Radical reductions of alkyl halides bearing electron withdrawing groups with N-heterocyclic carbene boranes

Article abstract of DOI:10.1039/c0ob01075h

1,3-Dimethylimidazol-2-ylidene borane and 2,4-dimethyl-1,2,4-triazol-3- ylidene borane are found to be useful reagents for the reduction of alkyl iodides and bromides bearing nearby electron withdrawing substituents. Signatures of radical chain reactions are seen in many cases, but ionic reductions may also be occurring with some substrates. The reagents are attractive because of their low molecular weight, their availability from inexpensive precursors, and their stability. Separation of the borane products from the target products is readily accomplished either with or without prior regeneration of the borane for later reuse. 2,4-Dimethyl-1,2,4-triazol-3-ylidene borane is versatile because both starting borane and its derived products can be removed by extraction with water.

Full text of DOI:10.1039/c0ob01075h

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PAPER
Radical reductions of alkyl halides bearing electron withdrawing groups with
N-heterocyclic carbene boranes†‡
Shau-Hua Ueng,a,b Louis Fensterbank,*a Emmanuel Lacoˆte,*a Max Malacria*a and Dennis P. Curran*b
Received 24th November 2010, Accepted 25th January 2011
DOI: 10.1039/c0ob01075h
1,3-Dimethylimidazol-2-ylidene borane and 2,4-dimethyl-1,2,4-triazol-3-ylidene borane are found to be
useful reagents for the reduction of alkyl iodides and bromides bearing nearby electron withdrawing
substituents. Signatures of radical chain reactions are seen in many cases, but ionic reductions may also
be occurring with some substrates. The reagents are attractive because of their low molecular weight,
their availability from inexpensive precursors, and their stability. Separation of the borane products
from the target products is readily accomplished either with or without prior regeneration of the borane
for later reuse. 2,4-Dimethyl-1,2,4-triazol-3-ylidene borane is versatile because both starting borane and
its derived products can be removed by extraction with water.
Introduction
Based on calculated bond dissociation energies, we recently
predicted that N-heterocyclic carbene boranes (hereafter, NHC-
boranes or NHC-BH3) could serve as radical hydrogen atom
donors.1a This prediction was then verified by the reduc-
tive deoxygenation of secondary xanthates with 1,3-bis-(2,6-
diisopropylphenyl)imidazol-2-ylidene borane 1 and 2-phenyl-
1,2,4-triazol-3-ylidene borane 2 (Fig. 1).1b Subsequent rate con-
stant measurements suggested that smaller and less expensive
1,3-dimethylimidazol-2-ylidene borane 3 and 2,4-dimethyl-1,2,4-
triazol-3-ylidene borane 4 would be better reagents than 1 and
2,2 and indeed xanthate reductions with these reagents proceeded
more rapidly and in higher yields.3 Proposed radical chain
mechanisms for the xanthate reductions have been supported by
EPR spectra of intermediate NHC-boryl radicals (NHC-BH2·)
and other experiments.4
In other radical chemistry, several NHC-boranes (includ-
ing those in Fig. 1) show promise as co-initiators in radi-
cal photopolymerizations.5 Preliminary EPR4 and laser flash
photolysis5 experiments showed that NHC-boryl radicals abstract
Fig. 1 Structures of N-heterocyclic carbene boranes used in reductive
halogen atoms from alkyl halides. This in turn suggests that
deoxygenation of xanthates and halides and a proposed radical chain
alkyl halides can be reduced by NHC-boranes by the radical
mechanism for reduction of halides.
aInstitut Parisien de Chimie Mole´culaire (UMR CNRS 7201) UPMC
chain mechanism shown in Fig. 1. However, we found that
chain propagation was not very efficient in radical reduction
of unfunctionalized alkyl iodides and bromides with 1 and 2.6
Instead, ionic reductions took place with suitable substrates
(for ex◦ample, 1◦-alkyl halides) by heating at high temperatures
(>100 C) without any initiator.7
Univ Paris 06,
4 place Jussieu, C. 229, 75005, Paris, France.
