Radical Hydrodehalogenation of Aryl Bromides and Chlorides with Sodium Hydride and 1,4-Dioxane

Article abstract of DOI:10.1002/anie.201706534

A practical method for radical chain reduction of various aryl bromides and chlorides is introduced. The thermal process uses NaH and 1,4-dioxane as reagents and 1,10-phenanthroline as an initiator. Hydrodehalogenation can be combined with typical cyclization reactions, proving the nature of the radical mechanism. These chain reactions proceed by electron catalysis. DFT calculations and mechanistic studies support the suggested mechanism.

Full text of DOI:10.1002/anie.201706534

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Accepted Article
Title: Radical Hydrodehalogenation of Aryl Bromides and Chlorides
with Sodium Hydride and 1,4-Dioxane
Authors: Tobias Hokamp, Abhishek Dewanji, Maximilian Lübbesmeyer,
Christian Mück-Lichtenfeld, Ernst-Ulrich Würthwein, and
Armido Studer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706534
Angew. Chem. 10.1002/ange.201706534
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http://dx.doi.org/10.1002/ange.201706534

10.1002/anie.201706534
Angewandte Chemie International Edition
COMMUNICATION
Radical Hydrodehalogenation of Aryl Bromides and Chlorides
with Sodium Hydride and 1,4-Dioxane
Tobias Hokamp, Abhishek Dewanji, Maximilian Lübbesmeyer, Christian Mück-Lichtenfeld, Ernst-Ulrich
Würthwein, and Armido Studer*
Abstract: A practical method for radical chain reduction of various
aryl bromides and chlorides is introduced. The thermal process uses
NaH and 1,4-dioxane as reagents and 1,10-phenanthroline as an
initiator. Hydrodehalogenation can be combined with typical
cyclization reactions proving the nature of the radical mechanism.
These chain reactions proceed by electron catalysis. DFT calculations
and mechanistic studies support the suggested mechanism.
Our design was based on the following facts: a) a C-H bond next
to a C-radical is acidified by the radical center by up to 25 pKa
units,[18] b) 1,4-dioxane is an efficient H-atom donor for reactive
aryl radicals, and c) aryl radicals are readily generated from aryl
halides by single electron reduction. These facts allowed
suggesting the working model depicted in Scheme 1. An aryl
radical (Ar·) generated upon SET reduction in the initiation step
will be reduced by 1,4-dioxane to give the target Ar-H along with
the dioxanyl radical A (step 1). A might be deprotonated by NaH
to radical anion B that should further react as a very strong SET
reductant to 1,4-dioxene formally liberating an electron.[17a] The
reduction potential of 1,4-dioxene was measured to be
below -2.7 V vs. ferrocene (see Supporting Information, SI)
documenting the reductive strength of its radical anion. Note that
deprotonation could also be achieved by other bases and NaH
was chosen because it is readily available and cheap. SET
reduction of the substrate aryl halide (step 3) proceeds to the
corresponding aryl halide radical anion that upon halide
An important task for chemists considering economy is the
development of simple and cost-efficient processes. Therefore,
discovering new reactivity for established and cheap textbook
reagents is highly demanded. Sodium hydride (NaH) is a versatile
and cheap reagent that has been heavily used.[1] It is readily
stored and commonly used as a Brønsted base.[1] NaH has also
found application as a reducing reagent, albeit less frequently.
Hence, non-enolizable carbonyl compounds,[2] disulfides,[3] aryl
iodides,[4] benzylic halides[5] and gem-dihalocyclopropanes[6] can
be reduced by NaH.
Recently, the Chiba group extended the application frame to the
reduction of nitriles, amides and imines.[7] They also applied NaH
to the ionic hydrodebromination of aryl bromides,[8a] and to amide-
directed C-H sodiation.[8b] This unprecedented NaH reactivity is
induced by LiI or NaI added in a stoichiometric or super-
stoichiometric amount.
fragmentation leads to the aryl radical (Ar·
) closing the chain.[19]
initiator
O
O
Ar
X
e
3
Radical hydrodehalogenation has been studied intensively since
it occurs under mild conditions and shows, as compared to metal-
halogen exchange processes[9] or other metal mediated
transformations,[10] higher functional group compatibility. The
radical approach allows the hydrodehalogenation to be combined
with a C-C bond forming step. Radical chain reducing reagents
developed include tin hydrides,[11] silanes,[12] silylated
O
[Ar X]
4
O
B
Na
H2
2
X
Ar
O
1
cyclohexadienes,[13]
N-heterocyclic
carbene-borane
NaH
complexes,[14] Na-alcoholates[15] and catechols[16] among others.
