Hantzsch Ester as a Photosensitizer for the Visible-Light-Induced Debromination of Vicinal Dibromo Compounds

Article abstract of DOI:10.1002/chem.201601819

The debromination of vicinal dibromo compounds to generate alkenes usually requires harsh reaction conditions and the addition of catalysts. Just recently the visible-light-induced debromination of vicinal dibromo compounds emerged as a possible alternative to commonly used methods, but the substrate scope of this reaction is limited and a photocatalyst is necessary for the successful conversion of the starting compounds. A catalyst-free visible-light-induced debromination of vicinal dibromo compounds with a base-activated Hantzsch ester as photosensitizer is reported. The method has a wide substrate scope and a broad functional-group compatibility.

Full text of DOI:10.1002/chem.201601819

DOI: 10.1002/chem.201601819
Communication
&
Organic Synthesis
Hantzsch Ester as a Photosensitizer for the Visible-Light-Induced
Debromination of Vicinal Dibromo Compounds
Wenxin Chen,[a] Huachen Tao,[a] Wenhao Huang,[a] Guoqiang Wang,[b] Shuhua Li,*[b]
Xu Cheng,*[a] and Guigen Li[a, c]
Abstract: The debromination of vicinal dibromo com-
pounds to generate alkenes usually requires harsh reac-
tion conditions and the addition of catalysts. Just recently
the visible-light-induced debromination of vicinal dibromo
compounds emerged as a possible alternative to com-
monly used methods, but the substrate scope of this reac-
tion is limited and a photocatalyst is necessary for the suc-
cessful conversion of the starting compounds. A catalyst-
free visible-light-induced debromination of vicinal dibro-
mo compounds with a base-activated Hantzsch ester as
photosensitizer is reported. The method has a wide sub-
strate scope and a broad functional-group compatibility.
Vicinal dibromination has been established as an efficient
method for the protection of CÀC double bonds in organic
synthesis, and the corresponding deprotection can be carried
Scheme 1. Visible-light-induced debromination to form alkenes.
out with metal reductants, however, harsh reaction conditions
are required; for example, the zinc-mediated reduction typical-
ly requires an acidic medium at elevated temperatures.[1] Fur-
demonstrated the Ru(bpy)3+-catalyzed debromination of vici-
thermore, the use of strongly reductive metal species limits
the functional-group compatibility.[2] Recently, the visible-light-
nal dibromocarbonyl compounds to yield a,b-unsaturated mol-
ecules.[5] Both of these methods require a costly transition-
induced mesolysis emerged as a powerful strategy for cleaving
CÀBr bonds.[3] Because the functional groups commonly found
metal catalyst. In 2014, the photoreductive debromination
in organic molecules do not absorb visible light, this reaction
requires a photocatalyst that acts as an electron shuttle
(Scheme 1).
using sexithiophene as photocatalyst was demonstrated by
Scaiano and co-workers, who focused on stilbene and a-car-
bonyl dibromo compounds.[6] Despite these reports, a general
strategy for unmasking alkenes, especially unfunctionalized
ones, from dibromo precursors has not been reported. In this
study, we investigated the catalyst-free visible-light-induced
deprotection of a wide range of substrates by a method that
uses the Hantzsch ester as light harvester, electron donor, and
hydrogen donor. Beyond its passive role as a reductant in
closed-shell[7] and open-shell chemistry,[8] the Hantzsch ester
can be self-activating with the help of an inorganic base to di-
rectly achieve an electron transfer.
In 1986, Willner et al reported the Ru(bpy)3+-catalyzed de-
protection of 1,2-dibromostilbene under visible-light irradiation
with NADH as the terminal reductant.[4] In 2011, Reiser et al
[a] W. Chen, H. Tao, W. Huang, Prof. Dr. X. Cheng, Prof. Dr. G. Li
Institute of Chemistry and Biomedical Sciences
School of Chemistry and Chemical Engineering
Nanjing University, Xianlin Road, 163, Nanjing, (China)
E-mail: chengxu@nju.edu.cn
[b] G. Wang, Prof. Dr. S. Li
We chose p-bromophenyl-1,2-dibromoethane (1a), a styrene
precursor, as a typical substrate for the optimization of the de-
bromination reaction to form the alkene 3a (Table 1). The de-
protection of 0.2 mmol 1a was carried out with 1.1 equiv of
the Hantzsch ester 2 and 1.1 equiv of a base under irradiation
with a 12 W white LED at room temperature. DMSO was found
to be the optimal solvent (Table 1, entry 6), but DMF also gave
good yields (Table 1, entry 1). In acetonitrile, DCM, THF, or tolu-
ene, the reaction did not run to completion (Table 1, entries 2–
Key Laboratory of Mesoscopic Chemistry of Ministry of Education
Institute of Theoretical and Computational Chemistry
School of Chemistry and Chemical Engineering
Nanjing University, Hankou Road, 22, Nanjing (China)
E-mail: shuhua@nju.edu.cn
[c] Prof. Dr. G. Li
Department of Chemistry and Biochemistry, Texas Tech University
Memorial Circle & Boston, TX 79409–1061 Lubbock(United States)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/chem.201601819.