E-mail: louis.fensterbank@upmc.fr, max.malacria@upmc.fr, emmanuel.
lacote@upmc.fr; Fax: +33 1 44 27 63 60; Tel: 33 1 44 27 25 47
bUniversity of Pittsburgh, Department of Chemistry, Pittsburgh, PA 15260,
USA. E-mail: curran@pitt.edu; Fax: 412-624-8611; Tel: 412-624-8240
† Dedicated to the memory of Professor Athelstan L. J. Beckwith, 1930–
2010
Like amine-boranes,8 NHC-boranes have a formal negative
charge on boron (see 1) and are presumably best paired in reactions
with radicals having electron withdrawing substituents due to
polar effects.9 Depending on location, such substituents may
‡ Electronic supplementary information (ESI) available: Contains a table
of experiments with admantyl iodide, experimental procedures for halide
precursors 5a, 5b, 9, 15a, 17, and 20, timecourse data of 11B NMR
experiments for recovery of borane 3 and copies of spectra of precursors
and products. See DOI: 10.1039/c0ob01075h
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by triethylborane (alone or with residual water),12 we conducted a
Table 1 Radical
reductions
glucofuranoses 5a and 5b
of
di-O-isopropylidene-3-halo-
similar experiment without adding 3 and by extending the reaction
1
time from 2 h to 18 h. However, 6 was observed by H NMR
spectroscopy in only 23% yield (entry 3), and most of the remaining
balance was starting iodide 5a and other unidentified products.
Reduction of bromide 5b with 3 under Conditions A also occurred
smoothly to provide 6 in 79% yield (entry 4). In contrast, 5b was
not reduced by 1 (entries 4 and 5).
In a typical AIBN experiment (Conditions B), a benzene
solution of borane 3 (1 equiv), iodide 5a (1 equiv.), and AIBN
(1 equiv.) was heated at 80 ◦C for 2 h. The product 6 was isolated
by flash chromatography in 63% yield. Likewise, bromide 5b was
Entry
Substrate
NHC borane
Conditionsa
Yield 6b
1
2
3
4
5
6
7
8
5a
5a
5a
5b
5b
5a
5b
5a
5b
5a
5a
5b
5b
3
1
A
A
A
A
A
B
B
C
C
C
C
C
C
77%
35%c
23%d
79%
S.M.
63%
77%
50%
73%
41%
57%e
72%
63%e
ˇ
t
reduced in 77% yield (entries 6 and 7). In a typical BuOOtBu
S
3
1
3
3
3
3
4
4
4
4
experiment (Conditions C), a benzene solution of borane 3 (1
equiv.), iodide 5a (1 equiv.), and di-tert-butyl peroxide (1 equiv.)
was placed in a tube and irradiated with a sunlamp for 1 h.
The lamp warmed the solution to an estimated temperature of
60 ◦C. After cooling and solvent evaporation, standard flash
chromatographic isolation provided 6 in 50% yield from iodide
5a and 73% yield from bromide 5b (entries 8 and 9).
9
10
11
12
13
Triazol-3-ylidene borane 4 is itself soluble in both benzene and
water. So we speculated that it might be possible to conduct
reactions in benzene and then to remove the borane-derived
products by water extraction. We chose the tBuOOtBu Conditions
(C) for these experiments because there are no non-volatile organic
products generated from the initiator (di-tert-butyl peroxide).
Pairs of experiments were conducted with iodide 5a and bromide
5b with standard purification by flash chromatography (entries 10
and 12) or with purification only by water extraction and solvent
evaporation (entries 11 and 13). The yields of 6 were comparable
(from 5a, 41% and 57%; from 5b, 72% and 63%), as were the
purities.
