Diols, acid catalyzed from epoxides

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Detailed kinetic studies comparing the chemical reactivity ofK-region vs. non-K-region arene oxides have yielded important information. In aqueous solution, the non-K-region epoxides of phenanthrene (the 1,2-oxide and 3,4-oxides) yielded exclusively phenols (the 1-phenol and 4-phenol, respectively, as major products) in an acid-catalyzed reaction, as do epoxides of lower arenes (Fig. 10.1). In contrast, the K-region epoxide (i.e., phenanthrene 9,10-oxide 10.29) gave at pH < 7 the 9-phenol and the 9,10-dihydro-9,10-diol (predominantly trans) in a ratio of ca. 3 1. Under these conditions, the formation of this dihydrodiol was found to result from trapping of the carbonium ion by H20 (Fig. 10.11, Pathway a). At pH > 9, the product formed was nearly ex-... 

Fig. 10.14. Reactivity ofdiol epoxides (Nu = H20, HCT, or another nucleophile), a) Hydrolytic reaction of diol epoxides to tetrols. b) Internal H-bonding in diol epoxides with syw-config-uration and rendering the distal C-atom more electrophilic (modified from [104]). c) General representation of proton-catalyzed (A-H = H+), general acid catalyzed (A-H = acid), or intra-molecularly catalyzed (A-H = syn-OW group) activation of the distal C-atom toward... 

Already in the first reports on olefin oxidation with the MTO/H2O2 system [3], it was noted that the formation of diols from the desired epoxides, caused by the Br0nsted acidity of the system, is a major drawback of this system. The solution for this problem was found in the same report by the addition of a nitrogen base. This method has been explored extensively since and has become an important factor in the MTO-catalyzed olefin epoxidation. 

Acid-catalyzed epoxide cleavage takes place by back-aide attack nucleophile on the protonated toxide in a manner analogous to the step of alkene bromtnation, in ndiich a cyclic brentORtunt ion is opened nucleophilic attack (Section 7.2l. When an opoxycycloalkane is opened aqueous acid, a frona-t -diol results just as a traR5 l,2-dibroiiiide from cycloalkene hrominaUun. 

Acid-catalyzed epoxide cleavage takes place by back-side attack of] nucleophile on the protonated epoxide in a manner analogous to the fn step of alkene bromination, in which a cyclic bromonium ion is opened 1 nucleophilic attack (Section 7.2). When an epoxycycloalkane is opened 1 aqueous acid, a rans-1,2-diol results, just as a from cycloalkene bromination. 

Epoxide rings are cleaved by treatment with acid just as other ethers are. The major difference is that epoxides react under much milder condition-because of ring strain. Dilute aqueous acid at room temperature is sufficiem to cause the hydrolysis of epoxides to 1,2-diols, also called vicina/glycols.(Th word vicinal means adjacent," and a glycol is a diol.) More than 3 millK tons of ethylene glycol, most of it used for automobile antifreeze, are produci each year in the United States by acid-catalyzed hydration of ethylene oxid Note that the name ethylene glycol refers to the glycol derived from ethylen just as ethylene oxide refers to the epoxide derived from ethylene. 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

Attempted peroxy acid epoxidation of the bicyclic ketone (31 equation 13) gave the lactone (33), instead of several possible rational alternatives. The epoxide (32) was implicated as an intermediate when it was independently synthesized from the epoxy alcohol, and shown to give (33) on treatment with aqueous acid.- A mechanism involving scission of the acyl bridgehead bond via the hydrated 1,1 -diol form of the ketone was proposed to account for the formation of this unexpected product. The rearrangement of the isolongifolene derivative (34 equation 14) appears to be mechanistically related. The product (35) is formed by brief treatment with dilute HCIO4 in dioxane as a mixture of isomers believed to arise by acid-catalyzed epimerization of the carbinol center. ... 

The acid-catalyzed hydrolysis of p-phenyl-substituted cyclohexene oxides 49a-d yields diols resulting from cis and "trans addition of water to the epoxide group (Scheme 14). It was initially reported that the cis/trans diol ratio correlates well with the electronic effect of the / -substituent, and varied from 7.5 93.5 for p-nitro-sub-stituted oxide 49d to 95.3 4.7 for p-methoxy-substituted epoxide 49a.56 58 Later work established that methoxy-substituted diols 50a and 51a underwent isomerization under the conditions of acid-catalyzed epoxide hydrolysis, and that the cis/ trans diol ratios for hydrolysis of 49a-c are quite similar.59... 

A mechanism that explains some of the more important observations in the acid-catalyzed hydrolysis of epoxides 49a-d is outlined in Scheme 15. The cis/trans diol product ratios from the acid-catalyzed hydrolysis of 49a-c, which have either hydrogen- or electron-donating groups in the para position of the phenyl ring, are 74 26, 83 17 and 65 35, respectively. An intermediate carbocation 52a is trapped by azide ion in the acid-catalyzed hydrolysis of 49a and the rate constant for reaction of 52a with water in 10 90 dioxane-water solvent is estimated, by the azide clock technique, to be 1.7 x 108 s 1. Azide ion also traps an intermediate 52b in the acid-catalyzed hydrolysis of 49b, but somewhat less efficiently. The rate constant ks for reaction of 52b with solvent is estimated to be 2 x 109 s-1. The somewhat greater reactivity of 52b compared to that of 52a is consistent with the observation that... 

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