Intramolecular photochemical cross-coupling reactions of 3-acyl-2-haloindoles and 2-chloropyrrole-3-carbaldehydes with substituted benzenes

Article abstract of DOI:10.1002/adsc.200900540

Highly efficient syntheses of indolo[2,1-a]isoquinolines, indolo[2,1-a][2]benzazepines, pyrrolo[2,1-a]isoquinolines and pyrrolo[1,2-a]benzazepines in excellent yields have been achieved by the intramolecular photochemical cross-coupling reactions of 3-acyl-2-halo-N- (ω-arylalkyl)indoles and 2-chloro-N-(ω-arylalkyl)pyrrole-3- carbaldehydes in acetone. A new heterocyclic ring system - pyrrolo[1,2-d][1,4] benzoxazepine - has also been constructed for the first time in this work by the photocyclization of 2-chloro-N-(2-phenoxyethyl)pyrrole-3-carbaldehyde.

Full text of DOI:10.1002/adsc.200900540

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DOI: 10.1002/adsc.200900540
Intramolecular Photochemical Cross-Coupling Reactions of 3-
Acyl-2-haloindoles and 2-Chloropyrrole-3-carbaldehydes with
Substituted Benzenes
Shen-Ci Lu,a Xi-Xia Zhang,a Zong-Jun Shi,a Yu-Wei Ren,a Bing Li,a
and Wei Zhanga,*
a
State Key Laboratory of Applied Organic Chemistry and College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-891-2582; e-mail: zhang_wei6275@yahoo.com
Received: July 31, 2009; Revised: October 18, 2009; Published online: November 18, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900540.
Abstract: Highly efficient syntheses of indolo
a]isoquinolines, indolo[2,1-a][2]benzazepines, pyr-
rolo[2,1-a]isoquinolines and pyrrolo[1,2-a]benzaze-
pines in excellent yields have been achieved by the
intramolecular photochemical cross-coupling reac-
tions of 3-acyl-2-halo-N-(w-arylalkyl)indoles and 2-
[2,1-
ners, rather harsh conditions and/or stoichiometric
amounts of expensive or moisture-sensitive organo-
metallic compounds are required. The direct arylation
AHCTUNGTRENNUNG
G
R
ACHTUNGTRENNUNG
À
by functionalization of an aryl C H bond, where hy-
drogen is replaced by the aryl group, has also
emerged, for example, Bu3SnH-mediated aryl radicals
substitution.[7] But there are also many disadvantages
associated with tributyltin hydride such as serious tox-
icity and problems of separation of tin residues from
organic products.[8–10] Thus, the search for alternative
methodologies has long been an attractive topic. Less
toxic and more expensive silanes[9] and germanes[10]
have been used as substitutes for Bu3SnH. Alterna-
tives were reported including intramolecular aryl radi-
chloro-N-(w-arylalkyl)pyrrole-3-carbaldehydes
acetone. A new heterocyclic ring system – pyrrolo-
[1,2-d][1,4]benzoxazepine – has also been construct-
in
A
ACHTUNGTRENNUNG
ed for the first time in this work by the photocycli-
zation of 2-chloro-N-(2-phenoxyethyl)pyrrole-3-car-
baldehyde.
Keywords: 2-chloro-N-(w-arylalkyl)indole-3-carbal-
dehydes; 2-chloro-N-(w-arylalkyl)pyrrole-3-carbal-
cal cyclizations mediated by the indiumACTHNUGRTENUNG(III) chloride-
dehydes; cross-coupling; indolo
N
sodium borohydride system[11] or by samarium(II)
iodide (SmI2)-HMPA in THF.[12] Nevertheless, the
simplest and most environmentally friendly protocol
has to be the generation of the s-aryl or heteroaryl
radical by UV-induced homolytic cleavage of an aryl
halide with subsequent intramolecular substitution
pines; indolo[2,1-a]isoquinolines; photocyclization
ACHTUNGTRENNUNG
The construction of aryl-aryl bonds has attracted onto a nearby aromatic ring. Recently Albini reported
much attention in the past decades because biaryls the metal-free intermolecular photocoupling reactions
are common structural components of many pharma- of amino- or hydroxy-substituted aryl halides with
ceutically and biologically active compounds.[1] Aryl- polyalkylbenzenes.[13] In comparison, the intramolecu-
aryl bonds could be formed through a large variety of lar photocoupling reactions which are used to con-
palladium- or nickel-catalyzed cross-coupling reac- struct polycyclic systems have been studied more ex-
tions. These reactions involve the inter- and intramo- tensively[14]. It is found, however, that the substrates
lecular coupling of a nucleophilic arylated organoele- used in the synthesis of polycycles are o-haloaryl-sub-
ment reagents [boron (Suzuki),[2] magnesium stituted arenes, few haloheteroaryl-substituted sub-
(Kumada),[3] silicon (Hiyama),[4] tin (Stille),[5] or zinc strates were considered in this synthetic strategy.[15]
(Negishi)[6]] with an electrophilic reagent such as aryl Therefore we are interested in the photocoupling re-
halides or triflates. Such versatile cross-coupling reac- actions of 1-aza-2-chloroarene-3-carbaldehydes with
tions have been highly successful in synthetic chemis- regard to the synthetic method, the reaction mecha-
try, but some limitations remain. Thus, activating nism and a few of works have been carried out re-
groups must be present on both arene coupling part- cently.[16] As a new progress in this program, we
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Shen-Ci Lu et al.
