Oxidation of unsaturated alcohols

Oxidation of unsaturated alcohols by Mn(III) pyrophosphate is also faster than that of saturated alcohols. ... 

Of particular interest are oxidations of unsaturated alcohols, for example, oxidation of cinnamyl alcohol to cinnamaldehyde,74,75 and special promoters have been added to increase selectivity (Fig. 6.13).75 Although the functions of these promoters are still not fully undestood, some authors attribute their increased selectivity to physical blocking of reaction sites. This blocking reduces the size of the active site ensemble and suppresses the tendency for alcohols to strongly adsorb and dissociate on Pt.75... 

V.C.8.1. Alkenes and Alcohol Functions. Although TS-1 and other titanosi-licates oxidize alcohols to the corresponding aldehydes and ketones, the rates are suppressed in the presence of compounds containing C=C bonds. CH3OH, for example, is not oxidized at all during epoxidations of alkene reactants. Higher alcohols, however, are partially oxidized. The oxidation of unsaturated alcohols in the presence of TS-1 is shown in Table XVII (193). 

Oxidation of unsaturated alcohols in the presence ofTS-1 effect of alkene structure on selectivity... 

We have studied the anodic oxidation of unsaturated alcohols using the controlled potential electrolysis (E = 1.9V vs SCE) in CH3CN-O.I mol/1 Et4NC104 solution in a divided cell [110]. The oxidation of 4-pentenol after consumption of 0.8 F/mol gave 2-methyltetrahydrofuran and tetrahydropyran as the major products. The oxidation of 5-pentenol gave 2-methyltetrahydro-pyran and oxepam, while the oxidation of 3-butenol under the same reaction conditions did not give the cyclic products. We rationalized this reaction as the electrongenerated acid (EGA) catalyzed intramolecular cyclization (Scheme 44). 

More recently, the Noyori group described an organic solvent- and haUde-free oxidation of alcohols with aqueous H202 . The catalyst system typically consists of Na2W04 and methyltrioctylammonium hydrogen sulfate, with a substrate-to-catalyst ratio of 50-500. Secondary alcohols are converted to ketones, whereas primary alcohols, in particular substituted benzyUc ones, are oxidized to aldehydes or carboxylic acid by selecting appropriate reaction conditions This system also catalyzed the chemoselective oxidation of unsaturated alcohols, the transformation exemplified in equation 65, with a marked prevalence for the hydroxy function. 

Table 5. Oxidation of unsaturated alcohols at the nickel hydroxide electrode to carboxylic acids...

Results of oxidation of unsaturated alcohols are shown in Table 3. Both 2-penten-1 -ol and 3-methyl-2-buten-1 -ol exhibited higher reactivity than cyclohexene. A decrease around 20-50% in catalytic activity of organically functionalized samples has been observed. This is probably due to the inhibition of access of the rather hydrophilic substrates to the Ti-active sites surrounded by the organic groups of increased hydrophobicity. It is noteworthy that the epoxidation was favorable for the organically functionalized samples whereas the alcohol oxidation was retarded. 

Table 3. Oxidation of unsaturated alcohols With H202 over Ti-MCM-41 and organically functionalized Ti-MCM-41 -R...

An interesting experimental modification of the standard protocol for the oxidation of unsaturated alcohols with active manganese dioxide, first described by Wald in 19 48,37 involves the percolation of a solution of the alcohol through a column of active MnO2.10c... 

Alkenes are normally inert to active M11O2 under the mild reaction conditions used in the oxidation of unsaturated alcohols. Nevertheless, occasionally minor amounts of enones resulting from the oxidation of alkenes at the allylic position are obtained.34b... 

Like active Mn02, BaMn04 is a solid that is used in excess as a suspension in an inert organic solvent like CH2CI2 for the oxidation of alcohols, producing a quicker oxidation of unsaturated alcohols than saturated ones. On the other hand, BaMn04 is a commercially available material that reportedly does not need a special activation and no different chemical behaviour has been communicated from samples of diverse origin. 

