Selective oxidation of unsaturated alcohols

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... 

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. 

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... 

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... 

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

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

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... 

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

The oxidation of alcohols to carbonyl compounds has been studied by several authors and a variety of methods have been used. Papers concerned vith such oxidations are illustrated (Scheme 3.26). Good results have been obtained using pyridinium chlor-ochromate (PCC) adsorbed onto silica gel for the selective oxidation of unsaturated substrates e.g. terpene [135] and furanyl derivatives [136]. Steroidal homoallylic alcohols can be converted to the corresponding 4-ene-3,6-diones using tetrapropylammo-nium per-ruthenate (TPAP) in catalytic amounts [137]. In this case, the oxidising agent is N-methyl morpholine N-oxide (NMO). 

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. 

Activated manganese dioxide, which is generally used for the selective oxidation of allylic alcohols, permitted the synthesis of an unsaturated (3-keto-arabino-hexopyranosyl)pyridine.13 New oxidative systems for alcohols, involving molecular sieve-assisted... 

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... 

However, a more detailed examination of the results obtained with the Pd(II) bathophenthroline and Pd(II) neocuproin complexes revealed a remarkable difference in the oxidation of the unsaturated alcohol substrate shown in Figure 5.17. With the former, the major product was derived from oxidation of the alkene double bond, while the latter afforded >99% selective oxidation of the alcohol moiety. This suggested that we were concerned with totally different types of catalyst. Indeed, further investigation revealed that the Pd(II) neocuproin complex dissociates completely to afford Pd nanoclusters which are the actual catalyst [85]. 

Figure 5.17 Selectivity in oxidation of unsaturated alcohol oxidation using the Pd-bathophenanthroline catalyst [82] versus the Pd-neocuproin catalyst [84]. For conditions see Eq. (5.12) and Figure 5.16.

Unsaturated alcohols generally undergo selective oxidation of the alcohol moiety (Eqs. (5.23) and (5.24)), but, when an allylic alcohol contained a reactive trisub-stituted double bond, selective epoxidation of the double bond was observed (Eq. (5.25)). 

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