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Table 1. Optimization of the debromination conditions.[a]
Table 2. Debromination reactions of 1b–l with the Hantzsch ester 2 to
form vinyl arenes 3b–l, and the E/Z-stability test of (Z)-stilbene (3m).[a]
Entry
Base
Solvent
Yield
[%][b]
Entry
Substrate
Product
Yield
[%][b]
1
2
3
4
5
6
7
8
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
CsF
NaHCO3
NaOH
DMF
84
55
18[c]
31[c]
9[c]
98
88
51
84
79
28
0[d]
0[e]
0
CH3CN
CH2Cl2
THF
1
2
3
95
86
80
toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
9
10
11
12
13
14
KOtBu
Na2CO3
Na2CO3
4
99
[f]
–
5
6
91
91
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.22 mmol), base (0.22 mmol),
solvent (1.5 mL), 12 W white LED, rt, 1.5 h. [b] Isolated yield. [c] Yield esti-
mated by GC-MS. [d] No visible-light irradiation. [e] No addition of
Hantzsch ester 2. [f] No addition of base.
7
8
99
99
5). Sodium carbonate, as well as KOtBu (potassium tert-butox-
ide), outperformed the other inorganic bases tested (Table 1,
entries 7–11). A 98% yield of 3a was achieved within 1.5 h
with sodium carbonate in DMSO. The reaction did not proceed
in the absence of the Hantzsch ester, light-irradiation, or base
(Table 1, entries 12–14).
9
99
With the optimized reaction conditions in hand (Table 1,
entry 6), we explored the substrate scope of the reaction
(Table 2). First, we screened a series of bromine adducts. To our
delight, the corresponding styrenes were obtained in good to
excellent yields from a variety of substrates (Table 2, entries 1–
9). Furthermore, phenyl boronic acid, a valuable synthetic
module, also underwent the transformation to 3g in 91% yield
(Table 2, entry 6). Other vinyl arenes, such as 2-vinylnaphtha-
lene 3h and 9- vinylanthracene 3i, were obtained in almost
quantitative yields (Table 2, entries 7 and 8). Stilbene was ob-
tained from its dibromo precursor in 95% yield (Table 2,
entry 10), and the alcohol 1l underwent the debromination re-
action to afford cinnamyl alcohol (3l) in 83% yield (Table 2,
entry 11). (Z)-stilbene 3m was used as a substrate with the
same debromination reaction conditions and recovered with-
out any E/Z isomerization.
10
11
12
95[c]
83[c]
100[d]
–
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.22 mmol), Na2CO3 (0.22 mmol),
DMSO (1.5 mL), 12 W white LED, rt, 1.5 h. [b] Isolated yield. [c] Only the E-
product was detected by GC-MS and 1H NMR spectroscopy. [d] No E/Z
isomerization.
Next we evaluated the use of the method for deprotection
reactions to form a,b-unsaturated products (Table 3). When we
irradiated the ester 1n and the lactone 1o under a 12 W white
LED in the presence of the Hantzsch ester 2 and Na2CO3, the
corresponding conjugated products were obtained in excellent
yields (Table 3, entries 1,2). Terminal alkenes 3p and 3q could
also be prepared (Table 3, entries 3,4), and the alkyne 3r could
be obtained similarly from the unsaturated ester 1r (Table 3,
entry 5). The amide 1s gave cinnamide (3s) smoothly in 96%
yield, and the trisubstituted E-cinnamide 3t could also be ob-
tained (Table 3, entries 6,7). Ketones, such as 1u and 1v, are
also suitable substrates; the desired a,b-unsaturated ketones
were isolated in 80% and 97% yields, respectively (Table 3, en-
tries 8,9). Substrates 1w and 1x, which bear metal-reductant-
sensitive functional groups, are also suitable candidates for the
described reaction (Table 3, entries 10,11). Several dibromo tri-
substituted alkenes were prepared and subjected to the de-
bromination reaction (Table 3, entries 12–14). Compound 1y,
with a syn/anti ratio of 54:46, gave the (E)-b-methyl cinnamate
as predominant product in 86% yield. This trend was also ob-
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served when substrate 1z was irradiated with visible light. The
syn-configurated compound 1aa gave the (E)-a-methyl cinna-
mate as the only detected product.