However, these compounds are either toxic, hazardous or are not
commercially available and hence require pre-formation.
Herein we present radical hydrodehalogenation using 1,4-
dioxane as a solvent that also occupies the role of a chain
reducing reagent in combination with cheap NaH as a base. We
will show that these reductions proceed via radical chain
processes under electron catalysis.[17]
O
A
H
O
O
H
Ar
H
Scheme 1. Working model.
We first addressed the proposed electron transfer cycle including
the key deprotonation step from A to B with DFT calculations for
Ar = Ph including solvent effects implicitly (B2PLYP-D3/def2-
TZVP(PCM-1,4-dioxane), see the SI for details). 1,4-dioxane acts
as a strong hydrogen atom donor to the aryl radical, generating A
and the product arene (for step 1, ΔG = -17.7 kcal/mol). The
deprotonation step (2) of A by hydride anion is exothermic (ΔG =
-8.7 kcal/mol), as is the electron transfer to the aryl halide (step 3,
-45.3 kcal/mol for PhBr, -36.6 kcal/mol for PhCl, -13.7 kcal/mol for
PhF). The radical anions ArX·- (X = Cl, Br) resemble complexes
[a]
M.Sc. T. Hokamp, Dr. A. Dewanji, M.Sc. Maximilian Lübbesmeyer,
Dr. Christian Mück-Lichtenfeld, Prof. Dr. E.-U. Würthwein, Prof. Dr.
A. Studer*
Organisch-Chemisches Institut, WWU Münster
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: studer@uni-muenster.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706534
Angewandte Chemie International Edition
COMMUNICATION
of the aryl radical with the respective halide ions and are in
equilibrium with the free radical (step 4, ΔGdiss = -5.0 kcal/mol for
PhBr, -4.8 kcal/mol for PhCl), whereas the structure of the radical
anion PhF·- is similar to that of PhF with a ΔGdiss = 9.4 kcal/mol.
The total free energy change along the complete cycle (-76.6/-
67.8/-36.2 kcal/mol for PhBr/PhCl/PhF, respectively) indicates the
strong driving force of the reaction, especially for the bromide
(2e, 75%). However, a significantly lower yield was achieved for
reduction of 1-bromo-3,5-di-tert-butylbenzene to 2f. 2g was
isolated in 73% yield from 1-bromonaphthalene and 2-
bromobiphenyl was reduced to 2h (95%). 2i was obtained in a
good yield (61%) from 2-bromodibenzofuran (1i) and 9-
bromophenanthrene 1j yielded phenanthrene (2j, 93%). 4-bromo-
N,N-diphenylaniline 1k furnished triphenyl amine (2k, 90%) but
species, which compensates other contributions not covered here, with the electron-poorer 4-bromobenzamide 1l yield significantly
e.g. the lattice energies of NaH vs. NaX.
dropped (2l, 45%). This is likely due to a less efficient Br-
fragmentation due to an increased stability of the intermediate aryl
radical anion.[21] Our method was also applicable to the
heteroarene 1m to give 2-phenylthiophene (2m, 75%).[22]
Motivated by the theoretical results, we chose 1-bromo-2-methyl-
naphthalene (1a) as test substrate using 1,10-phenanthroline as
an initiator.[20] Best result is achieved by treatment of 1a with 1,10-
phenanthroline (20 mol%) and NaH (3 equiv) in 1,4-dioxane at
140 °C for 24 hours and 2-methylnaphthalene (2a) was obtained
in 96% yield (Scheme 2). Other initiators and ethers tested
provided worse results (see SI). Attempted reduction in the
absence of the radical initiator did not provide 2a, indicating that
the process is likely radical in nature. The reaction also proceeds
smoothly, albeit less efficiently, when KOtBu is used as a base
confirming the hydride’s main role as a base.
