α-Diimine-Niobium Complex-Catalyzed Deoxychlorination of Benzyl Ethers with Silicon Tetrachloride

Article abstract of DOI:10.1021/acs.inorgchem.9b01784

α-Diimine niobium complexes serve as catalysts for deoxygenation of benzyl ethers by silicon tetrachloride (SiCl₄) to cleanly give two equivalents of the corresponding benzyl chlorides, where SiCl₄ has the dual function of oxygen scavenger and chloride source with the formation of a silyl ether or silica as the only byproduct. The reaction mechanism has two successive trans-etherification steps that are mediated by the niobium catalyst, first forming one equivalent of benzyl chloride along with the corresponding silyl ether intermediate that undergoes the same reaction pathway to give the second equivalent of benzyl chloride and silyl ether.

Full text of DOI:10.1021/acs.inorgchem.9b01784

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
α‑Diimine-Niobium Complex-Catalyzed Deoxychlorination of Benzyl
Ethers with Silicon Tetrachloride
Bernard F. Parker,†,‡ Hiromu Hosoya,† John Arnold,*,‡ Hayato Tsurugi,*,†
and Kazushi Mashima*,†
†Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
‡Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: α-Diimine niobium complexes serve as catalysts for
deoxygenation of benzyl ethers by silicon tetrachloride (SiCl4) to
cleanly give two equivalents of the corresponding benzyl chlorides,
where SiCl4 has the dual function of oxygen scavenger and chloride
source with the formation of a silyl ether or silica as the only
byproduct. The reaction mechanism has two successive trans-
etherification steps that are mediated by the niobium catalyst, first
forming one equivalent of benzyl chloride along with the
corresponding silyl ether intermediate that undergoes the same
reaction pathway to give the second equivalent of benzyl chloride and silyl ether.
INTRODUCTION
■
Ether linkages are one of the most fundamental bonds in
organic chemistry and are present in a huge range of both
natural and synthetic molecules. The Williamson ether
synthesis is a classic reaction discovered in the mid-19th
century, which is the synthesis of ethers by a simple
substitution reaction of alkyl halides. Since then, hundreds
more reactions have been developed to synthesize ethers;
current goals are improving the selective synthesis under mild
reaction conditions with high selectivity, high functional group
tolerance, and using cheaper reagents.1−3
In sharp contrast, the reverse reaction is considered to be
significantly more difficult, since deoxygenation of ethers back
to alkyl halides is generally thermodynamically unfavorable due
to the strength of the C−O bond and relative weakness of the
C−X bond formed in the alkyl halide products, as shown in
selected examples in Figure 1.4,5 One notable application of
such ether cleavage reactions has been the deprotection of
phenol derivatives;6 boron tribromide is a classic reagent for
the cleavage of methyl ethers of phenols as well as certain other
ethers.6,7 Other reactions with milder reagents or conditions
Figure 1. Selected ether deoxygenation reactions to alkyl halide
products.