Table 3. Debromination reactions of 1n–z and 1aa with 2 to the form
a,b-unsaturated compounds 3n–z and 3aa.[a]
After successful debromination of substrates with at least
one activated CÀBr bond, we extended the scope to dibromo
compounds derived from unfunctionalized alkenes (Table 4),
deprotection of which has not been achieved by means of visi-
ble-light-induced methods so far. The terminal alkene 1ab was
deprotected in 82% yield after 12 h (Table 4, entry 1). The ste-
roid 1ac was more reactive and was converted to 3b-chloro-5-
cholestene (3ac) in only 1.5 h (Table 4, entry 2). The long-chain
alkene 3ad was isolated in 65% yield (Table 4, entry 3), and
the allyl ester 1ae was also successfully debrominated in
almost quantitative yield after 24 h (Table 4, entry 4).
Entry
Substrate
Product
Yield
[%][b]
1
2
98[c]
96
To investigate the compatibility of the deprotection with an
extensive list of functionalities, we carried out a robustness
screening. In the presence of various spectator molecules, car-
rying the different functional groups, 1a was deprotected
under the standard conditions (Table 5). All tested molecules
(besides artemisin), such as piene, caryophyllene, taicros, nero-
lidol, phenol, nitrobenzene, phenylacetylene, phenyl cyclo-
propyl ketone, glycidol ether, 2-thiopheneethanol, and pyri-
dine, remained intact throughout the visible-light-induced re-
action, and had no influence on the product formation; the
alkene 3a was isolated in nearly quantitative yields. Even in
the presence of the structurally complex spectator molecule
artemisinin, the reaction gave a 54% yield of 3a, and the natu-
ral product survived the reaction. The lowered yield of 3a was
caused by elimination of HBr from 1a.[9]
To investigate the mechanism of this transformation, we per-
formed several experiments. Initially, we examined the possibil-
ity of a charge–transfer complex formation, by means of NMR
and UV/Vis spectroscopy. No significant interaction between
the Hantzsch ester and the dibromo species was observed,
suggesting that an intermolecular process was occurring. A
Stern–Volmer experiment showed that the substrate 1a did
3
4
95
98[d]
5
98
6
7
8
96[c]
98[c]
80[c]
9
97[c]
98[c]
81[c,e]
not quench the fluorescence generated by the Hantzsch ester
[10]
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&
2 in DMSO (Scheme 2a, ). Thus, the base not only scav-
enged HBr but also participated in the electron-transfer pro-
cess. When Na2CO3 was added to a solution of 1a and 2 in
DMSO, a weak but reproducible quenching of the fluorescence
*
was observed (Scheme 2a, ). This quenching was also ob-
served when 2 and Na2CO3 were mixed. Therefore, we propose
that the base plays a pivotal role in the reactivity of the
Hantzsch ester. The quenching was weak because the interac-
tions between 2 and Na2CO3 were limited by the poor solubili-
ty of the latter in DMSO.[11] On the basis of these results, we
propose the mechanism outlined in Scheme 2b. First, the
Hantzsch ester 2 harvests light with the help of the base. Sub-
sequently, the electron transfer (ET) from activated 2 to the di-
bromo substrate results in the mesolysis of one of the CÀBr
bonds, and then cleavage of the other CÀBr bond affords the
alkene and pyridine products. Because the Hantzsch ester was
excited by visible light almost identically in all the reactions,
the significantly different reactivities, for example 1a is 16
times faster than 1ae, suggest that the rate determining step
is downstream; the ET process from the Hantzsch ester to the
12
86 E
13
14
80 E
92[c]
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.22 mmol), Na2CO3 (0.22 mmol),
DMSO (1.5 mL), 12 W white LED, rt,1.5 h. [b] Isolated yield. [c] Only the E-
product was detected by GC-MS and 1H NMR spectroscopy. [d] 2.5 h.
[e] In DMF, 50 min.
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substrate or the radical fragmentation of the alkene.
