2245
Oxyfunctionalization of Hydrocarbons. 1. Protolytic Cleavage-Rearrangement Reactions of Tertiary Alkyl Hydroperoxides with Magic Acid George A. Olah,* David G. Parker,la Norihiko Yoneda,lBand Franco PelizzaIb Contribution from the Department of Chemistry, Case Western Reserue Uniuersity. Cleveland, Ohio 44106. Received August 25, 1975
Abstract: Magic acid catalyzed cleavage-rearrangement reactions of a series of tert-alkyl hydroperoxides were studied. Similar reactions were achieved by treating directly the corresponding tertiary alcohols with hydrogen peroxide and magic acid. For tert-butyl hydroperoxide both 0-0and C - 0 cleavage reactions were observed, depending on reaction conditions. The intermediate carboxonium ions generated from 0-0 cleavage reactions were examined by I3C N M R spectroscopy.
Introduction T h e reaction of hydrogen peroxide with tertiary alcohols in the presence of acid has long been used for preparation of hydroperoxides. For example, reaction of tert-butyl alcohol with hydrogen peroxide together with a catalytic amount of concentrated sulfuric acid at 0-5’ gives tert-butyl hydroperoxide in good yield (eq 1).2,3
ion 3; the latter immediately rearranges to the alkylated (arylated) ketone (carboxonium ion) 4 which then reacts further, depending on conditions employed (eq 6).
The reaction was assumed to proceed via a n SN1 pathway (eq 2), although in some cases such as that of 2-phenyl-
Ph-C-0,H‘ e
I\
“H CH, / 0-0-H
-
3
Rz\
I
Ph-C-OOH
( 3)
I
+ HOR,
,C=O
R3
e.g. H,O
R2\ ,C=O+ R, 4
CH,
butan-2-01, partial participation of an SNi mechanism (eq 3) could not be completely ruled o w 4 Hydroperoxides themselves react with acids, depending upon conditions, and acid-catalyzed rearrangement and cleavage reactions of particularly alkaryl hydroperoxides such as cumyl hydroperoxide have been extensively investigated.5 T h e initial step involves protonation a t either oxygen atom, leading to the two conjugate acids 1 and 2 (eq 4).
R,-C---OOH,
I
2
1 can undergo C - 0 heterolysis forming hydrogen peroxide and the corresponding carbenium ion (eq 5), while for 2 cleavage of the oxygen to oxygen bond results in formation of water and the corresponding highly energetic oxenium
Olah, Parker, Yoneda, Pelizza
Oxenium ions of type 3 have never been observed as longlived species in solution,6 and it may well be that a concerted alkyl (aryl) shift and elimination (eq 7 ) follows the initial protonation. Rj
-.
The relative migratory aptitude of the R group shows the trend phenyl > vinyl > cyclopentyl > propyl > ethyl > methyL5 Hydrogen was shown to be = propyl.1o tert-Butyl hydroperoxide is remarkably resistant to acidcatalyzed rearrangement. Sulfuric or perchloric acid in acetic acid failed to induce rearrangement, the reaction occurring via C - 0 heterolysis resulting in formation of 2-methylpropene and hydrogen peroxide.* In the presence of boron trifluoride, tert- butyl hydroperoxide yielded some acetone and methanol.’ More recently, however, Deno, using 9098% sulfuric acid at 25O, showed that exclusive 0-0 heterolysis occurred followed by methyl migration and hydrolysis, giving acetone and methanol.
(CH3)3COOH
H2SO4
(CH3)2CO (100%) t CH3OSO3H (100%) (8)
/
Reactions of tert-Alkyl Hydroperoxides with Magic Acid
2246 Table I.
,C
NMR Parameters of tert-Butyl Systemsa ' ,C chemical shift. uum Compound -c-0-CH,
I
CH,-C---OOH
I
tert-Butyl hydroperoxide 80.73 25.89 tert-Butyl peroxide 78.15 26.69 tert-Butyl alcohol 68.68 30.99 Di-tert-butyl etherb 76.3 33.8 a CDCl,, internal Me,%. b G. A. Olah and H. Lin, Synthesis, 315 (1975).
n+
I
CH,-C--OOH,
+
I
bH3
CH3
5
6
Interested in hydrocarbon functionalization reactions, we have now carried out a study of the reactions of tert-butyl hydroperoxide with superacids such as magic acid. W e found that by varying the amount of acid used both pathways (eq 5 and 6) could be observed. Our studies were also extended to the effect of strong acids on other tert-alkyl hydroperoxides by both direct and indirect methods. The latter involved reacting the corresponding tertiary alcohol in situ with hydrogen peroxide" and magic acid, while in the former, the hydroperoxide itself was added to the acid solution. Moreover, use of superacid media enabled direct observation of the carbocationic intermediates formed in the acid-catalyzed transformations of hydroperoxides, thus allowing to unequivocally establish the mechanism of the reactions.