for the cleavage of dimethyl ether and similar ethers have also
been developed that tolerate a wider range of functional
groups.8,9 In addition to deprotection reactions, ether cleavage
Metal halides have also been reported as reagents for ether
cleavage to give alkyl halides, where the metal acts as a halogen
transfer reagent. For example, bismuth can transfer a chlorine
atom to 2-methyl- and 2,5-dimethyltetrahydrofurans, where
the ring is preactivated with an acyl chloride (Figure 1a).15,16
Numerous other acylative metal-catalyzed ring-opening re-
can be used in its own right in synthesis and semisynthesis
from ether natural products and substrates, such as in the
depolymerization of lignin, which is a critical step in biomass
conversion to liquid fuels.10−12 In a similar manner, the
treatment and recycling of ethereal solvents, including benzyl
ether, by deoxygenation reactions are used to produce useful
chemical products from what would otherwise be waste
material.13,14
Received: June 15, 2019
© XXXX American Chemical Society
A
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Article
actions are known that rely on relieving ring strain, giving
esters rather than completely deoxygenated products.17,18
Tungsten hexachloride has been used to stoichiometrically
cleave many ethers, including benzyl ether, to form alkyl
chlorides (Figure 1d); a variety of other products including
alcohols and overhalogenated compounds are invariably
produced as well.19 Simple halogenating reagents such as
trimethylsilyl iodide or concentrated hydrobromic or hydro-
iodic acid are generally effective (Figure 1b and c), but these
aggressive reagents are typically required in excess and can
form a variety of different products depending on the
conditions used.3,20,21
Here, we present an ether cleavage reaction in which silicon
tetrachloride (SiCl4) or another silicon halide is used both as a
halogen source as well as an oxygen atom scavenger (Figure
1e). A niobium complex bearing a redox-active α-diimine
ligand catalyzes this reaction, although it can react stoichio-
metrically with ether substrates as well. The use of SiCl4 in this
role is especially attractive, as it forms silica as the only major
byproduct, which is inert and easily removed by filtration. This
reaction is selective for benzyl ethers, especially dibenzyl
ethers. The reaction takes place through two successive trans-
etherification reactions to silicon driven by Si−O bond
formation, both of which are catalyzed by niobium.
Table 1. Catalyst Screen for Benzyl Ether Deoxygenation
a
entry
catalyst
yield
1
2
3
4
5
6
7
8
1a
1b
1c
1d
2a
2b
3
88%
18%
20%
72%
36%
24%
78%
80%
57%
43%
47%
37%
20%
57%
NR
4
5
9
10
11
12
13
14
15
6
1a
b
NbCl5
TaCl5
NbCl3·2DME
none
a
b
All yields are NMR yields. NR = no reaction. 50 mol % SiCl4.
RESULTS AND DISCUSSION
■
phenyl substituents on the nitrogen atoms, similar complexes
with different atoms directly bonded to the niobium were also
tested. The tetrachloride complex (4), trichloro-oxo complex
(5), and monochloro-oxo complex LNbOCl (6) all likely
function as precatalysts, first forming 1, although lower yields
arise due to the inefficiency of these steps.
Group 5 metal salts were also tested for their reactivity
(entries 12−14). Benzyl chloride is produced in low yields in
these reactions, and a mixture of metal and silicon alkoxides are
observed also. The lower solubility of these salts is another
limitation for these reactions, and some oxychlorides that may
form during the reaction are also expected to be insoluble. This
reactivity is reminiscent of the reported reaction of ethers with
WCl6 and SbCl5, where the Lewis acidic metal alone acts as the
halogen source and oxygen acceptor.9,19 In the case of
tungsten, the reaction was stoichiometric, but it can be made
catalytic with the addition of SiCl4. Reactions with simple
metal salts are straightforward but less effective than the ligand-
supported catalysts (see Supporting Information, Table S1),
due to low solubility, incomplete reactions, and requiring
higher catalyst loadings.
Catalyst and Additive Screening. We have previously
reported that α-diimine complexes of niobium and tantalum
cleave activated carbon−halogen bonds, promoted by the
redox-active α-diimine ligand.22−25 A variety of niobium and
tantalum complexes with α-diimine ligands were screened for
their effectiveness as catalysts in ether deoxygenation (Figure
2, Table 1), in which 4-methoxybenzyl ether was used as the
Different solvents and conditions were examined to
determine optimal conditions for this deoxygenation reaction
(see Supporting Information, Table S2). Many noncoordinat-
ing solvents are suitable, with the reaction working in benzene,
toluene, chlorobenzene, dichloromethane, and chloroform;
unlike the others, the latter two performed well at room
temperature. In contrast, coordinating solvents including THF,
acetonitrile, and 1,4-dioxane are ineffective as they coordinate
to the niobium center, suppressing reactivity. In the case of
THF and acetonitrile, this manifests as a color change,
indicating a change of geometry upon their binding.22 Mixtures
of THF and noncoordinating solvents show diminished activity
rather than complete suppression of the reaction, confirming
that this binding is reversible and that such solvents do not
compete with benzyl ether or otherwise react with the catalyst.
