Oxidation of alcohols under non-aqueous conditions
(Rated as: excellent)
Oxidation of alcohols with benzyltriphenylphosphonium periodate under non-aqueous conditions.
Abdol Reza Hajipour, Shadpoul E. Mallakpour, Heshmat Allah Samimi
Synlett, no. 11, 2001, p. 1735-1738
A variety of alcohols were converted to their corresponding carbonyl compounds by using benzyltriphenylphosphonium periodate (I) as a useful reagent under non-aqueous conditions.
Key words: oxidation, benzyl alcohol, non-aqueous conditions
Oxidation of organic compounds under non-aqueous and aprotic conditions has an important role in modern organic synthesis. Most of them suffer at least from one of the following disadvantages:
1) high cost of preparations
2) long reaction times
6) no selectivity
8) dangerous procedures for their preparation
9) tedious work-up procedures
In continuation of our previous works, we introduce an efficient, mild and rapid method for the selective oxidation of alcohols to their corresponding carbonyl compounds by using benzyltriphenylphosphonium periodate (I) which is readily prepared by reaction of an aqueous solution of benzyltriphenylphosphonium chloride with NaIO4 solution at room temperature:
Ph3P+CH2PhCl- + NaIO4 --(H2O)--> Ph3P+CH2PhIO4-
The resulting white powder, which can be stored for months without loss of its reactivity, is soluble in acetonitrile, acetone, DMF, chloroform, ethyl acetate and DCM, and insoluble in CCl4, n-hexane and diethyl ether.
Here we wish to report the oxidation of aliphatic and benzylic alcohols to the corresponding carbonyl compounds in refluxing acetonitrile. The oxidation of alcohols with (I) in the presence of catalytic amounts of Lewis acid proceeds under non-aqueous conditions. The effect of various solvents such as cyclohexane, DCM, THF, CHCl3 and CH3CN on the oxidation of benzyl alcohol with (I) in the presence of AlCl3 was examined. TLC was used to monitor the rate of the oxidation. Only acetonitrile was a suitable solvent for this oxidation system (table 1).
Table 1 : Oxidation of benzyl alcohol with reagent I in different refluxing solvents
entry - solvent - time (min.) - yield (%)a,b
1 - cyclohexane - 180 - 50
2 - THF - 120 - 40
3 - methylene chloride - 120 - 60
4 - chloroform - 170 - 70
5 - acetonitrile - 90 - 100
a Monitored by TLC analysis.
b Oxidant/alcohol/AlCl3 (1:1:0.2)
Oxidation of benzyl alcohol to its corresponding aldehyde with reagent I in the presence of various Lewis acids such as ZnCl2, FeCl3, BiCl3 and AlCl3 were also examined in refluxing acetonitrile. Suprisingly, only AlCl3 was shown to be an effective catalyst for this purpose. The reaction in the presence of ZnCl2, FeCl3 and BiCl3 (0.5 mmol) proceeds with lower efficiency even with a higher molar ratio of the oxidant (1.5 mmol) in comparision with the amount of oxidant used in presence of AlCl3 (0.3 mmol). This could be the effect of hardness or high solubility of AlCl3 in comparision with the other Lewis acids, which have been used in these experiments. Alcohols (II) are oxidized to the corresponding carbonyl compounds (III) in refluxing acetonitrile in good to excellent yields; benzoin was converted to benzil in high yield :
PhCH2Ph3P+IO4- + R1-CH(OH)-R2 --(MeCN/AlCl3 reflux)--> R1-C(=O)-R2 R1,R2 = alkyl, aryl, H
When we tried to oxidize the allylic alcohols with this reagent several by-products were obtained. Therefore this reagent is not suitable for the oxidation of allylic alcohols. Table 3 shows the experimental results.
