Photo-ritter reaction of arylmethyl bromides in acetonitrile

Article abstract of DOI:10.1080/00397910903297555

The photo-Ritter reaction of five arylmethyl bromides can occur in acetonitrile to give acetamides. The intermediates, carbocations, which are formed from subsequent electron transfer between the radical pairs generated from initial homolytic cleavage of the C-Br bond, are trapped by acetonitrile, and subsequent hydrolysis generates the corresponding acetamides. Taylor & Francis Group, LLC.

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Photo-Ritter Reaction of Arylmethyl
Bromides in Acetonitrile
Nai-Min Bi a , Ming-Guang Ren a & Qin-Hua Song a
a Department of Chemistry, Joint Laboratory of Green Synthetic
Chemistry, University of Science and Technology of China, Hefei,
China
Published online: 05 Aug 2010.
To cite this article: Nai-Min Bi , Ming-Guang Ren & Qin-Hua Song (2010) Photo-Ritter Reaction of
Arylmethyl Bromides in Acetonitrile, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 40:17, 2617-2623, DOI: 10.1080/00397910903297555
To link to this article: http://dx.doi.org/10.1080/00397910903297555
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Synthetic Communications1, 40: 2617–2623, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903297555
PHOTO-RITTER REACTION OF ARYLMETHYL
BROMIDES IN ACETONITRILE
Nai-Min Bi, Ming-Guang Ren, and Qin-Hua Song
Department of Chemistry, Joint Laboratory of Green Synthetic Chemistry,
University of Science and Technology of China, Hefei, China
The photo-Ritter reaction of five arylmethyl bromides can occur in acetonitrile to give
acetamides. The intermediates, carbocations, which are formed from subsequent electron
transfer between the radical pairs generated from initial homolytic cleavage of the C–Br
bond, are trapped by acetonitrile, and subsequent hydrolysis generates the corresponding
acetamides.
Keywords: Acetamides; arylmethyl bromides; photolysis; Ritter reaction
INTRODUCTION
The Ritter reaction is a chemical reaction that transforms a nitrile into an
N-alkyl amide using various alkylating reagents, for example, strong acids and
alkenes[1] or alcohols.[2] In recent years, several alternative methodologies have been
developed for the Ritter reaction using various catalysts that can generate carboca-
tion intermediates. The main disadvantages of these methods are the use of toxic
catalysts or corrosive acids, and these catalysts cannot be reused. Recently, some
researchers have been making efforts to find recyclable catalysts.[3] The photo-Ritter
reaction is one alternative method that overcomes the disadvantages, and the
formation of intermediary carbocation undergoes either a photosensitized electron
transfer[4] or a direct photolysis.[5–7] The latter pathway is a more ‘‘green’’ synthetic
method. However, for halohydrocarbon as a substrate in the photo-Ritter reaction,
only a few examples were found, including alkyl halides,[5] aromatic alkenyl
bromides,[6] and an aryl halide.[7]
The photo-Ritter reaction of alkyl halides in acetonitrile was thought to
undergo the processes as follows. The substrate in the excited state, after absorbing
a photon, has an initial homolytic cleavage of the carbon–halogen bond to afford a
radical pair, which undergoes electron transfer to form an ion pair, including a
carbocation. The carbocation is trapped by acetonitrile, and subsequent hydrolysis
gives a Ritter product.[5]
Received April 10, 2009.
Address correspondence to Qin-Hua Song, Department of Chemistry, Joint Laboratory of Green
Synthetic Chemistry, University of Science and Technology of China, Hefei 230026, China. E-mail:
qhsong@ustc.edu.cn
2617

