Synthesis of Quinolines by Visible-Light Induced Radical Reaction of Vinyl Azides and α-Carbonyl Benzyl Bromides

Article abstract of DOI:10.1002/adsc.201500141

A visible-light induced radical reaction of vinyl azides and α-carbonyl benzyl bromides was developed, which provides an efficient route to polysubstituted quinolines via a C-C and C-N bond formation sequence.

Full text of DOI:10.1002/adsc.201500141

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DOI: 10.1002/adsc.201500141
Synthesis of Quinolines by Visible-Light Induced Radical Reac-
tion of Vinyl Azides and a-Carbonyl Benzyl Bromides
Qile Wang,+a,b Jun Huang,+a and Lei Zhoua,*
a
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, 135 Xingang West Road, Guangzhou 510275,
Peopleꢀs Republic of China
Phone: (+86)20-8411-0217
E-mail: zhoul39@mail.sysu.edu.cn
b
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA
+
Q. Wang and J. Huang contributed equally to this work.
Received: February 11, 2015; Revised: May 21, 2015; Published online: July 16, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500141.
Abstract: A visible-light induced radical reaction of
vinyl azides and a-carbonyl benzyl bromides was
developed, which provides an efficient route to
polysubstituted quinolines via a C C and C N
bond formation sequence.
duced radical reaction of vinyl azides with a-carbonyl
benzyl bromides (Scheme 1c).
In the past decades, iminyl radicals have been dem-
onstrated as valuable intermediates in organic trans-
formations, because they can be trapped by aromatic
ring or double bond to form a series of nitrogen con-
taining heterocycles.[9] Very recently, the first visible-
light promoted generation of iminyl radicals from acyl
oximes was reported by Zhang and Yu, which were
subsequently converted into pyridines, quinolines, and
phenanthridines through a homolytic aromatic substi-
tution (HAS) process.[10] However, this intramolecular
À
À
Keywords: iminyl radical; quinolines; vinyl azides;
visible light
Recently, visible-light photoredox catalysis has reaction requires several steps for the preparation of
emerged as a powerful tool in organic synthesis, the substrates, which limited its applications in the or-
which provides a green and sustainable process for ganic synthesis. Among the methods for the genera-
generating radical or ionic species.[1] Taking advantage tion of iminyl radicals,[11] the addition of carbon radi-
of the reduction potentials of a-halo carbonyl com-
pounds, generation of alkyl radicals through single-
electron transfer and their addition to alkenes,[2] al-
kynes,[3] isocyanides,[4] and aromatic rings[5] have been
reported. However, the use of vinyl azides as the radi-
cal acceptors in visible-light reactions remains a chal-
lenge, mainly because of their denitrogenative decom-
position to highly strained azirines via vinyl nitrene
intermediates.[6,7] For examples, Yoon recently ex-
ploited an energy transfer process for the formation
of 2H-azirines in the presence of a Ru photocatalyst
(Scheme 1a).[7a] Xiao and co-workers demonstrated
visible-light induced [3+2] cycloaddition between
vinyl azides and electron-deficient alkynes (Sche-
me 1b).[7b] We envisioned that employing judicious re-
action conditions, in particular a suitable radical pre-
cursor, could lead to the addition of radical to vinyl
azide prior to its decomposition. As a continuation of
our interest in heterocylces synthesis using photore-
Scheme 1. Formation of 2H-azridine versus the use of vinyl
dox catalysts,[8] herein we report a visible-light in- azide as a radical acceptor under visible-light conditions.
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Qile Wang et al.
cals to vinyl azides followed by the nitrogen extrusion 3aa sharply (Table 1, entries 6 and 7). Switching the
is attractive, which provides iminyl radical with con- base from K2HPO4 to NaOAc led to lower yield of
[12]
À
comitant formation of a new C C bond.
In this product, while only 17% yield of product was detect-
context, Chiba and co-workers developed an elegant ed when organic base iPr2NEt was employed (Table 1,
strategy for the synthesis of pyridines,[13] pyrroles[14] entries 8 and 9). The reaction delivered a slightly di-
and 6-perfluoroalkyl phenanthridine derivatives[15] minished yield of 3aa in the absence of 18-crown-6
using vinyl azide as a three-atom unit. However, the (Table 1, entry 10). However, the role of this additive
synthesis of quinolines has never been reported fol- in the reaction is not clear at this stage.
lowing this approach.[16] A visible-light induced reac-
Next, we then explored the scope and limitation of
tion of vinyl azides and a-carbonyl benzyl bromides the present visible-light induced radical reaction with
would not only provide a mild route to polysubstitut- various vinyl azides and benzyl bromides. As shown
ed quinolines, but it also represent the first example in Table 2, the reaction of vinyl azides 1a with a-me-
of using vinyl azide as a radical acceptor under visi- thoxycarbonyl benzyl bromides 2a–h proceeded
ble-light irradiation conditions.
