Sodium Hydroxide Catalyzed Dehydrocoupling of Alcohols with Hydrosilanes

Article abstract of DOI:10.1021/acs.orglett.6b01687

An O-Si bond construction protocol employing abundantly available and inexpensive NaOH as the catalyst is described. The method enables the cross-dehydrogenative coupling of an alcohol and hydrosilane to directly generate the corresponding silyl ether under mild conditions and without the production of stoichiometric salt byproducts. The scope of both coupling partners is excellent, positioning the method for use in complex molecule and materials science applications. A novel Si-based cross-coupling reagent is also reported.

Full text of DOI:10.1021/acs.orglett.6b01687

Letter
pubs.acs.org/OrgLett
Sodium Hydroxide Catalyzed Dehydrocoupling of Alcohols with
Hydrosilanes
Anton A. Toutov,† Kerry N. Betz,† Michael C. Haibach,† Andrew M. Romine, and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: An O−Si bond construction protocol employing
abundantly available and inexpensive NaOH as the catalyst is
described. The method enables the cross-dehydrogenative coupling
of an alcohol and hydrosilane to directly generate the corresponding
silyl ether under mild conditions and without the production of
stoichiometric salt byproducts. The scope of both coupling partners
is excellent, positioning the method for use in complex molecule and
materials science applications. A novel Si-based cross-coupling
reagent is also reported.
despite decades of work, the most straightforward method for the
construction of the O−Si bond is the treatment of alcohols with
moisture-sensitive chlorosilanes in the presence of nucleophilic
catalysts and a base to scavenge the HCl generated.12 Of course,
this method may have limited utility for acid-sensitive substrates.
Moreover, this classical method is unsuitable in certain especially
challenging silylations such as the silylene protection of 1,2-diols,
especially in the cases of hindered systems. In these cases, the use
of specialized substrate/silane combinations and particularly
reactive systems13 or toxic electrophilic silicon reagents14 is
necessary. As a result, the development of an effective and
convenient O−Si bond construction methodology which circum-
vents the production of stoichiometric salt byproducts and avoids
the use of toxic and moisture-sensitive electrophilic silicon
sources, while simultaneously improving the scope incomparison
to previous methods, would be of interest and significant practical
value to chemists working in a variety of fields.
During our prior studies of catalytic C−H silylation using
Earth-abundant alkali metal bases, a robustness evaluation
revealed that a compound containing an −OH group was
adventitiously silylated under certain conditions.4a This was not
entirely unanticipated since base-catalyzed methods for the
formation of O−Si bonds by dehydrocoupling with hydrosilanes
have been previously explored.15 However, base-catalyzed
systems reported thus far often use highly basic activators,15a−c
tend to require the use of additives such as crown-ethers to obtain
suitable reactivity,15a,c,d and can require unacceptably high
temperatures (i.e., >150 °C).10e From a practical utility
perspective, these reports have shown only moderate functional
group tolerance with respect to both the alcohol and hydrosilane
partner, rendering them generally unsuitable for complex
ur group has explored direct C−H bond functionalization
1
Oreactions enabled by catalysts based on Earth-abundant
alkali metals wherein neither coupling partner has been
prefunctionalized.2 Such dehydrogenative functionalization
methods are valuable from the perspective of atom- and step-
economy; however, they are intrinsically challenging.3 We
recently reported the direct C−H silylation of aromatic
heterocycles with hydrosilanes catalyzed by potassium tert-
butoxide (KO-t-Bu) alone.4 We sought to investigate the alkali-
metal catalysis concept for the construction of other silicon-
containing bond classes. We were thus drawn to the O−Si bond
due to its importance in protecting group chemistries,5 its utility
as a traceless directing group in organic synthesis,6 and its
prevalence in a number of important functional material classes.7
Silyl ether functionalities have also been implicated in enhancing
the pharmacokinetic properties of medicines.8
A large number ofcatalytic methods for the construction of O−
Si bonds have been developed: the direct silylation of alcohols by
transition-metal catalysis,9 Brønsted and Lewisacids/bases,10 and
catalytic hydrosilylation of carbonyl compounds11 have been the
most commonly employed protocols (Scheme 1a). However,
Scheme 1. Strategies for the Synthesis of Silyl Ethers
Received: June 10, 2016
Revised: October 11, 2016
© XXXX American Chemical Society
A
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Letter
molecule applications. Fluoride salts have also been shown to
catalyze the dehydrocoupling; however, reactions can be energy
intensive (i.e., temperatures between 0 and 180 °C), require
highly polar solvents (e.g., NMP), and are also generally reported
with modest substrate scope.10b−e,15f Nevertheless, an efficient
base-catalyzed (and ideally fluoride-free) strategy would be
attractive, potentially representing a convenient, low cost, and
transition-metal-free strategy for dehydrogenative silyl ether
synthesis.
