Alcohol electrophilic oxygenation

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

Cinnamyl alcohols substituted at the para carbon atom lend support to the mechanism in which the rate-determining step is the attack of the electrophilic oxygen atom the rate increases with the electron donicity of X, see Figure 14.7 [2]. 

These results are explained in terms of coordination of the nucleophilic hydroxy-(methoxy-, silyloxy-, amino-) functionality of the stereogenic center with the incoming electrophilic singlet oxygen (Scheme 24, right side, transition states C). Stereodifferentiation results from the preferred conformation of the ally lie alcohol for oxygen transfer, which is mainly determined by 1,3-allylic strain (threo-C favored over erythro-C). The experiments also showed that the optimal dihedral angle of the allylic alcohol (C=C—C—O) in the transition state lies between 90° and 130° and the newly formed double bond in the... 

Alkylated carboxonium ions have also been prepared by direct electrophilic oxygenations of alkanes, alcohols, and so on, by ozone or hydrogen peroxide in superacidic media606 [Eq. (3.82)]. 

Even electrophilic oxygenation of functionalized compounds has been achieved.599 Alcohols are oxidized to the corresponding keto-alcohols [Eq. (5.224)]. 

The electrophilic addition of bromine to aikenes is an oxidation. The starting alkene is at the alcohol oxidation level, but the product has two carbons at the alcohol oxidation level—the elimination reactions of dibromides to give alkynes that you met in the last chapter (p. 000) should convince you of this. There are a number of other oxidants containing electrophilic oxygen atoms that react with nucleophilic aikenes to produce epoxides (oxiranes). You can view epoxides as the oxygen analogues of bromonium ions, but unlike bromonium ions they are quite stable. 

The previous section described metal catalyzed epoxidation of allylic alcohols by alkyl hydroperoxides, and 193 was proposed as a model to predict the diastereoselectivity of these reactions,. In the cases presented, the reaction was diastereoselective but not enantioselective (sec. 1.4.F) and those epoxidation reactions generated racemic epoxides. To achieve asymmetric induction one must control both the relative orientation of the alkene relative to the peroxide and also the face of the substrate from which the electrophilic oxygen is delivered. Control of this type can be accomplished by providing a chiral ligand that will also coordinate to the metal catalyst, along with the peroxide and the alkene unit. There are two major asymmetric epoxidation reactions, one that can be applied only to allylic alcohols and is the prototype for asymmetric induction in these systems. The other is a procedure that can be applied to simple alkenes. Both procedures use a metal-catalyzed epoxidation that employs alkyl hydroperoxides, introduced in section 3.4.B.ii. 

The Sharpless asymmetric epoxidation (sec. 3.4.D.i) exploits this chelation effect because its selectivity arises from coordination of the allylic alcohol to a titanium complex in the presence of a chiral agent. The most effective additive was a tartaric acid ester (tartrate), and its presence led to high enantioselectivity in the epoxidation.23 An example is the conversion of allylic alcohol 40 to epoxy-alcohol 41, in Miyashita s synthesis of the Cg-Ci5 segment of (-t-)-discodermolide.24 in this reaction, the tartrate, the alkenyl alcohol, and the peroxide bind to titanium and provide facial selectivity for the transfer of oxygen from the peroxide to the alkene. Binding of the allylic alcohol to the metal is important for delivery of the electrophilic oxygen and... 

Because the protonation of ozone removes its dipolar nature, the electrophilic chemistry of HOs, a very efficient oxygenating electrophile, has no relevance to conventional ozone chemistry. The superacid-catalyzed reaction of isobutane with ozone giving acetone and methyl alcohol, the aliphatic equivalent of the industrially significant Hock-reaction of cumene, is illustrative. 

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