Hydrobromic acid-catalyzed Friedel-Crafts type reactions of naphthols

Article abstract of DOI:10.1039/c3ra45898a

Catalyzed by 10 mol% of hydrobromic acid, hydroxyalkylation and allylation-iodocyclization of naphthols proceeded smoothly under mild conditions. In contrast, other Bronsted acids tested, including HCl, HI, HBF₄, HPF₆, TFA, PTSA, H₂SO₄, TfOH etc., are almost ineffective under identical conditions or require harsh reaction conditions.

Full text of DOI:10.1039/c3ra45898a

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Hydrobromic acid-catalyzed Friedel–Crafts type
reactions of naphthols†
Cite this: RSC Adv., 2014, 4, 1559
*
*
Cong Xu, Hongjuan Yuan, Yingjie Liu, Mang Wang and Qun Liu
Received 17th October 2013
Accepted 15th November 2013
DOI: 10.1039/c3ra45898a
www.rsc.org/advances
Catalyzed by 10 mol% of hydrobromic acid, hydroxyalkylation and reaction of b-naphthol 1a with 4-nitrobenzaldehyde 2a had ꢀto
allylation-iodocyclization of naphthols proceeded smoothly under be performed in a Teon reaction vessel at 3.0 kbar and 60 C
mild conditions. In contrast, other Brønsted acids tested, including for 96 h to yield benzylidene biphenol 3a when using catalytic
HCl, HI, HBF4, HPF6, TFA, PTSA, H2SO4, TfOH etc., are almost inef- amounts of triuoromethanesulfonic acid (TfOH) as cata-
fective under identical conditions or require harsh reaction conditions. lyst.7a Indeed, in our experiments the hydroxyalkylation
reaction of 1a with 2a, performed in acetonitrile at ambient
conditions for 24 h (Table 1, entries 15ꢁ19), gave a trace
Acid-catalyzed/mediated Friedel–Cras (FC) type alkylation of
amount of 3a when 10 mol% of Brønsted acids, including
arenes with aldehydes, also known as hydroxyalkylation,1,2 is
one of the most important protocols for the functionalization of
aromatic compounds. In this eld, the reactions of electron-rich
arenes with aldehydes have been well studied, for example the
reaction of b-naphthols with aldehydes was successfully used
for efficient synthesis of xanthenes.3 These reactions smoothly
occurred in the presence of various readily available Brønsted
HBF4 (48% aqueous), HPF6 (65% aqueous), triuoroacetic
acid (TFA, 99%), p-methylbenzenesulfonic acid (PTSA, 99%)
or H2SO4 (98%), were used as catalysts, respectively. In the
case of TfOH (99%) as the catalyst (10 mol%), 3a was also
isolated only in 17% yield under the identical reaction
conditions (entry 20).
Surprisingly, benzylidene biphenol 3a could be obtained in
acids or Lewis acids through the activation of aldehydes and
high yield by using HBr (47% aqueous, 10 mol%) as the catalyst!
were found to be acidity-dependent.3
In this case, the reaction time was reduced to 7.0 h and the yield
On the other hand, counterion effect widely exists in chem-
of 3a reached to 76% (entry 1). In contrast, interestingly, HCl
ical transformations.4 Among them, halide effect has long been
(36% aqueous, 10 mol%) and HI (45% aqueous, 10 mol%) were
less effective (entries 3 and 5) even with 100 mol% of catalyst
recognized, especially in transition metal catalysis.5 In the eld
of aromatic electrophilic substitution reactions, the physical
(entries 4 and 6). These results indicated that there is no
properties of the counterion have been shown to dictate the
catalytic activity in the aromatic nitration and acetylation.6a
Counterion was also found to be able to tune both chemo-
selectivity and stereoselectivity of alkylation of indoles with b,
g-unsaturated a-keto esters.6b However, the signicantly
different catalysis existing between hydrobromic acid and other
hydrohalic acids has never been recoganized in acid-catalyzed
FC reactions. In this paper, we hope to disclose a unique cata-
lytic performance of HBr in hydroxyalkylation and allylation-
essential co-relation between the acidity of the reaction solution
and the efficiency of the reaction though an acidic environment
proved to be necessary (entry 1 versus entry 21). Satised yield of
3a could be obtained by using the combined HCl (36% aqueous,
10 mol%) and Bu4NBr (10 mol%) catalytic system (entry 22).
