Unsaturated alcohols, oxidation selective

Oxidation. This complex is made by passing gaseous fluorine into aqueous acetonitrile. It converts secondary alcohols to ketones (note that unsaturated alcohols are selectively oxidized to epoxy alcohols) and eventually to lactones. Phenols and polycyclic arenes are rapidly oxidized to give quinones. ... 

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.

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... 

Platinum oxide, Pt02, is used in the selective reduction of an unsatruated aldehyde to an unsaturated alcohol. 

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. 

Because V(V) is a better oxidant, it exhibits low selectivity in the oxidation of simple nonsubstituted alkenes and is used mainly in the epoxidation of unsaturated alcohols, just as Ti(IV).244... 

In fine chemical manufacturing, the application of promoted platinum catalysts is less known. Maxted and Akhar have reported that the addition of stannous, manganous, ceric and ferric chloride to platinum oxide (Adams catalyst) facilitates the hydrogenation of aldehydes, ketones and olefins (ref. 1). The selective hydrogenation of unsaturated aldehydes or ketones to unsaturated alcohols has been achieved by the addition of ferrous sulfate and zinc acetate to platinum catalysts (ref. 2). 

Although active M11O2 presents a very high selectivity for unsaturated alcohols versus saturated ones when it is employed under mild conditions, sometimes minor amounts of aldehydes or ketones resulting from the oxidation of saturated alcohols are obtained.69... 

During selective oxidations of 1,4- and 1,5-diols with active Mn02, sometimes the unreacting alcohol forms a lactol by interaction with the carbonyl group resulting from the oxidation of an unsaturated alcohol. This lactol can be further oxidized to a lactone.70... 

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... 

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

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 ... 

---