Epoxides acid-catalyzed hydrolysis

Diols can be prepared either by direct hydroxylation of an alkene with 0s04 followed by reduction with NaHSOj or by acid-catalyzed hydrolysis of an epoxide (Section 7.8). The 0s04 reaction occurs with syn stereochemistry to give a cis diol, and epoxide opening occurs with anti stereochemistry to give a trans diol. 

Epoxidation followed by acid-catalyzed hydrolysis constitutes a method for anti hydroxylation of a double bond. 

Figure 11.4 The overall result of epoxidation followed by acid-catalyzed hydrolysis is a stereospecific anti hydroxylation of the double bond. ds-2-Butene yields the enantiomeric 2,3-butanediols tra x-2-butene yields the meso compound.

Which alkene, c/s-2-butene or trans-2-butene, would you choose in order to prepare meso-2,3-butanediol by epoxidation followed by acid-catalyzed hydrolysis Which alkene would yield /rjeso-2,3,-butanediol by osmium tetraoxide hydroxylation ... 

Acid-catalyzed hydrolysis of an epoxide yields a trans diol (compound L) ... 

Acid-catalyzed hydrolysis of the epoxide yields a diol its stereochemistry corresponds to net anti hydroxylation of the double bond of the original alkene. 

EXAMPLE Acid-catalyzed hydrolysis of propylene oxide (epoxypropane). Step 1 Protonation of the epoxide. 

In Water In Section 8-13 we saw that acid-catalyzed hydrolysis of epoxides gives glycols with anti stereochemistry. The mechanism of this hydrolysis involves protonation of oxygen (forming a good leaving group), followed by Sn2 attack by water. Anti stereochemistry results from the back-side attack of water on the proto-nated epoxide. 

The reaction of an epoxide with hydroxide ion leads to the same product as the acid-catalyzed opening of the epoxide a 1,2-diol (glycol), with anti stereochemistry. In fact, either the acid-catalyzed or base-catalyzed reaction may be used to open an epoxide, but the acid-catalyzed reaction takes place under milder conditions. Unless there is an acid-sensitive functional group present, the acid-catalyzed hydrolysis is preferred. 

Rate coefficients of acid catalyzed hydrolysis of various epoxides in 0.69 M aqueous HC104 (tf0 = 0) at 0 °C [153]... 

Arrhenius parameters (as far as available) for the ring-opening reactions of ethylene oxide, propylene oxide, and isobutylene oxide. Overall values of ftCi and fcHcl obtained for propylene oxide have been split into rate coefficients for attack at primary carbon and attack at secondary carbon, utilizing gas chromatographic product analysis data [152]. (It is interesting to note that the results for attack at primary carbon are of the same order of magnitude as the corresponding values for ethylene oxide.) First-order rate coefficients at a constant acid concentration for the acid catalyzed hydrolysis of various epoxides [153] are collected in Table 10. Rate coefficients of the uncatalyzed and acid catalyzed reactions of ethylene oxide with various nucleophiles [151, 154] can be found in Table 11. 

The Tsi/EtO-functionalized polysiloxanes were cross-linked in the presence of trifluoromethanesulfonic acid generated in an UV-initiated decomposition of PhalOSChCFs. The process is based on acid-catalyzed hydrolysis of =SiOEt groups and formation of =SiOH functions and their subsequent condensation. We have found that the use of Ph2l0S02CF3 gave much better results than Ph2lBF4, widely used in photo-initiated polymerization of epoxides [8]. 

Cyclic sulfates can be prepared from cyclic but not acyclic carbohydrate units by reaction with sulfuryl chloride in pyridine [98] or phenyl chlorosulfate and sodium hydride [99]. The reactivity of cyclic sulfates is quite similar to that of epoxides. Thus, they can be used to form fluoro or azido derivatives of polysaccharides by highly regiospecific approaches, usually, but not always, leading to trans-diaxial products from which the sulfate can be easily removed by mild, acid-catalyzed hydrolysis [100,101]. 

Problem 28.9 The following reactions have all been found to yield a mixture of pinacol and pinacolone, and in the same proportions treatment of 3-amino-2,3-dimethyl-2-butanol with nitrous acid treatment of 3-chloro-2,3-dimethyl-2-butanol with aqueous silver ion and acid-catalyzed hydrolysis of the epoxide of 2,3-dimethyl-2-butene. What does this finding indicate about the mechanism of the pinacol rearrangement ... 

CH2CH= CH2 (c) Acid-catalyzed hydrolysis of epoxides (Section 18.8)... 

Mechanisms of acid-catalyzed hydrolysis of 1-phenylcyclohexene oxides, indene oxides and 1,2,3,4-tetrahydronaphthalene-l,2-epoxides 264... 

Kinetic and isotopic labeling studies by Long and Pritchard on the hydrolyses of ethylene oxide, propylene oxide and isobutylene oxide to their corresponding glycols provide additional insight on the hydrolysis mechanisms of simple aliphatic epoxides.23 In each case, three kinetically distinct mechanisms for their reactions in water solutions were observed acid-catalyzed hydrolysis, pH-independent hydrolysis and hydroxide-catalyzed hydrolysis. Thus, the rate law for hydrolyses of these simple... 

ACID-CATALYZED HYDROLYSIS OF ALIPHATIC EPOXIDES Simple primary and secondary epoxides... 

Several conclusions are drawn from the product studies of the reactions of propylene oxide and isobutylene oxide in H O.23 In the acid-catalyzed hydrolysis of propylene oxide, cleavage of the secondary C-O bond is favored by a factor of 2-3 over cleavage of the primary C-O bond. In the acid-catalyzed hydrolysis of isobutylene oxide, cleavage of the tertiary C-O bond is highly favored (>99%). These results show that in acid-catalyzed epoxide hydrolysis, cleavage of the C-O bond leading to... 

Simple tertiary carbocations react with water/trifluoroethanol (H20/TFE) solvent with an estimated rate constant of 1012s-1, which is somewhat faster than bulk solvent reorganizes.28 Simple primary and simple secondary carbocations are predicted to be even more reactive with nucleophilic solvent than tertiary carbocations. In water solutions, therefore, the rates at which simple primary and secondary carbocations are predicted to react with solvent would exceed the rate of a bond vibration ( 1013 s-1), and consequently they would not be sufficiently stable to exist as an intermediate. An A-2 mechanism would therefore be enforced on the acid-catalyzed hydrolysis of those epoxides that potentially undergo ring opening to form primary or secondary carbocations. 

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