I (20 mol%)
NaH (3 equiv)
Ar
X
Ar
H
1,4-dioxane
N
N
3
160 °C, 24 h
(X = Cl, F)
2
I
H
Ph
H
N
N
Me
N
I (20 mol%)
H
H
NaH (3 equiv)
2a (98%)
from Ar-Cl
2g (83%)
from Ar-Cl
2h (79%)
from Ar-Cl
2n
(50%)
Ar Br
1a m
Ar H
from Ar-Cl
1,4-dioxane
140 °C, 24 h
N
N
2a m
-
-
I
H
SMe
Ph
H
H
H
H
H
H
MeO
OMe
OMe
Me
H
2c
2o
2g
from Ar-F
2h
(54%)
from Ar-F
(27%)
(30%)
(37%)
from Ar-Cl[a]
from Ar-Cl[a]
OMe MeO
OMe
OMe
2b (70%)
2d (77%)
Scheme 3. Hydrodechlorination and hydrodefluorination (20 mol% of I for
chlorides and 40 mol% I for fluorides). Yield of isolated product given ([a] 20
mol% of additional I added after 6 h).
2a (96%)
2c (65%)
H
H
OMe
H
OBn
H
t-Bu
t-Bu
Due to the stronger C-Cl bond, reduction of chloroarenes is more
challenging and only few reports have appeared.[23] Pleasingly, at
160 °C hydrodechlorination occurred smoothly and 2a was
isolated in 98% yield (Scheme 3). High yields were achieved for
the reduction of 3g (83%), 3h (79%) and reduction of 3n afforded
terpyridine (2n, 50%). However, the more electron-rich
chloroarenes did not show good reactivity and products 2c and
2o were isolated in moderate yields. Although the
hydrodechlorination was not as efficient and general as the
hydrodebromination, it encouraged us to tackle fluoroarenes. For
two selected examples we managed the highly challenging C-F
bond cleavage: 1-fluoronaphthalene and 4-fluorobiphenyl
afforded the corresponding products 2g and 2h in 37% and 54%
yields.
2e
2d (81%)
(75%)
O
2f (41%)
2g
(73%)
Ph
NPh2
H
H
H
H
2h (95%)
2i
2j (93%)
(61%)
S
2k (90%)
O
NEt2
H
H
2l (45%)
2m
(75%)
Scheme 2. Substrate scope of the hydrodebromination (reactions were carried
out under argon atmosphere with 0.3 mmol of 1, 0.9 mmol of NaH and 0.06
mmol of I in 1,4-dioxane (3.0 mL). Yield of isolated product given).
As an advantage over the ionic NaH reduction,[8a] the radical chain
process allows combining the hydrodehalogenation with
a
cyclization reaction and we studied the reductive cyclization of
bromide 4 to 5a (Scheme 4). At 140 °C we found that a NaH-
promoted isomerization of the double bond in 4 occurred prior to
the hydrodebromination and phenyl prop-1-en-1-yl ether was
formed (E/Z mixture). However, at 120 °C, double bond
isomerization was suppressed and 5a was isolated in 84% yield
as mixture with the direct reduction product 5b.
Reaction scope was explored next: Electron-rich dimethoxy-
substituted bromobenzenes provided the arenes 2b (70%) and 2c
(65%) in good yields. Methoxy-substituted bromonaphthalenes
reacted with similar efficiency to give 2d (77% and 81%).
Switching from methyl to benzyl ethers did not affect the outcome
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10.1002/anie.201706534
Angewandte Chemie International Edition
COMMUNICATION
dispersed. When 1a is reacted with pre-washed NaH the amount
of D-incorporation increases slightly, indicating that the mineral oil
is a minor H-source. Therefore, we assume that the hydride may
act as a H-source likely via an SRN1 type process[24] where the
hydride in an unprecedented way acts as a nucleophile to trap the
C-radical to form the corresponding radical anion C which upon
oxidation leads the isolated hydrodehalogenation product (see
Scheme 5, bottom). This is further supported by the reductive
cyclization of 4 in 1,4-dioxane-d8 where no D-incorporation was
observed. For reduction of fluoronaphthalene, that had to be
conducted at much higher temperature, an aromatic nucleophilic
substitution by the hydride cannot be ruled out. 1,4-Dioxene
suggested as a byproduct could not be traced due to its instability
under the applied conditions.
