Iron-catalyzed regioselective hydroaryloxylation of C≡C Triple Bonds: An efficient synthesis of 2H-1-benzopyran derivatives

Article abstract of DOI:10.1002/adsc.200900483

An efficient, regioselective, iron-catalyzed intramolecular hydroaryloxylation of 2-propargylphenols or naphthols is reported. The reactions proceed through an endo-dig cyclization to afford benzopyran or naphthopyran derivatives in good to high yields us

Full text of DOI:10.1002/adsc.200900483

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DOI: 10.1002/adsc.200900483
ꢀ
Iron-Catalyzed Regioselective Hydroaryloxylation of C C Triple
Bonds: An Efficient Synthesis of 2H-1-Benzopyran Derivatives
Xiaobing Xu,a Jun Liu,a Linfeng Liang,a Hongfeng Li,a and Yanzhong Lia,*
a
Institute of Medicinal Chemistry, Department of Chemistry, East China Normal University, 3663 North Zhongshan Road,
Shanghai 200062, Peopleꢀs Republic of China
Fax : (+86)-021-6223-3969; e-mail: yzli@chem.ecnu.edu.cn
Received: July 11, 2009; Revised: September 30, 2009; Published online: October 28, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900483.
Abstract: An efficient, regioselective, iron-catalyzed
intramolecular hydroaryloxylation of 2-propargyl-
phenols or naphthols is reported. The reactions pro-
ceed through an endo-dig cyclization to afford ben-
zopyran or naphthopyran derivatives in good to
high yields using ironACTHNUTRGNEU(GN III) chloride as the catalyst
with the assistance of aniline in dimethylformamide
(DMF).
ly friendly and less expensive catalysts to accomplish
the carbon-heteroatom bond forming process is highly
À
À
desired. As a result, iron-catalyzed C C and C X
bond formation has attracted considerable attention,
because iron compounds are usually less toxic, low
priced and easy to synthesize. There have been a lot
À
of reports concerning the Fe-catalyzed C C forming
reactions with the assistance of Grignard reagents.[6,7]
À
À
In the past few years, Fe-catalyzed C C and C X
bond forming reactions without the utilization of
Grignard reagents have also appeared.[8,9] In the
course of our ongoing study on the development of
new, transition metal-catalyzed, selective heterocycle
forming protocols,[10] we found that iron-catalyzed
benzopyran ring formation was accomplished with
Keywords: benzopyrans; hydroaryloxylation; iron
catalysis; naphthopyrans; regioselective cyclization
The transition metal-catalyzed or -mediated intramo- high regioselectivity with the assistance of amines.
À
À
lecular O H addition across unsaturated C C bonds Herein we would like to present our results of the
represents one of the most effective and atom-eco- iron-catalyzed regioselective synthesis of 2H-1-benzo-
nomical routes to oxygen-containing heterocycles.[1] pyran derivatives by the intramolecular hydroaryloxy-
These catalysts include lanthanide,[1a–c] Pt,[1d,i] Ru,[1p] lation of C C triple bonds.
ꢀ
Au,[1d,g,m,r] Ag,[1k,n] Cu[1e] etc. Oxygen-containing heter-
ocyclic moieties such as benzopyran rings represent
For intramolecular 2-propargylphenol hydro-
ACHUTNGERNaNUG ryloxyACHTUTGNRENlNUGN ations, there exist two possible pathways,
key structural units of a variety of natural compounds namely exo-dig or endo-dig cyclization to five- or six-
with interesting biological activities.[2] Different syn- membered rings according to Baldwinꢀs rules[11]
thetic approaches to these compounds were reported (Scheme 1). Generally, metal-mediated hydroaryloxy-
in the literature, such as metal-catalyzed intramolecu- lation facilitates an exo-attack to give five-membered
lar ring closure,[1,3] acid-mediated condensation[4] and rings[1a–e,12] there is only one example of the six-mem-
others.[5] Due to the drawbacks of the reported meth- bered ring formation catalyzed by Ru.[1p] It is there-
ods, for example, low yields, requirement of a large fore attractive to develop a metal-catalyzed, regiose-
excess amount of one of the reagents or high catalyst lective hydroaryloxylation for six-membered ring for-
costs, the development of sustainable, environmental- mation.
Scheme 1.
