Metal-free oxidative coupling: Xanthone formation via direct annulation of 2-aryloxybenzaldehyde using tetrabutylammonium bromide as a promoter in aqueous medium

Article abstract of DOI:10.1002/adsc.201300488

A metal-free intramolecular annulation of 2-aryloxybenzaldehydes to xanthones is disclosed, which proceeds through the direct oxidative coupling of an aldehyde C-H bond and aromatic C-H bonds using tetrabutylammonium bromide (TBAB) as a promoter in aqueous medium. This strategy works smoothly in the presence of both electron-donating and electron-withdrawing groups, and displays good tolerance towards catalytically reactive substituents, thus promising further functionalizations of xanthone products.

Full text of DOI:10.1002/adsc.201300488

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DOI: 10.1002/adsc.201300488
Metal-Free Oxidative Coupling: Xanthone Formation via Direct
Annulation of 2-Aryloxybenzaldehyde using Tetrabutylammoni-
um Bromide as a Promoter in Aqueous Medium
Honghua Rao,a,* Xinyi Ma,a Qianzi Liu,a Zhongfeng Li,a Shengli Cao,a
and Chao-Jun Lib,*
a
Department of Chemistry, Capital Normal University, Beijing 100048, Peopleꢀs Republic of China
Fax : (+86)-10-6890-2493; e-mail: honghua.rao@gmail.com
Department of Chemistry, McGill University, Montreal, QC H3A 0B8, Canada
b
E-mail: cj.li@mcgill.ca
Received: June 7, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300488.
Abstract: A metal-free intramolecular annulation
of 2-aryloxybenzaldehydes to xanthones is dis-
closed, which proceeds through the direct oxidative
del–Crafts
ring
closure
of
2-aryloxybenzoyl
(pseudo)halide intermediates under anhydrous condi-
tions (Scheme 1a).[3] Also dehydration of 2-aryloxy-
benzoic acids (or precursors) in the presence of
strong acids (e.g., PPA,[4] MsOH-P2O5[5]) (Scheme 1b)
or dehydrofluorination of 2-hydroxy-2’-fluorobenzo-
phenones (Scheme 1c)[6] could realize the xanthone
skeleton as well. During the past decade, transition
À
coupling of an aldehyde C H bond and aromatic
À
C H bonds using tetrabutylammonium bromide
(TBAB) as a promoter in aqueous medium. This
strategy works smoothly in the presence of both
electron-donating and electron-withdrawing groups,
and displays good tolerance towards catalytically re-
active substituents, thus promising further function-
alizations of xanthone products.
À
metal-catalyzed C H activations have been success-
fully applied as an appealing and efficient protocol
for building the xanthone skeleton, especially with
the assistance of easily removable directing groups,
such as imine (Scheme 1d).[7] Although these synthet-
ic routes displayed remarkable efficiency towards
xanthone formation, they are usually strictly limited
to electron-rich arenes, and normally require harsh
conditions. Besides, most of these methods generally
require multistep procedures, and thus would suffer
from both step- and atom-economy points of view in
À
Keywords: acyl radicals; aldehyde C H bonds;
metal-free conditions; oxidative annulation; TBAB-
promoted reactions
Xanthones are the core structures of many naturally industrial and green chemistry.[2]
occurring compounds that have exhibited extraordina-
À
Recently, as an emerging field in C H activations,
À
ry biological and pharmaceutical activities (e.g., anti- oxidative couplings between C H bonds have become
cancer, antioxidant, antibacterial) (Figure 1).[1] There- a highly attractive coupling strategy due to its atom-
À
fore, construction of the xanthone skeleton has at- economic manner in realizing C C bonds. One typical
tracted considerable interest.[2] Most common synthe- example is the oxidative coupling of the aldehyde C
À
À
ses of xanthones typically proceed through the Frie- H bond with other C H bonds, in particular with the
2
[8]
À
Csp H bonds to afford ketones. For instance, in
early 2012, Li’s group established the first strategy for
xanthone formation via RhCl3-catalyzed intramolecu-
À
lar oxidative coupling of the aldehyde C H bond and
an aromatic C H bond in an atom- and step-econom-
À
ic way, although the reaction was conducted at 1608C
in chlorobenzene;[9a] and more recently, Studer’s
group reported the same oxidative coupling reaction
with the iron catalyst FeCp2 in CH3CN
(Scheme 1e).[9b] During the very recent years, direct
À
Figure 1. Naturally occurring xanthones.
