Recently, Pfitzner and Moffatt2 have reported that the oxidation of alcohols in the presence of phosphoric acid, dicyclohexylcarbodiimide, and dimethyl sulfoxide produced aldehydes and ketones. This reaction appears to involve dimethyl sulfoxide as a reactant and resembles an earlier report by Kornblum3 and Nace and Monagle,4 who obtained aldehydes from the reaction of primary halides or tosylates with dimethyl sulfoxide.
In continuation of our study of the reactions of alcohols in dimethyl sulfoxide,5 we wish to report the selective oxidation of a variety of benzyl alcohols to the corresponding aldehydes. This oxidation proceeded conveniently by refluxing the alcohol in dimethyl sulfoxide and was facilitated by passing a stream of air through the reaction medium. The reaction stopped at the aldehyde stage with over-oxidation to the acid occurring in only one case in 3% yield. In fact, when a solution of benzaldehyde in dimethyl sulfoxide was refluxed (190°C) for 24 hr. with air passing through the solution, only 1.6% benzoic acid was isolated with 87% benzaldehyde recovered. The unique feature of these observations is the role of dimethyl sulfoxide in inhibiting further oxidation of aldehydes yet permitting the oxidation of alcohols.
The following procedure is representative of this oxidation. A solution of benzyl alcohol (10.8 g., 0.10 mole) and dimethyl sulfoxide (54.6 g., 0.70 mole) was heated for 14 hr. at reflux with air passing through the solution. The mixture was cooled, diluted with water, extracted with ether, and the ether extract washed with water, dried, and distilled. The yield of pure benzaldehyde, bp 75-77°C (22 mmHg), n20D 1.5440, 2,4-dinitrophenylhydrazone mp 236-237°C, was 8.5g (80%). Table I contains a variety of examples which were subjected to these conditions for 4 to 48 hr. Unless stated otherwise, the yields in Table I represent isolated purified material.
Oxidation Of Benzyl Alcohols To Aldehydes
Although air facilitates the reaction, oxygen does not appear to be the oxidant. There was no oxygen uptake when the oxidation of benzyl alcohol was performed over a measured volume of oxygen under conditions which produced benzaldehyde in 60% yield. In this experiment dimethyl sulfide, identified by the mercuric chloride derivative, mp 146-148°C (lit.6 mp 150-151°C), was isolated in 48% yield while 65% dimethyl sulfide was recovered from the oxidation of benzyl alcohol described above in the representative procedure of oxidation. These results support the assignment of dimethyl sulfoxide as the oxidant; however, in the absence of oxygen the reaction failed (only 3% benzaldehyde was formed). In a second experiment under oxygen-free conditions t-butyl peroxide (0.001 mole) was added to a solution of benzyl alcohol (0.010 mole) and dimethyl sulfoxide (0.070 mole) at 190°C and a second portion of t-butyl peroxide (0.001 mole) was added 2 hr. later. After a total reaction time of 4 hr., the reaction mixture was processed and gave benzaldehyde in 60% yield. These data require the appearance of radicals in the mechanistic scheme of this oxidation.
In addition, experimental conditions qualitatively suggest that electron-releasing substituents para to the carbinol carbon enhance reaction while electron-withdrawing groups retard oxidation. These matters are currently under investigation.