Results and Discussion Preparation and Carbon-13 NMR Spectroscopic Analysis of Alkyl Hydroperoxides.The tert-alkyl hydroperoxides used in our study were usually prepared by the acid-catalyzed alkylation of hydrogen peroxide, and were analyzed as described by Hiatt.I2 Although this method is fairly reliable for hydroperoxide determinations, it tells nothing of the nature of any impurities present. Instrumental methods, e.g., ir, ' H N M R , R a m a n , and uv spectroscopy have also been employed for identification of impurities such as alcohols and peroxides contained in hydroperoxides. However, overlapping absorptions often make peak assignments difficult. FT I3C N M R spectroscopy was found to be a most convenient method for the analysis of alkyl hydroperoxides. T h e carbon shifts of the hydroperoxides are significantly different from those of the starting alcohols, as well as the corresponding peroxides. This is illustrated for the tertbutyl systems in Table I . W e were able to detect the presence of impurities a t levels of about 5% or less. For example, 1% of tert-butyl alcohol could be detected in tert-butyl peroxide after -400 transients. Furthermore, the I3C N M R method has the advantage of determining the nature of the impurities. I3C N M R parameters of the other hydroperoxides studied together with those for the corresponding alcohols are given in Table 11. Reaction of Alkyl Hydroperoxides with Magic Acid. tert-Butyl Hydroperoxide. Careful addition of an equimolar amount of tert-butyl hydroperoxide ( 5 ) in S O l C l F at -78' to a well-stirred solution of magic acid in S 0 2 C l F a t -78' gave a vigorous reaction. IH and I3C N M R spectra of the resultant clear pale yellow solution were consistent with reaction via 0-0 cleavage, as were the spectra obtained when a twofold excess of magic acid was employed. T h e latter spectra were more clearly defined as the protonated oxygenated species were "frozen out'' under these conditions, the reaction occurring as shown in Scheme I . The water formed in this reaction (particularly when the concentration of excess superacid is lower) can readily hydrolyze the dimethylmethoxycarbenium ion (6) yielding acJournal of the American Chemical Society
/
98.8
/
NMR 1, 27.01; 2, 32.2; 3, 24521; 4, 68.81 'H NMR 42, 3.37; 4, 5.20
13C
Table 11. "C NMR Chemical Shifts of tert-Alkyl Hydroperoxidesa
Compound
iyH
1
2
Peak number 3
4
84.16
32.14
24.44
9.35
71.0
36.56
28.70
8.77
85.14
34.08
21.0
17.6
73.05
38.88
26.44
11.77
92.1
36.86
24.88
23.59
79.7
41.3
28.33
24.18
77.28
35.47
26.0
24.9
22.53
69.76
39.66
29.67
26.0
22.88
85.11
28.75
20.7
8.0
72.92
33.91
25.9
8.29
5
I
CH,CH CH,&
I I CH CH
CH -C-OH
a In CDCL,, internal Me,Si. etone and methanol, both being observed as protonated species (Scheme 11). On reacting 5 with a fivefold excess of magic acid, ' H and I3C NMR showed formation of the trimethylcarbenium ionI4 (7), together with absorptions due to the intermediate ion 6, protonated methanol, and protonated acetone. Formation of 7 which occurred to the extent of 15% implies that with a large excess of magic acid C - 0 cleavage com-
April 14, 1976
2247 Scheme I1
NMR I, 31.6; 2, 30.21; 3, 248.53, 1, 6201 'H NMR 1,2,3.47; 4, 14.30, 1, 4.80, t, J = 4 Hz; 2, 9.50, q, J
= 4 Hz
petes effectively with 0-0 cleavage, as shown in Scheme 111. Scheme 111
+ H+
CH,-C-OOH
I
== CH,-~+-OH
==
I +
CHI
CH,
2
CHJ
I
CHi---C+
I
+ H20L
'CHi 7
NMR I, 335.67; 2, 47.78 'H NMR 2, 4.30
Whereas the reaction of tert- butyl hydroperoxide with perchloric acid yields products from C - 0 heterolysis8 the reaction with a twofold excess of magic acid gives exclusive 0-0 cleavage, while a fivefold excess of magic acid yields products via both 0-0 (85%) and C - 0 (15%) cleavage pathways. T h e differing reactivity can readily be explained as being due to the overall reversibility of the reaction shown in Scheme 111, i.e. CH
CHJ
I CH,-C--OOH I
+ H+
ple of 1:l magic acid shows the acid proton peak a t 6 12.2 and generally a hydronium ion impurity peak a t ca. 6 9.9. For the sample of magic acid used, the hydronium ion peak was -8% of the acid peak. Addition of 0.02 g of hydrogen peroxide (98%)19 to magic acid (1 g in S02CIF a t -78O) increased the relative intensity of the hydronium peak to 11% of the acid peak and this was increased to 18% by addition of a further 0.04 g of hydrogen peroxide with no observable broadening or appreciable change in chemical shift. However, on addition of a further 0.04 g of hydrogen peroxide to the acid solution, the 'H N M R spectrum showed broadening such that no peak was discernible from the baseline. Higher concentrations, ca. 0.1 g of hydrogen peroxide in 1 g of magic acid, show a single broad peak in the IH N M R spectrum a t -70' a t 6 10.7. T h e IH N M R results indicate that a t low concentrations (