BaMn04 is not only an interesting alternative for active M11O2 in the oxidation of allylic,79 benzylic80 and propargylic81 alcohols—when no selectivity is needed—but it can also be used for the selective oxidation of unsaturated alcohols in the presence of saturated ones in the same molecule.82... 

Subsequent authors confirmed the utility of DDQ in the oxidation of allylic94 alcohols and extended the scope of this oxidation to benzylic95 and propargylic96 ones. Nowadays, among quinones, DDQ is the preferred one for the oxidation of unsaturated alcohols because it possesses a very high... 

As early as in 1956, Braude et al.92 suggested that the selective oxidation of unsaturated alcohols with the quinone o-chloranil (82), can be explained by the intermediacy of a resonance-stabilized cation resulting from a hydride abstraction. Later, detailed mechanistic studies confirmed this hypothesis94c,95e in oxidations performed with the more common quinone DDQ. 

Normally, oxidations of unsaturated alcohols with DDQ are performed by stirring a solution of the alcohol in an organic solvent with DDQ at room temperature. The most common solvents for this reaction... 

Nomially, dry solvents are employed in the oxidation of unsaturated alcohols with DDQ. This is done because DDQ is decomposed by water.94c On the other hand, the use of wet solvents may not be deleterious, as a mixture of CH2CI2 and water is routinely employed for the deprotection of p-methoxybenzyl (PMB)10S and 3, 4-dimethoxybenzyl (DMPM)106 ethers with DDQ, and, when this deprotection leads to an unsaturated alcohol, a prolonged reaction allows a successful oxidation of the alcohol to a ketone.107... 

General Procedure for Selective Oxidation of Unsaturated Alcohols in Presence of Saturated Alcohols Using DDQ... 

DDQ is able to aromatize many cyclic compounds.122 Although, aro-matizations sometimes compete with the oxidation of unsaturated alcohols,123 they normally require harsh conditions and selective oxidations of unsaturated alcohols are possible.124... 

Tertiary amines normally resist the action of DDQ during the oxidation of unsaturated alcohols.129 There is no enough published data on the oxidation of unsaturated alcohols in the presence of secondary or primary amines to infer sufficient information regarding resistance of these amines. 

Because of the mechanism of action of DDQ, sulfides and selenides are expected not to react with DDQ during the mild conditions used in the oxidation of unsaturated alcohols. There is one published example in which an alcohol is oxidized with DDQ in the presence of a selenide.130... 

If heat is applied during the oxidation of unsaturated alcohols with DDQ, an over-oxidation resulting in the introduction of an unsaturation conjugated with the carbonyl group can happen.131... 

The oxidative potency of dichromates and chlorochromates decreases under less acidic conditions. This is so, for example, when a less acidic ammonium salt is included as counter-ion of a dichromate or chlorochromate anion. Thus, a number of ammonium dichromates and chlorochromates possessing a milder oxidative potency has been described with the specific purpose of allowing very selective oxidations of unsaturated alcohols in the presence of saturated ones. These selective dichromates and chlorochromates include bis(benzyltriethylammonium)dichromate,135 tetramethylethy-lenediammonium dichromate (TMEDADC),136 imidazolium dichromate (IDC),137 N, A -dimeth y I a m i n o py r id i n i u m chlorochromate (DMAPCC),138 l-(benzoylamino)-3-methylimidazolium chlorochromate (BAMICC)139 and butyltriphenylphosphonium chlorochromate (BTPPCC).140... 

The same principle of moderating the acidity in order to achieve a greater selectivity for the oxidation of unsaturated alcohols is applied in the use of PCC in a CH2C12 solution containing 2% of 3,5-dimethylpyrazole (DMP),141 complexes of -butylammonium chlorochromate (BACC) with 18-crown-6,142 and the solid support-bound 1-aminoimidazolium chlorochromate. 143... 

A number of other oxidants has been described for the selective oxidation of unsaturated alcohols. These include ... 

C6H5I(OAc)2 is used. Oxidation of unsaturated alcohols can be complex. With the exception of Ru02, almost any ruthenium complex is effective.1... 

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