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Oxidation of benzylic and some aliphatic alcohols 1,2-ethylenebis(triphenylphosphonium)chlorochromate (EBTPPCC): As a new, an efficient and mild oxidant
*Corresponding author. Tel./fax: +98 5118802153 m_gholizadeh@um.ac.ir (M. Gholizadeh) gholizadeh_mostafa@yahoo.com (M. Gholizadeh)
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Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
1,2-Ethylenebis(triphenylphosphonium)chlorochromate was quantitatively prepared and used for the oxidation of various benzylic and some aliphatic alcohols to their corresponding carbonyl compounds in water or acetonitrile and under microwave irradiation. This new oxidizing agent has advantages over similar oxidants in terms of the amount of used oxidant, short reaction time, simple work-up, and high yields. Furthermore, the oxidation of alcohols occurred under microwave irradiation many times and it produced the corresponding carbonyl compounds with high yields and short reaction times, when compared with the conventional method. Finally, the oxidation of different alcohols were accomplished in water under reflux condition with good yields as green chemistry.
Keywords
Ethylenebis(triphenylphosphonium)chlorochromate
Oxidation
Benzylic alcohols
Reflux condition
Microwave
Green chemistry
1 Introduction
Since the appearance of Collins reagent in 1968 (Collins et al., 1968), the development of new chromium(VI) oxidizing agent for the effective and selective oxidation of organic substrates, in particular alcohols, has attracted a great deal of continued interest in organic synthesis. Up to now several efficient chromium(VI) reagents such as pyridinium chlorochromate (Corey and Suggs, 1975), pyridinium dichromate (Corey and Schmidt, 1975), 2,2-bipyridiumchlorochromate (Guziec and Luzzio, 1980), 2,6-dicarboxylpyridinium chlorochromate (Tajbakhsh et al., 2002; Tajbakhsh et al., 2004), 2,6-dicarboxypyridinium fluorochromate (Tajbakhsh et al., 2005a,b), tetrabuthylphosphonium dichromate (Memarian et al., 2006), 1-methylimidazolium chlorochromate (Kim and Chang, 1978), 1-(benzoylamino)-3-methyl imidazolium chlorochromate (Martinez et al., 1995), ethylenebis(N-methylimidazolium)chlorochromate (Tajbakhsh et al., 2008), methyl triphenylphosphoniumhalochromate (Ghammamy et al., 2008), propyltriphenylphosphonium fluorotrioxochromate(VI) Javanshir et al., 2008, and tributhyl ammonium chlorochromate (Ghamamy and Mazaeery, 2005) have been introduced to improve the selectivity, the mildness and therefore, the effectiveness of the oxidant species, specially in the oxidation of complex and highly sensitive compounds.
The chlorochromate anion, while it combines with organophilic cation, becomes a very useful agent for the oxidation of organic compound in the nonaqueous solution. Although many chromium(VI) reagents are available for the oxidation of organic substrates, they have certain limitations such as instability, a need for an excess amount of the reagent, and poor selectivity about substrates. Therefore, there still exists a need for highly efficient and mild oxidizing agents.
Also, the microwave which enhanced organic reactions has gained popularity over the usual homogeneous and heterogeneous reactions, as they could be conducted rapidly and so, they produce pure products in quantitative yields without the use of solvents (Perreux and Loupy, 2001; Lindstroem et al., 2001).
We have found that EBTPPCC is an efficient and mild oxidizing agent for the oxidation of benzylic and some aliphatic alcohols. (EBTPP)ClCrO3 was easily prepared by the addition of an aqueous solution of 2-bromoethyl triphenylphosphonium bromide to a solution of CrO3 and HCl in water.
2 Experimental
All the alcohols were purchased from Fluka and Merck. The reactions were monitored by TLC using Silica gel plates. The products were identified by comparison of their 1H NMR, IR and GC spectra and physical data with those of authentic samples. 1H NMR spectra were measured by the Bruker AC 100 MHZ spectrometer with tetra methylsilane as an internal reference and DMSO-d6 and also CDCl3 as solvents. IR spectra were recorded by the Shimadzu FT-IR 8440S spectrophotometer. GC spectra were recorded by a Shimadzu 17a-GC spectrophotometer. Elemental analysis was performed on a LECO 250 instrument.
