Visible Light-Induced γ-Alkoxynitrile Synthesis via Three- Component Alkoxycyanomethylation of Alkenes

Article abstract of DOI:10.1002/adsc.201400548

A three-component alkoxycyanomethylation of alkenes is achieved using the iridium photoredox catalyst [fac-Ir(ppy)₃]. This catalytic radical difunctionalization accomplishes both alkylation and alkoxylation of alkenes in one pot. Various alcohols can serve as the alkoxy sources in this transformation. In addition, the introduced cyano group can undergo further transformations into various useful functional groups.

Full text of DOI:10.1002/adsc.201400548

UPDATES
DOI: 10.1002/adsc.201400548
Visible Light-Induced g-Alkoxynitrile Synthesis via Three-
Component Alkoxycyanomethylation of Alkenes
Hong Yi,+a Xu Zhang,+a Chu Qin,a Zhixiong Liao,a Jie Liu,a and Aiwen Leia,b,*
a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, Peopleꢀs Republic of China
Fax : (+86)-27-6875-4067; phone: (+86)-27-6875-4672; e-mail: aiwenlei@whu.edu.cn
National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang, Jiangxi 330022, Peopleꢀs
b
Republic of China
+
Hong Yi and Xu Zhang contributed equally to this work.
Received: June 4, 2014; Revised: July 10, 2014; Published online: September 11, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400548.
Abstract: A three-component alkoxycyanomethyla-
tion of alkenes is achieved using the iridium photo-
of mild conditions to generate free radicals from alkyl
halides is highly desirable.
redox catalyst [fac-Ir
(ppy)3]. This catalytic radical
Recently, visible light photoredox catalysis has re-
ceived widespread attention due to its low cost, easy
availability and environmental benignness.[5] Over the
years, various useful chemical transformations via visi-
ble light photoredox catalysis have been demonstrat-
ed.[6] The ruthenium(II) polypyridine complexes and
difunctionalization accomplishes both alkylation
and alkoxylation of alkenes in one pot. Various al-
cohols can serve as the alkoxy sources in this trans-
formation. In addition, the introduced cyano group
can undergo further transformations into various
useful functional groups.
cyclometalated iridiumACTHNUTRGENU(GN III) derivatives have been suc-
cessfully utilized to generate alkyl radicals from acti-
vated alkyl halides irradiated with visible light.[7]
Under visible light, the ATRA reactions or Heck-
type alkenylations which are usually proceeding with
transition metals, can be achieved with photoredox
catalysis under mild conditions.[8] According to the
proposed mechanism, a carbocation is one possible in-
Keywords: alkyl halides; photocatalysis; radical di-
functionalization; radical reaction; visible light
Free radical generation from alkyl halides (mostly termediate. If a nucleophile is introduced, the key in-
alkyl bromides and iodides) is a useful method to termediate carbocation could be trapped by a suitable
access reactive intermediates, which have widespread nucleophile and yield the three-component product of
usage in various domains of organic synthesis.[1] These alkene difunctionalization.[9] Using this photo-induced
À
carbon-centered radicals are typically prepared via C Br activation strategy, we successfully achieved
halide abstraction with organotin hydrides (X3SnH, a visible light-induced g-alkoxynitrile synthesis via the
usually present in stoichiometric quantities), which three-component alkoxycyanomethylation of alkenes
have dominated free radical chemistry and are exten- (Scheme 1).
sively used in radical ring closures, ring expansions
We started our evaluation of the reaction parame-
and cascade reactions.[2] However, stannane reagents ters with 4-tert-butylstyrene (1a) and bromoacetoni-
are notorious for their toxicity, hazardous handling trile (2a) as the model substrates. The anticipated di-
and disposal of toxic garbage. Thus, many non-stan- functionlization product 4a could be achieved in 90%
nane based protocols have been developed,[3] among yield in the presence of 0.5 mol% [fac-Ir
ACTHNUTRGNEU(NG ppy)3],
which transition metal-mediated free radical genera- 2.0 equivalents NaHCO3 using methanol (3a) as both
tion through reductive scission of the carbon-halogen the nuclephile and solvent under a 25 W household
bond represents an attractive approach.[4] Despite the fluorescent lamp. In the control experiments, no de-
fact that some processes have been successfully sired product was observed without either photoredox
achieved using transition metals, this single-electron catalyst or light indicating that the photoredox cataly-
transfer (SET) process always requires high tempera- sis is essential to this process (Table 1, entries 2 and
ture or other harsh condition which limits further ap- 3). Considering that other nucleophiles, such as iso-
plication of this methodology. Thus, the development propyl alcohol, may not be a good solvent for this
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Hong Yi et al.
Table 2. The scope of the olefin in the synthesis of g-alkoxy-
nitriles.[a]
AHCTUNGTRENNUNG
À
Scheme 1. C Br bond activation by photoredox catalysis
under visible light and successive three-component reac-
tions.
Table 1. Optimization of the reaction conditions for the visi-
ble light-induced synthesis of g-alkoxynitriles.