Scheme 2. NaOH-Catalyzed Dehydrogenative Si−H/O−H
a,e
Cross Coupling: Scope of the Hydrosilane
Conversely, the Lewis acid tris(pentafluorophenyl)borane
(B(C6F5)3) has also been shown to be a catalyst for the
dehydrogenative silylation of alcohols with hydrosilanes.10a This
method proceeds under mild conditions and has a useful
functional group tolerance. However, the presence of Lewis
basic nitrogen functionalities dramatically lowers the reactivity or
leads to complete catalyst deactivation. Moreover, cyclic ethers
such as THF derivatives are ring-opened in the course of the
reaction except in rare special cases. These issues and others limit
the utility of Lewis acid catalyzed methods, especially in alkaloid
naturalproduct synthesisandthederivatization andmanipulation
of pharmaceutical targets.
We sought to develop a cross-dehydrogenative O−Si bond
coupling reaction that employs a mild and inexpensive catalyst,
operates under mild conditions in the absence of costly additives,
and shows superior functional group tolerance to prior strategies.
Thus, inspired by the work of others and encouraged by our
previous results in silylation catalysis,4 we herein report a practical
and general cross-dehydrogenative O−Si bond construction
methodology catalyzed by NaOH alone (Scheme 1b). The
protocol has excellent functional group compatibility, proceeds
well in the presence of Lewis basic moieties and sensitive
functionalities on the substrate, satisfies sterically demanding as
well as fragile hydrosilanes, and operates under mild conditions.
The silyl ether products are formed in high yield, and H2 is
generated as the byproduct.
a
Reactions performed with 0.5 mmol of starting material and 0.5 mL
b
of THF at the prescribed temperature. A 1:1 mixture of DMF/THF
was used as the solvent. 3.0 equiv of hydrosilane and 20 mol %
NaOH. The reaction was conducted for 24 h. Yield of isolated
material after purification. Yield based on a theoretical maximum of
c
d
e
f
0.25 mmol of Si-tethered product.
catalyzed cross-dehydrogenative coupling is amenable to
substrates containing aromatic (Table 1, entries 1−12, 21, and
22) as well as aliphatic (entries 13−20) moieties. The reaction
proceeds well in the presence of arenes bearing halides (entries 2
and 3), nitro- (entry 4), ether (entry 6), and alkyl (entry 10)
functionalities leading to the corresponding silyl ethers in
generally high yields.
Electron-rich aromatic heterocycles (entries 8 and 9) are
likewise excellent substrates for the dehydrocoupling providing
the corresponding silyl ethers in high yield. A 2° allylic alcohol
(entry 13) and a 1° propargylic alcohol (entry 14) also react well,
with no reduction or hydrosilylation detected. Cyclopropanes are
also tolerated (entry 15). Functionalities that deactivate organic
and organometallic Lewis acid catalysts such as pyridines (entry
7) and those that are acid-sensitive such as an epoxide (entry 16)
are silylated in 97% and 72% yield, respectively. The 3° alcohol 1-
adamantol (entry 17) also undergoes dehydrosilylation, affording
the silyl ether product in excellent yield. Wewere surprised to find
that functionalities such as an aromatic methyl ester (entry 5)19
and a phthalimide (entry 11),20 which are known to readily
undergo hydrosilylation or direct reduction in the presence of
mixtures comprising Lewis bases and hydrosilanes, react well
under the reaction conditions with little (5−7%) hydrosilylation
detected only in the latter case. Alkyl−CF3 and alkyl−Cl moieties
likewise reactwell (entries 19and 20). ArOH fragments such asp-
methoxyphenol and BINOL are silylated in good yields (entries
21 and 22).
