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J.Org. Chem., Vol. 41, No. 14,1976
Reaction of tert-Butyl Hydroperoxide Anion with Dimethyl Sulfoxide. On the Pathway of the Superoxide-Alkyl Halide Reaction’ Morton J. Gibian* and Timothy Ungermann Department of Chemistry, University of California, Riverside, California 92502 Received November 10,1975 Superoxide ion ( O y - ) has been of considerable recent interest because of its biochemical ubiquity (all aerobic cells produce it) and as a species of relatively unexplored chemical reactivitya2It has the potential of acting as a redox (oxidizing or reducing) agent, or as a nucleophile (or base). With alkyl halides, it apparently acts in its nucleophilic capacity. Two original studies were of electrochemically generated 02.- reacting under slightly basic conditions with the butyl halide^;^ hydroperoxides (by GC) and dialkyl peroxides were reported as respective products in Me2S03a and DMF.3b Recently, two reports utilizing KO2 solubilized with crown ether showed that stereochemical inversion occurs a t the reacting carbon atom of alkyl halides and t ~ s y l a t e s .However, ~,~ San Filippo et al.* (using RXKO2:18-crown-6 of 3.3:101 mole ratio in Me2SO) found primarily alcohols (some olefin) as the major end products, while Johnson and Nidy5 (all reagents 1:l:l using dicyclohexyl-18-crown-6in benzene, one experiment with 0.1 equiv of crown ether in benzene) found primarily dialkyl peroxides (plus olefin and a little alcohol in some cases). At first it seemed that the reagent ratios (particularly K02:RX) might be causing the difference in products, in that the prior workers mentioned that in benzene or HMPA similar products but in lower yield resulted. T h a t is not the case, however, and we report here on the extremely rapid reaction of alkyl hydroperoxide ion, a likely intermediate in the 0 2 . - plus alkyl halide reaction, with Me2SO to give a quantitative (by GC) yield of corresponding alcohol and dimethyl sulfone.
Experimental Section Solvents were reagent grade, dried over molecular sieve. Distillation of MezSO from desiccant did not reduce its water content below that as supplied in Baker Analyzed material. KOz, from Alfa, was used as obtained after thorough grinding to a fine powder in a dry atmosphere (glove bag). Crown ether (18-crown-6)was synthesized as reported by Gokel et a1.,6 and peroxides were distilled Lucidol products. All reactions were run at room temperature by adding ground KO2 or base to solvent with or without crown ether, followed by further grinding and dispersing with a spatula, addition of peroxide, and then vigorous stirring throughout. Several workup procedures were employed. tert-Butyl alcohol is not well extracted into organic solvent from MezSO/water, even after acidification (tert-butyl hydroperoxide is also poorly extracted).For the runs in MezSO, the addition of water at the end of a run (whether KOH, KzC03, or KOz) did not affect the alcohol and peroxide GC peak areas in any case. Apparently the activity of the water in MezSO (“dry”)is sufficient to keep at least 95% of the OH groups protonated. For the toluene runs 1 equiv (to base) of concentrated HCl was added to terminate reaction, the reaction mixture was thoroughly shaken, and an aliquot was removed for analysis. All analyses were on GC, and product identificationwas by retention volume.
Results and Discussion The simplest explanation for the difference in products might have been that in the alcohol-producing study excess KO2 reacted with product dialkyl peroxide, reducing it to alcohol and forming 0 2 . However, we found that 0.17 M ditert-butyl peroxide was completely stable when stirred at room temperature in Me2SO (20 ml) in the presence of 0.05 M 18-crown-6 and 10 mmol of well-dispersed KO2 for up to a day. This experiment was repeated several times under various conditions, another set being included in Table I.7
Notes Since the product in this case is stable, and the same initial inverting 02.- displacement very likely occurs in both solvents, an intermediate must be somehow diverted in one of the systems (KOz/crown ether is stable in MezSO for long times). Pathways involving bimolecular peroxy radical reaction are unlikely for two reasons. First, the concentration of the peroxy radicals v i s - h i s other potential exothermic reactants (02.-, RX hydrogens, solvent) is very low, so that even given the high rate constant for recombination it is unlikely to be a major route. Second, such a mechanism fails to account for the dramatic effect of solvent on products. Alkyl hydroperoxide anion is a second likely intermediate on the route to dialkyl peroxide, being produced by reduction of the initially formed alkyl peroxy radical by 02’-. The peroxide anion might be intercepted by either excess 02.- or solvent rather than participating in a subsequent displacement on halide, and we here report data showing that such a competitive situation is reasonable. Table I, presenting representative data from a variety of experiments, indicates that in mixtures of tert- butyl hydroperoxide, KO2, and MezSO with crown ether the only reaction observed is the base-catalyzed transformation of hydroperoxide and MezSO to tert-butyl alcohol and dimethyl sulfone. I n toluene, no loss of hydroperoxide occurs with KO2 or crown ether or both together. While hydroperoxide itself is stable in MezSO crown ether, the addition of just base (K2CO3 or KOH) causes a rapid quantitative transformation to alcohol and sulfone (eq 1 and 2). ROOH B t ROO-+ BH’ (1) BH’ ROO(CHJ2S0 B ROH (CH,)BO, (2a) or ROO(CHJ2SO RO(CH&SO, (2b) ROBH’ ROH B The reaction with KO2, since it does not occur in 2 X lo4 the time in toluene, would seem to be eq 2 in which B is 02’- and BH+ is HOO.. The equilibrium in eq 1would lie far to the left, but a t least one of the species on the right is then extremely reactive in Me2SO and not in toluene.8 From Table I it is clear that both crown ether and base are required to catalyze the hydroperoxide reaction with Me2SO. KOH is more effective than K2C03, and KO2 is apparently more like KOH. Several entries show that crown ether is required to solubilize the base, and that the rate increases as its concentration is raised. Two runs, one with KO2 and the other with KOH, were found to produce quantitative yields of tert- butyl alcohol and dimethyl sulfone. The analogous oxidation of sulfoxides with H202 and organic peracids has been known for some time, and involves 0 atom transfer with formation of a covalent intermediate adduct.g I t has been shown that two processes can occur:1° a slower one in neutral or acidic medium (pH independent) that involves attack by H202 or peracid itself, and a rapid reaction of HO2- or peracid anion on sulfur that is dependent on the pK, involved. Alkyl hydroperoxides have been investigated in neutral and acidic medium, and give oxidation of sulfides to the sulfoxide stage only.ll With transition metal catalyst sulfones are formed.12 In a study of the autoxidation of triphenylmethane, Russell and Bemis13 found that triphenylmethyl hydroperoxide, in a base-requiring process, yielded triphenylmethylcarbinol in 95% yield in Me2SO (they also found MezSOz), but not in HMPA or DMF. An attractive pathway for the hydroperoxide reaction, based on that for H202 and peracids, with sulfoxide is shown in Scheme L’Crown ether catalyzes this process by solubilizing the cations. There is a basis for preferring the process in eq 5 over that in eq 4 (or 2a over 2b) because in addition to an excess of base (KO2, K2CO3, KOH) in each reaction in Table I,
+
+
+
+
+
-+
--
+
+ +
,
J.Org. Chem., Vol. 41, No. 14,1976 2501
Notes
Table I. Reaction of tert-Butyl Hydroperoxide in the Presence of Various Bases and Crown Ether Solventa
t-BuOOH, M
3
MezSO MezSO Me2SO
0.17,2.5 2.5 2.5
4
MezSO
0.17
1 2
.05
