One-Pot oxidative Passerini reaction of alcohols using a magnetically recyclable TEMPO under metal- and halogen- Free conditions

Article abstract of DOI:10.1002/adsc.201200449

A novel domino oxidative Passerini threecomponent reaction (OP-3CR) has been successfully developed with either primary or secondary alcohols instead of their corresponding aldehydes or ketones by using a recyclable magnetic core-shell nanoparticle suppor

Full text of DOI:10.1002/adsc.201200449

FULL PAPERS
DOI: 10.1002/adsc.201200449
One-Pot Oxidative Passerini Reaction of Alcohols Using a
Magnetically Recyclable TEMPO under Metal- and Halogen-
Free Conditions
Babak Karimia,* and Elham Farhangia
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45137-66731, Gava Zang,
Zanjan, Iran
Fax : (+98)-241-4214949; e-mail: karimi@iasbs.ac.ir
Received: May 31, 2012; Revised: October 19, 2012; Published online: February 1, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200449.
Abstract: A novel domino oxidative Passerini three- metal- and halogen-free reaction conditions. It
component reaction (OP-3CR) has been successfully should be also noted that there has been no prece-
developed with either primary or secondary alcohols dent example of oxidative OP-3CR of secondary
instead of their corresponding aldehydes or ketones alcohols to their corresponding a-acyloxy carbox-
by using a recyclable magnetic core-shell nanoparti-
ACHTUNGTRENaNUNG mides. The catalyst can be magnetically recycled
cle supported TEMPO (2,2,6,6-tetramethyl-piperi- and successfully reused in 14 subsequent reaction
din-1-oxyl) (MNST, 1). It has been shown that cycles with only slight decreases of its catalytic activi-
MNST is a highly effective and recyclable catalyst ty.
system in the metal- and halogen-free aerobic, oxida-
tive, three-component Passerini reaction. To the best
of our knowledge this method can be considered as Keywords: aerobic oxidation; alcohols; magnetic
the first example of a OP-3CR of alcohols under nanoparticles; Passerini reaction; supported catalysts
Introduction
To address this issue, very recently it has been shown
that replacing aldehydes with alcohols could signifi-
Multicomponent reactions (MCRs) are among the cantly widen the versatility and the scope of well-
most powerful synthetic methods owing to the effi- known aldehyde-based multicomponent reactions
ciency with which products are formed from three or through domino oxidation/P-3CR.[7] Nevertheless, the
more reactants in a one-pot, simultaneous reaction.[1] successful examples using this useful strategy are very
In this regard, isocyanide-based multicomponent reac- limited and most of them are still using transition
tions (IMCRs) have been investigated intensively metal catalysts. In addition, to the best of our knowl-
during the past two decades and many innovative var- edge, there is currently no report of employing a recy-
iations have been developed which lead to the facile clable catalyst in domino aerobic oxidation/P-3CR
and atom-economic synthesis of a large collection of (OP-3CR). In recent years, we have demonstrates
important synthetic building blocks.[2]
that both SBA-15-functionalized TEMPO (2,2,6,6-tet-
Of particular importance in this area is the Passeri- ramethyl-piperidin-1-oxyl)[8] and magnetic core-shell
ni three-component reaction (P-3CR),[3] involving the nanoparticles supported TEMPO[9] are highly effec-
condensation of an isocyanide, an aldehyde or ketone, tive and recyclable in the metal-free aerobic oxidation
and a carboxylic acid followed by the Mummꢀs rear- of alcohols under mild reaction conditions. Encour-
rangement, leading to a-acyloxy amide scaffolds.[4,5] aged by the interesting work of Zhu et al. regarding
For this reason, much effort has been devoted to the the successful application of the homogeneous cata-
development of new alternatives of this extremely lyst system TEMPO/CuCl2 in the tandem aerobic oxi-
useful transformation.[6] However, in the majority of dation of alcohols coupled with a P-3CR,[10] we were
the reported P-3CR protocols, commercially available initially very interested to investigate the performance
aldehydes constitute the main starting materials. This of our heterogeneous TEMPO-based catalyst systems
is unfortunate since the use of aldehydes as the main in the same transformation under metal- and halogen-
reaction partner limits the versatility of this reaction. free conditions. However, among our immobilized
508
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One-Pot Oxidative Passerini Reaction of Alcohols Using a Magnetically Recyclable TEMPO
factory yields of 5a (Table 1, entries 2 and 3). It is im-
portant to note that while the same reaction using the
homogeneous TEMPO/CuCl2 catalyst system[10] re-
sulted in lower product yields of 75% within 24 h, it
was certainly employed at a much higher catalyst
loading. Repeated experiments show that the results
are reproducible, and more importantly the same
yields can also be obtained when catalyst 1 is used
after aging for as long as five months.