2618
N.-M. BI, M.-G. REN, AND Q.-H. SONG
Chart 1. Structures of arylmethyl bromides, 1a–e, in this work.
Herein, we report the photo-Ritter reaction of five arylmethyl bromides
including benzyl bromide (1a), 1-(bromomethyl)naphthalene (1b), 2-(bromomethyl)
naphthalene (1c), 2-(bromomethyl)anthraquinone (1d), and 2-(1-bromoethyl)
anthraquinone (1e) in acetonitrile (Chart 1). Photolysis of the five compounds in
acetonitrile can efficiently generate N-arylmethyl amides.
RESULTS AND DISCUSSION
To investigate photolysis of arylmethyl bromides, acetonitrile solutions of 1a–e
were prepared in a quartz tube for 1a, a Pyrex tube for 1b and 1c, and a Pyrex flask
for 1d and 1e. Under a 300-W high-pressure mercury lamp, acetonitrile solutions of
1a–c were irradiated, and benzylacetamide (2a), 1-acetamidomethylnaphthalene (2b),
and 2-acetamidomethylnaphthalene (2c) were obtained in the yields of 42%, 42%, and
63%, respectively. Compounds 1d and 1e in acetonitrile were irradiated with a 500-W
Xe lamp, giving 2-acetamidomethylanthraquinone (2d) and 2-(1-acetamidoethyl)
anthraquinone (2e) in yields of 72% and 40%, respectively. The selection of the exci-
tation light source and a reactor depends on absorption spectra of arylmethyl bromides,
that is, the absorption band of 1a in the far-ultraviolet region and a longer-band region
for 1d and 1e as compared with 1b and 1c. Based on these photoproducts, the photoche-
mical reactions can be inferred to be a photoinduced Ritter reaction. In the photoche-
mical reaction, the formed carbocation intermediates are trapped by acetonitrile and
subsequent hydrolysis by a trace of water in acetonitrile or during the separation
process, generating the corresponding amides (2) (Scheme 1).
Scheme 1. Photoinduced Ritter reaction of arylmethyl bromides (1a–e) in acetonitrile.

PHOTO-RITTER REACTION OF ARYLMETHYL BROMIDES
2619
Competing ionic and radical photobehavior has been observed for a number of
alkyl halides.[5] Irradiation of alkyl halides in a variety of media afforded predomi-
nantly the nucleophilic substitution product, accompanied by the reduction product
norbornane. The results have been explained in terms of initial homolytic cleavage of
the carbon–halogen bond to afford a radical pair, which undergoes competing (a)
diffusion and hydrogen abstraction to give reduction products and (b) electron
transfer (ET) to afford an ion pair. The ion pair, in turn, gives rise ultimately to
the substitution and elimination products.[5] The competition between ionic and
radical pathways was further demonstrated through investigating photolysis of
1-fluoro-2-halo-1,2-diphenylethanes solutions of tetrahydrofuran (THF), acetoni-
trile, and cyclohexane.[8] In the hydrolysis system of 1e, besides the Ritter product
(2e), a lesser amount of reduction product 2-ethylanthraquinone (3, 5%), nucleophi-
lic substitution product 2-(1-hydroxyethyl)anthraquinone (4, 6%), and the elimin-
ation product 2-vinylanthraquinone (5, 30%) in excellent yield were also obtained
(Scheme 2). Furthermore, the radical product 3 decreases with increasing solvent
polarity. These results can be well explained with this mechanism.
In general, acetonitrile as a solvent does not undergo nucleophilic addition to
radical cation species formed from photoinduced ET reactions.[8,9] However, we
observed that the photolysis of the arylmethyl bromides gave the corresponding
Ritter products in good yields. When the photo-Ritter reactions were preformed
in wet acetonitrile, the yields and conversion rates of amides become lower because
of the nucleophilic addition of water and gave corresponding hydrolysis products
(Table 1). This confirms the formation of carbocaion in photolysis of the arylmethyl
bromides.
To measure the quantum yields, acetonitrile solutions (1 mM) of 1a–e were
prepared in quartz cuvettes with a Teflon stopper, sealed after bubbling with high
pure nitrogen for 15 min, and then irradiated with the monochromatic light at their
maximum absorption wavelength from a fluorescence spectrometer with a 20-nm
slit. After a certain time of irradiation, the irradiated solution was taken out for
HPLC analysis. The irradiation time was selected in terms of their photochemical
efficiencies from the synthesis. The products were identified by co-injection with
known samples. The concentrations of products in the irradiated solution were
analyzed, and conversion rates were obtained, as listed in Table 1. Data show that
2d and 2a have good values and that 2e formed in a poor conversion rate. The
poor conversion rate of 2e results from the elimination reaction, and there is no
Scheme 2. Proposed mechanism for photochemical reactions of arylmethyl bromides in acetonitrile.

