Hydrodehalogenation of alkyl halides catalyzed by a trichloroniobium complex with a redox active α-diimine ligand

Article abstract of DOI:10.1039/c9cc03268a

A high-valent d⁰ niobium(v) complex, (α-diimine)NbCl₃ (1), bearing a dianionic redox-active α-diimine ligand served as a catalyst for a hydrodehalogenation reaction of alkyl halides in the presence of PhSiH₃. During the catalytic reaction, the redox-active α-diimine ligand allowed the complex to reversibly release and accept one-electron through switching its coordination mode between a dianionic folded form and a monoanionic planar one.

Full text of DOI:10.1039/c9cc03268a

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DOI: 10.1039/C9CC03268A
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Hydrodehalogenation of Alkyl Halides Catalyzed by a
Trichloroniobium Complex with a Redox Active -
Diimine Ligand†
Received 00th January
20xx,
Accepted 00th January
20xx
Haruka Nishiyama,a Hiromu Hosoya,a Bernard F. Parker,a,b John Arnold,b,* Hayato Tsurugi,a,* and
DOI:
10.1039/x0xx00000x
Kazushi Mashimaa,*
www.rsc.org/
A high-valent d0 niobium(V) complex, (-diimine)NbCl3 (1), reported the catalytic application of group 5 metal complexes with
bearing a dianionic redox-active -diimine ligand served as a redox active -diimine ligands for radical addition of alkyl halides
catalyst for a hydrodehalogenation reaction of alkyl halides in the to alkenes,8 in which the -diimine ligand assisted the reductive
presence of PhSiH3. During the catalytic reaction, the redox- cleavage of the carbon-halogen bonds as well as the in situ-
active -diimine ligand allowed the complex to reversibly release generated metal-halogen bonds via reversibly transferring one-
and accept one-electron through switching its coordination mode electron between the metal center and the -diimine ligand.
between a dianionic folded form and a monoanionic planar one. Herein, we report that a hydrodehalogenation reaction of alkyl
halides is catalyzed by a niobium complex bearing a redox active
-diimine ligand (N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-
dimethyl-1,3-butadiene) in the presence of 1 equiv of PhSiH3, in
Replacement of a halogen atom in organic compounds by a
hydrogen atom, a so-called hydrodehalogenation reaction, is one
which the -diimine ligand plays an important role for assisting
of the straightforward methods to decompose useful but toxic
the cleavage of the carbon-halogen bonds of organic halides to the
organic halides, whose disposal is otherwise problematic due to
carbon-hydrogen bond by safety silanes, though the handling of
their high resistance to degradation under ambient conditions.1
this needs under strictly anaerobic conditions.
Alkaline metals and alkaline earth metals as well as their
We started searching for the optimal catalyst system by treating
corresponding organometallic reagents have been typically
1,1,1,3-tetrachloropropane, as a model substrate, with 1 equiv of
utilized for converting the carbon-halogen bond to a carbon-
hydrogen bond in the presence of organic and inorganic hydrogen
different silanes in the presence of (-diimine)NbCl3 (1, 3 mol%)
in C6D6 at 120 °C for 2 hours, and the results are summarized in
sources, from which a stoichiometric amount of metal salt is
Table 1.
The reaction with PhSiH3 (1 equiv) gave a
inevitably generated. Organotin hydrides, typically nBu3SnH, are
effective for metal-salt free hydrodehalogenation in the presence
of a radical initiator; however, both organotin hydrides and their
hydrodehalogenated product, 1,1,3-trichloropropane, in 85% yield
along with PhSiH2Cl and PhSiHCl2 as byproducts (entry 1). Other
secondary and tertiary silanes such as Ph2SiH2, Ph3SiH, and
Et3SiH were less effective hydrogen sources (entries 2—4). The
yield of the hydrodehalogenated product increased upon using 3
equiv of Et3SiH (entry 5). NaBH4 has been reported as a hydrogen
source for early transition metal-catalyzed hydrodehalogenation,
but was ineffective here (entry 6).7a Another borane, HB(pin)
(entry 7), was also checked, producing only a trace amount of
product. When 3 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-
cyclohexadiene (Si-Me-CHD)9-11 was used as a masked silane
instead of PhSiH3, 1,1,3-trichloropropane was obtained in 45%
yield along with the formation of chlorotrimethylsilane and
trimethylsilyl-toluene (entry 8), although Si-Me-CHD served as an
effective hydrogen source for the hydrodehalogenation of 1,1,1,3-
tetrachloropropane catalyzed by paddlewheel dimolybdenum
complexes.7f As a result, we selected the best reaction conditions
of PhSiH3 (1 equiv) at 120 °C for hydrodehalogenation.