Entry
Nucleophiles
Solvent
Base
Yield [%][f]
1[a]
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
i-PrOH
i-PrOH
i-PrOH
MeOH
MeOH
MeOH
DMF
DMSO
DCM
CH3CN
acetone
THF
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
K3PO4
90[g]
n/d
n/d
11
2
91
85
53
63
15
2[b]
3[c]
[a]
Standard reaction conditions: 0.50 mmol olefin, 1.0 mmol
bromoacetonitrile, 0.0025 mmol [fac-Ir(III)ppy)3],
AHCTUNGTRENNUNG
4[d]
5[d]
6[d]
7[d]
8[d]
9[d]
10[d]
11[e]
12[e]
1.0 mmol NaHCO3, 2.0 mL MeOH, hv (incandescent),
24 h. Isolated yield.
1.5 mol% [fac-IrACHTUNTRGENN(GU III)ACHTUGNTREN(NUGN ppy)3], 48 h.
The reaction time was 48 h.
1.0 mol% catalyst, blue LED, 12 h.
[b]
[c]
[d]
DCM
DCM
DCM
27
64[g]
lem, giving the desired difunctionalization product in
moderate yield (Table 1, entry 12).
[a]
Reaction conditions: 0.50 mmol olefin, 1.0 mmol bromo-
ACHTUNGTRENNUNGacetonitrile, 2.0 mL solvent (solvent was used as nucleo-
Then, we investigated different olefins under the
established conditions and the results are summarized
in Table 2. This reaction was successfully amenable to
a wide range of substituted styrenes, and moderate to
excellent yields were achieved with substrates bearing
functional groups such as t-Bu (4a), Me (4b), F (4c),
Cl (4d), Br (4e), OMe (4f). In addition, styrene (1g)
and 2-methylstrene (1h) also worked well to give the
products in 87% (4g) and 80% (4h) yield, respective-
ly. However, styrenes substituted with electron-with-
phile), 0.0025 mmol [fac-Ir
Without catalyst.
ACHTUNGTERN(NUNG III)ppy)3], 1.0 mmol base.
[b]
[c]
[d]
[e]
[f]
Without light.
Solvent:nucleophile=1.8 mL:0.2 mL.
CH2Cl2:i-PrOH (1.0 mL:1.0 mL).
GC yield with biphenyl as the inner reference.
Isolated yield.
[g]
photoredox reaction, further optimization of various drawing groups (CF3 or CN) did not furnish any de-
co-solvents was carried out (Table 1, entries 4–9). To sired product. The 1,1-disubstituted olefins (4i, 4j)
our delight, the same result was obtained when using were also suitable for this reaction and the corre-
DCM as the co-solvent (Table 1, entries 1 and 6). sponding products were achieved. Surprisingly, 3,4-di-
However, replacing MeOH with isopropyl alcohol led hydro-2H-pyran underwent the transformation to
to a relative lower yield (Table 1, entry 10). Fortu- achieve a 52% yield (4k, about 1:1 dr). However,
nately, changing the base to K3PO4 and modifying the trans-stilbene and allylbenzene did not afford the de-
ratio of nucleophile and co-solvent solved the prob- sired product under the standard conditions.
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UPDATES
Visible Light-Induced g-Alkoxynitrile Synthesis
Table 3. The scope of the alcohols in the synthesis of g-alk-
ACHTUNGTRENNUNG
oxynitriles.[a]
Scheme 2. The visible light-induced g-nitrile synthesis using
aniline as the nucleophile.
Entry
Product 4
R
Alcohol
Yield[d] [%]
1[b]
2[b]
3
4a
4l
4m
4n
Me
Et
n-Pr
n-Bu
3a
3b
3c
3d
90
89
63
71
4
5
4o
33
69
6
7[c]
4p
4q
i-Pr
3f
3g
69
40
8[c]
4r
3h
54
[a]
Standard reaction conditions: 0.50 mmol olefin, 1.0 mmol
bromoacetonitrile, 0.0025 mmol Ir(ppy)3, 1.0 mL CH2Cl2,
Scheme 3. The visible light-induced g-alkoxynitrile synthesis
AHCTUNGTRENNUNG
using 2-bromopropanenitrile.
1.0 mL ROH, 1.0 mmolK3PO4, 24 h.
Without co-solvent and the base was NaHCO3.
1.0 mol% catalyst, 48 h.
[b]
[c]
[d]
Isolated yield.