Having the optimal reaction conditions for transi-
tion-metal- and halogen-free catalysis, we then at-
tempted to study the scope of this new domino aero-
bic oxidation/P-3CR for other alcohol–isocyanide–car-
boxylic acid combinations.
As depicted in Table 1, benzyl alcohol can be oxi-
datively coupled with various combinations of carbox-
ylic acids and isocyanides, giving the corresponding
OP-3CR adducts in good to excellent yields under the
Scheme 1. Oxidative Passerini reaction (OP-3CR) of alco-
hols using MNST
TEMPO catalysts, we have chosen to employ magnet- optimized reaction conditions (Table 1, entries 1–7).
ic core-shell nanoparticles supported TEMPO be- Among the isocyanides tested, benzyl isocyanide is
cause it would allow easy separation of the catalyst the most reactive in the case of domino oxidative
from the reaction mixture by simple applying an ex- P-3CR of benzyl alcohol (Table 1, entries 1, 6, and 7).
ternal magnetic field.[11]
Similarly, the use of various types of primary benzylic
alcohol with electron-donating and electron-with-
drawing groups provided the corresponding OP-3CR
product with excellent selectivity and moderate to ex-
cellent yields after chromatography (Table 1, en-
Results and Discussion
The magnetic core-shell nanoparticles supported tries 8-17). Notably, the reaction of 2-phenyl ethanol
TEMPO 1 (MNST) was initially synthesized accord- as a model for aliphatic alcohols with benzoic acid
ing to our previously reported protocol with slight and different types of isocyanides afforded the corre-
modifications.[9,12] We initially attempted to investigate sponding a-acyloxy carboxamides in moderate to
a model of the OP-3CR, which involved the mixing of good yields with excellent selectivities (Table 1, en-
equimolar portions of benzyl alcohol and benzoic acid tries 18–20). The reaction also showed good generality
with benzyl isocyanide in toluene (2 mL) in the pres- for the oxidative coupling of other primary aliphatic
ence of 1 (1 mol%) and tert-butyl nitrite[13] (TBN, alcohols such as 3-phenylpropanol, n-butanol, and
10 mol%) to afforded 1-benzylamino-1-oxo-2-phenyl- also n-hexanol with a series of selected aliphatic and
ethane-2-yl benzoate 5a (R1 =R2 =Ph, R3 =PhCH2) at aromatic acids and isocyanides, affording their corre-
room temperature in order to validate our hypothesis sponding a-acyloxy carboxamides in moderate to
(Scheme 1).
good yields (Table 1, entries 21–28).
The reaction media was chosen as toluene in con-
Encouraged by these results, we then managed to
sideration of the higher rate and selectivity of P-3CR use various types of secondary alcohols in the same
in comparison with those carried out under protic re- transformation. Gratifyingly, we found that our proto-
action conditions.[14] The possible impact of NaNO2 col is also very effective in the oxidative P-3CR of
(10 mol%) instead of TBN has also been investigated benzylic, cyclic and aliphatic secondary alcohols with
and it was found that TBN displayed the highest in- benzyl isocyanide by employing either aromatic or ali-
fluence for the present oxidative P-3CR under the de- phatic carboxylic acids to afford the corresponding a-
scribed conditions (Table S1, Supporting Informa- acyloxy carboxamide adducts in good yields, (Table 1,
tion). Our preliminary investigations also revealed entry 29–33). These results further confirm the capa-
that the quantity of benzyl isocyanide and TBN were bility and reliability of our system in the described
vital for the described domino oxidative P-3CR to OP-3CR protocol.
