Hafnium triflate as an efficient catalyst for direct Friedel-Crafts reactions of chromene hemiacetals

Article abstract of DOI:10.1002/adsc.201000930

The direct Friedel-Crafts reaction of chromene hemiacetals with indoles, furans and sterically hindered anilines has been accomplished with high selectivity and excellent yields in the presence of a catalytic amount of hafnium triflate [Hf(OTf)₄

Full text of DOI:10.1002/adsc.201000930

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DOI: 10.1002/adsc.201000930
Hafnium Triflate as an Efficient Catalyst for Direct Friedel–Crafts
Reactions of Chromene Hemiacetals
Yan-Chao Wu,a,* Hui-Jing Li,a Li Liu,a Nicolas Demoulin,b Zhe Liu,a Dong Wang,a
and Yong-Jun Chena,*
a
Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory for Molecular Recognition and
Function; Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Peopleꢀs Republic of China
Fax : (+86) 10-6255-4449; e-mail: ycwu@iccas.ac.cn or yjchen@iccas.ac.cn
Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
b
Received: December 9, 2011; Revised: January 31, 2011; Published online: April 12, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000930.
Abstract: The direct Friedel–Crafts reaction of
chromene hemiacetals with indoles, furans and ster-
ically hindered anilines has been accomplished with
high selectivity and excellent yields in the presence
changes of the characteristic photochemical and pho-
tophysical properties in response to external optical,
chemical and thermal stimulation.[4] The multistate/
multifunctional chemical system existing in chromene
hemiacetals makes their direct derivatization a con-
siderable synthetic challenge. In contrast to chromene
hemiacetals, the derivatization of oligosaccharide
hemiacetals has been extensively studied due to the
growing importance of synthetic oligosaccharides in
glycobiology.[5] Traditionally, a glycosidic coupling
usually consists of functionalization of an anomeric
hydroxy group of an oligosaccharide hemiacetal to a
latent leaving group (LG), and subsequently activa-
tion and coupling of the resulting intermediate glyco-
syl donor with a nucleophilic glycosyl accepter (Nu-
H) in the presence of various catalysts or promoters.[6]
Recently, it has been shown that the two steps above
can be accomplished in one-pot by a dehydrative cou-
pling strategy, in which the anomeric hydroxy group
can be employed directly as the glycosyl donor by the
assistance of a suitable oxosulfonium agent.[7] The
of a catalytic amount of hafnium triflate [HfACTHUNRGTNEUNG(OTf)4,
0.1 mol%, 0–408C]. The mild conditions tolerate
various sensitive functional and protecting groups,
and the products were confirmed unambiguously
from their spectra and by single-crystal X-ray analy-
sis. This direct Friedel–Crafts reaction of chromene
hemiacetals should inspire and encourage the con-
sideration of hafnium triflate in the development of
mild reaction conditions for the efficient derivatiza-
tion of hemiacetal-containing compounds.
Keywords: chromenes; Friedel–Crafts reaction; haf-
nium; hemiacetals; selectivity
Intramolecular cyclic hemiacetals have been a subject direct derivatization of oligosaccharide hemiacetals
of consistent interest due to the presence of their served to stimulate our interest in the direct derivati-
structural motifs in a large number of natural prod- zation of chromene hemiacetals, which are relatively
ucts such as chromenylium compounds and oligosac- less studied in comparison with oligosaccharide hemi-
charides, which include glucosamines, glucoses, chitin acetals. Herein, we would like to report that HfACHTUNGTRENNUNG(OTf)4
and chitosan oligosaccharides, etc. As members of the is a highly efficient catalyst for the direct Friedel–
positive species of chromenylium compounds, antho- Crafts reaction of chromene hemiacetals under mild
cyanins have long been considered as ubiquitous pig- conditions.
ments responsible for the various colors in fruits and
flowers,[1] effective food colorants[2] and pharmaceuti- hydroisoquinoline alkaloids,[8] we observed that hafni-
cals.[3] Moreover, chromenylium compounds have at- um triflate [Hf
(OTf)4] is an efficient catalyst for trans-
During our syntheses of antitumor antibiotic tetra-
AHCTUNGTRENNUNG
tracted increasing attention in molecular information forming the labile hemiaminal to an amino thioether
processing, optical memories and logic gates due to moiety, a key intermediates in our total syntheses of
the reversible structural transformations among chro- (À)-quinocarcin[8a] and lemonomycin amide.[8b] How-
mene hemiacetal forms, ring-opened forms and chro- ever, a large variety of Brønsted acids and Lewis
menylium salt forms that accompanied significant acids led either to no reaction, or, if forcing condi-
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Yan-Chao Wu et al.
