ASCII by Aurelius, HTML by Rhodium
Calcium hypochlorite, a relatively stable, and easily stored and used solid hypochlorite oxidant, was found to oxidize secondary alcohols to ketones in excellent yields. Primary alcohols gave esters where both the acid and the alcohol portions of the ester were derived from the alcohol. Ethers were oxidized to esters though only in moderate yields.
While carrying out studies on the Grob-type cleavage of gamma-hydroxysulphones2,3 we attempted the preparation of the corresponding hypochlorite by the action of sodium hypochlorite on 2-tosylmethyl cyclohexanol4 Instead of the hypochlorite, an excellent yield of the corresponding ketone (2-tosylmethylcyclohexanone) was obtained. Subsequent to this conversion, we independently found that this method was general for converting secondary alcohols to ketones by the use of sodium hypochlorite or commercially available Chlorox solutions. Though our conditions differed somewhat, the results were essentially the same as those reported by Stevens5.
Table 1
The Oxidation of Secondary Alcohols Using Ca and Na Hypochlorite
Run |
Substrate | Product | Yielda Ca(OCl)2 |
Yield NaOCl |
Ref. |
1 |
l-Menthol | l-Menthone | 98% |
98% | |
2 |
Borneol | Camphor | 98% |
99% | |
3 |
Norborneol | Norcamphor | 92% | - | |
4 |
Cyclohexanol | Cyclohexanone | 91% |
98% | |
5 |
2-tosylmethyl- cyclohexanol4 | 2-tosylmethyl- cyclohexanonec |
98% |
98% | - |
6 |
3,5-dimethyl- cyclohexanol | 3,5-dimethyl- cyclohexanone |
93% | - | |
7 |
5-cholesten-3-ol | 4-cholesten-3-one | 91% |
91% | |
8 |
3-pentanol | 3-pentanone | 97% | - | |
9 |
3-pentanol | 3-pentanone | - |
87% | |
10 |
2-octanol | 2-octanone | 80% |
99% | |
11 |
diphenylcarbinol | benzolphenone | 98% | - | |
12 |
2-tosylmethyl- 1-phenylethanol4 | α-tosylmethyl- acetophenoned | - |
98% | - |
The instability of sodium hypochlorite solutions, however, led us to consider other hypochlorite reagents which would be more stable and easier to handle. Calcium hypochlorite6 is a commercially available solid and is inexpensive. Since this reagent does not decompose significantly when stored without light in a desiccator7, carrying out oxidations by weighing the required amount of solid oxidant represented a more convenient method than using solutions which would frequently have to be titrated. We now wish to report our results concerning the use of Calcium hypochlorite as an oxidant.
Oxidations of secondary alcohols with this reagent proceed smoothly, and in excellent yields at 0°C in a solvent containing acetic acid. Our results are given in Table 1 for twelve compounds along with our initial results using sodium hypochlorite.8
A general procedure is outlined for the oxidation of l-menthol to l-menthone:
Thus, Menthol (3g, 19mmol) was dissolved in acetonitrile:acetic acid (3:2, 25ml) and added dropwise over a period of 10 minutes to a cooled (0°C) and stirred solution of calcium hypochlorite (1.84g, 12.7mmol) in water (40ml). Stirring was continued for 1 hr after which water (40ml) was added. The solution was extracted with DCM (4x30ml) and the organic layers washed with 10% sodium bicarbonate followed by an aqueous wash. After drying with Magnesium sulfate and evaporation of solvent, the crude product was distilled affording menthone (2.89g, 98%). The spectra (IR and NMR) were identical with those of authentic material.9,10
Table 2
Oxidation of Primary Alcohols and Ethers
Using Calcium and Sodium Hypochlorite.
Run | Substrate | Product | Yielda Ca(OCl)2 |
Yield NaOCl |
Ref. |
1 |
Benzyl alcohol | benzaldehyde | 98% |
98% | |
2 |
1-pentanol | pentyl pentanoate |
83% |
91% | |
3 |
1-hexanol | hexyl hexanoate |
98% |
98% | |
4 |
3-methylbutanol | 3-methylbutyl isovalerate |
76% |
87% | |
5 |
ethyl alcohol | ethyl acetate | --- | ||
6 |
ethyl ether | ethyl acetate | --- | --- |
|
7 |
butyl ether | butyl butanoate |
40% | --- | |
8 |
THF | γ-butyrolactone | 68% | --- |
|
9 |
Tetrahydropyran | δ-valerolactone | 56%c | --- |
Oxidation of primary alcohols under identical conditions gave an aldehyde only in the case of benzyl alcohol11. Other primary alcohols gave esters as in Table 2. This table also includes our results on the oxidation of ethers to esters. Though the yields were not nearly as good for the alcohols, the data is reported because of the unusual and potentially useful transformation12. The ethers were oxidized under similar conditions as the alcohols except that the reactions were carried out at RT for 4-16 hours. Heating does not seem to increase yield.
We are presently carrying out studies to improve the yields on the ether to ester transformation and to utilize the primary alcohol oxidation for the preparation of lactones from α,ω-diols.