proceed well, with 2 equivalents and 10 mol%, respec-
tively, shown to be optimum, and the described a- phatic carboxylic acids equally participated in the
acyloxy carboxamide 5a was isolated in 84% yields present protocol to furnish the corresponding a-acyl-
after 24 h (Table 1, entry 1). Notably, in the case of oxy amides with comparable yields and excellent se-
It is worth mentioning that both aromatic and ali-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
using somewhat lower amounts of TBN (8 mol%), it lectivity (Table 1, entries 6, 7, 9, 14, 17, 21–27, and
is necessary to significantly increase the loading of 30). Both aromatic and aliphatic isocyanides including
MNST up to 50% (1.5 mol%) in order to obtain satis- alicyclic isocyanides such as benzyl, cyclohexyl, tert-
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Babak Karimi and Elham Farhangi
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Table 1. Synthesis of a-acyloxy carboxamides via domino oxidative Passerini reaction (OP-3CR) of alcohols using MNST
1 as recyclable catalyst.[a]
Entry Alcohol
Carboxylic Acid
Isocyanide
Product
Product/Yield [%][b]
1
5a/84
2
3
4
5
5a/87[c]
5a/70[d]
5b/71
5c/74
6
7
8
5d/92[e]
5e/80
5f/94
9
5g/78
5h/57
10
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One-Pot Oxidative Passerini Reaction of Alcohols Using a Magnetically Recyclable TEMPO
Table 1. (Continued)
Entry Alcohol
Carboxylic Acid
Isocyanide
Product
Product/Yield [%][b]
11
5i/73
12
5j/74
13
14
15
5k/88
5l/87
5m/80
16
17
18
5n/81
5o/76
5p/90
19
5q/58
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Babak Karimi and Elham Farhangi
Product/Yield [%][b]
5r/79
FULL PAPERS
Table 1. (Continued)
Entry Alcohol
Carboxylic Acid
Isocyanide
Product
20
21
22
23
5s/85
5t/67
5u/74
24
25
26
5v/72
5w/83
5x/76
27
28
5y/61
5z/79
29
5z1/53[f]
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One-Pot Oxidative Passerini Reaction of Alcohols Using a Magnetically Recyclable TEMPO
Table 1. (Continued)
Entry Alcohol
Carboxylic Acid
Isocyanide
Product
Product/Yield [%][b]
30
5z2/59[f]
31
32
5z3/68[f]
5z4/64[f]
5z5/46[f]
33
[a]
Reaction conditions for primary alcohols: molar ratios 2:3:4=1:1:2 mmol, toluene (2 mL), MNST 1 (1 mol%), TBN
(10 mol%), O2 balloon (ꢀ1 atm), room temperature, 24 h.
[b]
[c]
[d]
[e]
[f]
All yields refer to isolated pure products after column chromatography unless otherwise stated.
MNST 1 (1.5 mol%), TBN (8 mol%).
Reaction conditions: MNST 1 (1 mol%), TBN (8 mol%).
Catalyst recycled and reused in 14 consecutive reaction runs.
Reaction conditions for secondary alcohols: molar ratios 2:3:4=1:1:2 mmol, toluene (2 mL), MNST 1 (1 mol%), TBN
(10 mol%), O2 balloon (ꢀ1 atm.), room temperature, 36 h.
butyl, and 1,1,3,3-tetramethylbutyl isocyanide partici- under mild conditions, the catalyst was magnetically
pated well in the present protocol. It is also very in- isolated within just 5 seconds (Figure 1), washed thor-
teresting to note that in all tested examples, this oughly with H2O and EtOH, and finally used directly
method showed high selectivity towards the presented in 14 subsequent domino oxidative P-3CR runs.
products, and no detectable amounts of P-3CR by-
Under the described conditions, the catalyst exhib-
products arising from the over-oxidation of starting ited high but slowly decreasing activity in 14 consecu-
alcohols to the corresponding carboxylic acids were tive runs (Figure 2).
observed. To the best of our knowledge this method
can be considered as the first example of domino oxi-
dative P-3CR of alcohols under metal- and halogen-
free reaction conditions. It should also be noted that
there has been no precedent example of an oxidative
P-3CR of secondary alcohols to their corresponding
a-acyloxy carboxamides.