2.1 Preparation of ethylenebis(triphenylphosphonium)bromide (EBTPPB)
Three general methods were used for the preparation of EBTPPCC. In one (method A) a mixture of triphenylphosphine (8.76 g, 30 mmol) and 1,2-dibromoethane (0.861 ml, 10 mmol) was stirred in DMF (10 ml) at 120–150 °C for 4 h. After cooling the mixture, water (10 ml) was added to the solution. Then the white solid1 was produced and also, the mixture was filtered. In addition the collected filtrates was evaporated on the rotary evaporator. Finally, the crystal of ethylenebis(triphenylphosphonium)bromide were obtained.
Secondly, (methodB): a mixture of triphenylphosphine (20 mmol, 5.84 g) and 1,2-dibromoethane (10 mmol, 0.861 g) was magnetically stirred under solvent-free condition and at 150 °C for 6–7 h. After cooling the mixture, water (10 ml) was added to the solution. Then the mixture was filtered, and so, evaporated the solvent on the rotary evaporator. Finally the white crystals of EBTPPB were collected.
Finally, the best method (Method C) is using a quantity of catalytic SbF5, that is added to 1,2-dibromoethane for separating bromine, then triphenylphosphine was added to those in order to give EBTPPB (80%, 5.69 g). Mp: 321–323 °C.
1H NMR, (CDCl3), (δ, 3.4 ppm, S, CH2), (7.76, S, aromatic hydrogens).
IR(KBr)nm, 590nm(w), 720nm(m), 760 nm(S), 1150nm(m), 1375nm(m), 1440nm(S), 1580–1640nm(m), 3300nm(h).
2.2 Preparation of ethylenebis(triphenylphosphonium)chlorochromate (EBTPPCC)
To a solution of (10 mmol, 0.99 g)CrO3 in a 100 ml beaker, hydrochloric acid 6 M (15 ml) was added over a period of 0.5 h and after that, stirring was continued for 0.5 h. A clear orange solution was obtained. Then (5 mmol, 3.56 g) ethylenebis(triphenylphosphonium)bromide in 25 ml water was added to the solution of CrO3 in 6 N HCl. The reaction mixture was cooled to 0 °C and the yellow-orange form was collected by filtration, then it was separated and dried under vacuum for 24 h to give EBTPPCC (80.46%, 3.25 g). Mp: 319 °C.
2.3 Oxidation of alcohols: general procedure
To a solution of benzylic alcohol (1 mmol) in acetonitrile (or water), (10 ml) ethylenebis(triphenylphosphonium)chlorochromate (1 mmol, 0.823 g) was added and the mixture was magnetically stirred under reflux condition for 65–200 min. The progress of the reaction was monitored by TLC (eluent: n-hexane/EtoAC 5:2). After completion of the reaction, the solvent was evaporated and diethyl ether (10 ml) was added to the residue. The supernatant was decanted and the insoluble residue was washed by diethyl ether (3 × 10 ml). The combined ether extracts were concentrated under reduced pressure and the crude product was purified by column chromatography.
2.4 General procedure for the oxidation of alcohols under microwave irradiation
In microwave a mixture of an alcohol (1 mmol) and EBTPPCC (1 mmol, 0.823 g) was irradiated in closed vessel in a microwave oven (operating at 1000 W) for 3–9 min, and work-up was continued as mentioned above to get the corresponding aldehyde or ketone (Table 2). The mole ratio of substrate to the oxidant: 1:1.5, under reflux condition.