The success of the water extraction procedure for removal
of starting borane 4 and reagent-derived products is pleasing,
especially since we are not completely sure what the reagent
byproducts are. Speculation on what may occur is shown in
Fig. 2.
a Conditions, A: Et3B (1 equiv.), PhH, rt, 2 h. B: AIBN (1 equiv.), PhH,
80 ◦C, 2 h. C: (tBuO)2 (1 equiv.), hn, 60 ◦C, 1 h. b Isolated by flash
chromatography unless otherwise indicated. c Based on a 62% isolated
yield of a 1.3/1 mixture of 6 and 5a. d Reaction time extended to 18 h.
e Isolated by washing with water and solvent evaporation only.
also discourage ionic reactions of the boranes with the halide
precursors. Herein we show that NHC-borane reductions of alkyl
halides with nearby electron withdrawing groups are both possible
and potentially useful. However, the conditions needed to obtain
good yields suggest that the radical chains are relatively short.
Smaller boranes 3 and 4 are the preferred reagents, and the boron-
derived products in reductions of 4 can be removed simply by
water extraction. The recycling of reagent 3 is also demonstrated.
These reagents are white solids that are stable to air and water.
Results and Discussion
We selected 3-iodo- (5a) and 3-bromo- (5b) di-O-isopropylidene
glucofuranoses as representative hindered secondary halides
that would resist ionic reduction but might be sensitive to
radical reductions with carbene boranes. Both features follow
because the halides are surrounded by electron withdrawing
groups. The two halides were reduced at room temperature with
triethylborane/air10 (Conditions A), by heating at 80 ◦C with
◦
Fig. 2 Understanding the successful water workup procedure with 4;
possible fates of the borane-derived by-products.
AIBN (Conditions B) and by photolysis at about 60 C with di-
tert-butyl peroxide (Conditions C). All reactions were conducted
in benzene at 0.1 M substrate concentration. The results of this
series of experiments are summarized in Table 1.
Based on experiments with 113a (also see below), we expect that
the boron halide 7a or 7b is the primary reaction product. It
is not clear whether these halides are soluble in water or not.
However, they may decompose when water is added to generate the
N,N-dimethyltriazolium salt 8 and inorganic boron.13b Whatever
is formed during the reductions from triazolylidene borane 4, it is
evidently more soluble in water than benzene.
We next selected NHC-borane 3 to survey the scope of
reductions with a series of substrates, and the results of these
reactions are summarized in Table 2.
In a typical experiment under Conditions A, a flask containing
a benzene solution of borane 3 (1 equiv.), iodide 5a (1 equiv.),
and Et3B (1 equiv.) was sealed and the stopper was pierced with
a needle to admit ambient air. After 2 h at room temperature,
the solvent was evaporated and the crude product was purified by
flash chromatography to provide 6 in 77% yield (Table 1, entry
1). A similar reduction with borane 1 provided 62% of a mixture
of product 6 and starting iodide 5a in a ratio of 1.3/1 (entry 2,
35% corrected yield of 6).11 To control for background reduction
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Table 2 Reductions of iodides and bromides bearing electron withdrawing substituents with NHC-borane 3
Entry
Substrate
Cond.a
Equiv. 3
Products, yieldsb
1
2
9
9
B
C
2
1
10, 67%
10, 71%
3
11
D
1
12, 70%c
4
5
13
13
A
B
2
1
14, 83%d,e
14, 76%d,f
6
7
8
9
15a, X = I
15a, X = I
15b, X = Br
15b, X = Br
B
C
B
C
2
2
2
2
16, 61%
16, 60%
16, 63%
16, 77%
10
11
12
17
17
17
B
1
1
1
18, 11%
18, 39%
18, 42%
19, 58%
19, 36%
19, 27%
Bg
Ch
13
14
20
20
Bg
Cg
1
1
21, 44%i
21, 56%i
22, 28%
22, 17%
◦
a Conditions, A: Et3B (1 equiv.), PhH, rt, 2 h. B: AIBN (1 equiv.), PhH, 80 ◦C, 2 h. C: (tBuO)2 (1 equiv.), hn, 60 C, 1 h D, PhH, rt, 1 h. b Isolated by
flash chromatography unless otherwise indicated. c Yield was determined by 1H NMR analysis, the isolated yield was 60%. d Yield was determined by GC
analysis. e Reduction without the NHC-borane gave 14 in 21% yield. f Reduction without AIBN gave 14 in 23% yield. g 0.02 M. h 0.01 M. i 4.5/1 trans/cis.