First, to elucidate the factors behind the varied reac-
tivity, cyclic voltammetry experiments for different di-
bromo substrates were carried out (Scheme 2c);
a large difference of the reduction potential between
1a (E1/2red =À0.80 V vs. SCE) and 1ae (E1/2red =À1.87 V
vs. SCE) was observed. Therefore, 1a will gain elec-
trons more easily and react faster than 1ae in the ET
step. Second, because the proposed radical inter-
mediates A (1a and 1ae) are too transient for any
chemical analysis, computational studies were carried
out for the step from A to the final alkenes (3a and
3ae, respectively) at the UB3LYP/6-311G(d,p) level to
optimize the geometries of all stationary points.[12]
(Scheme 2d). The optimized structure of A-1ae
shows that its CÀBr bond (2.51 ꢁ) is longer than that
of A-1a (2.08 ꢁ) and a “conventional” CÀBr bond
(1.90 ꢁ).[13] Meanwhile, the overall DG for these reac-
tions were À55.0 and À60.7 kcalmolÀ1 for A-1a and
A-1ae, respectively, indicating that the second cleav-
age of CÀBr is quite smooth and A-1ae will be even
closer to the product than A-1a. These results sup-
port that the first ET transfer from the Hantzsch ester
to dibromo compounds is the rate determining step.
To demonstrate the potential of the described procedure,
the debromination reaction employing the Hantzsch ester 2
and substrate 1a was scaled up to several grams. The reaction
reached full conversion after 8 h with an isolated yield of 98%
also using the standard 12 W LED irradiation (Scheme 3).
Table 4. Debromination reactions of 1ab–ae with 2 to form unfunctionalized al-
kenes 3ab–ae.[a]
Entry Substrate
Product
t
Yield
[h] [%][b]
1
12 82
2
1.5 90
3
4
72 65
24 99
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.22 mmol), Na2CO3 (0.22 mmol), DMSO
(1.5 mL), 12 W white LED, rt. [b] Isolated yield.
Scheme 3. Gram scale debromination reaction of 1a and 2.
In conclusion, we developed an efficient method for the de-
protection of vicinal dibromo compounds to afford alkenes.
With the help of an inorganic base, an Hantzsch ester acts as
a self-activating reductant under visible-light irradiation. The
tolerated substrates include styrene, a,b-unsaturated com-
pounds, and unfunctionalized alkenes. Robustness screening
demonstrated that this method is compatible with a great vari-
ety of functional groups. Mechanistic experiments revealed
that the base is critical for the activation of the Hantzsch ester.
Experimental Section
A flask equipped with a rubber septum and a magnetic stirrer was
charged with the dibromo substrate 1a (0.2 mmol), the Hantzsch
ester 2 (0.22 mmol), Na2CO3 (0.22 mmol), and DMSO (1.5 mL) under
nitrogen. The mixture was irradiated with a 12 W white LED at am-
bient temperature for 1.5 h and then concentrated by under re-
duced pressure on rotavapor. The residue was purified by silica gel
Scheme 2. Plausible mechanism for the debromination reaction. a) Proposed
reaction mechanism (ET=electron transfer). b) Fluorescence-intensity-ratio
measurements of 1a (p-bromophenyl-1,2-dibromoethane) and the Hantzsch
ester 2 at varying concentrations. c) CV measurements with 1a and 1ae.
d) Computational studies with transient reaction intermediates A-1a and A-
1ae.
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[%][b]
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[%][b]
99
99
99
98
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[9] We observed that the artemisinine enhanced the fluorescent of exciet-
ed Hantzsch ester, instead of quenching it. This might account for the
low yield of debromination reaction that rely on the ET quenching
pathway. For details, see Supporting Information.
99
98
98
99
95
54
[a] Reaction conditions: 1a (0.2 mmol),
2
(0.22 mmol), Na2CO3
(0.22 mmol), DMSO (1.5 mL), 12 W white LED, rt. [b] Isolated yield of 3a;
the intactness of the spectator molecules was detected by GC-MS and
NMR spectroscopy.
column chromatography with petroleum ether/AcOEt =1:1, v/v as
the eluent to give 3a.
[10] For a recent report showing that an a-carbonyl bromide can quench
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0.55 gLÀ1 at ambient temperature.
Acknowledgements
[12] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) C. Lee, W. Yang, R. G.
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We are grateful for grants from the National Science Founda-
tion of China (21572099, 21332005), the Natural Science Foun-
dation of Jiangsu Province (BK20151379), the Ph.D. Programs
Foundation of the Ministry of Education of China
(20130091120047), and SRF for the Returned Overseas Chinese
Scholars (ROCS), State Education Ministry (SEM).
Keywords: alkenes · catalyst-free reactions · debromination ·
Hantzsch ester · visible light
Received: April 18, 2016
Published online on && &&, 0000
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COMMUNICATION
&
Organic Synthesis
W. Chen, H. Tao, W. Huang, G. Wang,
S. Li,* X. Cheng,* G. Li
&& – &&
Hantzsch Ester as a Photosensitizer
for the Visible-Light-Induced
Debromination of Vicinal Dibromo
Compounds
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