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Table 1. Survey of conditions for the direct Friedel–Crafts
reaction of chromene hemiacetal 1a with indole (2a).[a]
and NiCl2 displayed some efficiency for this reaction,
which afforded Friedel–Crafts adduct 3a in low to
moderate yields under the same conditions (entries 6–
8). When stoichiometric amounts of CuCl2, ZnCl2,
TiCl3, NbCl5 and CrCl3 were used, treatment of 1a
with 2a in dichloromethane at room temperature for
10 h afforded 3a in high yields (entries 9–13). Howev-
er, the use of lower loadings of these Lewis acids dra-
matically diminished the efficiency (entries 9–13). The
reaction was very complex when 10 mol% AlCl3 was
used as the catalyst (entry 14). Instead, the direct
Friedel–Crafts reactions of 1a with 2a could be ac-
complished in high yields in the presence of 10 mol%
Entry
Catalysts/Promoters
0.1 mol% Hf(OTf)4
Time
Yield
1
2
3
4
5
6
7
8
G
0.5 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
10 h
10 h
10 h
10 h
10 h
0.5 h
4 h
99%
0
0
0
100 mol% LiCl
100 mol% NaCl
100 mol% CaCl2
100 mol% MnCl2
100 mol% MgSO4
100 mol% CoCl2
100 mol% NiCl2
100 mol% CuCl2
100 mol% ZnCl2
100 mol% TiCl3
100 mol% NbCl5
100 mol% CrCl3
10 mol% AlCl3
10 mol% FeCl3
of FeCl3 and InACTHNURGTNEUNG(OTf)3 under the same conditions
0
41%
60%
30%
within 4 h (entries 15 and 16). Unsurprisingly, de-
creasing yields were observed when the catalyst load-
ings were decreased from 10 mol% to 1 mol% (i.e.,
93–95% versus 24–60%, entries 15 and 16). Although
9
97% (43%)[b]
99% (45%)[b]
99% (62%)[b]
99% (60%)[b]
99% (51%)[b]
10
11
12
13
14
15
16
17
18
the reaction went smoothly in the presence of
[11]
1 mol% of YbCAHTGNURTEN(NNUG OTf)3 and ScACHTNURTGEN(NGUN OTf)3, HfCAHTNUGTNER(NUGN OTf)4 was
the best catalyst for this reaction when the catalyst
loadings were decreased to 0.1 mol% (entry 1 and en-
tries 17 and 18). These results are in good agreement
[c]
–
93% (24%)[d]
95% (60%)[d]
98% (73%)[e]
99% (81%)[e]
with Ishiiꢀs observation that HfACTHNUTRGNENUG(OTf)4, superior to the
10 mol% In
1 mol% Yb(OTf)3
1 mol% Sc(OTf)3
A
4 h
1 h
1 h
N
other selected Lewis acids and Brønsted acids, was
the most active catalyst for the activation of the hy-
droxy groups of various benzyl alcohols.[12,13] The
highly effective hydroxy group activation of HfACHTUNGTRENNUNG(OTf)4
would be of significant importance for the derivatiza-
tion of hemiacetals with inert nucleophiles as hemi-
C
[a]
General conditions: 1a (1.0 equiv.) and 2a (1.1 equiv.) in
CH2Cl2 (c=1.0M) at room temperature.
The yield in brackets corresponds to 10 mol% catalyst
[b]
loading.
Complex.
[c]
[d]
AHCTUNGTRENNUNG
The yield in brackets corresponds to 1 mol% catalyst
loading.
AHCTUNRTGEG(NUNN OTf)4 has the potential to
[e]
The yield in brackets corresponds to 0.1 mol% catalyst
loading.
