studies suggest that the C-H bond cleavage is the turnover-limiting step of the catalytic cycle. This silylation of secondary C-H bonds is only 40-50 times slower than the analogous silylation of primary C-H bonds.

Full text of DOI:10.1021/ja5026479

Communication
pubs.acs.org/JACS
Iridium-Catalyzed Regioselective Silylation of Secondary Alkyl C−H
Bonds for the Synthesis of 1,3-Diols
Bijie Li,†,‡ Matthias Driess,‡ and John F. Hartwig*,†
†Department of Chemistry, University of California, Berkeley, California 94720, United States
‡Department of Chemistry, Metalorganics and Inorganic Materials, Technische Universitat Berlin, 10623 Berlin, Germany
̈
S
* Supporting Information
Table 1. Development of Ir-Catalyzed Hydroxyl-Directed
a
ABSTRACT: We report Ir-catalyzed intramolecular silyl-
Silylation of a Secondary C−H Bond
ation of secondary alkyl C−H bonds. (Hydrido)silyl
ethers, generated in situ by dehydrogenative coupling of
a tertiary or conformationally restricted secondary alcohol
with diethylsilane, undergo regioselective silylation at a
secondary C−H bond γ to the hydroxyl group. Oxidation
of the resulting oxasilolanes in the same vessel generates
1,3-diols. This method provides a strategy to synthesize
1,3-diols through a hydroxyl-directed, functionalization of
secondary alkyl C−H bonds. Mechanistic studies suggest
that the C−H bond cleavage is the turnover-limiting step
of the catalytic cycle. This silylation of secondary C−H
bonds is only 40−50 times slower than the analogous
silylation of primary C−H bonds.
elective functionalizations of C−H bonds by transition-
Smetal complexes are creating new strategies for the efficient
synthesis of functionalized organic molecules.1 Among various
a
2a (0.5 mmol), nbe (0.6 mmol), [Ir(OMe)(COD)]2 (2 mol%), and
C−H functionalization reactions, the silylation of C−H bonds is
particularly valuable.2 The organosilane products of these
reactions are stable enough to be isolated readily in pure
form,3 but reactive enough to be converted to useful products
through various selective transformations.4
ligand (4.8 mol%) in THF at 120 °C for 24 h. Conversion and yield
were determined by GC using n-dodecane as an internal standard. nbe
= norbornene.
process provides the opportunity to create an overall stereo-
selective C−H bond oxidation. However, the few examples of
silylation of unactivated secondary C−H bonds have occurred in
low yields under harsh reaction conditions.11,12 Here we report
that the catalyst formed from [Ir(OMe)(COD)]2 and 3,4,7,8-
tetramethyl-1,10-phenanthroline (Me4Phen) transforms secon-
dary C−H bonds of tertiary and conformationally restricted
secondary alcohols to secondary organosilanes in good yields
with high site selectivity and stereoselectivity. This silylation
reaction, along with subsequent oxidation at the C−Si bond,
provides a method to convert tertiary and secondary alcohols to
products in which the newly generated functionality is a
secondary alcohol within a 1,3-diol. Previously reported methods
for the direct conversion of alcohols to 1,3-diols based on radical
reactions favor hydroxylation at benzylic and tertiary C−H
bonds.15
Although a variety of catalysts have been developed for
silylation of aromatic C−H bonds,5−8 catalytic methods for
silylation of aliphatic C−H bonds are limited.9−14 Silylation of
C−H bonds at a benzylic position9 or adjacent to a hetero-
atom10,11 occurs in the presence of a directing group, and
silylation of unactivated C−H bonds at the terminal position of a
triorganosilane occurs intramolecularly in the presence of
catalysts containing Pt,6a Rh,12 or Ir.13 One author’s group
recently reported an Ir-catalyzed, hydroxyl-directed silylation of
terminal alkyl C−H bonds of alcohols and ketones.14 In this
study, high-yielding reactions were limited to those functionaliz-
ing primary C−H bonds. The related silylation of secondary C−
H bonds, which are sterically more demanding and less reactive
than primary C−H bonds, is undeveloped.