Furthermore, CuBr2 was proved less effective (entry 23), and
Cu(OAc)2 or CuCl2 was ineffective (entries 24 and 25). A
comparison of different solvents indicated that acetonitrile was
the best choice (entry 1 versus entries 7ꢁ13). It was also found
iodocyclization of naphthols.
that a good yield of 3a (71%) could be obtained by using acetic
In most cases, the reaction of naphthols with aldehydes
acid as the solvent in the presence of catalytic amount of HBr
required either at least stoichiometric amount of acid cata-
(entry 14).
lysts or harsh reaction conditions.1,3,7 For example, the
Unlike the well known acidity-dependent Friedel–Cras
hydroxyalkylations,1–3 the remarkable catalytic performance of
Department of Chemistry, Northeast Normal University, 5268 Renmin Street, China.
HBr (entry 1 versus entries 3ꢁ20) indicates that the bromide
E-mail: wangm452@nenu.edu.cn; liuqun@nenu.edu.cn; Fax: +86 43185099759
anion is more than a spectator in the above HBr-catalyzed
† Electronic supplementary information (ESI) available. For ESI see DOI:
process. It may reveal that simple Brønsted acid HBr acts as a
10.1039/c3ra45898a
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Table 1 Screening of catalysts and reaction conditionsa
for 24 h (Scheme 1A, (4)). In this reaction, product 3e0 was
formed in 21% yield by a hydroxyalkylation-Ritter reaction
cascade.10 Furthermore, it was noted that benzylidene biphenol
3f could be obtained in 71% yield by the reaction of 2a with b-
methoxynaphthalene 1c in the presence of 10 mol% of HBr for
24 h. However, no 3f was detected when HCl or HI was used as
catalyst, respectively (Scheme 1A, (5)). The successful synthesis
of 3f indicates that the free-OH of naphthols is not necessary in
the HBr-catalyzed reactions. In comparison, a phenol hydroxyl
group is required for the generation of, for example, reactive
magnesium phenoxide in FC reactions of phenols.11
As presented above, we observed a signicantly catalytic
power of bromide anion in FC type reactions. Different from
those conventional hydroxyalkylation usually involving high
reaction temperature, solvent-free, microwave, or in the pres-
ence of excess amounts of acid catalysts,3 the mild reaction
conditions of our HBr-catalyzed reaction of b-naphthols with
aldehydes (Scheme 1) can avoid further dehydration and thus
easily afford biphenol derivatives 3 (the key skeleton of ble-
pharismins)12 as the products.
It has been proved that a-naphthol is much less reactive than
b-naphthol in hydroxyalkylation and related reactions.3a In our
research, however, a-naphthol 1d was found to be suitable
substrate for the HBr-catalyzed hydroxyalkylation. The reactions
of 1d with selected aromatic aldehydes could afford biphenol
products 3g–j and their isomers 3g0–j0 in good to high yields
under the identical reaction conditions (Scheme 1B), while poor
regioselectivities were observed likely due to the similar nucle-
ophilicity of the orto- and para-position of phenols. On the other
hand, 3g and 3g0 were obtained in very low yields with HCl or HI
as catalyst, respectively (Scheme 1B, (1)). The crucial and unique
catalytic role of HBr in FC reaction could also be appreciated by
other observations, for example, the hydroxyalkylation-Ritter
reaction of p-cresol 1e with 2a (Scheme 1C).
Entry
Catalyst (equiv.)