The substrates were selected to have one or more electron with-
drawing groups, sometimes conjugated to the radical precursor
(11, 13, 17, 20), other times operating only by inductive effects
(9, 15a,b). Conditions were the same as A–C above (benzene, 0.1
M) unless otherwise indicated. In several cases, higher yields were
obtained with 2 equiv. of borane 3, so this variable is also featured
in Table 2. Reported yields are again isolated unless otherwise
indicated.
Reduction of 2-bromo-2-methyl-1-phenyl-1-propanone 13 pro-
vided 2-methyl-1-phenyl-1- propanone 14 in 83% yield under Et3B
initiation (A, entry 4) and 76% yield under AIBN initiation (B,
entry 5). Complementary control experiments were conducted for
each of these two experiments (see Table footnotes). When the
NHC-borane was omitted from entry 4, the yield of 14 was reduced
to 21%. And when the AIBN was omitted from entry 5, the yield
of 14 was reduced to 23%.
Reduction of tetra-O-acetyl-6-iodo-b-D-glucopyranose 9 un-
der Conditions B (AIBN, heat) and C (tBuOOtBu, photolysis)
provided 10 in 67% and 71% yield, respectively. In the first
experiment with 3-iodo-3,4-dihydrocoumarin 11, we observed
that reduction was occurring at rt even before any initiator was
added. The reduction was complete after 1 h (Conditions D), and
3,4-dihydrocoumarin 12 was formed in 70% according to NMR
spectroscopic analysis (entry 3). Flash chromatography provided
pure 12 in 60% yield.
Reduction
of
N-allyl-N-(2-haloethyl)-4-methyl-benzene-
sulfonamide as either the iodide 15a or the bromide 15b provided
good yields of cyclized product 16 under either AIBN (B)
or tBuOOtBu (C) conditions (entries 6–9, yields 60–77%).
There was no evidence for formation of the directly reduced
product
(N-allyl-N-ethyl-4-methylbenzene-sulfonamide)
in
any of these experiments. In contrast, reductions of N-allyl-
N-(2-bromoethanoyl)-4-methylbenzene-sulfonamide 17 and
the propanoyl analog 2014 gave mixtures of directly reduced
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and cyclized/reduced products under all conditions studied.
Reduction of 17 under AIBN conditions (B) provided 11% of 18
and 58% of 19 at 0.1 M and 39% of 18 and 36% of 19 at 0.02 M.
boron product at (d -23.7, broad). Standard workup and flash
chromatography returned 3 in pure form in 84% yield.
To extend this proof-of-principle to an actual reduction, we
irradiated a solution of 13, 3 and tBuOOtBu in an NMR tube in
C6D6 (Conditions C). After 15 min, 13 was consumed and the
only observable products were 14 (from the 1H NMR spectrum),
boron bromide 23b (from the 11B NMR spectrum, d -22.5, triplet),
and few excess of NHC-borane 3. A solution of NaBH4 (2 equiv.)
was added gradually. After 3 h, the reaction mixture was worked
up and the crude product was purified by flash chromatography
to return 3 in 77% yield. These results suggest that it should be
generally possible to recover borane 3 from preparative halide
reductions.
t
A BuOOtBu (C) experiment at 0.01 M provided 42% of 18 and
27% of 19. Reduction of 20 with AIBN (B) at 0.02 M gave 44%
of 21 and 28% of 22 while a tBuOOtBu (C) reduction at the same
concentration gave the same two products in 56% and 17% yield.
The results in Table 2 confirm that radical reductions of halides
bearing electron withdrawing groups can indeed be practical. The
control experiments with 13 and the exclusive (from 15a,b) or
partial (from 17 and 20) formation of cyclized products support
a radical chain mechanism. However, it is not clear that a radical
chain mechanism can account for all of the results in Table 2.