smoothly, producing the corresponding quinolines
Our initial efforts to realize the transformation 3aa–3ah in moderate to good yields (Table 2, en-
commenced with the reaction of radical precursor 2a tries 1–8). Various functional groups including halo-
and vinyl azide 1a. When the solution of 1a and 2a in gen (F, Cl, Br), trifluoromethyl, methoxy, nitro, and 4-
1:1 mixture of MeCN and MeOH was irradiated with bromo phenyl were tolerant of the reaction condi-
a 5 W blue LED at room temperature for 18 h in the tions. When 4-(dibromomethyl) benzyl bromide 2i
presence of K2HPO4, 18-crown-6 and Ir(ppy)3, the de- was employed as the substrate, the reaction afforded
sired quinoline 3aa was obtained in a yield of 77% quinoline 3ai in 60% yield with the conversion of the
(Table 1, entry 1). In the control experiments, no gem-dibromo group to aldehyde (Table 2, entry 9).[17]
product was observed in the absence of either catalyst The standard reaction conditions could be applied to
or the light (Table 1, entries 2 and 3). Ru(bpy)3Cl2 1-naphthyl a-bromo acetic ester, albeit producing 3aj
and eosin Y were also promising catalysts for the re- with low efficiency (Table 2, entry 10). Notably, 2-pyr-
actions, but both of them were less efficient compar- idyl a-bromo ester 2k was also a suitable substrate
ing to Ir(ppy)3 (Table 1, entries 4 and 5). We noted for the reaction, furnishing the desired 1,5-naphthyri-
that the reaction was significantly affected by the sol- dine 3ak in the yield of 61% (Table 2, entry 11). The
vent, the use of MeCN or DMF decreased yields of reaction also worked efficiently when only one ortho
hydrogen atom was available in a-methoxycarbonyl
benzyl bromide 2l, yielding 3al in 65% yield
Table 1. Optimization of reaction conditions.[a]
(Table 2, entry 12). Interestingly, a sole regioisomer
1
3am was obtained based on the H NMR analysis of
the unpurified mixture when meta-Cl substituted
benzyl bromide 2m was used as the substrate
(Table 2, entry 13). It was found that a-bromo ketone
also underwent the reaction with 1a smoothly to give
the quinoline 3an in a yield of 63% (Table 2,
entry 14). In the case of ethyl 2-bromo-2-phenylace-
tate, 3aa was isolated as the product in 57% yield
owing to the trans esterification with MeOH (Table 2,
entry 15). Encouraged by this result, 2-bromo-2-phe-
nylacetic acid was directly used as the substrate; the
reaction also gave the esterified product 3aa in the
yield of 44% (Table 2, entry 16).
In the next step, we chose a-methoxycarbonyl
benzyl bromide 2a as the substrate for the reactions
with various vinyl azides. As shown in Table 3, the re-
action was not significantly affected by the substituent
on the benzene ring of vinyl azides; both electron-do-
nating and electron-withdrawing groups were well tol-
erated. In another case, methoxycarbonyl-substituted
vinyl azide 2h was transformed into the desired 2,4-
substituted quiniline 3ha in moderate yield. Unfortu-
nately, the reaction of alkyl vinyl azide was unsuccess-
ful, which might be attributed to its fast decomposi-
tion under visible-light conditions.
Entry
Conditions
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
standard conditions
no Ir(ppy)3
no light
Ru(bpy)3Cl2 instead of Ir(ppy)3
Eosin Y instead of Ir(ppy)3
MeCN as the solvent
DMF as the solvent
NaOAc as the base
iPr2NEt as the base
no 18-crown-6
77
0
0
35
41
38
26
45
17
65
[a]
Standard conditions: 1a (0.20 mmol), 2a (0.60 mmol),
Ir(ppy)3 (1.0 mol%), K2HPO4 (4 equiv), 18-crown-6
(2 equiv) in dry MeCN and MeOH (1:1, 1 mL in total)
under the irradiation of a 5 W blue LED at room tem-
perature for 18 h.
[b]
Yields were determined by 1H NMR using 1,3,5-trime-
thoxybenzene as the internal standard.
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Synthesis of Quinolines by Visible-Light Induced Radical Reaction
Table 2. Visible-light induced reaction of vinyl azide 1a with
various a-carbonyl benzyl bromides.[a]
Table 3. Visible-light induced reaction of a-methoxycarbonyl
benzyl bromides 2a with various vinyl azides.[a,b]
[a]
All the reactions were carried out under the conditions
as described in Table 2.
Isolated yield.
[b]
[a]
Reaction conditions: 1a (0.20 mmol), 2 (0.60 mmol),
Ir(ppy)3 (1.0 mol%), K2HPO4 (4 equiv), 18-crown-6
(2 equiv), dry MeCN and MeOH (1:1, 1 mL in total), 5
W blue LED, Ar, r.t., 18 h.
Scheme 2. Stern–Volmer quenching and control experi-
ments.