Substrate Scope and Limitations. A wide variety of
substrates were tested under the optimized deoxygenation
Figure 2. α-Diimine metal catalyst structure.
model substrate due to its high reactivity at room temperature.
Several variants of the α-diimine (DAD) ligand with different
substituents suggest that a sterically bulky ligand is optimal,
and the similar naphthalene-based BIAN ligand (1d) is also
effective. Analogous complexes of the other group 5 metals
tantalum (2) and vanadium (3) can catalyze the reaction as
well but are less effective, and side reactions are observed.
Having identified the optimal α-diimine ligand bearing
methyl substituents on the imine carbons and diisopropyl-
B
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reaction conditions to determine substrate scope and discover
reactivity patterns. The reaction rate and yield strongly depend
on the substituents on the benzyl group, where electron-
donating substituents (Table 2, entries 2−5) increase the
Table 3. Substrate Scope of Other Ethers
Table 2. Substrate Scope of Symmetric Benzyl Ethers
a
b
Reaction performed at room temperature. No product, side
c
reactions occur. Yields are NMR yields (isolated yields in
parentheses). NR = no reaction.
a
b
Mixture of ( ) and meso substrate. Reaction performed at room
c
temperature. All yields are NMR yields. NR = no reaction.
reaction rate, while electron-withdrawing groups on the
aromatic ring (entries 7−9) hinder the reaction, resulting in
either lower yields or no reaction. The trifluoromethyl-
substituted benzyl ether substrate (entry 10) undergoes side
reactions, resulting in multiple unidentified products. Similar
niobium complexes are known to be effective at defluorination
of CF3 groups, although in the present case the fluorine-
containing products have not been identified.26
Unsymmetrical substrates containing only one benzyl group
(Table 3, entries 4−8) also react, although generally with
slower reaction rates and lower product yields. The initial
activation step of the first C−O bond appears to proceed
normally, but the reaction may then need to proceed through
an alternate pathway to form side products containing
nonbenzylic alkoxide groups (see below for details). Diaryl
and nonbenzylic dialkyl ethers show no reactivity, even with
prolonged heating or in a neat substrate. This unusually high
selectivity suggests that the benzyl group plays a unique role
such as radical stabilization, consistent with ethers bearing
substituents on the α-carbon showing much higher reactivity,
most notably α-propylbenzyl ether (Table 3, entry 2), where
the reaction is very rapid, forming silica at room temperature
within minutes.
The other reagent that is critical to this reaction is the silicon
halide. Silicon tetrachloride was found to be superior to other
silicon halides tested, including dichlorodimethylsilane,
trimethylsilyl chloride, and others (see Table S3 in Supporting
Information). The silicon halide is both the source of the
halogen atom that is incorporated into the product
(trimethylsilyl bromide can be used to form the analogous
benzyl bromide) as well the oxygen atom acceptor, forming
strong Si−O bonds to give the only byproduct of the reaction.
When SiCl4 is used, silica precipitates in the reaction mixture,
leading to simplified workup and purification procedures. An
excess of SiCl4 is needed for high yields and a smooth reaction,
due to the trapping of some unreacted halogen atoms in the
C
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solid as it is formed. Therefore, a 50% excess of SiCl4 was used
in reactions (corresponding to 75 mol % with respect to
substrate or 300 mol % Cl atoms), leading to the satisfactory
results. The nominal product of one reaction cycle is one unit
of the silyl ether (Cl3Si)2O from two equivalents of SiCl4;
however, once formed, the mixed oxygen-chlorine species
rearrange and continue reacting to ultimately form variable
composition silicon oxyhalide oligomers, polymers, and
network solids. This type of product mixture has been
previously reported as the byproduct of phosphine oxide
deoxygenation and other partial hydrolysis and condensation
reactions.27
as it disproportionates into the bis(benzyl) product
(BnO)2SiCl2 and SiCl4; see Supporting Information Figures
S3 and S4). However, the rate of this process is greatly
increased when the niobium catalyst is present, and the silyl
ether C formed at this point in the reaction does not proceed
to form benzyl chloride in the absence of a catalyst.