For example, iodocoumarin 11 is reduced without initiator, but
in all other cases relatively large amounts of initiator are needed
to consume the precursors. And directly reduced products 19 and
22 were isolated from the reductions of 17 and 20, even though
NHC-borane 3◦is a relatively modest hydrogen atom donor (at
least towards 2 -alkyl radicals).2 So some of the products from
these experiments could originate from ionic reductions. If so,
then NHC-borane 3 must be much more reactive in such reactions
than previously studied 1.7
Conclusions
This study begins to establish the scope and utility of radical
reductions of halides by NHC-boranes. Unlike xanthates, unfunc-
tionalized alkyl halides are not reduced very efficiently by existing
NHC-borane reagents under radical chain conditions. However,
ionic reductions can be expected for suitably reactive substrates.7
In contrast, halides bearing electron withdrawing groups can be
reduced in good yields by reagents 3 and 4 by initiation with
Et3B/air, with AIBN/heat or with tBuOOtBu/photolysis.
Compared to related reductions with current standard reagents
like tributyltin hydride15 and tris-trimethylsilyl silicon hydride,16
the new reagents have both advantages and disadvantages. On
the down side, the scope of halide reductions with 3 and 4 does
not begin to approach that of Bu3SnH or even (TMS)3SiH.6
These reagents reduce all sorts of halides (diverse alkyl, aryl,
alkenyl, etc.) rapidly and in high yields. Typically, large amounts
of initiator are not needed. In contrast, the scope of radical
reductions with 3 and 4 seems limited to alkyl halides with electron
withdrawing substituents. And even with such substrates, large
amounts of initiator (1 equiv.) and sometimes even excess borane
(up to 2 equiv.) speed the reaction and improve the yields. These
observations suggest that the chains with 3 and 4 are relatively
short.
To close the study, we briefly investigated the prospects for
recovery of the NHC-borane 3 after halide reduction. This is
possible in principle because 3 is a robust white solid, stable under
ambient laboratory conditions, flash chromatography, moisture,
etc. The results of these experiments are summarized in Fig. 3.
On the up side, for those substrates that do work, NHC-boranes
3 and 4 have a lot of practical advantages. Having only second row
elements, these reagents have low molecular weights (110 and 111
amu, respectively) and lack the toxicity detractions of tin. They
are stable solids that are easy to handle, react, separate from target
products, and recover for reuse. And they are made in simple
processes from inexpensive precursors. This goes especially for
3, which has recently been made from the common, inexpensive
ingredients N-methyl imidazole, dimethyl carbonate and borane-
dimethyl sulfide.17 Because of these attractive features, further
study of the scope of radical reductions with existing NHC-
boranes and study of new and potentially improved boranes are
worthwhile objectives.
Fig. 3 Characterizing and recycling boron halides 23a,b.
First, a solution of NHC-borane 3 in C6D6 in an NMR tube
was treated with 0.5 equiv. of diiodine (I2). Bubbling ceased
1
after 10 min, and both H and 11B NMR spectroscopic analysis
showed rapid and essentially quantitative formation of a new
product assigned as boron iodide 23a (d -32.7, triplet in 11B
NMR spectrum), see Supporting Information‡. To the solution
containing 23a was added 2 equiv. of a 2 M solution of NaBH4
in triethyleneglycol dimethyl ether. After 30 min, the resonance
for 23a had disappeared and that for 3 (d -36.4, quartet) had
reappeared along with another small resonance of an unknown
Experimental
General Remarks
The following compounds were prepared by previously published
procedures: carbene-borane complexes 1, 3, 4;1–4 and halide
precursors 11,18 and 15b.19,20
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Conditions A, General procedure for triethylborane initiated free
radical reactions
dimethylether), and the conversion was monitored by 11B NMR
spectroscopy (Figure S2, Supporting Information‡). To the reac-
tion solution was added 0.1 mL of the solution of NaBH4 (2 M
in triethyleneglycol dimethylether), and then another 0.05 mL of
the solution of NaBH4 (2 M in triethyleneglycol dimethylether).
The NMR tube was kept at room temperature for 3 h, and
the conversion was complete. The reaction mixture was loaded
to a silica gel column for flash chromatography (elution with
CH2Cl2 : MeOH = 99 : 1) to give the starting borane complex 3
as a white solid (8.5 mg, 77%).