In addition, 2-bromopropanenitrile (containing b hy-
drogens) also reacted very well (Scheme 3) giving the
desired product in 88% yield (4t, dr=1:1.3) under the
We next applied the optimized reaction conditions standard conditions. However, 2-bromo-2-methylpro-
to examine different alcohols (Table 3). Reactions of panenitrile and 2-bromo-2-phenylacetonitrile gave
1a with primary alcohols such as MeOH, EtOH, n- only traces of product mainly because of the debromi-
PrOH and n-BuOH gave the expected products in nation of the bromo nitriles. To obtain some insights
good to excellent yields (4a, 4l–4o). The secondary al- into the mechanism, luminescent quenching experi-
cohol was also suitable for this reaction (4p). In addi- ments were carried out. As shown in Figure 1, the
tion, the sterically hindered tertiary alcohols resulted fluorescence quenching process between IrACHTUNGTRENNUNG(ppy)3*
in slightly lower yields (4q, 4r). Although tertiary al- and BrCH2CN was obviously observed. Therefore,
cohols led to relatively lower yields, the present pho- the above luminescent quenching experiments con-
tocatalytic alkoxycyanomethylation is highly efficient firmed that the single electron transfer (SET) process
and regioselective, regardless of the oxygen nucleo- between Ir
philes. The reactions can smoothly introduce a cyano-
methyl group into an alkene and construct ether func-
tionalities. To further expand the application of this
methodology, different nucleophilic partners were
also tried. For the thiols (para-toluenethiol, n-butyl
mercaptan, tert-butyl mercaptan), they all underwent
hydrothiolation of alkenes, which had been previously
reported by Yoon and co-workers.[10] When the nucle-
ophile was an alkylamine such as benzylamine or
ACHTUGNRTEN(NUGN ppy)3* and BrCH2CN could go through an
butylACHTUNGTRENNUNGamine, the alkene did not react at all because of
the nucleophilic substitution reactions between amine
and bromoacetonitrile. Fortunately, aniline could
afford the three-component reaction product in 22%
yield (Scheme 2). When using a carboxylate (like am-
monium formate, sodium acetate) as nucleophile, the
Heck-type products were mainly obtained.
Figure 1. Luminescent quenching experiments.
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UPDATES
Hong Yi et al.
the three-component coupling product 4. The atom
transfer radical addition (ATRA) followed by nucleo-
philic attack by the alcohol with the assistance of base
could be another possible reaction pathway. Although
we cannot completely exclude this possibility, ATRA-
type product is not observed during this process. Ad-
ditionally, there was no ATRA-type product as well
and mainly Heck-type product was formed under the
standard conditions without the alcohol. As a result,
the ATRA reaction pathway is not favored.
In conclusion, we have developed a three-compo-
nent alkoxycyanomethylation of alkenes using the
photoredox catalyst [fac-IrACHTNUGRTENUNG(ppy)3]. This highly regio-
selective reaction can proceed smoothly under mild
conditions (room temperature under sunlight or
a household lamp). Both cyanomethylation and al-
koxylation of alkenes are achieved in one pot with
this catalytic radical difunctionalization. Various alco-
Figure 2. Time profile of the photocatalytic reaction with
and without visible light.
oxidative quenching process. The time profile of pho- hols can serve as the alkoxy sources. In addition, the
tocatalytic reaction shown in Figure 2 indicated that introduced cyano group can undergo further transfor-
continuous irradiation of visible light is essential to mations into various useful functional groups.
achieve this photocatalytic transformation.
Based on the previous reports[8c,9b] and experimen-
tal results, a plausible reaction mechanism was pro- Experimental Section
posed in Scheme 4. Firstly, the photocatalyst
A
ACHTUNGTRENNUNG(III)ACHTUNGTRENNUNG(ppy)3] is excited by visible light irradiation General Procedure
to
generate
the
excited
species
A dried Schlenk tube equipped with a stir bar was loaded
with sodium bicarbonate (84 mg, 1.0 mmol, 2.0 equivalents)
and fac-IrACTHNUGRTENUNG(ppy)3 (1.6 mg, 0.0025 mmol, 0.5 mol%). The tube
G
G
ACHTUNGTRENNUNG(ppy)3]*, which then undergoes single elec-
tron transfer (SET) process to bromoacetonitrile gen-
erating cyanomethyl radical 5, a bromide anion and
was evacuated and refilled with nitrogen before alkene
(0.5 mmol), bromoacetonitrile (120 mg, 1.0 mmol, 2.0 equiv-
alents) were added through a syringe. Then 2.0 mL solvent
were added into the reaction tube via a syringe. The reac-
tion mixture was degassed by the freeze-pump-thaw method
and then irradiated with a 24 W fluorescent household light
bulb (distance app. 10 cm) for 24 h. After the completion of
the reaction, the pure product was obtained by flash column
chromatography on silica gel.
E
ACHTUNGTRENNUNG(ppy)3]. The generated cyanomethyl radical
À
AHCTUNGTRENNUNG
(path a) or bromoacetonitrile (path b) to form the
carbocation intermediate 7 through the single elec-
tron transfer (SET) process. Subsequent nucleophilic
attack of alcohol on the carbocation intermediate and
final deprotonation in the presence of base generated
Acknowledgements
This work was supported by the 973 Program
(2011CB808600, 2012CB725302), the National Natural Sci-
ence Foundation of China (21390400, 21025206, 21272180
and 21302148), and the Research Fund for the Doctoral Pro-
gram of Higher Education of China (20120141130002) and
the Program for Changjiang Scholars and Innovative Re-
search Team in University (IRT1030). The Program of Intro-
ducing Talents of Discipline to Universities of China (111
Program) is also appreciated.
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ACHTUNGTRENNUNGoxynitriles
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