Palladium-catalyzed regioselective ortho-acylation of azoxybenzenes with aldehyde derivatives

Article abstract of DOI:10.1002/adsc.201400594

An efficient strategy for the regioselective ortho-acylation of azoxybenzenes with various aldehydes in the presence of palladium catalysts has been developed and furnishes good to excellent yields. The reaction proceeds smoothly and can tolerate a variety of functional groups.

Full text of DOI:10.1002/adsc.201400594

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DOI: 10.1002/adsc.201400594
Palladium-Catalyzed Regioselective ortho-Acylation of
Azoxybenzenes with Aldehyde Derivatives
Meng Sun,a,* Le-Kai Hou,a Xiang-Xiang Chen,a Xiao-Juan Yang,a Wei Sun,a
and Yu-Shi Zanga
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of
Chemistry & Materials Science, Northwest University, Xi’an 710069, Peopleꢀs Republic of China
E-mail: sunmeng@nwu.edu.cn
Received: June 19, 2014; Revised: August 5, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400594.
Abstract: An efficient strategy for the regioselec-
tive ortho-acylation of azoxybenzenes with various
aldehydes in the presence of palladium catalysts has
been developed and furnishes good to excellent
yields. The reaction proceeds smoothly and can tol-
erate a variety of functional groups.
ous methods for the synthesis of azoxy compounds
have been developed,[17] exploration in the functional-
ization of azoxybenzenes is very limited. Very recent-
À
ly, as part of the development of group-directed C H
bond activation, Wang and co-workers first reported
a Pd-catalyzed functionalization of azoxybenzenes
with oxocarboxylic acids,[14a] and subsequently devel-
oped a Ru-catalyzed alkenylation of azoxybenze-
nes.[14b] As far as we know, there are only the two
Keywords: ortho-acylation; aldehydes; azoxyben-
À
zenes; C H bond cleavage; palladium catalysis
À
above reports on C H bond functionalization of
azoxy compounds. Considering the importance of
azoxy compounds and the significant bioactive motifs
of aryl ketones,[18] it is necessary to develop a new
À
During the development of methods for organic syn- method to realize C H bond activation of azoxyben-
thesis, the emergence of direct functionalization of zenes, and the coupling partner should be very cheap
À
unreactive C H bonds by transition metal catalysts and accessible. Pursuing our interest in the cleavage
provides an atom-economic strategy, which dramati- of inert chemical bonds,[19] herein we report a direct,
cally shortens the pathway to the construction of novel and efficient approach to establish a strategy
carbon-carbon or carbon-heteroatom bonds and has for the ortho-acylation of a number of different azoxy
achieved great progress in the synthesis of many compounds via nitrogen atom-directed Pd-catalyzed
useful polyfunctional compounds.[1,2] Generally, most C H bond activation with various commercially ac-
À
of the established approaches to regio- and chemose- cessible aldehydes (Scheme 1).
À
lective C H bond cleavage involving directing group-
Our initial investigation focused on the model reac-
assisted activation have been extensively investigated, tion of azoxybenzene (1a, 1.0 equiv.) and benzalde-
with groups such as acetyl,[3] acetamino,[4] carboxylic hyde (2a, 2.0 equiv.) with TBHP (3.0 equiv.) in the
acid,[5] ester,[6] aldehydes,[7] ketones,[8] nitriles,[9] presence of 10 mol% Pd(OAc)2 in DCE at 608C for
oxazolyl,[10] pyridyl[11] and imino[12] moieties. Hence, 24 h (Table 1, entry 1). To our delight, the desired
the elegant combination of suitable transition metals product 3a was isolated in 28% yield, and further
À
and directing groups is very crucial to realize C H screening of additives showed an obvious increase in
bond cleavage and further transformations. To the yield when trifluoroacetic acid was added (Table 1,
best of our knowledge, azoxy compounds with two di- entry 2). Furthermore, different Pd catalysts were also
recting groups[13] (N or O), which represents an ex- investigated, among which Pd(TFA)2 possessed the
treme challenge in the control of two different kinds highest catalytic activity, and the corresponding prod-
À
of C H bond activation, have rarely been studied in uct was obtained in 85% yield (Table 1, entries 3–6).
the field of C H functionalization.