A mild, catalytic silylation of secondary C−H bonds directed
by common functional groups, such as alcohols or ketones,
would be synthetically useful because methylene C−H bonds are
abundant in organic molecules, and the strategy of using a silyl
ether derived from an alcohol or ketone for C−H bond silylation
would create site-selective functionalizations of methylene C−H
bonds. Oxidation of the C−H silylation product would install a
secondary alcohol containing a stereogenic center. Such a
To identify reaction conditions for the silylation of secondary
C−H bonds, we studied reactions of alcohols containing long
alkyl chains. Dehydrogenative coupling of tripropylcarbinol (1a)
with diethylsilane catalyzed by 0.25 mol% [Ir(OMe)(COD)]2
Received: March 15, 2014
Published: April 15, 2014
© 2014 American Chemical Society
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a
formed the (hydrido)silyl ether 2a. In the presence of an Ir
catalyst and the 4,4′-di-tert-butylbipyridine ligand, intramolecu-
lar cyclization provided the five-membered oxasilolane product
3a in 76% yield (entry 1, Table 1).
Table 2. Examples of the Silylation of Secondary C−H Bonds
The electronic properties of the ligands had a strong influence
on the yield. When dichlorophenanthroline L2 was used as a
ligand, a lower yield of 55% of oxasilolane 3a was obtained (entry
2). In contrast, when the strongly electron-donating Me4Phen
(L4) was used, full conversion of the substrate and a high yield of
the product (94%) were observed (entry 4).
The yield of product from the silylation of a secondary C−H
bond depends on the structural properties of the alcohol and the
substituents on the silane. Reaction of the (hydrido)silyl ether
derived from an acyclic secondary alcohol did not provide
significant amounts of cyclization product (entry 5). However,
reaction of a tertiary alcohol formed the oxasilole product in high
yield. Further studies on secondary alcohols are discussed below.
Cyclization of a (hydrido)silyl ether containing dimethyl groups
on the Si was similar to cyclization of the diethyl-substituted
silane (entry 6). However, the silyl ether containing bulky
isopropyl groups on the Si did not react (entry 7).
After identifying an efficient catalyst and appropriate
substrates for the silylation of secondary alkyl C−H bonds, we
sought to identify conditions to oxidize the silylation product to
form the corresponding 1,3-diol. Reaction of the secondary alkyl
oxasilolanes under conditions we used previously to oxidize
primary alkyl oxasilolanes did not furnish the desired diol.14
However, heating the crude reaction mixture with tert-butyl
hydroperoxide (TBHP), cesium hydroxide monohydrate, and
tetrabutylammonium fluoride (TBAF) in a DMF solution
converted the silylation product to the desired 1,3-diol (Scheme
1). These conditions were developed previously for the oxidation
of sterically hindered silanes, and the reaction proceeds with
retention of the configuration of the alkylsilane.16
a
Conditions: 1 (1.0 equiv), Et2SiH2 (1.5 equiv), [Ir(cod)OMe]2
(0.25−0.50 mol%), THF, room temperature (rt); removal of volatiles,
then [Ir(cod)OMe]2 (2 mol%), Me4Phen (4.8 mol%), nbe (1.2 equiv),
b
THF, 120 °C. Isolated yields are reported. Yield for the silylation
product. Starting from 1p in which R = TBS.
c
the cyclohexyl ring also affect the regioselectivity of the silylation
reaction. The 4-tert-butylcyclohexanol derivative 1i and the 4,4-
dimethylcyclohexanol derivative 1j, like the 4-phenylcyclo-
hexanol 1f, reacted to form 4i and 4j from silylation at the side
chain rather than silylation at the cyclohexyl ring. Similar steric
control was observed in a bicyclic system. Reaction of 9-decalinol
gave a single hydroxylation product 4k resulting from reaction at
the C2 position. A methylene carbon at the C-5 position
hindered reaction at the other possible silylation site (the C-4
position).
Scheme 1. Oxidation of Oxasilolane 3a
In addition to cyclohexanols containing a linear side chain,
cyclohexanols bearing cyclic side chains underwent silylation in
good yields (4l, 4m). Reaction of cyclobutyl-substituted
cyclohexanol 1l gave a single cis isomer, while cyclohexyl-
substituted cyclohexanol 1m gave mainly the trans isomer. The
relative configuration of the stereogenic centers in the major
isomer of 4m was confirmed by X-ray crystallography.
