Unsaturated alcohols cleavage

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

Cleavage of fi-chlorotetrahydrofurans. Sml2 cleaves either cis- or trans- -chloro-a-methyltetrahydrofurans (1) at reflux to give the (E)-unsaturated alcohol 2. This cleavage can provide (E)-dienols (4) and (E)-enynols (6). 

Note that similar cleavage of a-alkyl-p-chlorotetrahydropyrans is best effected with sodium, which gives almost pure (E)-unsaturated alcohols from either the cis-or the tram-isomer. 

XXXV) and (XXXVI) indicates the occurrence of heterolytic cleavage of the C-3-C-4 bond of the amine (XXXIV). In the presence of an excess of allylzinc bromide, only the amines (XXXIV) and (XXXVI) are isolated. Similar types of reactions have been observed in the reactions of allylic organozinc reagents with unsaturated alcohols (333). The reaction of allylzinc bromide with the branched alcohol (XXXVII) forms both the allylcarbinol (XXXVIII) and the rearranged linear alcohol (XXXIX) (333). 

Similar ring-closure reactions of unsaturated alcohols or carboxylic acids have been achieved with the phenylselenium (C6H5Se+) species if generated by photooxidative cleavage of diphenyl diselenide105. 

Unsaturated alcohols generally behave like their saturated analogs. However, the stability of the allyl system decreases the probability of cleavage adjacent to the double bond. 

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

Allyl phenyl telluriums, prepared from allyl halides and benzenetellurolate, experience oxidative cleavage of the allyl group when treated with hydrogen peroxide, tcrr.-butyl hydroperoxide, sodium periodate, oxygen, or air The allyl group is converted to unsaturated alcohols, aldehydes, and ketones. Before elimination from the molecule the phenyltelluro group is probably oxidized to a telluroxide or a tellurinic ester ... 

The cleavage of double bonds in unsaturated alcohols (including saccharides) [87], phenols [86, 989], unsaturated ketones [1110], and unsaturated acids [840, 842] and esters [5i] are discussed in the appropriate sections. 

Oxidations with peimanganates are suitable for the preparation of carboxylic acids from saturated and benzylic alcohols. Unsaturated alcohols may suffer cleavage of double bonds. Conventional oxidations are carried out in aqueous media, usually in the presence of alkali hydroxides. Thus... 

The Sn2 mechanism is ruled out for reaction between the tertiary halide, r-BuBr, and radical anions derived from the more easialy reduced compounds cinnamonitrile (9) ethyl cinnamate (12a), methyl styryl ketone (23a), and phenyl styryl ketone (20a). Reduction of the activated alkenes in the presence of an excess of r-BuBr leads to mixtures of products where a r-Bu group has been introduced in a- or j0-position or in the phenyl ring. For 9 and 12a small amounts of butylated hydrodimers were obtained in addition, and for the enone 23a formation of the unsaturated alcohol with introduction of the /-Bu group at C-1 was a major product [192]. In this case the mechanism is unambiguously reduction of the activated alkene followed by electron transfer to r-BuBr concerted with halide cleavage, in... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

---