Unsaturated esters oxidation

Hiese unsaturated esters, hy oxidation, permit of the elimination of the CHO group and the preparation of aldehydes or acids with fewer oarbon atoms. 

Only strong oxidants can react dangerously with saturated esters. Unsaturated esters have often been involved, but the reaction mainly concerns non-saturation the ester group can increase the risk by boosting the double bond reactivity. Therefore, reference to unsaturated esters will be concerned with this type of reaction. 

A wide range of organic substrates can undergo an oxidative carbonylation reaction. Depending on reaction conditions, alkenes have been converted into -chloroalkanoyl chlorides (oxidative chloro-chlorocarbonylation) [1,2], succinic diesters (oxidative dialkoxycarbonylation) [3-20], a,/J-unsaturated esters [21,22] (oxidative monoalkoxycarbonylation), or /J-alkoxyalkanoic esters [11] (oxidative alkoxy-alkoxycarbonylation), according to Eqs. 10-13. 

Acetoxylation of fS,y-unsaturated esters.3 / ,y-Unsaturated esters are oxidized by PdCI2-CuCl-02 to y-keto esters in 20 40% yield. Reaction of these esters with PdCl2 and KOAc and pentyl nitrite (oxidizes Pd(0)] under oxygen leads to y-aceloxy-a,/ -unsaturated esters (45- 55% yield). The double bond has the (reconfiguration (equation I). 

The structure of the methyl ester m.p. 174° C. (XL) was proven (5) mainly by the fact that the corresponding dibasic acid (XLI R = H) and its dimethyl ester (XLI R = Me) are oxidized with periodate and that the dimethyl ester is oxidized by manganese dioxide to a trisubstituted a,/ -unsaturated ketone (XLII) which regenerates (XLII R = Me) with sodium borohydride. 

There is some interesting selectivity in this sequence. Only one of the three groups next to the alkene is oxidized and only one ( -) isomer of the enal is formed. No position next to the unsaturated ester is oxidized. All these decisions are taken in the initial cycloaddition step. The most nucleophilic double bond uses its more nucleophilic end to attack SeOi at selenium. The cycloaddition uses the HOMO (7t) of the alkene to attack the LUMO (rc of Se=0). Meanwhile the HOMO (Jt) of Se-0 attacks the LUMO (C-H a ) of the allylic system. 

Although Smh is more chemoselective than traditional dissolving metal reagents, it does react with sulfoxides, epoxides, the conjugated double bonds of unsaturated ketones, aldehydes and esters, alkyl bromides, iodides and p-toluenesulfonates. It does not, however, reduce carboxylic acids, esters, phosphine oxides or alkyl chlorides. In common with most dissolving metal systems, ketones with an a-hetero substituent suffer loss of the substituent rather than reduction of the carbonyl group. ... 

Functional groups tend to deactivate the catalysts but reduction can be achieved by working at 100 atm H2 and 20°C under these conditions, diolefins (e.g., vinylcyclohexene, 1,5-hexadiene) are reduced to paraffins (ethylcyclohexane, hexane), and unsaturated ketones or esters (mesityl oxide, vinyl acetate) are reduced (to methyl (isobutyl) ketone or ethyl acetate). 

The standard method for the conversion of a-thio-esters into a,/3-unsaturated esters involves oxidation to the corresponding sulphoxide followed by pyrolysis. An alternative approach, which avoids the use of an oxidant, is by formation of a sulphonium salt using CFjSOaCHzSiMea followed by ylide formation using caesium fluoride and rearrangement (Scheme 44). This method can run into difficulties if the starting thio-ester already contains an olefinic bond. 

P-Keto-esters are readily converted into unsaturated esters via oxidation of the a-thiolo-ester (121) and an elimination reaction (Scheme 73). The scheme has also been applied to the synthesis of dienes the key step is the deacylation of an intermediate a-thiolo-P-keto-ester. 

Trost has described in full yet another method for the introduction of a double bond adjacent to an ester function. Treatment of the ester enolate with diphenyl-(or dimethyl-) disulphide affords the a-thioester which may be smoothly converted to the a/3-unsaturated ester following oxidation (Nal04 or w-ClC H CO3H) to the sulphoxide and thermal elimination at 110 "C (for an alternative, cf. ref. 98) the method gives acceptable yields. 

It has been found that there are marked differences in the behaviour of non-conjugated and conjugated unsaturated fatty acids and their esters toward oxidation. The reactions of the two types of compounds are therefore considered separately. 

Recently macromonomers derived from polycondensates have been prepared and studied by Percec. Poly(phenylene oxide) and poly(ether sulfone) have been functionalized with terminal methacrylic ester unsaturations by means of phase transfer catalysis, using the functions located at the chain ends of the polycondensates. 

P-Diketones and p-keto esters are oxidatively fluorinated with DAST to furnish a,p-difluoro-a,p-unsaturated ketones and esters, respectively (eq 14). ... 

The most common situation studied is that of a film reacting with some species in solution in the substrate, such as in the case of the hydrolysis of ester monolayers and of the oxidation of an unsaturated long-chain acid by aqueous permanganate. As a result of the reaction, the film species may be altered to the extent that its area per molecule is different or may be fragmented so that the products are soluble. One may thus follow the change in area at constant film pressure or the change in film pressure at constant area (much as with homogeneous gas reactions) in either case concomitant measurements may be made of the surface potential. 

It may be pointed out that dehydration of p hydroxy esters with fused potassium hydrogen sulphate, acetic anhydride, phosphoric oxide or with tliionyl chloride in benzeue solution leads to ap unsiiturated esters containing some PY-unsaturated ester the proportion of the latter depends not only upon the structure of the ester but also upon the dehydrating agent used. Elehydration occasionally occurs during the reaction itself or upon attempted distillation. 

Allyllic ether 53 is oxidized regioselectively to the /3-alkoxy ketone 54, which is converted into the a,/i-unsaturated ketone 55 and used for annulation[99]. The ester of homoallylic alcohol 56 is oxidized mainlv to the 7-acetoxy ketone 57[99]. 

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