We also tested the functional group tolerance of the silylation
of secondary C−H bonds. Substrates bearing methoxy,
benzyloxy, siloxy, and acetal groups underwent silylation and
oxidation to provide the diols in good yields (4n−4p, 4r). The
TBS group in the silylation product was cleaved during oxidation,
producing the triol product (4p). In contrast to these silylation
reactions, silylation of alcohols containing ester (4q) and amide
groups did not occur.
In addition to the silylation of tertiary alcohols, we studied the
silylation of secondary alcohols, and these results revealed the
importance of conformation to the rate of C−H bond silylation.
Reaction of the (hydrido)silyl ether derived from 4-heptanol did
not provide significant amounts (<10%) of product (vide supra).
To understand the origin of the lower yield of reactions of this
secondary alcohol, relative to those of the reactions of the
analogous tertiary alcohols, we analyzed the silylation of
Through this sequence of silylation and oxidation, a variety of
1,3-diols were prepared from tertiary alcohols (Table 2).
Tributylcarbinol underwent silylation of the secondary C−H
bond (4b), even though the C−H bond γ to the alcohol is slightly
more hindered than that in tripropylcarbinol (4a). In addition to
acyclic tertiary alcohols, cyclic tertiary alcohols underwent
silylation at a secondary C−H bond. For example, 1-propyl-
cyclohexanol underwent hydroxylation in good yield to give two
constitutional isomers (4c, 4d) in a 1.6:1 ratio, resulting from
hydroxylation of the side chain and hydroxylation of the axial C−
H bond at the C-3 position, respectively.
Further reactions probed the effect of steric properties of the
substrate on the regioselectivity of the silylation process.
Reaction of a cyclohexanol containing a bulky tert-butyl group
on the side chain led to a single product (4e) resulting from
cleavage of the axial C−H bond on the cyclohexyl ring. In
contrast, reaction of cyclohexanols containing a phenyl group on
the cyclohexyl ring led to silylation of the side chains as the only
product (4f−4h). In this case, the cis phenyl group hindered
reaction at the axial C−H bonds. tert-Butyl and methyl groups on
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sequence of C−H silylation and oxidation was conducted with a
tertiary alcohol derived from cholesterol. The hydroxyl-directed
C−H functionalization reaction occurred at the side chain in
good yield to afford the corresponding 1,3-diol product (Scheme
3). The high regioselectivity of this reaction should allow
selective oxidation of a target C−H bond in a complex scaffold.
Scheme 2. Relative Rates for Silylation and Redistribution
Scheme 3. Functionalization of a Natural Product Derivative
(hydrido)silyl ethers derived from 4-heptanol and 4-methyl-4-
heptanol (Scheme 2). The (hydrido)silyl ether (2s) derived from
4-heptanol mainly underwent silane redistribution to give the
corresponding bis(alkoxy)diethylsilane.17 The (hydrido)silyl
ether (2t) derived from 4-methyl-4-heptanol gave products of
silane redistribution and silylation in a 1.2:1 ratio. The ratio of the
initial rates of formation of the product from silane redistribution
of 2s and 2t was 1:2.0 0.2. This result shows that redistribution
of the silyl ether derived from the secondary alcohol is slower than
that of the silyl ether derived from the tertiary alcohol. Thus, the
relative rates for redistribution reactions do not cause the yield
for silylation of a tertiary alcohol to be higher than that for
silylation of the analogous secondary alcohol; rather, it is the
faster rate of C−H silylation of the tertiary alcohol that leads to
the higher yield. This increased rate of reaction of the tertiary
alcohol is a manifestation of the Thorpe−Ingold effect;
cyclization of the gem-disubstituted reactant occurs faster than
cyclization of the monosubstituted or unsubstituted analogues.18
In the case of the (hydrido)silyl ether derived from a tertiary
alcohol, steric repulsion between the gem-dialkyl groups and both
the silyl ether and the third alkyl group will favor a conformation
of the silyliridium intermediate in which the secondary C−H
bond approaches the Ir center for C−H bond cleavage.
To gain insight into the mechanism, we measured the kinetic
isotope effect (KIE). The initial rates of two separate reactions
were measured with non-deuterated and deuterated substrates
2h and 2h-d4. These reactions revealed a primary KIE of 2.0 0.1
(Scheme 4). These data indicate that C−H cleavage is the rate-
limiting step of the silylation of secondary C−H bonds.