Observation of Reaction Progress. Reactions were
monitored over time by NMR spectroscopy, using unsub-
stituted benzyl ether and heating to 80 °C with varying
concentrations of reagents and catalysts (see Supporting
Information Figures S5−S7). The reaction is first-order with
respect to the catalyst; however, it shows no dependence on
both the substrate and silicon reagent concentrations. This is
supportive of a unimolecular rate-determining step, where an
initial activation or coordination of the substrate takes place
before the C−O bond cleavage.
Proposed Reaction Mechanism. On the basis of
observations of this deoxygenation reaction with a variety of
different substrates and conditions, we propose a reaction
pathway as shown in Scheme 1. Dibenzyl ethers are activated
Scheme 1. Proposed Catalytic Cycle of Benzyl Ether
Deoxygenation
To elucidate the mechanism for the C−O bond activation
step, a “one-pot Hammett” analysis was attempted, using a
mixture of two substrates.29 Unsubstituted benzyl ether and a
substituted benzyl ether in approximately equal molar amounts
were allowed to react for a short period of time and the
product yields measured. Although some substrates show the
trend that electron-withdrawing substituents decrease the
reaction rate and electron-donating ones increase it, other
substrates do not follow a typical Hammett free energy
relationship. Notably, 4-phenylbenzyl ether reacts almost one
order of magnitude faster than the unsubstituted ether;
however, it is expected to have virtually no effect on an ionic
mechanism (see Supporting Information Table S4). Con-
sequently, we propose that the intramolecular C−O activation
step does not easily fall into a simply ionic or radical
mechanism, and the benzyl ether sterics probably play an
important role as well in orienting the ether, which is not
accounted for in the Hammett analysis.
Alkoxide complex B, here denoted as compound 7, is
unstable and decomposes over several hours. Further proof
that the two C−O bond activation reactions are independent
from each other came from their reactivity of some asymmetric
ethers such as the mono(4-methyoxy) substituted benzyl ether
(eq 2). Only 4-methoxybenzyl chloride was produced cleanly,
then the reaction stopped and the other benzyl chloride was
not immediately observed.
in two steps, each effectively a trans-etherification reaction to
form a silyl ether. In both of these steps, the ether reversibly
coordinates to the niobium, although the coordinated ether is
labile due to steric hindrance. This can be observed as a color
change as well as by NMR, but only at very large [ether]/[Nb]
ratios (see Supporting Information Figures S1 and S2). The
niobium species A then performs an intramolecular C−O bond
cleavage (the rate-determining step), to release one equivalent
of benzyl chloride, leaving the alkoxide B. Byproducts arising
from radical coupling, H• abstraction, or reaction with the
solvent are never observed, suggesting that no free radicals are
formed in this process. Subsequently, SiCl4 reacts to form a
silyl ether, releasing the alkoxide and regenerating the initial
niobium trichloride complex 1a. The second trans-ether-
ification proceeds in the same manner to form a second
equivalent of benzyl chloride via silyl ether-coordinated species
D and siloxyniobium species E.
The isolation of 7 was attempted, including by trapping with
other ligands or using several chelating ethers as substrates;
however, in all cases, 7 is unstable at room temperature,
decomposing within several hours at most. However, its
reactivity is consistent with the assigned structure (Scheme 2).