be used as an efficient catalyst for the activation of
some hydroxy groups under mild conditions. During
our thioacetalization and transthioacetalization reac-
tions were used, to destruction of the molecules. The tions of various carbonyl compounds,[13g] we found
specific hydroxyl-philicity of HfACHTUNTGRENNUG(OTf)4 led us to hy- that the reaction conditions [0.1 mol% HfACHTUNGTRENNUNG(OTf)4,
pothesize that it could potentially act as an efficient CH2Cl2, room temperature] tolerate a large variety of
catalyst for the direct derivatization of chromene functional and protecting groups such as nitro, cyano,
hemiacetals.[9,10] As expected, simply stirring a di- hydroxy of phenols, amino, benzyloxycarbonyl (Cbz),
chloromethane solution of chromene hemiacetal 1a allyloxycarbonyl (Alloc) and benzyloxyl. Moreover,
and indole (2a) at room temperature in the presence tert-butyldimethylsilyl (TBS) and tert-butylcarbonyl
of 0.1 mol% of Hf
Friedel–Crafts adduct 3a in 99% yield (Table 1, found to be tolerated under these conditions.[13g] The
entry 1). This reaction, completed within 0.5 hour, is results suggests that 0.1 mol% of Hf(OTf)4 might be
extremely easy to perform without the need of using used as a practical catalyst for the direct Friedel–
ACHTUNGERTN(NUNG OTf)4 afforded the corresponding (Boc), Brønsted acid-sensitive groups, were also
AHCTUNGTRENNUNG
anhydrous solvents and an inert atmosphere.
Crafts reaction of chromene hemiacetals.
Many other conditions were also screened for com-
With the optimized reaction conditions in hand, the
parison, and representative results are shown in scope of the reaction with respect to chromene hemi-
Table 1. LiCl, NaCl, CaCl2 and MnCl2 were not effec- acetals 1 and various nucleophiles 2 was subsequently
tive promoters for the Friedel–Crafts reaction of chro- investigated (Table 2). Reaction of chromene hemi-
mene hemiacetal 1a with indole (2a). Treatment of 1a
ACHTUNGTRENaNUGN cetal 1a with non-highly-deactivated indoles 2a–c af-
and 2a in dichloromethane at room temperature for forded Friedel–Crafts adducts 3a–c in 97–99% yields
24 h in the presence of these salts did not generate within less than 1 hour (Table 1, entries 1–3). Highly
any products and the starting materials were recov- deactivated indoles 2c–f reacted smoothly with 1a to
ered (Table 1, entries 1–5). Otherwise, MgSO4, CoCl2 generate the corresponding products 3d–f in high
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Adv. Synth. Catal. 2011, 353, 907 – 912

Hafnium Triflate as an Efficient Catalyst for Direct Friedel–Crafts Reactions
(OTf)4-catalyzed direct Friedel–Crafts of chromene hemiacetals 1 with nucleophiles 2.[a]
Table 2. Hf
ACHTUNGTRENNUNG
[a]
[b]
General conditions: 1 (1.0 equiv.), 2 (1.1 equiv.) and Hf
ACHTUNGTRENNUNG
[c]
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909

Yan-Chao Wu et al.
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yields under the standard conditions, albeit with a light, the direct Friedel–Crafts reaction of chromene
longer reaction time (entries 4–6). Chromene hemi- hemiacetals with indoles, furans and sterically hin-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
conditions to afford the Friedel–Crafts products 3g–i the potential by-products were not detected in these
in 89–98% yields (entries 7–9). There was an obvious reactions. The structures of Friedel–Crafts products
decrease in yields with 2-methylindole (2g) in compar- 3a–t were confirmed from their spectra and supported
ison to indole (2a), reflecting the steric factor effect by the X-ray crystallographic analysis of 3a (see Sup-
on the Friedel–Crafts reaction (i.e., 88% versus 99%, porting Information).[18]
entries 1 and 10). Flavylium hemiacetal 1e,[9] one of
Cytotoxicity tests (in vitro) indicated that com-
labile chromene hemiacetals,[1–4] reacted uneventfully pound 3t exhibits selective antibiotic activity against a
with indole (2a) to afford Friedel–Crafts product 3k human breast cancer cell line MCF7 (IC50 =0.8 mM),
in 79% yield under the standard conditions, albeit at which would widen the structural diversity of this an-
a relatively lower reaction temperature (entries 1–11). titumor target and confirm the perspectives of further
Similarly, by treatment of the simple chromene acetal investigations in this area.