I (20 mol%)
NaH (3 equiv)
O
O
O
H
+
1,4-dioxane, 120 °C
Br
84%
H
5a/5b (20:1)
4
O
I (20 mol%)
NaH (3 equiv)
O
O
+
1,4-dioxane, 120 °C
Br
H
58%
H
6
7a/7b (20:1)
O
O
+
I (20 mol%)
NaH (3 equiv)
H
9b
O
H
9a
1,4-dioxane, 130 °C
72%
O
I (20 mol%)
NaH (3 equiv)
Br
H
+
8
1,4-dioxane-d 8, 140 °C
10
Cl
D/H
70%
9a 9b 10
3g
2g
(D/H = 64:36)
/
/
(5.8 : 1 : 1.5)
I
(20 mol%)
I (20 mol%)
NaH (3 equiv)
NaH (3 equiv)
+
Me
Me
1,4-dioxane-d 8, 120 °C
70%
1,4-dioxane, 120 °C
76%
H
Br
Br
1a
D/H
H
2a (D/H = 43:57)
11
12a/12b (9:1)
(D/H = 63:37)[a]
I (40 mol%)
NaH (3 equiv)
I
(20 mol%)
O
O
NaH (3 equiv)
1,4-dioxane-d 8, 160 °C
49%
1,4-dioxane, 130 °C
70%
Br
13
F
D/H
H
2g
(D/H = 23:77)
14
I (20 mol%)
NaH (3 equiv)
O
O
O
Scheme 4. Reductive radical cyclizations.
+
1,4-dioxane-d 8, 120 °C
75%
Br
H
H
4
5a/5b (8:1)[a]
In analogy, 6 was converted to dihydrobenzopyran (7a) which was
formed along with the direct reduction product 7b. Allyloxy
benzene 8 reacted at 130 °C via reductive 5-exo-cyclization to 9a.
The 6-endo-product 10 and the non-cyclized product 9b were
formed in significant amounts. Reaction of bromide 11 provided
cyclization product 12a and its non-cyclized isomer 12b (76%).
Finally, substrate 13 containing an internal alkene furnished
exclusively the exo-cyclization product 14 (70%). Reductive
cyclizations on chloroarenes were lower yielding: 11-Cl provided
the cyclization products 12a,b in 41% along with unreacted
starting material.
To verify the source of the H-atom, we reacted 1-
chloronaphthalene (3g), 1-bromo-2-methylnaphthalene (1a) and
1-fluoronaphthalene with NaH in 1,4-dioxane-d8. In all cases
deuteration of products 2g and 2a was not complete (Scheme 5).
On one hand, these experiments show that intermediate aryl
radicals are partially reduced via D-atom abstraction from the
solvent, in agreement with the suggested mechanism (Scheme 1).
On the other hand, these experiments also indicate that other H-
sources have to be considered such as NaH. We therefore
replaced NaH by KOtBu and found for the deuterodehalogenation
of 1a a D/H-ratio of 87:13 in 2a (see SI). We ascribe the hydrogen
content to residual hydrogen in the perdeuterated dioxane
(approx. 1%) considering isotope effects. Additional H-
incorporation might derive from mineral oil in which the NaH is
H
Ar-H
Ar-H
Ar
C
SRN1 mechanism
Scheme 5. Deuterodehalogenation in perdeuterated dioxane. ([a] NaH was
washed with hexane to remove mineral oil.)
In summary, a method for radical chain reduction of various aryl
bromides was introduced. NaH and 1,4-dioxane act as reagent
couple for the hydrodehalogenation. Both reagents are cheap and
commercially available. The method is also applicable to the more
challenging reduction of aryl chlorides and even selected aryl
fluorides are reduced. Harnessing the potential of radical
chemistry, it was also shown that reductive cyclization can be
achieved with this novel approach.
Experimental Section
Experimental procedures and characterization data for all compounds are
provided in the Supporting Information.
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COMMUNICATION
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Acknowledgements
We thank the WWU Münster and the European Research Council
(advanced grant agreement no. 692640) for financial support.
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Keywords: Sodium Hydride • Hydrodehalogenation • Low-cost •
Radical anion • SRN
1
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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T. Hokamp, A. Dewanji, Maximilian
Lübbesmeyer, Christian Mück-
Lichtenfeld, E.-U. Würthwein and A.
Studer*
Page No. – Page No.
Radical Hydrodehalogenation of Aryl
Bromides and Chlorides with Sodium
Hydride and 1,4-Dioxane
It is the combination! NaH in combination with 1,4-dioxane is a reagent couple for
radical chain reduction of various aryl halides. Hydrodehalogenation is initiated by
1,10-phenanthroline (phen) at elevated temperature and can be combined with a
typical radical cyclization reaction. Processes proceed via electron catalysis.
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