A number of mechanistic pathways for Lewis base activation of
silanes in the context of related dehydogenative couplings have
been proposed.12,21−23 Most recently, Oestreich and co-workers
investigated the mechanism of a related dehydrogenative silyl
ethersynthesisusingaKO-t-Bu-basedsystem.15c Inthatinsightful
study, theauthorsproposetheformationofakeypentacoordinate
potassium silicate by interaction of KO-t-Bu with hydrosilane
under similar conditions. An analogous activation mode is likely
also occurring in our reported system. It is interesting to note that
Unfunctionalized benzyl alcohol was chosen as the model
substrate along with PhMe2SiH as the hydrosilane (Scheme 2).
The reaction proceeded in THF as the solvent with 10 mol %
NaOHasthecatalysttoafford94%yieldofthedesiredbenzylsilyl
ether 2a at ambient temperature. The reaction can be performed
without regard for air and moisture with an identical 94% yield;
however, siloxane (PhMe2Si)2O is generated as a byproduct,
likely arising from the double silylation of H2O.
We next evaluated the scope of the hydrosilane and observed
that a large scope of hydrosilanes were amenable to the NaOH-
catalyzed reaction (Scheme 2). Monohydrosilanes with small-
and medium-sized aryl (2a and 2c) and alkyl (2b) substituents
gave thecorresponding silyl ethersinhighyields. Themuchlarger
TBDMS and TIPS groups could also be introduced, yielding 2d
and 2f, respectively; however, it was found that the use of DMF as
a cosolvent was necessary to obtain good yields in these cases.16
The use of the bulky dihydrosilane di-tert-butylsilane led to
efficient monosilylation, generating 2e in high yield; however, the
use of less sterically hindered dihydrosilanes led exclusively to Si-
tethered species 2h and 2i at ambient temperature. The method
tolerates even fragile disilanes such as H−Si2(CH3)5 leading to
product 2g and could thus be useful for applications to silicon
materials.17 Finally, the reaction of diethylsilane with water under
NaOH catalysis results in the formation of cyclic siloxanes, with
trisiloxane 2j being the major product (by GC analysis).18 With
thesedatainhand, weproceededtoexplorethealcoholscope, and
a wide variety of hydroxyl-containing small molecules proved to
be excellent substrates in this reaction (Table 1). The NaOH-
B
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Letter
a
,d,
e
a,b
Table 1. Scope of the Alcohol Partner
Scheme 3. Applications to Directing Group Chemistry
a
Reaction performed with 0.5 mmol of starting material and 0.5 mL of
b
THF. Yield of isolated material after purification.
PhSiMe(pin), where Me is chosen as a nontransferrable group,
and evaluate its suitability as an aryl-transfer reagent. This would
be beneficial given the increased abundance and lower cost of
silicon relative to boron24,25 and the potential for improved
stability or overall utility of the silicon reagent.24,26 However, one-
step preparation of PhSiMe(pin) would involve a silylene
protection of a 1,2-diol, which has proven challenging, especially
in the case of sterically hindered diols such as pinacol even using
standard electrophilic silicon sources (i.e., [Si]−Cl).13,14,15d,27,28
This is normally due to either poor cyclizat-ion reactivity,
uncontrollable oligomerization, or rearrangements.13,14,15d As a
result, and to the best of our knowledge, this cyclic siloxane
compound has only been prepared by refluxing pinacol in THF
for 24 h with (a) the dichlorosilane in the presence of
stoichiometric pyridine (a yield was not reported)27 or (b) the
dihydrosilane in the presence of a catalytic quantity of Cp2TiCl2/
n-BuLi, providing the product in 67% isolated yield.28
Fortunately, it was found that the NaOH-catalyzed dehy-
drosilylation is an excellent strategy for the synthesis of cyclic
siloxanes. The method was successful in providing access to
silicon-protected diols with varying ring sizes and functionalities
at silicon from the corresponding diols and hydrosilanes enabling
thehigh-yieldingpreparationof9and10. Itwasnoticed, however,
that 10 was hydrolytically unstable and could not be isolated.