2620
N.-M. BI, M.-G. REN, AND Q.-H. SONG
Table 1. Conversion rates of products acetamides in phoyolysis of arylmethy bromides in dry acetonitrilea
Substrates
kb (nm)
Timec (min)
Ritter product
Conv. (%, Urel)
1a
1b
1c
1d
1e
250
290
275
326
350
60
30
30
20
10
2a
2b
2c
2d
2e
78 (5)
53 (1)
42 (1)
92 (25)
25 (3)
a[Substrate] ¼ 1 mM, determined by high-performance liquid chromatography (HPLC) directly on the
crude product mixture.
bIrradiation wavelengths, which are maxima absorption peaks of arylmethyl bromides.
cIrradiation time.
competition for the other four arymethyl bromides because there is no b-hydrogen.
Furthermore, the intensity of irradiation light was measured using potassium
ferrioxalate actinometry.[10] Based on the yields and the light intensity, the relative
quantum yields (Urel) were calculated shown in Table 1. The value of 1d is the
highest, 25 times higher than those of 1b and 1c.
In general, a photo-Ritter reaction gives a relatively low yield. In this work,
photolysis of the arylmethyl bromides in dry acetonitrile was observed to give
relative good yields and good conversion rates except for 1e. Because of the compe-
tition of the elimination, the photochemical reaction of 1e gives a poor conversion
rate of Ritter product 2e. In addition, the conversion rate would be decreased in
the presence of a stronger nucleophile, such as water.
EXPERIMENTAL
General Methods
1
Melting points were uncorrected. H and 13C NMR spectra were determined
with a Bruker AV300 spectrometer. 1H NMR were run at 300 MHz, and 13C
NMR spectra were recorded at 75 MHz, under full proton decoupling. Chemical
shifts are given in parts per million from tetramethylsilane (TMS) as the internal
reference. Fourier transform–infrared (FT-IR) spectra were carried out on a Bruker
Vector22 IR spectrometer. Ultraviolet–visible (UV-vis) absorption spectra were
recorded with a Shimadzu UV-2401PC UV–vis spectrometer. Mass spectra were
obtained with a Micromass GCF TOF mass spectrometer. HPLC analysis was
performed on an HP Agilent (1100 series) HPLC system with C-18 reverse-phase
columns (250 mm ꢀ 4.6 mm ꢀ 5 mm).
Synthesis of Arylmethyl Bromides 1a–e
Methyl aromatic compounds (toluene, 1-methylnaphthalene, 2-methylnaphtha-
lene, 2-methylanthraquinone, and 2-ethylanthraquinone) (20 mmol), NBS (20 mmol),
and azobisisobutyronitrile (AIBN) (3 mmol) were dissolved in CCl4 (30 mL) and
refluxed for 7 h. The crude product was obtained by filtration and purified by
recrystallizing it in ethanol to give the corresponding arylmethyl bromides.