n
reaction by-products such as Bu3SnX are highly toxic.2,3 As an
environmentally friendly and less toxic hydrodehalogenation
reaction, heterogeneous precious metal catalysts show high
activity for hydrodehalogenation in combination with dihydrogen
gas.4,5 Recent requirements have favoured the use of cheap and
earth abundant non-noble metal catalysts,6 and some early
transition metal catalysts also exhibited activity for the
hydrodehalogenation reaction due to their high reactivity in
reductively cleaving carbon-halogen bonds.7
We recently
a Department of Chemistry, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan
b Department of Chemistry, University of California, Berkeley,
California 94720, United States
E-mail: mashima@chem.es.osaka-u.ac.jp; tsurugi@chem.es.osaka-u.ac.jp
arnold@berkeley.edu
;
† Electronic Supplementary Information (ESI) available: Experimental data for all
new compounds. See DOI: 10.1039/x0xx00000x
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Table 1 Hydrodehalogenation of 1,1,1,3-tetrachloropropane
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a
DOI: 10.1039/C9CC03268A
Table 2 Substrate scope of hydrodehalogenation reaction
H source
entry H source
yield (%)a entry
yield (%)a
73
1
2
3
4
PhSiH3
Ph2SiH2
Ph3SiH
85
66
44
47
5
6
7
8
Et3SiHb
NaBH4
n.d.
HB(pin)
trace
45
Et3SiH
Si-Me-CHDb
a
1
b
Determined by H NMR with respect to an internal standard (C6Me6).
3
equiv.
We found that the catalytic hydrodehalogenation reaction is
applicable to polyhaloalkanes, allyl bromide, and benzyl bromide
derivatives in combination with PhSiH3 as a reductant in C6D6 in
sealed J-young NMR tubes, and the results are shown in Table 2.
Debromination of dibromomethane afforded bromomethane in
72% yield. Allyl bromide was selectively converted to propene in
good yield, in which no hydrosilylation of the C=C moiety was
observed. Hydrodehalogenation of benzyl bromide derivatives
produced the corresponding aromatic compounds in moderate
yield; 4-methylbenzyl bromide was reduced to p-xylene, and 4-
trifluoromethylbenzyl bromide was transformed to 4-
trifluoromethyltoluene after 14 h without defluorination from the
CF3 moiety, although the products were contaminated with radical
coupling dimers. For these benzyl bromide derivatives, using of
Si-Me-CHD as the hydrogen source instead of PhSiH3, p-xylene
and 4-trifluoromethyltoluene were obtained in better yield without
forming any radical coupling dimers. Both cyclohexyl chloride
and bromide were converted to cyclohexane in 62% and >99%
yield, respectively. Cyclopropylmethyl bromide was converted to
1-butene in 43% yield, in which the initially formed
cyclopropylmethyl radical was transformed to the corresponding
ring-opening isomer,12 with no contamination with the ring-
opened isomer of cyclopropylmethyl bromide, 4-bromo-1-butene
(vide infra). Hydrodehalogenation of gem-dihalocyclopropanes
gave the monodehalogenated halocyclopropanes in moderate to
good yield.
a
All reactions were carried out in J-young NMR tubes, and yields was
determined with respect to an internal standard (1,4-dioxane) in the 1H NMR
spectra. b Si-Me-CHD (3 equiv) was used. c 10 mol% catalyst and 5 equiv of
PhSiH3 were used for 0.5 h. d 6 mol% catalyst was used.
When NbCl3 was used as the catalyst instead of 1 for the
hydrodehalogenation of bromomethylcyclopropane, NbCl3 acted
as an initiator for the radical reaction to afford 4-bromobutene and
1-butene. This mixture arises from a radical chain reaction of the
butenyl radical, generated by the ring-opening of a free
cyclopropymethyl radical, which then abstracts either a bromine
atom from a second (bromomethyl)cyclopropane molecule or a
hydrogen atom from the silane (Scheme 1). This is in sharp
contrast to the selective formation of 1-butene catalyzed by 1 as
shown in Table 2, where the catalyst moderates the reactivity and
suppresses free radical reactivity instead of just initiating a radical
chain reaction. A similar result was observed for NbCl3(DME).