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
HBr (0.1)
HBr (0.05)
HCl (0.1)
HCl (1.0)
HI (0.1)
HI (1.0)
HBr (0.1)
HBr (0.1)
HBr (0.1)
HBr (0.1)
HBr (0.1)
HBr (0.1)
HBr (0.1)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
CH2Cl2
DCE
Et2O
7.0
24
24
24
24
24
24
24
24
24
24
24
24
16
24
24
24
24
24
24
24
24
24
24
24
76
36
Trace
30
8.0
43
Nrc
Nrc
Trace
5.0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
THF
DME
C6H6
6.0
13
19
71
Trace
5.0
HBr (0.1)
AcOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
HBF4 (0.1)
HPF6 (0.1)
TFA (0.1)
Trace
Trace
Trace
17
PTSA (0.1)
H2SO4 (0.1)
TfOH(0.1)
Bu4NBr (0.1)
HCl (0.1) Bu4NBr (0.1)
CuBr2 (0.1)
CuCl2 (0.1)
Cu(OAc)2 (0.1)
Nrc
68
19
Nrc
Nrc
a
Conditions: 1a (2.0 mmol), 2a (1.1 mmol), solvent (10 mL), rt.
To gain further insight into the unique catalytic behavior
observed, a typical Friedel–Cras alkylation of naphthols with
allyl iodide was next tried under the catalysis of HBr. As a result,
b
Isolated yields. c No reaction.
2-(iodomethyl)-1,2-dihydronaphtho [2,1-b]furans
4
were
dual catalyst8 in contrast to previous conventional activation obtained in good yields by using excess of allyl iodide (10 equiv.)
pathways (activation of aldehydes with acids).2,7 According to in the presence of HBr (100 mol%) at 60 ꢀC for 8.0 h (Scheme 2).
our work on the special catalysis of bromide anion for the By comparison, HI was a less effective catalyst and HCl was
condensation of ketene dithioacetals with aldehydes,9 we envi- almost inefficient (Scheme 2). Fousteris and his co-workers
sioned that there may be exist a catalytic activation of b-naph- developed a green process for the synthesis of 2-(iodomethyl)
thol by bromide anion in the present reaction though the dihydro-benzofurans via water promoted iodocyclization of 2-
mechanistic details are not clear at the moment.
allylphenols.13 Clearly, our procedure provides a more direct
Further evidence for the unique catalytic activity was route to dihydro-benzofurans via HBr-catalyzed allylation of
obtained by the reaction of 1a with 4-chlorobenzaldehyde 2b. commercially available phenols with allyl iodide and sub-
Benzylidene biphenol 3b was isolated in 79% yield with HBr sequential iodocyclization (Scheme 2).
(10 mol%) as the catalyst at room temperature for 8.0 h, while
As we known, the classical Friedel–Cras reactions usually
HCl (10 mol%) and HI (10 mol%) showed inefficient catalysis require an acidic catalyst for the activation of electrophiles.1–3,7
for this procedure even aer 24 h (Scheme 1A, (1)). In addition, Differently, the Friedel–Cras type reactions of phenols pre-
the catalytic activity of HBr was also observed from the reaction sented in this communication are seem to be both proton and
of 1a with benzaldehyde 2c and formaldehyde 2d (Scheme 1A, anion-dependent. The catalytic mechanism of HBr is not yet
(2) and (3)), respectively, although a lower yield of 3c (39%) was clear at this stage. Recently, the unique catalysis of bromide
obtained from less electrophilic 2c. In the case of the reaction of anion was also found in metal-free diamination of alkenes14a
less nucleophilic 6-bromo-b-naphthol 1b with 2a, products 3e and copper-catalyzed oxidative aromatization of 2-cyclohexen-1-
and 3e0 were afforded in 73% total yield at elevated temperature ones.14b The specially catalytic phenomenons of bromide anion,
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Scheme 1 HBr-catalyzed Friedel–Crafts hydroxyalkylation.
phenols with aldehydes afford biphenol derivatives in the
presence of only a catalytic amount of HBr under very mild
reaction conditions. In addition, 2-(iodomethyl)dihydro-
benzofurans can also be synthesized in a single step directly
from commercially available phenols and allyl iodide under the
similar metal-free and good laboratory practice conditions.
Although this catalytic procedure is still limited in scope, the
special catalytic power of bromide anion may indicate a new
anion catalytic version and deserves more attention in further
research.
Acknowledgements
We gratefully acknowledge National Natural Sciences Founda-
tion of China (21172031, 21272034 and 21372040), New Century
Excellent Talents in Chinese University (NCET-11-0613) and
Research Fund for the Doctoral Program of Higher Education of
China (20120043110004) for nancial support.
Scheme 2 HBr-mediated tandem allylation/iodocyclization reaction.
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