[14]
À
It was very important to demonstrate that the reac-
Azoxy compounds, which are key materials in elec- tion temperature is very crucial to this transformation,
tronic devices, have wonderful liquid crystalline prop- and increasing or lowering the temperature sup-
erties,[15] and have also wide applications in dyes, pressed the efficiency, for example, increasing the
polymer inhibitors and stabilizers.[16] Although numer- temperature to 808C brought a distinct reduction in
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Scheme 1. Pd-catalyzed ortho-acylation of azoxybenzenes with benzaldehydes.
Table 1. Optimization of the reaction conditions.[a]
dure was also effective under air conditions, although
the isolated yield was slightly lower (Table 1,
entry 18). Finally, the optimized conditions for the C
H ortho-functionalization of azoxybenzenes were de-
termined to be 608C for 24 h in DCE with 10 mol%
Pd(TFA)2 as the catalyst and 3.0 equiv. of TBHP as
the oxidant.[20]
À
To explore the applicability of this protocol, various
aromatic aldehydes were surveyed and the results are
À
summarized in Table 2. To our delight, the C H acy-
lation of azoxybenzenes with different aldehydes
could proceed smoothly and moderate to excellent
yields were obtained for most cases. The substitution
on the aromatic ring demonstrated that no significant
electronic effect of the para-substituted azoxyben-
zenes was present (Table 2, 3ab, 3af–3ah, 3ak), except
for 4-cyanobenzaldehyde and 4-nitrobenzaldehyde,
which showed obvious deleterious effects (Table 2, 3ai
and 3aj). It is worth noting that the bromo group re-
mained intact in this procedure with an excellent
yield, which could be used for further transformations
into other important structures. And fortunately,
a representative structure of 3ah was confirmed by X-
ray single crystal analysis (see the Supporting Infor-
mation for more details). Surprisingly, the steric hin-
drance effect was not observed when meta- and ortho-
substituted aromatic aldehydes were involved in the
reaction, and the corresponding products were ob-
tained in 81% (3ac) and 80% (3ad) yields, respective-
ly. Also, 1-naphthaldehyde was tested in the reaction
and the yield of the product was acceptable (Table 2,
3ae). Encouraged by the above results, an aliphatic al-
dehyde, such as n-butyraldehyde, was evaluated and
a moderate yield of the product was isolated (Table 2,
[a]
All the reactions were carried out in the presence of
0.2 mmol of 1a, 0.4 mmol of 2a and 0.6 mmol of TBHP in
1.0 mL DCE at 608C under Ar.
Isolated yields.
1.0 equiv. of TFA was added.
At 408C.
At 808C.
Under air conditions.
[b]
[c]
[d]
[e]
[f]
yield (Table 1, entries 7 and 8). It should be noted 3am). However, when heterocyclic-substituted alde-
that this transformation works well only in the pres- hydes were exposed under the reaction conditions,
ence of DCE, however, other solvents such as tolu- the corresponding products were obtained in low effi-
ene, dioxane, MeCN, or AcOH were less effective or ciency. For example, the product 3an was isolated in
impeded the reaction (Table 1, entries 9–12). After- 20% yield, which did not change even with higher
wards, we also tested this transformation under neat temperature or longer reaction time.