functionalizations of the aldehyde C H bond have
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Scheme 1. Traditional and transition metal-catalyzed xanthone formation.
been explored under metal-free conditions, wherein benzene, and CH3CN, but they were even less effec-
direct esterifications[10] and amidations[10e,11] in organic tive (entries 3–5). Inspiringly, when Cu
(OAc)2·H2O
solvents were the major subjects. Also, to the best of was used as an alternative to Cu(OAc)2, the yield in-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
our knowledge, there are few examples on the direct creased to 31%, indicating that H2O might improve
À
oxidative couplings between the aldehyde C H bond the efficiency of this oxidative coupling (entry 6).
and Csp H bonds without metal additives or cata- Thus a 70% aqueous solution of TBHP was em-
2
À
lysts,[12] or even no cases reported in an environment- ployed, and fortunately, the reaction afforded 40% of
friendly medium. So the development of effective and the desired xanthone (2a) (entry 7). Furthermore,
much more environmentally benign methods for such when using CH3CN/H2O (1:1, v/v ratio) as the co-sol-
reactivity is desirable, especially for xanthone forma- vent, a 45% yield of xanthone was obtained. Intrigu-
tion. Herein, we present a concise route to xanthones ingly, if the reaction was conducted without Cu-
by using TBAB as a promoter in aqueous medium
ACHTUNGTREN(UNNG OAc)2·H2O and solvents in air, we observed a slightly
without any metal additives or catalysts, which pro- increase of the yield, while the yield was reduced dra-
ceeds through the intramolecular oxidative coupling matically under an O2 atmosphere (entries 9 and 10).
À
À
of the aldehyde C H bond and aromatic C H bonds As such, the following optimizations were performed
in 2-aryloxybenzaldehydes (Scheme 2). Furthermore, under an air atmosphere. To ensure that the reaction
this transformation displays good tolerance towards proceeded in a homogeneous way in TBHP (aqueous
catalytically reactive substituents, such as aryl halide, solution), quaternary ammonium salts, such as TBAB
nitrile, quinolyl, naphthyl, and aminoacetyl groups.
and CTAB, were added to the reaction system, and
At the outset of our investigations on the oxidative indeed TBAB exhibited a more positive influence on
À
coupling between the aldehyde C H bond and aro- this oxidative coupling (entries 11 and 12). As indicat-
matic C H bonds to obtain xanthones, we treated 2- ed above, additional H2O facilitated this transforma-
À
phenoxybenzaldehyde (1a) with TBHP (in decane) in tion (entry 13). It is also worth noting that reducing
CH3CN at 1208C by using common Lewis acid cata- the amount of TBAB to 0.5 equivalent afforded xan-
lysts, such as FeCl3 or InACHTUNRTGNEUNG(acac)3. However, none of thone in 69% yield (entries 14 and 15). Besides, carry-
the catalysts showed a significant effect towards this ing out the reaction under N2 using TBAB (0.5 equiv.)
reaction, as expected (Table 1, entries 1 and 2). Since and TBHP (aqueous solution) revealed similar reac-
CuACHTUNGTRENNUNG(OAc)2 has been widely utilized as an efficient, tivity as under an air atmosphere (entry 16). Howev-
convenient and inexpensive catalyst (or additive) in er, if the reaction was conducted without TBHP (70%
[13]
À
oxidative coupling of C H bonds,
hyde C H bonds,
including alde- aqueous solution) or TBAB under N2, the xanthone
[8f,14]
À
we attempted to carry out the was realized in nearly no yield (entries 17 and 18).
reaction in the presence of Cu
(OAc)2 and TBHP (in These results indicated that TBAB was most likely in-
decane) in different solvents, such as toluene, chloro- volved in the oxidative annulation process as a pro-
moter. Other endeavours to improve the reaction effi-
ciency were undertaken, such as using TBP as the oxi-
dant and lowering the reaction temperature, but all of
them decreased the yield greatly (entries 19 and 20).
If TBHP (70% aqueous solution) was reduced to
2.5 equivalents, 68% of the desired xanthone was ob-
tained unless the reaction time was prolonged to 36 h
(entry 21). Likewise, a much longer reaction time was
Scheme 2. Metal-free TBAB-promoted oxidative coupling of
aldehyde C H bond/ArCÀH bond to xanthones.