Scheme
1
Hydrodehalogenation
reaction
of
bromomethylcyclopropane catalyzed by NbCl3
Next, we carried out kinetic studies for the
hydrodehalogenation reaction.13 Under the optimized conditions,
kobs for the reaction progress indicated a linear dependence on the
concentration of the catalyst 1. In the presence of a large excess
of PhSiH3, 1,1,1,3-tetrachloropropane showed pseudo first-order
kinetic behavior. Similarly, pseudo first-order kinetics was also
2 | J. Name., 2012, 00, 1-3
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observed for PhSiH3 in the presence of large excess 1,1,1,3-
tetrachloropropane. The reaction progress followed overall
second-order kinetics with respect to the concentrations of both
substrates (Figure 1). We further analyzed the rate dependence on
the reaction temperature to estimate the activation parameters for
the reaction of 1,1,1,3-tetrachlropropane and PhSiH3 catalyzed by
1. Eyring analysis derived from the kinetic data gave the
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DOI: 10.1039/C9CC03268A
thermodynamic parameters of H‡ = 126(7) kcal/mol, S‡
=
2.18(5) e.u., and G‡ (298 K) = 123(13) kcal/mol. The entropy
value was nearly zero, suggesting no involvement of a free radical
during the catalytic cycle. In addition, the reaction using
Ph2SiH2/Ph2SiD2 showed a KIE of 1.72, corresponding to the rate-
determining step involving H-abstraction by organic radicals.
Scheme 2. Proposed catalytic cycle.
We alternatively prepared a tetrachloroniobium complex 2
with a monoanionic -diimine ligand by treating NbCl5 with the
-diimine ligand in the presence of Si-Me-CHD (0.5 equiv) (eq
1).7b Since the complex 2 is analogous to the tetrachloroniobium
moiety of C in Scheme 2, we treated 2 with PhSiH3 (3 equiv) in
1
Fig. 1 A second-order kinetic behavior on the concentration of C6D6 to give 1 in 80% yield (determined by H NMR) together
1,1,1,3-tetarachlropropane and PhSiH3 for the hydrodehalogenation with the formation of ClSiH2Ph (eq 2), indicating that PhSiH3
reaction of 1,1,1,3-tetrachlropropane with PhSiH3 (A = PhSiH3, B = worked as not only a hydrogen source for the hydrodehalogenation
1,1,1,3-tetrachlropropane, P = product).
reaction but also a reductant for the niobium complex 2. Under
the reaction condition, further transformation of 1 by the
Based on the kinetic study, we propose a mechanism for the remaining PhSiH3 was not observed.
hydrodehalogenation reaction catalyzed by 1. As shown in
Scheme 2, the initial step is the coordination of the alkyl halide to
the niobium center of 1 to afford a niobium(IV) species A, in
which one electron is transferred from the -diimine ligand to the
niobium(V) center. This behaviour of the redox-active ligand is
similar to our previous results on radical addition reactions.8b
Subsequent reductive cleavage of the carbon-halogen bond in A
via a one-electron transfer from the niobium(IV) center to the alkyl
halide affords a niobium(V) species B. A hydrogen atom is then
abstracted from PhSiH3 to form the dehalogenated product as well
as the niobium(V) species C, which is consistent with first-order
rate dependence on both alkyl halides and phenylsilane. Finally,
disproportionation of C produces XSiPhH2 and regenerates 1.
During the catalytic reaction, the redox-active -diimine ligand
In summary, we have developed a hydrodehalogenation
accelerates the disproportionation of C by accepting one electron.
The observed primary kinetic isotope effect for deuterated and
non-deuterated silanes is consistent with the involvement of
hydrogen abstraction in the overall rate-determining step.
reaction of haloalkanes using PhSiH3 as a reductant as well as a
hydrogen source. Although the catalyst 1 contained a high-valent
d0 niobium center, the -diimine ligand played an important role
in releasing one electron from the dianionic ligand to the alkyl
halides to a generate carbon radical as the initial step of the
catalytic reaction. Further study on the reductive transformation
of organic compounds by 1 and similar niobium complexes
bearing an -diimine ligand is ongoing in our laboratory.
H. N. thanks the financial support by the JSPS Research Fellow-ships for
Young Scientists. K.M. acknowledged financial supports by JSPS
KAKENHI Grant Numbers 15H05808 and 15K21707 in Precisely
Designed Catalysts with Customized Scaffolding (No. 2702). J.A. thank
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2017, 53, 13157. b) P. Suman, S. Rej, S. Kando, H. VTieswurAurgticileanOdnliKne.
the United States National Science Foundation (Grant No. CHE-
1465188) for support.
Mashima, J. Org. Chem. 2018, 83, 2409.
12 M. Newcomb, Tetrahedron 1993, 49, 1151.
13 Details for the kinetic study, see ESI.
DOI: 10.1039/C9CC03268A
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TOC used only:
We have developed a hydrodehalogenation reaction of haloalkanes using PhSiH3 as a reductant
as well as a hydrogen source. The -diimine ligand on the niobium center plays an important
role in releasing one electron from the dianionic ligand to the alkyl halides to a generate carbon
radical as the initial step of the catalytic reaction.