One-pot synthesis benzils from aldehydes via nhc-catalyzed benzoin dimerization under metal-free conditions in water

Article abstract of DOI:10.4067/S0717-97072011000200008

A simple and convenient one-pot procedure is reported for the synthesis of 1,2-diketones from corresponding benzoin-type condensation reaction of aromatic aldehydes in water with N,N-dialkylbenzimidazolium salt as condensation catalyst and air as oxidizing reagent.

Full text of DOI:10.4067/S0717-97072011000200008

J. Chil. Chem. Soc., 56, Nº 2 (2011)
ONE-POT SYNTHESIS BENZILS FROM ALDEHYDES VIA NHC-CATALYZED BENZOIN DIMERIZATION
UNDER METAL-FREE CONDITIONS IN WATER
XIAOXIN BI1, LINTAO WU1, CHAOGUO YAN1, XIAOBI JING11*, HONGXIANG ZHU2
1College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China
2School of Light Industrial and Food Engineering, Guangxi University, Nanning, P. R.China
(Received: June 23, 2010 - Accepted: May 12, 2011)
ABSTRACT
A simple and convenient one-pot procedure is reported for the synthesis of 1,2-diketones from corresponding benzoin-type condensation reaction of aromatic
aldehydes in water with N,N-dialkylbenzimidazolium salt as condensation catalyst and air as oxidizing reagent.
Keywords: benzils; benzoin condensation; benzimidazolium salts; metal-free oxidation
zimidazolium salts, solvent and time by using the reaction of benzaldehyde
as an comparing reference. (Scheme 2) The results are summarized in Table
1 and Table 2.
From the Table 1, it can be seen that the octyl substituted benzimidazole(2e)
is the best catalyst in this reaction and from table 2, it can be seen that water is
the best solvent for this reaction.
INTRODUCTION
Benzils have received a great deal of attention because of their applications
as photosensitive reagents in photocurable coatings,1 as starting materials
in organic and pharmaceutical synthesis,2 and also as biologically active
compounds.3 Conventionally benzils are prepared by coupling of acid chlorides
with SmI2,4 and oxidation of mono ketones with SeO2.5 Oxidation of acetylenes
with NaIO4/RuO26, Tl(NO )37, and KMnO48 has been reported to afford
α-diketones in lower yield3s. Some complex and expensive reagents, such
as 4-Me-TEMPO,9a (4-AcNH-TEMPO)-OTs-TsOH,9b 4-phCO -TEMPO,9c
n-Bu4N[MoO(O)2(C5H4N(O)O2)],10
H2O2-[C5H5N-(CH2)15CH3]32{PO4[W(O)
(O )]4},11 can oxidize 1,2-diols to 1,2-diketones. Benzils can also be prepared
by2 the oxidation of corresponding benzoins with oxidizing agents such as
CuSO4/Py, Bi2O3/H+, HNO3 in aq. solution,12 RuO413. And all these methods
have its drawbacks in terms of expensive regents, difficult reaction conditions
and low yields. Recently, we reported our work on the synthesis of benzils
using FeCl3 as oxidizing reagent14, Herein, we disclose our new result.
Results and Discussion
In the past decade, there has been a dramatic increase in the use of
N-heterocyclic carbenes (NHCs) in synthetic organic chemistry. NHCs are
versatile ligands that can be used in homogenous transition metal catalysis.15
Furthermore, considerable attention has been paid to the use of NHCs as
organocatalysts.16 In 2006, Iwamoto17 reported that benzoin condensation
reaction can be catalyzed by N,N-dialkylbenzimidazole in water to yield
α-hydroxyketones. We repeatedthisreactionandfoundthat ifroom temperature
stirring was replaced by refluxing, the yield of benzoin was increased to 92%
and the reaction time was only 0.5 h. (Scheme 1)
Scheme 2 Route to benzil from aldehyde through benzoin dimerization
under metal-free conditions
Table 1: Synthesis of benzils catalyzed by varying benzimidazolium salt
Entry
1
Catalyst
2a
R
Time(h)
2
Yield%
CH2CH3
32
2
3
4
5
6
7
2b
2c
2d
2e
2f
CH2CH2CH3
CH2(CH2)2CH3
CH2(CH2)3CH3
CH2(CH2)6CH3
CH2(CH2)10CH3
CH2(CH2)14CH3
2
2
2
2
2
2
55
63
72
85
82
80
2g
Table 2 Synthesis of benzils in different solvents.
Entry
Solvent
CH3CH2OH
CH3OH
H2O
Time(h)
Yield%
1
2
3
4
2
2
2
2
67
36
85
55
Scheme 1: Benzoin reaction catalyzed by benzimidazolium salts in water
by refluxing.
THF
But when we prolonged the reaction time to 1h, the yield of the α-hydroxy
ketone was decreased and the amount of byproduct which is benzil was
increased. If the reaction was repeated in the atmosphere of nitrogen, a small
amount (<5%) of benzil was formed. So it can be concluded that in the reaction
process some of α-hydroxy ketones was oxidized to benzil by the oxygen in
the air. So we prolonged the reaction time to 2 h and obtained benzil in good
yield (85%).
Under optimized conditions18 (catalyzed of 2e in water), the reactivity of
a series of aromatic aldehydes in this reaction was examined (Scheme 3), the
results are summarized in Table 3. From the table it can be seen that aromatic
aldehyde bearing chloro-, bromo- and methyl substituent afforded the desired
benzils in higher yields within shorter reaction time. While the reactions of
3- or 4-nitrobenzaldehyde or p-hydroxyl benzaldehyde were considerably slow
and the yields of corresponding benzils were very low. Furfural also gave the
product of furil with good yield.
To optimize the reaction condition we examined this domino reaction with
varying ben-
e-mail: jingxiaobie@yahoo.com.kn
663