PHOTO-RITTER REACTION OF ARYLMETHYL BROMIDES
2621
Synthetic Procedure of 2a–c
Arylmethyl bromides 1a–c was dissolved in dry acetonitrile, ꢁ30 mM, and
placed in a Pyrex tube. The solution was bubbled with very pure nitrogen for
15 min and then was irradiated with a 300-W high-pressure lamp. The reaction
followed by thin-layer chromatography (TLC). When TLC showed a maximum
concentration of the Ritter product, the irradiation was stopped. The solvent in
the reaction mixture was concentrated by rotary evaporation. The residue was
subjected to TLC to get the crude product and further purified by recrystallization
with petroleum ether to give 2a–c.
Synthetic Procedure of 2d–e
Compounds 1d (60 mg, 0.2 mmol) and 1e (126 mg, 0.4 mmol) were dissolved in
dry acetonitrile (20 mL) and placed in a flask. Under a nitrogen atmosphere, the sol-
ution was irradiated with a 500-W Xe lamp, and the reaction was followed by TLC.
When TLC showed a maximum concentration of the Ritter product, the irradiation
was stopped. The irradiated solution was concentrated. The residue was dissolved in
dichloromethane (DCM) and subjected to column chromatography to give 2d and 2e.
Selected Data
N-Benzylacetamide (2a).[11] Irradiation for 3.5 h, yield 42%; Rf ¼ 0.31
(petroleum ether=ethyl acetate 1:1); mp 60–62 ꢂC; 1H NMR (300MHz, CDCl3)
d ¼ 2.02 (s, 3H, CH3), 4.43 (d, J ¼ 5.4 Hz, 2H, NHCH2), 5.79 (b, 1H, NH),
13
=
7.26–7.36 (m, 5H, HAr); C NMR (75 MHz, CDCl3) d ¼ 170.1 (C O), 138.4, 128.8,
128.0, 127.6, 43.9 (CH2), 23.3 (CH3); IR (KBr): n (cmꢃ1) ¼ 3297s, 1644s, 1552s,
743m, 696m. TOFMS (EI) calcd. for (Mþ) C9H11NO: 149.0841; found 149.0847.
1-(Acetamidomethyl)naphthalene (2b).[11b,12] Irradiation for 3 h, yield 42%;
Rf ¼ 0.48 (ethyl acetate); mp 124–126 ꢂC; 1H NMR (300 MHz, CDCl3): d ¼ 2.00 (s, 3H,
CH3), 4.88 (d, J ¼ 5.4 Hz, 2H, NHCH2), 5.70 (b, 1H, NH), 7.42–7.56 (m, 4H, HAr),
7.80–8.02 (m, 3H, HAr); 13C NMR (75 MHz, CDCl3): d ¼ 169.9 (C O), 134.0, 133.6,
=
131.5, 128.9, 128.7, 126.8, 126.7, 126.1, 125.5, 123.6, 41.9 (CH2), 23.2 (CH3); IR
(KBr): n (cmꢃ1) ¼ 3278s, 1640s, 1558s, 1428m, 1371m 1294s, 1035m, 797s. TOFM
(EI) calcd. for (Mþ) C13H13NO: 199.0997; found 199.0996.
2-(Acetamidomethyl)naphthalene (2c).[11b] Irradiation for 2.5 h, yield 63%;
1
Rf ¼ 0.43 (ethyl acetate); mp 121–123 ꢂC; H NMR (300 MHz, CDCl3): d ¼ 2.06 (s,
13
3H, CH3), 4.59 (d, J ¼ 4.5 Hz, 2H, NHCH2), 5.86 (b, 1H, NH), 7.38–7.51 (m, 3H,
=
HAr), 7.71–7.83 (m, 4H, HAr); C NMR (75 MHz, CDCl3): d ¼ 170.2 (C O),
135.8, 133.5, 132.9, 128.7, 127.8, 126.5, 126.4, 126.1, 44.0 (CH2), 23.4 (CH3); IR
(KBr): n (cmꢃ1) ¼ 3264s, 1640s, 1558s, 1434m, 1371m, 1291s, 1029m, 817s, 797s.
TOFM (EI) calcd. for (Mþ) C13H13NO: 199.0997; found 199.0991.
2-(Acetamidomethyl)anthraquinone (2d). Irradiation for 1 h, yield 72%;
1
Rf ¼ 0.35 (ethyl acetate); mp 226–228 ꢂC; H NMR (300 MHz, DMSO-d6) d ¼ 1.93
13
(s, 3H, CH3), 4.45 (d, J ¼ 5.4 Hz, 2H, NHCH2), 7.78–8.23 (m, 7H, HAr), 8.59 (s,
=
=
1H, NH); C NMR (75 MHz, DMSO-d6) d ¼ 182.5(C O), 182.2 (C O), 169.6

2622
N.-M. BI, M.-G. REN, AND Q.-H. SONG
=
(C O), 147.0, 134.5(2C), 133.1, 133.0, 131.7, 127.0, 126.7(2C), 125.0, 42.0 (CH2),
22.6 (CH3); IR (KBr): n (cmꢃ1) ¼ 3437s, 1673s, 1592s, 1534s, 1332s, 1236m, 846m,
706s. TOFMS (EI) calcd. for (Mþ) C17H13NO3: 279.0895; found 279.0887.
2-(1-Acetamidoethyl)anthraquinone (2e). Irradiation for 3.5 h, yield 40%;
1
Rf ¼ 0.4 (ethyl acetate); mp 193–195ꢂC; H NMR (300 MHz, CDCl3) d ¼ 1.56 (d,
J ¼ 6.9 Hz, 3H, CHCH3), 2.05 (s, 3H, COCH3), 5.24 (quart, J ¼ 6.9 Hz, 1H,
NHCHCH3), 5.87 (d, J ¼ 6.9Hz, 1H, NHCH), 7.73–7.82 (m, 3H, HAr), 8.23–8.31 (m,
13
=
=
=
4H, HAr); C NMR (75MHz, CDCl3) d ¼ 182.6 (C O), 182.2 (C O), 168.7 (C O),
152.1, 134.6, 134.5, 133.2, 133.0, 132.3, 131.7, 127.2, 126.8, 126.7, 123.9, 48.0
(NHCHCH3), 22.6 (CH3), 22.0 (CH3); IR (KBr): n (cmꢃ1) ¼ 1675s, 1591s, 1324s,
1284s, 931m, 711s. TOFMS (EI) calcd. for (Mþ) C18H15NO3: 293.1052; found 293.1059.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (Grant Nos.
20972149 and 30870581).
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