Scheme 4. Kinetic Isotope Effect on the Silylation Reaction
With these data in mind, we hypothesized that secondary
alcohols containing substituents β to the hydroxyl groups should
undergo silylation faster than secondary alcohols that have linear
substituents. Consistent with this hypothesis, dicyclohexyl-
methanol and dicyclopentylmethanol, both of which contain
substituents at the β-position of the hydroxyl groups, were
converted to the corresponding diols in good yields and good
diastereoselectivity by the combination of silylation and
subsequent oxidation (eqs 1 and 2). The relative configuration
In addition, we sought to obtain data on the relative rates for
oxidation of primary and secondary C−H bonds in hydrosilyl
ethers. Having observed the silylation of both primary and
secondary C−H bonds, we could measure the relative rates for
silylation at these two types of positions on an alkyl chain in
separate molecules and in the same molecule. In one experiment,
we measured the initial rates of silylation of silyl ethers 2x and 2f
separately under the same conditions. Silylation at the secondary
C−H bond was 49 2 times slower than silylation at the primary
C−H bond (Scheme 5). In a second experiment, we conducted
the reaction of silyl ether 2y containing one ethyl and two propyl
substituents that would establish an intramolecular competition
between primary and secondary C−H bonds. Again, the primary
C−H bond underwent silylation preferentially in the presence of
secondary C−H bonds (eq 3); in this case the products from the
silylation of primary and secondary C−H bonds were obtained in
a 41 2:1 ratio. These results suggest that silylation of a
secondary C−H bond is less facile than silylation of a primary
C−H bond, but only by a factor of 40−50.
of the major products was determined by X-ray crystallography
of their 4-nitrobenzoyl ester derivatives (see SI for details). The
functionalization of secondary C−H bonds located β to a ketone
also occurred after conversion of the ketone to the corresponding
silyl ethers by Ir-catalyzed hydrosilylation. For example,
dicyclohexyl ketone was converted to 4u in 59% yield and 10:1
diastereoselectivity by a sequence comprising hydrosilylation,
secondary C−H bond silylation, and oxidation.
In summary, we have developed an Ir-catalyzed site-selective
silylation of unactivated secondary C−H bonds. After oxidation
of the silylation products, 1,3-diols were obtained from the
corresponding alcohols. Methods for direct conversion of
alcohols to 1,3-diols are limited,15 and previously reported
This hydroxyl-directed silylation has potential for applications
to the functionalization of complex molecules. For example, the
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methods based on radical reactions strongly favor hydroxylation
at benzylic and tertiary C−H bonds. Previously reported Ir-
catalyzed silylation reactions were limited to those occurring at
primary C−H bonds.14 Now, we have shown that a secondary
C−H bond can undergo hydroxylation by a sequence of C−H
silylation and Tamao−Fleming oxidation reactions and that this
sequence can be conducted site selectively and diastereo-
selectively. Moreover, the 40:1 ratio of the rate for functionaliz-
ation of a primary C−H bond vs a secondary C−H bond gives
rise to selective reactions at a primary position, but allows
reactions to occur at a secondary position when a primary C−H
bond is not located γ to the alcohol. Further studies of the
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currently in progress.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jhartwig@berkeley.edu
■
(9) (a) Kakiuchi, F.; Tsuchiya, K.; Matsumoto, M.; Mizushima, E.;
Chatani, N. J. Am. Chem. Soc. 2004, 126, 12792. (b) Mita, T.; Michigami,
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledged financial support by the Einstein
Foundation Berlin (J.F.H.), the NSF (01213409) (J.F.H.), and
the Cluster of Excellence UniCat (financed by the DFG and
administered by the TU Berlin to M.D.) for research support. We
thank Dr. Antonio DiPasquale for assistance with crystallo-
graphic data. B.L. thanks Drs. Qian Li, Ming Chen, and Wenyong
Chen for insightful discussions.
(13) Ghavtadze, N.; Melkonyan, F. S.; Gulevich, A. V.; Huang, C.;
Gevorgyan, V. Nat. Chem. 2014, 6, 122.
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