The addition of SiCl4 and heat is needed to continue the
reaction and produce the Bn-O-SiCl3 ether, with similar
reactivity to other silyl halides. Additionally, when a solution
containing 7 was quenched with a small amount of water,
Benzyl-trimethylsilyl ether and benzyl-trityl ether (Table 3,
entries 3 and 8), both containing Lewis acid-labile C−O
bonds, are protected benzyl alcohols.28 These react with SiCl4
at room temperature in the absence of any catalyst (eq 1) to
form the benzyl-O-SiCl3 ether C, in the same way as the first
step of the proposed catalytic cycle (C cannot be isolated pure
D
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Scheme 2. Reactivity and Decomposition of 7
benzyl alcohol was produced and isolated from the product
mixture. The similar 4-methoxy/4-cyano substituted asym-
metric ether reacts in the same way, but the cyano-substituted
7 is even less reactive.
The stoichiometric reaction of the symmetric benzyl ether
with the niobium complex in the absence of SiCl4 does
produce approximately 1.8 equiv of benzyl chloride over 48 h
along with the niobium oxomono(chloride) 6 along with other
niobium oxido species, Nb2O5, as well as a free and partially
hydrolyzed DAD ligand.30 Compound 6 can also be
synthesized from 5, the niobium oxotrichloride, using our
group’s silicon-based reducing agents (Scheme 3).22,31 When
Figure 3. Geometry and oxidation state changes at niobium.
O bond cleavage to form 7, the Nb atom and ligand are best
described as Nb(IV) and L•−, where the ligand donates less
electron density to niobium due to the electron-rich alkoxide
(see Supporting Information Figure S1). This is seen as a color
change to a deep emerald-green color and lack of an NMR
signal, consistent with the previously reported LNbCl4
complex 4 with a radical anion on the ligand as the source
of the green color.22,23
Scheme 3. Formation of 6 from 7 (top) and Independent
Synthesis of 6 (Middle) and Deoxygenation Back to 1a
Since the precise mechanisms for the important reaction
steps are not known, the effect of different alkyl groups on the
ligand (Figure 2) is not obvious. However, larger groups on
both the backbone and nitrogen atoms are important for high
yields (1a and 1d). The increased electron-donating ability of
these ligands may help stabilize 7 and favor the formation of
the Nb(IV) center, as 1a has shown to be especially reactive in
other redox-active roles.22,25 However, steric protection
afforded by these ligands may play a more significant role in
suppressing side reactions such as radical addition to the ligand
backbone or H-abstraction from the ligand.23 In addition,
ligand lability appears to be a source of side reactions as the
free ligand has been observed in reaction mixtures at long
reaction times (see above).
The deoxygenation reaction is typically very clean, forming
only benzyl chloride under the optimized catalytic conditions
with excess SiCl4. In control experiments, when stoichiometric
(50 mol %; 200 mol % Cl atoms per benzyl ether) or
substoichiometric amounts are used, side reactions are
observed at long reaction times. Most of these decomposition
products are unidentified, but a few of them are known to arise
from ligand hydrolysis or H-abstraction of the backbone
methyl group, both of which are reported for these types of
ligands.23,32−34 By using an excess of the cheap silicon reagent,
these reactions are effectively suppressed.
an excess of SiCl4 is added to 6 or mixtures containing 6 from
either source, deoxygenation occurs to regenerate 1a.
However, this step is slow, and several unidentified side
reactions also take place throughout this pathway. Although it
could contribute to the overall catalytic reaction, it is much
slower and less efficient and does not play a meaningful role in
the catalytic cycle.