J. Chil. Chem. Soc., 56, Nº 2 (2011)
REFERENCES AND NOTES
1. (a) Matsushita Electric Industrial Co. Ltd., Jpn. Kokai Tokyo Koho, 1981;
(b) Chem. Abstr. 1981, 95, 188163u.
2. J. Mattay, J. Runsink, J. Org. Chem. 50, 2815, (1985).
3. L. R. Hillis, R. C. Ronald, J. Org. Chem. 50, 470, (1985).
4. J. Souppe, J.-L Namy, H. B. Kagan, Tetrahedron Lett. 25, 2869, (1984).
5. N. Rabjohn, Org. React. 5, 331, (1949).
6. R. Zibuck, D. Seebach, Helv. Chim. Acta. 71, 237, (1988).
7. R. McKillop, O. H. Oldenziel, B. P. Swann, E. C. Taylor, R. L. Robey, J.
Am. Chem. Soc. 95, 1296, (1973).
Scheme 3: One-pot synthesis of a-diketones from aldehydes.
Table 3: The synthesis of benzils in one-pot reaction.
8. D. G. Lee, V. S. Chang, Synthesis, 462, (1978).
9. (a) P. L. Anelli, S. Banfi, F. Montanari, S. Quici, J. Org. Chem. 54, 2970,
(1989). (b) M. G. Banwell, V. S. Bridges J. R.Dupuche, S. L. Richards, J.
M. Walter, J. Org. Chem., 59, 6338. (1994). (c) J. Inokuchi, S. Matsumoto,
T. Nishiyama, S. Torii, Synlett, 57, (1990).
10. S. Campestrini, F. D. Furia, G. Modena, J. Org. Chem. 55, 3658, (1990).
11. T. Iwahama, S. Sakaguchi, Y. Nishiyama, Y. Ishii, Tetrahedron Lett. 36,
1523, (1995).
Entry
Ar
Product
3a
Time(h)
Yield%
85
Mp(°C)
95-96
1
2
Ph
2
2
4-Me-C6H4
4-MeO-C6H4
4-OH-C6H4
4-Cl-C6H4
3-Br-C6H4
4-Br-C6H4
4-NO2-C6H4
3-Cl-C6H4
3-NO2-C6H4
Furan-2-yl
3b
80
103-105
130-131
---
3
3c
2.5
4
75
12. (a) W. Rigby, J. Chem. Soc. 793, (1951). (b) H. T. Clarke, E. E. Dreger,
Org. Synth. Coll. 1, 87, (1941).
4
3d
none
94
13. S. Baskaran, J. Das, S. Chandrasekaran, J. Org. Chem. 54, 5182, (1989).
14. X. B. Jing, X. Pan, Z. Li, Y. C. Shi, Syn. Commn. 39, 492, (2009).
15. (a) S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev. 109, 3612,
(2009). (b) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 47, 3122,
(2008). (c) Topics in Organometallic Chemistry, Vol. 21; Glorius, F. Ed.;
Springer: Berlin, 2007; (d) R. E. Douthwaite, Coord. Chem. Rev. 251, 702,
(2007). (e) E. A. B. Kantchev, C. J. O’Brien, M. G. Organ, Angew. Chem.
Int. Ed. 46, 2768, (2007). (f) W. A. Herrmann, Angew. Chem. Int. Ed. 41,
1290, (2002).
5
3e
1.5
1.5
1.5
4
195-198
122-124
224-226
---
6
3f
86
7
3g
80
8
3h
trace
90
9
3i
1.5
4
114-116
---
10
11
3j
trace
85
16. (a) D. Enders, O. Niemeier, A. Henseler, Chem. Rev., 107, 5606, (2007).
(b) H. Clavier, S. P. Nolan, Annu. Rep. Prog. Chem., Scet. B. 103, 193,
(2007).
3k
1.5
165-166
17. K. Iwamoto; M. Hamaya. Tetrahedron Lett. 47, 7175, (2006).
CONCLUSIONS
In conclusion, as described above, we developed a one-pot synthesis of
benzils by the domino reaction of aldehydes with benzimidazolium salt as
catalyst and the oxygen in the air as oxidizing reagent. The present reaction
with its mild and metal-free conditions opens a novel green entry to synthesis
of benzils by simple procedure.
ACKNOWLEDGMENTS
This research was supported by the Cultivation and Construction Found
for the State Key Subject of Physical Chemistry.
664