Entries
Substrate
Product
Yield (%)a (Time)
Acetonitrile
MWb
Water
1
96 (80 min)
100 (3.5 min)
73 (90 min)
2
98 (45 min)
98 (3 min)
76 (45 min)
3
90 (120 min)
99 (5 min)
80 (135 min)
4
98 (80 min)
100 (4 min)
80 (80 min)
5
88 (160 min)
100 (6 min)
80 (150 min)
6
80 (150 min)
85 (4 min)
78 (180 min)
7
78 (180 min)
90 (9 min)
80 (200 min)
8
95 (45 min)
95 (3 min)
90 (60 min)
9
66 (400 min)
95 (12 min)
57 (420 min)
10
69 (320 min)
98 (8.5 min)
67 (350 min)
11
63 (400 min)
90 (10 min)
60 (420 min)
12
95 (40 min)
100 (5 min)
73 (40 min)
13
95 (30 min)
95 (2 min)
75 (40 min)
14
95 (120 min)
100 (5 min)
80 (120 min)
15
80 (450 min)
90 (7 min)
75 (420 min)
16
75 (360 min)
85 (20 min)
85 (20 min)
17
68 (400 min)
78 (18 min)
65 (420 min)
18
65 (430 min)
70 (20 min)
65 (480 min)
3 Results and discussion
We have synthesized ethylenebis(triphenylphosphonium)chlorochromate (EBTPPCC) and studied its oxidizing properties for alcohols under different reaction conditions. This compound is easily and cheaply prepared from triphenylphosphine and 1,2-dibromoethane followed by treatment with CrO3 in the presence of HCl with high yield. It is completely soluble in DMF, acetone and acetonitrile, and insoluble in methylenechloride, n-hexane and diethyl ether. One more advantage of EBTPPCC in the comparison of the PCC and similar Cr(VI) reagents that is much less hygroscopic and also, is not decomposition at room temperature. The chromium content of the reagent and the calculated results are in very good agreement. A wide variety of alcohols such as benzylic, allylic and aliphatic were treated with 1 equivalent of EBTPPCC in refluxing CH3CN. In order to find the optimum condition for the oxidation of benzylic alcohols with (EBTPP)ClCrO3, we have chosen 4-chlorobenzyl alcohol as a model substrate. We have accomplished the oxidation in the different solvents such as n-hexane, acetone, DMF, THF, chloroform, dichloromethane and also acetonitrile under reflux and room temperatures and with different molar ratios of oxidants. The results are shown in Table 1.
Entries
Solvent
Reaction time (min)
Yield (%)b
1
CH3CN
45
98
2
Acetone
70
80
3
THF
70
80
4
CH2Cl3
70
56
5
CHCl3
70
65
6
n-Hexane
70
40
7
DMF
180
70
8
CH3CN
90
60c
9
Water
45
76
In order to show the applicability and generality of this method, we have examined the reaction of benzylic and aliphatic alcohols with (EBTPP)ClCrO3 in refluxing acetonitrile and water (Scheme 1). The results are shown in Table 2.
As shown in Table 2, benzylic alcohols were oxidized to the corresponding aldehydes or ketones with good to excellent yields (Table 2, entries 1–15) No significant byproducts, such as over oxidation to the respective carboxylic acids, were observed. The oxidation of allylic alcohols was somewhat slower. Thus cinnamyl alcohol has been required 7.30 h, for completion of the reaction (Table 2, entry 15). It is an evidence from Table 2 which was shown the exception for all the nitro benzyl alcohols (Table 2, entries 9–11). The selectivity of this reagent was indicated by oxidation of benzylic alcohols in the presence of aliphatic primary and secondary alcohols. The treatment of 1-propanole or 2-propanole and also cyclohexanole under refluxing acetonitrile in the presence of EBTPPCC were oxidized to the corresponding carbonyl compounds in 6–8 h (Table 2, entries 15–18).
Generally the rings had electron releasing groups which were reactive toward an electrophile and this oxidant. On the contrary the rings contained electron withdrawing groups which were unreactive toward this reagent. The functional groups such as nitro were also unreactive to this reagent and electrophiles (Table 2, entry 9–11).
The effect of halogen substituent upon an aromatic ring is the predominance regarded as being an electron donating group via resonance of л charge to the ring. It is observed experimentally that in the presence of halogens they greatly activate the aromatic ring toward this reagent and an electrophilic attack (Table 2, entries 2–5).
We have also conducted these oxidation reactions under microwave irradiation and refluxing water. When we carried out the oxidation reaction under microwave irradiation, oxidation of benzylic alcohols occurred in an even much shorter time 3–12 min, with excellent yields (Table 2); all reactions produced better yields in comparison to conventional conditions. In addition reactions in water produced good yields in comparison to some organic solvent but the yield was less than refluxing acetonitrile (Table 2). Also, those aliphatic alcohols were remained unchanged (Table 2, entries 16–18). However, water is too important in the green chemistry.
4 Conclusion
In conclusion, we have shown that EBTPPCC is an efficient and inexpensive reagent for the selective oxidation of benzylic alcohols. Allylic and benzylic alcohols under microwave irradiation are oxidized in a much shorter reaction time in comparison to refluxing in the acetonitrile. Over oxidation to the corresponding carboxylic acid was not observed at all. Interestingly, the α,β-unsaturated alcohols underwent oxidation very efficiently without an effect on the olefinic bond and the reaction is an essentially chemoselective. Furthermore, functional groups such as nitro were also inert to this reagent and no byproduct was observed.
Acknowledgements
The authors are grateful to the Ferdowsi University of Mashhad and the Sabzevar Tarbiat Moallem University Research Councils for the financial support of this work.
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