Oxidation manganese dioxide, benzyl alcohol

Manganese dioxide oxidizes allylic and benzylic alcohols faster than primary saturated alcohols, but primary and secondary allylic alcohols react at about the same rate. This use of manganese dioxide is particularly important.i Oxidation of benzylic alcohols is also facile and a secondary benzylic alcohol is oxidized faster than a primary saturated alcohol. The secondary benzylic alcohol group in 102 was oxidized to give aryl ketone 103 (94% yield) in preference to reaction at the primary aliphatic hydroxyl. 

For example, oxidation of the allylic alcohol 33 gave the a, -unsaturated aldehyde 34, used in a synthesis of the macrolactone bafilomycin Ai (6.30). Chemose-lective oxidation of allylic or benzylic alcohols can be achieved in the presence of aliphatic alcohols. Thus, in a synthesis of the alkaloid galanthamine, treatment of the diol 35 with manganese dioxide promoted selective oxidation of the benzylic alcohol to give the aldehyde 36 (6.31). 

Selective oxidation of the benzylic alcohol of the diol 6 to give the ketol 7 is possible by using the oxidant manganese dioxide in a neutral solvent such as acetone at room temperature. Alternatively, silver carbonate on celite heated in a solvent such as acetone or benzene is effective (see Section 6.2). 

It is sometimes required to oxidize the C-1 methyl group of a substituted isoquinoline to an aldehyde function. Such a transformation is usually carried out using selenium dioxide, in which case anhydrous conditions using dioxane as solvent often give superior yields. An alternate route, however, involves initial formation of the isoquinoline A-oxide followed by acetylation as indicated below. The final step is a manganese dioxide oxidation of a benzylic alcohol. ... 

Selective Oxidation of a Benzylic Alcohol with Manganese Dioxide H OH... 

Manganese dioxide (Mn02) supported on silica provides an expeditious and high-yield route to carbonyl compounds. Benzyl alcohols are selectively oxidized to carbonyl compounds by use of 35 % Mn02 doped silica under MW irradiation conditions (Scheme 6.28) [96]. 

Alkyl groups can be oxidized, as for example the benzyl group in compound (148), to a benzoyl group by permanganate or (better) by selenium dioxide.42 Similarly, the benzyl alcohol in compound 149 is oxidized by manganese dioxide.87 The thiol 150, treated with ferricyanide, gives the disulfide 151.65 Oxidation of thiol 150 with nitric acid is thought to... 

Manganese dioxide very soon became a widely used standard oxidant for the transformation of allylic and benzylic alcohols into aldehydes and ketones.4 It offers very mild conditions and is extremely selective for allylic and benzylic alcohols when it is not employed at a high temperature. On the other hand, the work-up of oxidations with M11O2 is very simple, involving just filtration of suspended solid and elimination of solvent. 

The selective oxidation of benzylic and allylic alcohols with active manganese dioxide in the presence of saturated alcohols is normally carried out by stirring or shaking a solution of the alcohol in an organic solvent in the presence of 5-20 equivalents of suspended active M11O2. 

Oxidation of sensitive divinyl alcohol 20 to dienone 4 is achieved by treatment with activated manganese dioxide. Commercially available active Mn02 21 is a synthetic nonstoichiometric hydrated material. This reagent provides mild conditions for oxidation of allylic, propargylic, and benzylic alcohols.10... 

Conjugated—a,/)-unsaturated or aromatic—aldehydes are oxidized by manganese dioxide in the presence of one equivalent of sodium cyanide to give an acyl cyanide (Formula C in Figure 17.18) via an intermediate cyanohydrin (B in Figure 17.18), which is also either an allylic or a benzylic alcohol. Alcohols of this kind can be oxidized by a relatively weak oxi-... 

To oxidize the benzylic alcohol function of compound 20, a mixture of this compound and a large excess of manganese dioxide is stirred in dichloro-methane at room temperature under argon for 1.5 h. After a short work-up, compound 21 may be obtained quantitatively (Scheme 9.16). 

Manganese dioxide is extensively used as an oxidizing agent for the oxidation of allylic alcohols to the corresponding aldehydes. Benzylic and unactivated alcohols are also oxidized by Mn02-... 

A useful addition to the roster of oxidants converting primary alcohols into aldehydes is manganese dioxide, which can be prepared by several methods [805, 806, 807, 808, 809, 810]. It is used as a suspension in petroleum ether [8< 5, 808], ether [806, 808, 811], hexane [806, 811], benzene [808, 813], chloroform [808, 811, 813], and carbon tetrachloride [808, 813]. The oxidations are carried out at room temperature and are especially suitable for allylic and benzylic alcohols, which are oxidized more readily than the saturated alcohols [808, 810], Yields vary widely depending on the substrate, on the ratio of the substrate to oxidant (which in turn depends on the particle size of the oxide [810]), on the solvent, and on the reaction time 811] (equation 214). 

Oxidation of oxygen functions. DDQ is useful for the selective oxidation of allylic " and benzylic alcohols. The reagent has the advantage of not showing the variability encountered with active manganese dioxide. It dehydrogenates 2-hydroxymethylene-3 ketosteroids in about 50% yield. The steroid is treated in dioxane solution at room temperature with 1.1 -1.5 equivalents of DDQ for 1 -5 min.. 

Activated manganese dioxide (MnO2) reliably oxidizes acetylenic, allylic, and benzylic alcohols to aldehydes and ketones. Saturated primary and secondary alcohols are also oxidized, albeit more slowly. The two main concerns are the activity of the manganese dioxide and the slow filtration of salts after the reaction. Activated MnOi is available commercially or may be prepared. 

Oxidation of allylic or benzylic alcohols. Manganese dioxide prepared by ozoniza-tion of manganese (II) nitrate is an effective agent for oxidation of allylic and benzylic alcohols.38... 

Preparation [1, 637-638, after citation of ref. 7]. Belew and Tek-Ling7a have prepared an active form of manganese dioxide by ozonization of an aqueous solution of manganese (11) nitrate containing perchloric acid. This material oxidizes benzyl alcohol to bcnzaldehyde in 82% yield. 

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