À
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Metal-Free Oxidative Coupling: Xanthone Formation via Direct Annulation of 2-Aryloxybenzaldehyde
Table 1. TBAB-promoted xanthone formations under metal-free conditions: optimization of reaction conditions.
Entry
Cat. (mol%)
FeCl3 (20)
Additive (equiv.)
Oxidant
Solvent
Yield [%][b]
1
2
3
4
5
6
7
–
–
TBHPdecane
TBHPdecane
TBHPdecane
TBHPdecane
TBHPdecane
TBHPdecane
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
TBHPaq
–
CH3CN
CH3CN
toluene
PhCl
CH3CN
CH3CN
CH3CN
CH3CN/H2O
–
–
–
–
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
28
26
18
11
21
31
40
45
49
<10
58
53
63
In
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
(acac)3 (20)
Cu
Cu
Cu
Cu
Cu
Cu
–
N
8[c]
9[d]
10[e]
11[d]
12[d]
13[d]
14[d]
15[d]
16
–
TBAB
CTAB
TBAB
TBAB (1.0)
TBAB (0.5)
TBAB (0.5)
TBAB
67
69
68
17
18
NA[f]
Trace
19
–
TBHPaq
TBP
19[d]
20[d]
21[d]
22[d]
TBAB (0.5)
TBAB (0.5)
TBAB (0.5)
TBAB (0.2)
TBHPaq
TBHPaq
TBHPaq
59[g]
68[h]
67[i]
H2O
[a]
Reaction conditions: 1a (0.20 mmol), additive (1.5 equiv.), solvent (0.10 mL) and oxidant (0.20 mL), 1208C, under nitro-
gen unless otherwise noted.
Yields were determined by H NMR spectroscopy using mesitylene as an internal standard.
CH3CN/H2O (1:1, v/v ratio).
Under an air atmosphere.
Under an O2 atmosphere.
With being 1a recovered in 95% yield.
Reaction temperature: 1008C.
TBHP (70% aqueous solution, 2.5 equiv.); reaction time: 36 h.
TBHP (70% aqueous solution, 2.5 equiv.); reaction time: 40 h. TBHP=tert-butyl hydroperoxide, TBHPdecane =TBHP
(5.5M in decane), TBHPaq =TBHP (70% aqueous solution), TBAB=tetrabutylammonium bromide, CTAB=cetyltrime-
thylammonium bromide, TBP=tert-butyl peroxide.
[b]
[c]
[d]
[e]
[f]
1
[g]
[h]
[i]
also required to achieve similar reaction efficiency (2b–2d), tert-butyl (2e), methoxy (2f–2h), phenoxy
when reducing the amount of TBAB to 0.2 equiva- (2i), aminoacetyl (2j) and naphthyl (2k) groups. Par-
lents (entry 22).
ticularly, when 2-(4’-phenoxyphenoxy)-benzaldehyde
With the optimized reaction conditions in hand, the was introduced into the reaction system, the aldehyde
À
substrate scope was explored at 1208C for 24 h under C H bond selectively coupled with the 2-phenoxy
an air atmosphere by using 50 mol% of TBAB as the part to give xanthone 2i while leaving the 4’-phenoxy
promoter, TBHP (0.20 mL, 70% aqueous solution) as part intact, which showed that the oxidative coupling
the oxidant, and H2O (0.10 mL) as the additional sol- protocol favours the intramolecular route. Moreover,
vent. As summarized in Scheme 3, this TBAB-pro- the reaction tolerated a range of electron-withdrawing
moted oxidative annulation was successfully per- groups with the oxidative annulation occurring
formed with different substituents at the para-, meta-, smoothly, such as phenyl (2l and 2m), halogen (2n),
or ortho- position of the aryloxy unit, affording the trifluoromethyl (2o), cyano (2p), and quinolyl (2q), all
desired xanthones in moderate to excellent yields. of which promise further functionalizations of the
Obviously, the reaction proceeded well with electron- products. Notably, the substrate with m-CF3 yielded
donating groups at the aryloxy part, such as methyl a sole product 2o (with the para-position to CF3 as
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Scheme 4. Investigations on the mechanism.
a 2-Cl (2r) or 4-F (2s) group also underwent the
metal-free annulation smoothly to afford the desired
xanthones in 76% or 75% yield, respectively. In addi-
tion, this strategy could facilitate the syntheses of thi-
oxanthones, such as 2-methylthioxanthone (2t).