Table 1. Photocyclization of 1a in different solvents.[a]
COMMUNICATIONS
report here a highly efficient synthesis of indoloACTHUNTGRNEUNG[2,1-
a]isoquinolines and the like (Scheme 1).
We found that the simple irradiation of 2-chloro-N-
(w-arylalkyl)-3-acylindoles 1 in acetone could afford
Entry Solvent
Time Conversion Yield
[h]
[%][b]
[%][c]
1
2
3
4
5
MeCN
AcOEt
CH2Cl2
CH2Cl2 +10%Me2CO 14
Me2CO+0.1N aque-
16
16
16
80
82
86
92
99
84
84
86
92
95
Scheme 1. Photocyclization of 2-chloro-N-(w-arylalkyl)-3-
acylindoles in acetone.
8
ous Na2CO3
[a]
Compound 1a (0.283 g, 1 mmol) was dissolved in differ-
ent solvents (2ꢂ50 mL). The solution was irradiated at
tetracyclic [2,1-a] fused heterocycles 2 in excellent
yields (Scheme 1). Whereas the Bu3SnH-mediated
aryl radical cyclization of similar precursors was often
accompanied with halogen-reduced uncyclized pro-
ducts.[7b]
lꢀ300 nm with
a medium-pressure mercury lamp
(500 W) under an argon atmosphere at ambient tempera-
ture.
[b]
[c]
Conversion was calculated on the basis of 1a.
Yield of isolated product based on consumed 1a.
Indolo
azepines are well known for their potent biological
and pharmacological activities.[17] Different methods
ACHTUNGTRENNUNG[2,1-a]isoquinolines and indoloAHCTUNGTRNEN[UGN 2,1-a][2]benz-
ACHTUNGTRENNUNG
It is noticeable that the photoreactions of substrates
for the synthesis of these compounds have been re- 1a–e bearing a phenylalkyl or a phenylalkenyl group
ported. The usual methods for the synthesis of indolo- all afforded the corresponding cyclization products
ACHTUNGTRENNUNG[2,1-a]isoquinolines are ring closure of the indole ring 3a–e in excellent yields (Table 2, entries 1–5), and no
of functionallized isoquinolines, for example, nickel- intramolecular indolyl-alkenyl coupling products were
mediated intramolecular coupling of tetrahydroisoqui- detected. Photoreactions of substrates 1f and 1j with
noline with aryl chlorides,[18] Bu3SnH-mediated intra- an electron-withdrawing group on the phenyl group
molecular coupling of dihydroisoquinolines aryl bro- did not give positive results (entries 6 and 10). In con-
mides,[7a] and ring closure of the isoquinoline ring of trast, an electron-donating group on the phenyl group
functionallized indoles, for example, the norbornene- in 1g, 1h and 1i could lead to an increase in both the
mediated palladium-catalyzed tandem alkylation and yields of the products and the rate of the photoreac-
arylation of N-(2-bromoethyl)indoles,[19] and Bu3SnH- tions (entries 7–9). However, an attempt to extend
mediated intramolecular coupling of 2-bromo-N-(w- the range of substrates to 1-benzyl-2-chloroindole-3-
arylalkyl)indole-3-carbaldehydes.[7b] In comparison carbaldehyde was unsuccessful (Table 2, entry 11),
with these methods, the advantages of our new syn- which was consistent with that observed in the
thesis over the Bu3SnH-mediated cyclization of 1-(w- Bu3SnH-mediated cyclization of 1-benzyl-2-bromoin-
arylalkyl)-2-bromoindoles-3-carbaldehydes are not dole-3-carbaldehyde.[7b] With 2-chloro-N-(w-arylalk-
only the high yields with no reductive products found, yl)-3-acetylindoles 1l–n as substrates (Table 2, en-
but also the simple and environmentally friendly pro- tries 12–14), similar results were obtained. Compara-
tocol.