conditions which only afforded the desired product in
Next, we tested the scope of different azoxy com-
56% yield (Table 1, entry 13). Meanwhile, the oxi- pounds in this procedure with benzaldehyde under
dants also played a very important role in this catalyt- our best conditions. The results demonstrated that
ic procedure, and other common oxidants reduced or azoxy derivatives with electron-donating groups gave
failed to participate in this reaction (Table 1, en- better yields than those with electron-withdrawing
tries 14–17). It is gratifying that this catalytic proce- groups. For example, 4,4’-dimethylazoxybenzene and
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Pd-Catalyzed Regioselective ortho-Acylation of Azoxybenzenes
Table 2. Scope of the ortho-acylation of azoxybenzene with
Table 3. Scope of azoxy compounds with benzaldehyde.[a,b]
aldehydes.[a,b]
[a]
All the reactions were carried out in the presence of
0.2 mmol of 1, 0.4 mmol of 2a and 0.6 mmol of TBHP in
1.0 mL DCE at 608C under Ar.
Isolated yields.
[b]
on the regions of the N and O atoms, the real direct-
ing group is N, which is generally considered to be
a better coordinating atom than O when complexed
with Pd(II). So it is reasonable to assume first that
the reaction is probably initiated by N-assisted ortho-
selective cyclometalation on the arene ring by
Pd(TFA)2, followed by reaction with the acyl radicals,
which were generated in situ by abstraction of hydro-
gen atom of aldehydes. Second, the intermediate II
was formed via oxidation of Pd(II) to Pd(IV) or di-
meric Pd(III).[23] Finally, reductive elimination of acyl
and aryl groups provides the ortho-acylated products
[a]
All the reactions were carried out in the presence of
0.2 mmol of 1a, 0.4 mmol of 2 and 0.6 mmol of TBHP in
1.0 mL DCE at 608C under Ar.
[b]
Isolated yields.
4,4’-azoxydianisole provided the desired products in and the active Pd(II) was regenerated. To prove the
excellent yields (Table 3, 3ba and 3ca), while sharply existence of radical intermediates, TEMPO (2,2,6,6-
reduced yields were observed with electron-deficient tetramethyl-piperidyl-1-oxyl) was added as a radical
substituents at the para-positions of azoxybenzenes, scavenger and the reaction was suppressed complete-
such as F and Cl (Table 3, 3da and 3ea).[21] Unfortu- ly.[24]
nately, other electron-deficient groups, such as CF3
and CO2Me were not beneficial for this transforma- as liver X receptor agonists[22,25] and a previous
tion, and almost no desired products were isolated.
report,[14a] the various ortho-acylation products can be
Due to the unique biological activity of indazoles
On the basis of our experimental results and previ- easily transformed into the corresponding indazoles
ous literature,[14,22] a plausible mechanism pathway for under reduction conditions, which afforded a more ef-
À
the Pd-catalyzed C H functionalization of azoxyben- ficient and economical method to construct the back-
zenes is depicted in Scheme 2. Although two kinds of bone of indazoles (Scheme 3).
À
potential C H bonds exist in the azoxybenzene based
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Experimental Section
General Procedure
An oven-dried 10-mL screw-capped vial was charged with
Pd(TFA)2 (6.6 mg, 0.02 mmol, 10 mol%), azoxybenzene
(39.6 mg, 0.2 mmol, 1.0 equiv.), and benzaldehyde (42.4 mg,
0.4 mmol, 2.0 equiv.) under a gentle stream of argon. Then
TBHP (108 mL, 5.5M, 0.6 mmol, 3.0 equiv.) and DCE
(1 mL) were added to the vial by a syringe. The vessel was
heated in an oil bath at 608C for 24 h followed by cooling.
The contents were subjected to flash chromatography to
give the corresponding product (85%) as a pale yellow oil.
The purified material was dried under an oil-pump vacuum.
Acknowledgements
We are grateful to the Northwest University (PR13089,
NF14020) for financial support.
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À
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6 Palladium-Catalyzed Regioselective ortho-Acylation of
Azoxybenzenes with Aldehyde Derivatives
Adv. Synth. Catal. 2014, 356, 1 – 6
Meng Sun,* Le-Kai Hou, Xiang-Xiang Chen,
Xiao-Juan Yang, Wei Sun, Yu-Shi Zang
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