1f[11] with 2a under the above conditions (entry 11),
In summary, HfACTHUNGTERN(NUG OTf)4 is an excellent catalyst for
compound 3l was obtained in good yield (entry 12).[14] the direct Friedel–Crafts reaction of labile chromene
The treatment of N-methylindole (2h) with 1a afford- hemiacetals with indoles, furans and sterically hin-
ed Friedel–Crafts product 3m in 99% yield within dered anilines. The reaction takes place at room tem-
0.5 hour under the standard conditions, indicating that perature and displays high selectivity and high effi-
À
the N H functionality is not a requisite in these reac- ciency. This is of significant importance as chromene
tions (entries 1–13). Unsurprisingly, N-Bs-indole (2i) hemiacetals are usually unstable under harsh reaction
reacted smoothly with 1a to give Friedel–Crafts conditions. The HfACTHNUGTRENUNG(OTf)4 catalytic system tolerates
adduct 3n in 84% yield under the standard conditions, various sensitive functional and protecting groups,
albeit at a slightly higher reaction temperature and thus we are exploiting this catalytic system for
(entry 14). The reaction could proceed successfully at the mild derivatization of different hydroxy function-
the 2-position of indoles when the 3-position possess- alities along with their applications in the syntheses of
es a substituent. For example, 3-cyanomethylindole natural products as well as functional compounds.
(2j) reacted smoothly with 1a to give Friedel–Crafts
adduct 3o in 81% yield under the standard conditions,
albeit at a slightly higher reaction temperature Experimental Section
(entry 15). 2-Methylfuran (2k) was also reacted with
chromene hemiacetals (1a–c) to afford the corre- Procedure for the HfACHTNUTRGNEUNG(OTf)4-Catalyzed Direct
sponding Friedel–Crafts adducts 3p–r in high yields Friedel–Crafts of Chromene Hemiacetals
with a longer reaction time (entries 16–18). In con-
trast to 2-methylfuran, sterically hindered anilines 2l
and 2m could react with chromene hemiacetal 1a to
afford the desired products 3s and 3t in excellent
yields within just 0.5 hour (entries 19 and 20), indicat-
ing that there is an electronic effect on the Friedel–
Crafts reaction.
Potentially, indoles and anilines might react with
chromene hemiacetals through amination or Friedel–
Crafts reaction, and a double Friedel–Crafts reaction
of the hemiacetal functionalities might be also possi-
ble. Moreover, the nucleophilic attack on the allylic
hydroxy units, known as Michael acceptor units, of
chromene hemiacetals might take place at the a- and
g-carbons. It is noteworthy that direct Friedel–Crafts
reactions of allylic and alkynylic alcohols are ideal
chemical transformations due to the corresponding
green, economic and environmentally benign proper-
ties and that water iss generated as the only side
product. This is attracting much attention in the syn-
thetic community, and various Lewis acids,[15] Brønst-
ed acids[16] and organocatalysts[17] have been explored
A solution of chromene hemiacetal 1a (338.4 mg, 1.0 mmol),
indole (2a, 128.9 mg, 1.1 mmol), and HfACTHNURTGNENG(U OTf)4 (0.8 mg,
0.001 mmol) in CH2Cl2 (1 mL) was stirred at room tempera-
ture for 0.5 hour. The reaction mixture was filtered through
a short pad of Celite, and the filtrate was concentrated
under reduced pressure. The residue was purified by flash
column chromatography on silica gel to afford Friedel–
Crafts product 3a as a white solid; yield: 433.2 mg (99%);
mp 244–2458C; 1H NMR (300 MHz, CDCl3): d=8.14 (s,
1H), 7.98 (t, J=4.5 Hz, 1H), 7.50–7.40 (m, 5H), 7.28–7.14
(m, 5H), 7.09 (d, J=2.1 Hz, 1H), 7.00 (d, J=8.4 Hz, 1H),
6.34 (s, 1H), 3.77 (s, 3H), 1.18 (s, 9H); 13C NMR (75 MHz,
CDCl3): d=171.7, 150.6, 144.0, 138.2, 137.9, 136.6, 128.8,
128.5, 128.2, 126.8, 125.1, 124.2, 122.9, 122.5, 121.7, 121.1,
120.7, 120.3, 116.6, 115.0, 111.4, 78.8, 53.0, 34.2, 31.4; FT-IR
(KBr): n=3315, 2957, 1736, 1251, 743 cmÀ1; anal. calcd. for
C29H27NO3: C 79.61, H 6.22, N 3.20; found: C 79.57, H 6.49,
N 3.23.
Acknowledgements
We thank the National Natural Science Foundation of China,
for such direct Friedel–Crafts reactions. To our de- Ministry of Science and Technology (No. 2009ZX09501-006)
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Hafnium Triflate as an Efficient Catalyst for Direct Friedel–Crafts Reactions
and the Chinese Academy of Sciences for the financial sup-
port
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graphic data of 3a for this paper. These data can be ob-
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Adv. Synth. Catal. 2011, 353, 907 – 912