Ligand Participation. Throughout the catalytic cycle, the
transformations taking place at the niobium atom are all redox-
neutral, and no formal oxidation state changes occur. However,
the geometry and redox-active nature of the DAD ligand play a
critical role in the reaction by varying the extent of electron
donation from the ligand (Figure 3). In the niobium trichloride
catalyst 1a, niobium is in the +5 oxidation state with a
dianionic ligand, with additional π-donation from the ligand.22
When the ether coordinates to form intermediate A, the ligand
reverts to simple κ2 coordination to make a 6-coordinate
complex with a corresponding color change to yellow; after C−
CONCLUSIONS
■
α-Diimine supported niobium complexes are efficient catalysts
for the deoxygenation of benzyl ethers to two equivalents of
benzyl chloride, either stoichiometrically or catalytically with
silicon tetrachloride or another silicon halide as the halogen
source and oxygen scavenger. The use of SiCl4 is especially
advantageous as the only byproduct of the reaction is silica,
which is easily separated by simple filtration. The reaction is
selective for benzyl ethers, with those containing electron-
withdrawing groups showing reduced reactivity and ones with
electron-donating groups or α-carbon substituents showing
faster reaction rates and yields.
E
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Through kinetic observations and substrate reactivity
patterns, the deoxygenation reaction normally takes place
through two successive trans-etherification steps, where the
first C−O bond is activated to form benzyl chloride and a silyl
ether with SiCl4. The other C−O bond is activated in the same
way, although these steps occur at different rates for
asymmetric ethers. Although no formal redox transformations
occur throughout the catalytic cycle, the redox-active ligand
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electron donation to the niobium.
Ongoing work is aimed at exploring further halogenation
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01784.
Catalyst screening and reaction condition optimization,
interactions of complex 1a with benzyl ether, benzyl-O-
SiCl3 intermediate (C) spectra, kinetic studies and one-
pot Hammett analysis, NMR spectra of catalytic reaction
mixtures or products, experimental section (PDF)
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Acylative Cleavage of 2,5-Dimethyltetrahydrofuran: A Substrate
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Paver, S. P. Bi(III) Halides as Efficient Catalysts for the O-Acylative
Cleavage of Tetrahydrofurans: An Expeditious Entry to Tetralins.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: mashima@chem.es.osaka-u.ac.jp.
*E-mail: tsurugi@chem.es.osaka-u.ac.jp.
*E-mail: arnold@berkeley.edu.
ORCID
John Arnold: 0000-0001-9671-227X
Hayato Tsurugi: 0000-0003-2942-7705
Kazushi Mashima: 0000-0002-1316-2995
(18) Fotie, J.; Adolph, B. R.; Bhatt, S. V.; Grimm, C. C.
Palladium(II) Acetate Catalyzed Acylative Cleavage of Cyclic and
Acyclic Ethers under Neat Conditions. Tetrahedron Lett. 2017, 58
(49), 4648−4651.
(19) Bortoluzzi, M.; Marchetti, F.; Pampaloni, G.; Zacchini, S.
Reactivity of [WCl6] with Ethers: A Joint Computational,
Spectroscopic and Crystallographic Study. Eur. J. Inorg. Chem. 2016,
2016 (19), 3169−3177.
(20) Jung, M. E.; Lyster, M. A. Quantitative Dealkylation of Alkyl
Ethers via Treatment with Trimethylsilyl Iodide. A New Method for
Ether Hydrolysis. J. Org. Chem. 1977, 42 (23), 3761−3764.
(21) Jung, M. E.; Martinelli, M. J.; Olah, G. A.; Surya Prakash, G. K.;
Hu, J. Iodotrimethylsilane. In Encyclopedia of Reagents for Organic
Synthesis; John Wiley & Sons, Ltd: Chichester, UK, 2005; Vol. 3, p
533.
(22) Nishiyama, H.; Ikeda, H.; Saito, T.; Kriegel, B.; Tsurugi, H.;
Arnold, J.; Mashima, K. Structural and Electronic Noninnocence of α-
Diimine Ligands on Niobium for Reductive C-Cl Bond Activation
and Catalytic Radical Addition Reactions. J. Am. Chem. Soc. 2017, 139
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.M. acknowledges financial support by JSPS KAKENHI
Grant Numbers 15H05808 and 15K21707 in Precisely
Designed Catalysts with Customized Scaffolding (No. 2702).
J.A. thanks the United States National Science Foundation
(Grant No. CHE-1465188) for support.
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