For practical applications of heterogeneous systems,
a long catalyst lifetime and the rapid reusability of
catalyst are very important. Magnetic separation pro-
vides a convenient method for removing and recycling
magnetized species by applying an appropriate exter-
nal magnet, which minimizes the loss of catalyst
during the separation stage. Our preliminary investi-
gations showed that the catalyst provides excellent re-
cycling properties. After the completion of first reac-
tion of benzyl alcohol, 4-methoxyphenylacetic acid
and benzyl isocyanide to afford the 1-benzylamino-1-
oxo-2-phenylethane-2-yl 4-methoxyphenylacetate 5d ed TEMPO catalyst 1 (MNST) within 5 seconds.
Figure 1. Easy separation of magnetic nanoparticle-support-
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Babak Karimi and Elham Farhangi
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nanoparticles supported TEMPO (MNST, 1) under
metal- and halogen-free conditions. This catalyst
system shows excellent selectivity and good to excel-
lent yields for a range of a-acyloxy carboxamides
through domino aerobic oxidation of alcohols fol-
lowed by P-3CR using different combinations of car-
boxylic acids and isocyanides such as aromatic, ali-
phatic and cyclic ones. The present catalyst system
showed superior catalytic activity than those previous-
ly reported for oxidative P-3CR especially in terms of
catalyst loading and its metal- and halogen-free fea-
tures. The magnetically recyclable catalyst shown high
catalytic activity and excellent reusability under tran-
sition metal- and halogen-free reaction conditions and
can be used in 14 subsequent domino oxidative
P-3CR runs albeit with slightly decreasing catalytic
activity. Besides the easy recovery and reusability, the
possibility to perform the transformation in the ab-
Figure 2. Reusability of MNST catalyst in the oxidative
Passerini reactions (OP-3CR) after 24 h.
The transmission electron microscopic (TEM) sence of a transition metal and a halogen make this
images of catalyst before and after recycling experi- protocol a valid candidate towards the goal of green
ments illustrate that no significant change of the cata- chemistry. We speculate that this catalyst system will
lyst had occurred during the catalysis and the recy- open a new challenging area in the design of new
cling stages (Figure 3). This suggests that both the types of domino oxidative multi-component reactions
morphology and core-shell structure of the catalyst (DOMCRs) under mild conditions.
remain almost unchanged even after multiple reaction
cycles.
Experimental Section
Conclusions
General Catalytic Procedure
In conclusion, we have developed a novel domino,
aerobic, oxidative Passerini three-component reaction
A mixture of MNST (1 mol%), t-BuONO (TBN, 10 mol%),
carboxylic acid (1.0 mmol, 1.0 equiv.) and alcohol (1 mmol,
of alcohols using highly recyclable magnetic core-shell 1.0 equiv.) in dry toluene (2 mL) was prepared in a 10-mL
Figure 3. TEM images of MNST before (left; scale bar=100 nm) and after (right; scale bar=100 nm) recycling experiments.
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One-Pot Oxidative Passerini Reaction of Alcohols Using a Magnetically Recyclable TEMPO
two-necked flask. Then the isocyanide compound (2 mmol,
2 equiv.) was added to the reaction mixture. The flask was
purged with pure oxygen (filled balloon) and the resulting
mixture was vigorously stirred at room temperature for 24 h
under an oxygen atmosphere. After this time, the mixture
was diluted with CH2Cl2 (10 mL) and the catalyst was sepa-
rated by an external magnet. An aqueous sodium hydroxide
solution (0.2M, 10 mL) was added to the organic layer
which was then extracted with CH2Cl2 (3ꢂ10 mL) and dried
over sodium sulfate, filtered and the excess of solvent was
removed under reduced pressure to give the corresponding
product. The crude product was purified by means of chro-
matographic purification (silica gel, n-hexane/EtOAc) to
afford the corresponding pure product. The separated cata-
lyst was successively washed with H2O and EtOH, and then
re-used directly for subsequent reaction runs without further
purification.
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Acknowledgements
We gratefully acknowledge the IASBS Research Council and
financial support from the Iranian National Science Founda-
tion (INSF). We also appreciate Prof. Hojatollah Vali from
McGill University for the TEM images.
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