Finally, reacting the commercially available PhMeSiH2 with
pinacol resulted, upon addition of the NaOH catalyst, in the
immediate and vigorous evolution of hydrogen. The reaction is
complete in 20 h at ambient temperature, giving a high yield (2.19
g, 93% yield) of the corresponding colorless liquid PhSiMe(pin)
11 (Scheme 4) after purification by distillation. Note that since
the silylation protocol described in this paper produces
stoichiometric H2, the reaction, especially when performed on a
a
Reactions performed with 0.5 mmol of starting material and 0.5 mL
b
of THF at the prescribed temperature. 2:3 DMF/THF used as the
solvent. 3.0 equiv of hydrosilane, 20 mol % of NaOH, DME (1.0 M)
solvent. [Si] = PhMe2SiH. Yield of isolated material after
purification. Base-catalyzed transesterification occurs to a minor
extent. See the Supporting Information. 1,2-Dimethoxyethane is used
as the solvent; 3 equiv of PhMe2SiH.
c
d
e
f
g
NaOH is a much weaker base than KO-t-Bu so the exact nature of
the interaction will require further study. This may explain the
successful silylation of more sensitive examples such as 4s under
our conditions.
Scheme 4. Silylene Protection of Diols and Synthesis of
a,b
PhSiMe(pin)
With ascope ofsubstrates established, we turned toemployour
NaOH-catalyzed method tothe synthesis of silyl ethers that could
be used in directing group chemistry and cross-coupling
reactions. For example, the reaction of phenol (3r) with the
bulky and commercially available di-tert-butylsilane 5 with 10 mol
% of NaOH furnishes the corresponding di-tert-buylhydrosilyl
ether 6 in high yield and without undesirable double activation of
the Si−H bond (Scheme 3). Gevorgyan has reported that silane 6
is elaborated by palladium-catalyzed C−H functionalization
reactions such as ortho-oxidation to generate catechols (Scheme
3,6→7)6b andortho-alkenylationtoaccessα-hydroxystyrenes(6
→ 8) via the corresponding silanol.6a
Cognizant of the importance of heteroatom-substituted
arylsilanes in C−C and C−X bond-forming reactions,12,21b,24
we sought to employ the catalytic O−Si bond construction
method for the expedient and cost-efficient synthesis of novel
cross-coupling reagents.24 We aimed to construct the silicon
analogue of the aryl boronic acid pinacol ester PhB(pin),
a
Reactions performed on a 1.0 mmol scale, except for the synthesis of
b
11, which is performed on a 10 mmol scale. Yields are of isolated
material after purification. Quantitative conversion by GC, but
product cannot be isolated.
c
C
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Letter
a,b
Scheme 5. Application of PhSiMe(pin) to Aryl Transfer
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Cu-mediated reaction performed on a 0.5 mmol scale. Yields are of
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vented system and under conditions that are appropriately
suitable and safe (see the Supporting Information for more
details). As with 9 and 10, no oligomers, polymers, or uncyclized
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N-arylation product 13 in 71% yield (Scheme 5).30
In summary, we have developed a mild and general cross-
dehydrogenative O−Si bond construction protocol using NaOH
as the catalyst. The reaction is robust, scalable, shows exceptional
scope, and even enables the silylene protection of challenging
diols. The convenience and breadth of the chemistry should make
this method accessible to chemists and nonchemists alike for the
synthesis of silyl ethers in a myriad of contexts and across a wide
variety of fields.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b01687.
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(15) (a) Le Bideau, F.; Coradin, T.; Henique, J.; Samuel, E. Chem.
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Full experimental details and characterization of new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: rhg@caltech.edu.
Author Contributions
†A.A.T., K.N.B., and M.C.H. contributed equally.
Notes
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The authors declare no competing financial interest.
(19) Revunova, K.; Nikonov, G. I. Chem. - Eur. J. 2014, 20, 839.
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ACKNOWLEDGMENTS
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Chemistry; Wiley: New York, 2000; p 569.
This work was supported by the NSF under the CCI Center for
Selective C−H Functionalization (CHE-1205646) and under
CHE-1212767. A.A.T. is grateful to the Resnick Sustainability
Institute at Caltech and to Dow Chemical for a predoctoral
fellowship and to NSERC for a PGS D fellowship. We thank S.
Virgil and the Caltech Center for Catalysis and Chemical
Synthesis for access to analytical equipment. M. Shahgoli and N.
Torian (Caltech) are acknowledged for assistance with high-
resolution mass spectrometry.
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