tively, the rate of photoreaction of 2-chloro-3-methyl-
We first investigated the photoreaction of 2-chloro- 1-phenethylindole (1o) was much slow although the
N-(2-phenylethyl)indole-3-carbaldehyde (1a) in differ- corresponding cyclization product 2o (Table 2,
ent solvents. It was found that the photocyclization re- entry 15) could also be obtained after long irradiation.
action in acetonitrile, ethyl acetate or dichlorome- In fact, no cyclization product 2o was detected after
thane was slow and the addition of acetone could effi- long irradiation of the same substrate 1a in dichloro-
ciently accelerate the reaction and increase the yield methane at lꢀ300 nm. These results indicated that
as shown in Table 1. This result may be ascribed to acetone could catalyze the cyclization reaction and
the photosensitization effect of acetone because the the presence of an acyl group was favorable for the
ET of acetone (ET =326 kJmolÀ1)[20a] is higher than cyclization reaction.
that of indole-3-carbaldehyde (ET =275 kJmolÀ1).[20b]
Therefore acetone was selected as solvent in all pho- 1-phenethylindoles (1a, 1a-2 and 1a-3) were investi-
tocyclization reactions of 1a–o. gated as shown in Table 3. Contrary to expected reac-
The photoreactions of different halogen-substituted
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Table 2. Photocyclization of 2-chloro-N-(w-arylalkyl)-3-acylindoles.[a]
Entry
Substrate
n
Y
R
R2
Time [h]
Conversion [%][b]
Product
Yield [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1
1
1
1
1
1
1
1
1
1
0
1
1
1
1
CH2
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
COCH3
COCH3
COCH3
CH3
H
H
H
H
H
H
H
H
OCH3
NO2
H
H
H
10
10
10
10
12
16
10
10
8
10
16
12
12
10
16
99
86
99
89
96
0
86
94
96
0
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
95
90
94
88
96
0
92
95
96
0
CH2CH2
CHCH3
C=CH2
CH=CH
C=O
CONCH3
CH2O
CH2O
CH2O
CH2
9
10
11
12
13
14
15
0
0
CH2
90
84
93
34
87
84
95
91
CH2CH2
CH2O
CH2
H
H
[a]
Compound 1a–o (1.0 mmol) was dissolved in acetone (2ꢂ50 mL) containing 0.1N aqueous Na2CO3 (2 mL). The solution
was irradiated at lꢀ300 nm with a medium-pressure mercury lamp
ACHTUNGTREN(NUNG 500 W) under an argon atmosphere at ambient tem-
perature.
[b]
[c]
Conversion was calculated on the basis of the substrate.
Yield of isolated product based on the consumed substrate.
Table 3. Photoreactions of different halogen-substituted-N-phenethylindole-3-carbaldehydes.[a]
Entry
Substrate
Solvent
X
Y
Time [h]
Conversion [%][b]
Product
Yield [%][c]
2a
2a-1
1
2
3
1a
Me2CO
Me2CO
Me2CO
CH2Cl2 +50%C6H12
CH2Cl2 +50%C6H12
CH2Cl2 +50%C6H12
Cl
Br
I
Cl
Br
I
CH2
CH2
CH2
CH2
CH2
CH2
10
16
16
16
16
16
99
82
62
80
58
45
2a
2a
2a
2a
2a+2a-1
2a+2a-1
95
92
90
83
70
44
1a-2
1a-3
1a
1a-2
1a-3
4[d]
5[d]
6[d]
trace
5
23
[a]
Substrate (1.0 mmol) was dissolved in acetone (2ꢂ50 mL) containing 0.1N aqueous Na2CO3 (2 mL). The solution was ir-
radiated at lꢀ300 nm with a medium-pressure mercury lamp (500 W) under an argon atmosphere at ambient tempera-
ture.
Conversion was calculated on the basis of the substrate.
Yield of isolated product based on the consumed substrate.
Substrate (1.0 mmol) was dissolved in dichloromethane (2ꢂ25 mL) and cyclohexane (2ꢂ25 mL). The solution was irradi-
ated at lꢀ300 nm with a medium-pressure mercury lamp (500 W) under an argon atmosphere at ambient temperature.
[b]
[c]
[d]
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Shen-Ci Lu et al.
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Scheme 2. A plausible photoreaction mechanism of 2-halogen-N-(2-phenylethyl) indole-3-carbaldehydes in acetone.
À
tivity of 1a-3>1a-2>1a based in a simple C X bond zenes are iodobenzene>bromobenzene>chloroben-
homolysis mechanism (the C X bond dissociation en- zene,[24] which is also contrary to our observations.