The functional groups such as NO2 and MeO decrease the rate of the reaction. This could be the effect of producing complexes of these functional groups with AlCl3 (Table 3)
Table 3 : Oxidation of alcohols IIa-v with reagent I in acetonitrile under refluxing conditions.a
Alcohols - Time (h) - Yield (%) - mp or bp/Torr found - mp or bp/Torr reported
2a PhCH2OH - 1:30 - 78 - 176-177/760 - 178-179/760
2b 2-MeOC6H4CH2OH - 4:10 - 82 - 36-39 - 37-39
2c 3-MeOC6H4CH2OH - 4:00 - 90 - 100-103/10 - 143/50
2d 4-MeOC6H4CH2OH - 3:45 - 86 - 118-120/13 - 248/760
2f 2,5-(MeO)2C6H4CH2OH - 3:55 - 90 - 50-52 - 49-52
2g 3,4-(MeO)2C6H4CH2OH - 3:20 - 86 - 41-43 - 42-45
2h 4-NO2C6H4CH2OH - 6:00 - 84 - 105-107 - 105-108
2i 3-NO2C6H4CH2OH - 6:20 - 81 - 56-59 - 57-59
2j 2-NH2C6H4CH2OH - 7:00 - 82 - 38-40 - 39-40
2k 2-ClC6H4CH2OH - 3:30 - 90 - 212-214/760 - 209-215/760
2l 4-ClC6H4CH(CH3)OH - 3:10 - 83 - 230/760 - 232/760
2m 4-BrC6H4CH(CH2Br)OH - 9:00 - 90 - 107-108 - 108-110
2n PhCH(CH3)OH - 9:25 - 95 - 200/760 - 202/760
2o Ph2CHOH - 8:40 - 89 - 49-50 - 49-51
2p PhCOCH(OH)Ph - 2:00 - 83 - 93-94 - 94-95
2q Ph(CH2)2OH - 14:00 - 81 - 193-196/760 - 195/760
2r Ph(CH2)3OH - 14:00 - 88 - 103-105/13 - 104-105/13
2s CH3(CH2)2CH2OH - 3:50 - 95 - 76-79 - 78-80
2t cyclohexanol - 7:00 - 100 - 153-155/760 - 155/760
2u menthol - 8:00 - 87 - 208-210/760 - 207-210/760
2v beta-citronellol - 9:00 - 82 - 207-208/760 - 207-208/760
a All compounds were characterised by comparing with known samples.
Table 4 : Reaction of Benzyl alcohol under refluxing conditions.
Reagent - solvent - Lewis acid - time (min.) - yielda (%)
1 BTPPPI - CH3CN - none - 220 - 30
2 NaIO4 - CH3CN - none - 220 - no reaction
3 NaIO4 - CH3CN/H2O - none - 220 - 20
4b NaIO4-BTPPPI - CH3CN - none - 180 - 10
5c NaIO4 - CH3CN - AlCl3 - 120 - 30
6c BTPPPI - CH3CN - AlCl3 - 90 - 100
a Monitored by TLC
b BTPPPI was used as catalytic amount (0.2 mmol)
c Oxidant/alcohol/Lewis acid (1:1:0.3).
Preparation of benzyltriphenylphosphonium periodate (I)
A solution of benzyltriphenylphosphonium chloride (10.3 gr, 26 mmol) in 50 ml of water was prepared, then NaIO4 (5.56 gr, 26 mmol) in 50 ml of water was added dropwise to the above solution and stirred for 20 min at RT. The resulting precipitate was filtered and washed with cooled distilled H2O (50 ml) and dried in a dessicator under vacuum over CaCl2 to afford a white powder (13.9 gr, 98% yield), which decomposed at 138-140°C to a dark-brown material.
Oxidation of the alcohols II to the corresponding carbonyl compounds III with reagent I in refluxing CH3CN in the presence of AlCl3, general procedure.
In a round-bottomed flask, a solution of the alcohol II (1 mmol) in CH3CN (10 ml) was treated with PhCH2P+Ph3IO4- (0.54 gr, 1 mmol) and AlCl3 (0.04 gr, 0.3 mmol) and refluxed for 1.30-14 h. TLC (cyclohexane/EtOAc, 8:2) was used to monitor the reaction progress. The reaction mixture was cooled to RT and the solid filtered off and washed with 15 ml of CH3CN. The filtrates were evaporated on a rotary evaporator and the resulting crude material was purified by column chromatography on silica gel with an appropriate eluent or distillation in vacuum to afford pure carbonyl compounds in 78-100% yields (table 3).
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