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We began our investigation with 1-(1,3-diphenyl- entry 1). Addition of 2.0 equiv. of Na2CO3 resulted in
prop-2-ynyl)naphthalen-2-ol (1a), which was synthe- the formation of the exo-dig cyclizaiton product 3a in
sized according to a modified literature method.[12b] 70% yield, 2a was not detected (Table 1, entry 2). It is
Treatment of 1a with Fe catalyst and additives was se- interesting to note when 2.0 equiv. of triethylamine
lected as the prototypical case to screen the experi- were used, both 5- and 6-membered ring compounds
mental conditions. The results are summarized in were produced in 63% yield with a ratio 1:1(Table 1,
Table 1. We first carried out the reaction using FeCl3 entry 3). Changing the amine to N-methylbenzen-
(10 mol%) as the catalyst without any additives in
ACHTUNGTRENaNUNG mine furnished the desired naphthopyran 2a in 36%
DMF at 1358C, the endo-dig cyclization product 2a yield without the detection of 3a (Table 1, entry 4).
was obtained in 15% as the sole product (Table 1, The use of aniline gave rise to 2a as the only product
Table 1. Optimization of reaction conditions for regioselective formation of 2a.
[a]
Isolated yields.
2a:3a=1:1.
2a:3a=4:5.
The reaction mixture was complicated.
[b]
[c]
[d]
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Iron-Catalyzed Regioselective Hydroaryloxylation of C C Triple Bonds
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in 43% yield (Table 1, entry 5). Increasing FeCl3 to OMe) aryl group gave rise to the corresponding 2e in
20 mol% resulted in the formation of 2a in 52% yield 64% yield, the regioselectivity of naphthopyran to
(Table 1, entry 6). The structure of 2a was unequivo- naphthofuran was 87:13 (Table 2, entry 5). When 1k
cally confirmed by X-ray crystal analysis (Figure 1), with a similar electron-donating (m-OMe) aryl group
was employed, the corresponding naphthopyran was
produced in 56% yield with a better regioselectivity
of 95:5 (Table 2, entry 11). The naphthol bearing a (p-
Me) aryl group 1f furnished the desired naphthopyran
2f in 72% isolated yield (Table 2, entry 6). It seems
that naphthols with an electron-withdrawing group at
the propargylic position gave better yields in the
endo-dig cyclization products.
The regioselective effect of the substituents on the
triple bond was also investigated. Naphthols with an
electron-withdrawing (p-Cl) aryl group on the triple
bond, e.g., 1d, gave the endo-dig compound 2d in
60% yield as the sole product (Table 2, entry 4), 1h
with an electron-donating (p-OMe) aryl group gave
rise to the corresponding 2h in lower yield and selecti-
vity(Table 2, entry 8).
The substrate scope of the reaction could be ex-
tended to various phenols, the desired benzopyrans
were obtained in good to high yields with high regio-
selectivity. When the propargylphenol with an elec-
Figure 1. The X-ray crystal structure of 2a.
tron-donating (p-OMe) aryl group at the propargylic
position 1c was used, the reaction was completed
cleanly to afford 2c in 72% yield (Table 2, entry 3).
which clearly shows the six-membered pyran ring. Dimethylarylphenol 1i furnished 2i in 67% yield
One equivalent or 20 mol% of aniline gave the simi- (Table 2, entry 9). Interestingly, the diynyldiol 1j gave
lar results as 2.0 equiv. (Table 1, entries 9 and 10). the corresponding tricyclic compound 2j as two dia-
Other solvents such as DMA, DMSO, 1,4-dioxane or stereoisomers in good yield (Table 2, entry 10).
toluene failed to accomplish the hydroaryloxylation
In order to further probe the scope of this process,
reaction or gave lower yields of the naphthopyran, re- we also examined the alkyl-substituted phenols and
spectively (Table 1, entries 13–16). A control reaction less substituted substrates under the optimized reac-
in the absence of FeCl3 led to the formation of both tion conditions, the results are shown in Table 3.