Triethylborane (1 M solution in hexane, 0.1 mmol) was added to
a solution of the borane (0.1 mmol) and the substrate (0.1 mmol)
in benzene (1 mL). The septum was pierced with a needle to admit
ambient air. The colorless solution was stirred for 2 h. Then the
solvent was evaporated and the crude product was purified by flash
column chromatography.
Conditions B, General procedure for AIBN initiated free radical
reactions
Acknowledgements
AIBN (0.1 mmol) was added to a solution of the borane (0.1 mmol)
and the substrate (0.1 mmol) in deoxygenated benzene (1 mL).
The colorless solution was refluxed for 2 h. After cooling to room
temperature, the solvent was removed and the residue was purified
by flash column chromatography.
This work was supported by grants from the US National Science
Foundation (CHE-0645998), UPMC, CNRS, and the French
Agence Nationale de la Recherche (ANR, BLAN0309 Radicaux
Verts, and 08-CEXC-011-01 Borane).
Conditions C, General procedure for di-tert-butylperoxide initiated
free radical reactions
Notes and references
´
1 (a) S.-H. Ueng, M. Makhlouf Brahmi, E. Derat, L. Fensterbank, E.
Di-tert-butylperoxide (0.1 mmol) was added to a solution of the
borane (0.1 mmol) and the substrate (0.1 mmol) in deoxygenated
benzene (1 mL). The colorless solution was charged in a quartz
NMR tube and irradiated with a sunlamp for 1 h. The air
temperature around the NMR tube was measured as 60 ◦C. After
1 h, the mixture was cooled, the solvent was evaporated, and the
crude product was purified by flash column chromatography.
Water extraction procedure: After irradiation of the reaction
solution with a sunlamp for 1 h, the reaction solution was diluted
with 30 mL of pentane and washed with 30 mL of water (¥3). The
organic layer was dried (MgSO4), and concentrated to give the
desired product.
Lacoˆte, M. Malacria and D. P. Curran, J. Am. Chem. Soc., 2008, 130,
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2 A. Solovyev, S.-H. Ueng, J. Monot, L. Fensterbank, M. Malacria, E.
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3 S.-H. Ueng, L. Fensterbank, E. Lacoˆte, M. Malacria and D. P. Curran,
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4 (a) S.-H. Ueng, A. Solovyev, X. Yuan, S. J. Geib, L. Fensterbank, E.
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6 To exemplify the point, the results of series of pilot reductions of
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Borane recovery after the reaction with iodine
A mixture of iodine (12.7 mg, 0.05 mmol) and dimethylimidazol-
2-ylidene borane 3 (11.0 mg, 0.1 mmol) in 1 mL of benzene-d6
was charged in a quartz NMR tube and kept at room temperature
for 10 min until the generation of bubbles ceased. The reaction
progress was monitored by 1H and 11B NMR spectroscopy
and 100% conversion was observed (Figure S1, Supporting
Information‡). To the reaction mixture was added 0.1 mL of
the solution of NaBH4 (2 M in triethyleneglycol dimethylether),
and the conversion of BH2I to borane was complete after 30
min. The reaction mixture was loaded onto a silica gel column
for flash chromatography (elution with CH2Cl2 : MeOH = 99 : 1)
to give the starting borane complex 3 as a white solid (9.2 mg,
84%).
9 B. P. Roberts, Chem. Soc. Rev., 1999, 28, 25–35.
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11 A similar reaction has recently been reported as a control experiment:
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Table 2, entry 10.
Procedure for recovery of starting borane 3 after halide reduction
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and dimethylimidazolylidene borane 3 (11.0 mg, 0.1 mmol) in
1 mL of benzene-d6 was charged in a quartz NMR tube and
irradiated by a sunlamp for 15 min. The air temperature around
the NMR tube increased to 60 ◦C. The reaction progress was
monitored by 1H and 11B NMR spectroscopy and 100% conversion
was observed after 15 min irradiation. To the reaction mixture was
added 0.05 mL of the solution of NaBH4 (2 M in triethyleneglycol
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3420 | Org. Biomol. Chem., 2011, 9, 3415–3420
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The Royal Society of Chemistry 2011
©