[b]
Isolated yields.
ence of 2H-azirine 4. It is known that the excited
To gain insight to the reaction mechanism, we per- Ir(ppy)3* species is a strong reductant (E1/2 IV/*III =
formed Stern–Volmer quenching experiments using À1.73 V vs SCE), capable of donating an electron to
the employed substrates. As shown in Scheme 2, both a-ester benzyl bromide 2a (E1/2red =À0.69 V vs
vinyl azide 1a and a-methoxycarbonyl benzyl bro- SCE).[18] However, the single-electron transfer be-
mide 2a displayed luminescence quenching of the tween vinyl azide 1a (À1.74 V, vs SCE in MeCN)[19]
Ir(ppy)3* excited state, whereas
a decrease in and Ir(ppy)3* is difficult. In the control experiment,
Ir(ppy)3* luminescence was not observed in the pres- 85% yield of 2H-azirine 4 was isolated when MeCN
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solution of vinyl azide 1a was irradiated with a 5 W tion, we detected the dimerization (major) and deha-
blue LED in the absence of a-methoxycarbonyl logenation (minor) of 2 as the byproducts.[21] The
benzyl bromides (Scheme 2, eq. 1). These results indi- result indicates that benzyl bromide 2 might act as the
cate that the luminescence quenching of the Ir(ppy)3* oxidant in the dehydrogenation step, and the forma-
by 1a might be realized through an energy transfer tion of 3 is not a consequence of oxidation during the
process, and the formation of 2H-azirines is compet- work-up procedure.[22] On this basis, a plausible path-
ing with the radical addition to vinyl azides. In term way for the dehydrogenation of 2H-quinoline E is
of the relatively high oxidative potential of 2H-azirine postulated. Deprotonation of E in the presence of
4 (+1.69 V)[20] and the luminescence quenching ex- base generates F, which might form a EDA (electron
periments, we speculated 2H-azirines could not act as donor–acceptor) complex with a-carbonyl bromide to
electron donors. The control experiment also revealed trigger the reduction of 2, giving radical G and A.[23]
that treatment of 4 and a-methoxycarbonyl benzyl The oxidation of G by Ir(IV) leads to cation H, which
bromide 2a under standard reaction conditions did delivered quinolines 3 via deprotonation. Another
not give any quinoline product (Scheme 2, eq. 2). possible route to quinolines is the tautomerization of
These experiments provide strong evidence that the E to give Hantzsch ester type of 2H-quinoline, which
reaction is initiated by an oxidative quenching pro- is oxidized by the cooperation of photocatalyst and a-
cess.
carbonyl bromide. Although secondary amines have
A tentative mechanism for this visible light-induced relative high oxidative potentials, this pathway cannot
radical reaction of vinyl azides and a-carbonyl benzyl be ruled out completely (see Scheme S1 in supporting
bromides is described in Scheme 3. First, photoexcita- information).[24]
tion of IrIII by visible light generates triplet IrIII* (E1/2
In conclusion, we have developed an efficient
IV/*III =À1.73 V vs SCE), which is oxidatively method for the synthesis of quinolines via sequential
quenched by a-carbonyl bromide 2 (E1/2red =À0.69 V, C C and C N bond formation. This reaction uses
vs SCE) with the generation of Ir(IV) complex and vinyl azides and benzyl bromides as the starting mate-
radical A. The addition of radical A to vinyl azide rials with tolerance of a wide range of functional
1 affords iminyl radical B with the extrusion of nitro- groups. More importantly, this study demonstrates the
gen, followed by an intramolecular radical cyclization use of vinyl azide as the radical acceptor under visi-
to give the cyclized radical intermediate C. Electron ble-light conditions, which opens up the possibilities
transfer from C to Ir(IV) regenerates the catalyst to construct various nitrogen-heterocyclic frameworks
with formation of the cation D, which yields 2H-quin- based on iminyl radicals by using photoredox cataly-
oline E via deprotonation with the aid of base. Final- sis.
À
À
ly, dehydrogenation of E forms quinoline 3, driven by
the aromatization of the final product. In this reac-
Experimental Section
General Procedure for the Synthesis of Quinolines 3
In a 10 mL snap vial were placed Ir(ppy)3 (1.0 mol%),
K2HPO4 (4 equiv), 18-crown-6 (2 equiv), vinyl azide
1
(0.20 mmol), a-methoxycarbonyl benzyl bromides 2
(0.60 mmol) and 1 mL of CH3OH/CH3CN (1:1) under
argon. After stirring at room temperature with the irradia-
tion of a 5 W blue LED for 18 h, the reaction mixture was
filtered through a short path of silica gel. The volatile com-
pounds were removed in vacuum and the residue was puri-
fied by column chromatography (SiO2) with hexane/ethyl
acetate.
Acknowledgements
We are grateful to the funds from the National Natural Sci-
ence Foundation of China (21472249 and 21202207), the
Pearl River S&T Nova Program of Guangzhou
(2013J2200017), and the Fundamental Research Funds for
the Central Universities (14lgzd05).
Scheme 3. Proposed reaction mechanism.
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