À
À
À
À
ergies DCÀX for C Cl, C Br and C I are 339, 289, The photoreactions of 1a, 1a-2 and 1a-3 in cyclohex-
222 kJmolÀ1, respectively[21]), the observed reactivity ane were also examined to detect if the free indolyl
is 1a>1a-2>1a-3 in acetone because the time needed radical could be formed (Table 3, entries 4–6). The
to complete the photoreactions of 1a, 1a-2 and 1a-3 great amount of reductive deiodination product 2a-1
increased greatly. Another unmatchable point to could be separated besides the cyclization product 2a
these results is the fact that the excited energy of 1a in the photoreaction of 1a-3. Compound 2a-1 was ob-
(ET =275 kJmolÀ1)[20b] is insufficient to cleave C Cl viously derived from the H-abstraction of a free in-
À
À
and C Br bonds effectively. These results rule out a dolyl radical from the solvent. In comparison, 1a and
simple homolysis mechanism for 1a and 1a-2 and 1a-2 gave the photocyclization product 2a as the main
show the presence of some factor which provides a product. These results further confirmed the radical
much lower energy path for cyclization. We believe complexation mechanism for the photocyclization of
this factor to be a type of anchimeric assistance 1a and 1a-2.
whereby the phenyl ring can, through its p-cloud,
In order to expand the scope of this photocycliza-
complex the developing radical centres in the stretch- tion reaction, we further investigated the photoreac-
À
ing C X bond and thus lower the transition state tions of 2-chloro-N-(w-arylalkyl)pyrrole-3-carbalde-
energy for reaction as depicted in Scheme 2. Chlorine hydes in acetone. As shown in Table 4, the results of
and bromine atoms are electron-deficient species with the photoreactions were very similar to those of 2-
high electron affinities[22] and their complexation by chloro-N-(w-arylalkyl)indole-3-carbaldehydes.
All
1
aromatic molecules is well known and the anchimeric products were fully identified by H, 13C NMR and
À
assistance to C X bond cleavage by radical complexa- MS, and the structure of 4a was further confirmed by
tion has also proved by dynamic methods[23a] and flash X-ray crystallography (Figure 1).[25]
photolysis studies.[23b,c] The mechanism affords an ex-
planation why the chloro-compound cyclizes faster
In summary, highly efficient syntheses of indolo-
[2,1-a]isoquinolines, indolo[2,1-a][2]benzazepines,
pyrrolo[2,1-a]isoquinolines and pyrrolo[1,2-a]benzaze-
U
ACHTUNGTRENNUNG
than the bromo-compound even though DCÀCl
>
N
ACHTUNGTRENNUNG
DCÀBr, since the transition state 5 is reached earlier pines have been developed. To the best of our knowl-
along the reaction coordinate for X=C1 than for X= edge, this is the first report for the synthesis in excel-
Br due to the higher electron affinity of the chlorine lent yields of these compounds via photoinduced de-
atom. Photocyclization of 1a-3 may probably proceed halogenation and cyclization of 2-halo-N-(w-arylalk-
À
via homolysis of the C I bond to give a free indolyl
ACHTUNGTRENyNUGN l)indole-3-carbaldehydes and 2-chloro-N-[3-(w-aryl-
radical and an iodine atom and subsequent coupling
ACHTUNGTRENaNUNG lkyl)]pyrrole-3-carbaldehydes in acetone. The
of the indolyl radical with an aryl group. The electron advantages of this reaction over the Bu3SnH-mediat-
transfer mechanism for photocyclizations of 1a, 1a-2 ed cyclization of 1-(w-phenylalkyl)-2-bromoindoles-3-
and 1a-3 can also be ruled out because the reported carbaldehydes are not only the high yields with no re-
dissociation rates for radical-anions from haloben- ductive products formed, but also the simple and en-
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Table 4. Photocyclization of 2-chloro-N-(w-arylalkyl)pyrroles-3-carbaldehydes.[a]
Entry
Substrate
Y
R2
Time [h]
Conversion [%][b]
Product
Yield [%][c]
1
2
3
4
3a
3b
3c
3d
CH2
H
H
H
OCH3
6
8
8
6
99
90
95
97
4a
4b
4c
4d
95
91
93
94
CH2CH2
CH2O
CH2O
[a]
Compound 3a–d (1.0 mmol) was dissolved in acetone (2ꢂ50 mL) containing 0.1N aqueous Na2CO3 (2 mL). The solution
was irradiated at lꢀ300 nm with a medium-pressure mercury lamp (500 W) under an argon atmosphere at ambient tem-
perature.
Conversion was calculated on the basis of the substrate.
Yield of isolated product based on the consumed substrate.
[b]
[c]
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant No. 20872056 and J0730425) for financial support.
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Adv. Synth. Catal. 2009, 351, 2839 – 2844