When investigating the reaction mechanism for this
TBAB-promoted oxidative coupling under metal-free
conditions in aqueous medium, a radical acylation is
the preferred consideration. In order to gain some in-
sights into the transformation, benzaldehyde was sub-
jected to our reaction system at 408C to see whether
TBAB-TBHP could work as an acyl radical- genera-
tor. To our delight, tert-butyl perbenzoate (most prob-
ably formed via an acyl radical[10c]) was detected in
about 10% yield, indicating that our TBAB-TBHP
system could act as the acyl radical-generating system
as expected (Scheme 4a). Then 2-phenoxybenzalde-
hyde (1a) was introduced into a metal-free acyl radi-
cal initiator system, PhI
ACHTUNGTRENUN(G OTf)2-NaN3 (or PhIACHTUNGTRENNUNG(OTf)2-
TMSN3), which could be utilized in the oxidative cou-
2
À
À
pling of aldehyde C H bond/Csp H bonds via a radi-
cal process.[12a] And indeed, it afforded xanthone in
about 30% (or 20%) yield (Scheme 4b). On the other
hand, the desired reaction did not occur when the
radical-trapper TEMPO (2,2,6,6-tetramethyl-1-piperi-
dinyloxy) or BHT (2,6-di-tert-butyl-4-methylphenol)
was employed under the standard conditions
(Scheme 4c). Therefore, we can infer that the reaction
most probably involves the acyl radical intermediate
and an intramolecular annulation process.
Scheme 3. TBAB-promoted xanthone formation. General re-
action conditions: 1 (0.20 mmol), TBAB (0.10 mmol), TBHP
(70% aqueous solution, 0.20 mL), H2O (0.10 mL), 1208C,
24 h, in air. Isolated yields were given.
On the basis of the above investigations and previ-
the only acylation site), and the one with m-OMe af- ous work,[10c,e] a tentative mechanism for this TBAB-
forded 2ha and 2hb in 2.5:1 ratio (with the para-posi- promoted direct oxidative coupling chemistry in aque-
tion to OMe as the major acylation site), indicating ous medium is depicted in Scheme 5. Initially, with
that this oxidative coupling process might be influ- the promotion of TBAB, tert-butoxyl radical A was
enced by steric effects. And importantly, the direct generated, and then trapped H· from aldehyde to
annulation
of
2-(quinolin-8-yloxy)-benzaldehyde form acyl radical B. The ensuing radical B could be
could realize a 53% yield of xanthone 2q, which well-suited to add to the aryloxy unit to get radical C,
might exhibit certain biological activity.[15] To expand which would form intermediate D via an SET process.
the substrate scope, the benzaldehyde unit bearing Finally, the previously formed OHÀ would act as the
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Metal-Free Oxidative Coupling: Xanthone Formation via Direct Annulation of 2-Aryloxybenzaldehyde
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (Grant No. 20972099), Beijing Municipal Education
Commission Foundation (Grant No. KZ201210028035), and
Capital Normal University for support of our research.
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Experimental Section
General Procedure
To a round-bottom Schlenk flask (10 mL) was added 2-ary-
loxybenzaldehyde (0.20 mmol), tetrabutylammonium bro-
mide (TBAB, 0.10 mmol), H2O (0.10 mL) and TBHP (70%
aqueous solution, 0.20 mL) sequentially. Then the flask was
sealed directly, placed into an oil bath, and the mixture
stirred at 1208C. After 24 h, the resulting mixture was
cooled to room temperature, anhydrous Na2SO4 was added,
and the mixture filtered through a short silica gel pad, and
washed with ethyl acetate. After concentration under
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petroleum ether/ethyl acetate as the eluent (or purified by
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Metal-Free Oxidative Coupling: Xanthone Formation via
Direct Annulation of 2-Aryloxybenzaldehyde using
Tetrabutylammonium Bromide as a Promoter in Aqueous
Medium
7
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Honghua Rao,* Xinyi Ma, Qianzi Liu, Zhongfeng Li,
Shengli Cao, Chao-Jun Li*
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