5- and 6-membered rings (Table 1, entry 8). Switching When a propargylnaphthol with a butyl group at the
to butan-1-amine afforded the 5-memberd ring com- triple bond, 1m, was used, only the desired six-mem-
pound as the sole product (Table 1, entries 17–19). It bered ring was detected after 24 h, although the yield
is interesting to note that the addition of aniline or was low (ca.10%). This could be improved by using a
butylamine resulted in completely different products, higher catalyst loading (100 mol%), however, the se-
and the optimized reaction conditions for 6-mem- lectivity was decreased to 3:1 (Table 3, entry 1). When
bered ring formation were to use 20 mol% FeCl3 as a naphthol with a propyl group at the propargylic po-
the catalyst with the addition of 20 mol% to 2.0 equiv. sition, 1n, was employed, neither 5- nor 6-membered
of aniline in DMF at 1358C.
ring compound could be obtained with most of the
Having established an effective catalytic system for starting material being recovered, increasing FeCl3 to
the hydroalkoxylation reactions, we next synthesized 50 mol% resulted in the formation of only five-mem-
a variety of 2-propargylphenol derivatives to explore bered ring product 3n in low yield (Table 3, entry 2).
the scope of the regioselective cyclization reaction The phenol bearing a terminal alkyne, 1o, gave rise to
under the optimized conditions. Representative re- the production of both benzopyran (2o) and benzofur-
sults are shown in Table 2. The reaction was applica- an (3o) in moderate yields with a ratio 3:1(Table 3,
ble to various propargylnaphthols and phenols. Naph- entry 3). A trimethylsilyl-substituted phenol, 1p, re-
thol 1b with an electron-withdrawing (p-Cl) aryl acted smoothly to produce the respective benzopyran
group at the propargylic position cyclized smoothly to and benzofuran in 23% and 39% yields, respectively
give the naphthopyran 2b in 80% yield (Table 2, (Table 3, entry 4). When a phenol without any sub-
entry 2), the electron-withdrawing (o-Cl) aryl group stituents at the propargylic position, namely 2-(3-phe-
naphthol 1g afforded 2g in 72% yield (Table 2, nylprop-2-ynyl)phenol, was employed, no reaction oc-
entry 7). Whilst 1e with an electron-donating (p- curred after 24 h. It seems that substrates without any
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Table 2. FeCl3-catalyzed regioselective formation of benzopyrans.
[a]
[b]
[c]
[d]
[e]
Isolated yields.
The regioselectivity of the crude mixture was 2:3=91:9.
2:3=87:13.
2:3=95:5.
Products were isolated as a mixture of 2+3.
substituents or with an alkyl group at the propargylic argylphenols to benzopyrans, good to excellent regio-
position have low reactivity and prefer to give the 5- selectivities were achieved when diaryl-substituted
membered ring compound, while substrates with an phenols were employed. The reactions proceed to
aryl group at the propargylic position result in the for- afford the benzopyran or naphthopyran derivatives in
mation of 6-membered rings with good to excellent good to high yields using FeCl3 as the catalyst with
regioselectivity.
the assistance of aniline in DMF. Further applications
In summary, we have reported an efficient, regiose- of this novel iron-catalyzed hydroaryloxylation proce-
lective, intramolecular hydroaryloxylation of 2-prop-
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Iron-Catalyzed Regioselective Hydroaryloxylation of C C Triple Bonds
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Table 3. FeCl3-catalyzed cyclization reactions of alkyl-substituted phenols.
[a]
Isolated yields; unless otherwise noted, reactions were carried out using 20 mol% FeCl3,
2.0 equiv. PhNH2 at 1358C in DMF for 24 h.
100 mol% FeCl3 and 4.0 equiv. PhNH2 were used.
50 mol% FeCl3 were used.
[b]
[c]
dure to extend the scope of synthetic utility of the re- Acknowledgements
action are under progress in our group.
We are grateful to the National Natural Science Foundation
of China (No. 20572025 and 20872037) for financial support.
We also thank the Lab of Organic Functional Molecules, the
Sino-French Institute of ECNU for support.
Experimental Section
Typical Procedure for the Fe-Catalyzed Formation of
1,3-Diphenyl-3H-benzo[f]chromene (2a)
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A 20-mL Schlenk tube was charged with naphthol 1a
(104 mg, 0.5 mmol) and FeCl3 (16 mg, 0.1 mmol). The reac-
tion mixture was heated to 1358C for 5 h and then cooled
down to room temperature. The mixture was quenched with
3M HCl and extracted with dichloromethane (3ꢂ15 mL).
The extract was washed with brine and dried over MgSO4.
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was purified by chromatography on silica gel to give the
white solid product 1,3-diphenyl-3H-benzo[f]chromene (2a);
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Me4Si): d=5.71 (d, J=4.0 Hz, 1H), 5.96 (d, J=4.0 Hz, 1H),
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141.02, 154.03; HR-MS: m/z=334.1362, calcd for C25H18O:
334.1358.
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