Alkenes with aldehydes, free-radical

Compounds with weak C—H bonds can add to alkenes by a free-radical chain mechanism. Compounds that can add to alkenes in this way include RCHO compounds (aldehydes, formates, etc.) and 1,3-dicarbonyl compounds. In the initiation part of the mechanism, an initiator radical abstracts H- from the weak C—H bond to give an alkyl radical. In the propagation part of the mechanism, the alkyl radical adds across the C=C rrbond, and then the new radical abstracts H- from the weak C—H bond to give the product and regenerate the first alkyl radical. 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

Free radical hydrosilylation of poly(phenylsilanes) (294) with alkenes, aldehydes or ketones promoted by 2,2 -azo(bisisobutyronitrile) (AIBN) provides functional polysilanes (295) possessing a variety of properties (equation 111)292. 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

As a further example the four hydroperoxides obtained in the autoxidation of oleate would be expected to give either the aldehydes and radical esters shown in the following equation or, alternatively, the oo-oxoesters and alkane and alkene radicals if the fi scission takes place on the other C-C bond. The free radicals can then react with neutral molecules or inactivate one another (Figure 2.12). 

Lewis acids, such as SnCU, also catalyze the reaction, in which case the species that adds to the alkenes is H2C O SnCl4. " Montmorillonite KIO clay containing zinc (IV) has been used to promote the reaction. " The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form homoallylic alcohols. A typical example is the reaction of methylenecyclohexane with an aryl aldehyde to give 89. 

It has long been known that unsymmetrical ketones can be prepared by the reaction of aldehydes with alkenes under free-radical reaction conditions. Recently the revision of this chemistry has been reported by the Roberts group [42], They introduced thiols as a polarity reversal catalyst for the addition of aldehydes to alkenes. Thiyl radicals are electrophilic, and therefore a polar Sh2 type transition state for the hydrogen transfer step from an aldehyde would be ideal in this situation. Indeed, the addition of aldehydes to a variety of alkenes can be effected by... 

The functionalization of polymers is another useful feature of the hydrosilation reaction. The introduction of highly fluorinated alkyl chains by the hydrosilation of Si-H groups of the polymer with fluorinated alkenes is a typical example. Poly(phenylsilanes) obtained by the dehydrocoupling polymerization of phenylsilane undergo AIBN initiated free radical hydrosilation with alkenes, ketones and aldehydes. Cross-linking and branching of polymers can also be easily accomplished using hydrosilation. 

Stoicheiometric hydroformylation of alkenes with [MnH(CO)s] has recently been reported. Treatment of 1,2-diphenyl 3,3-dimethylcyclo-propene in hexane under CO with [MnH(CO)s] at 55 °C gave a mixture of cis-(S7%) and trans-(l37o) aldehydes together with some alkane. The proposed mechanism is shown in Scheme 3. A chemically induced dynamic nuclear polarization (CIDNP) effect was observed in the H n.m.r. spectrum during the reaction and this was ascribed to the initial formation of an alkyl radical (10). The final step, reaction of a metal hydride with a metal acyl to give free aldehyde, is similar to that proposed in Co-catalysed hydro-... 

Aldehydes.- Terminal alkenes can be converted into aldehydes by free-radical addition of ihioplienol, chlorination with N( S, and hydrolysis (equation I). t he cltloro stdiides can also lx converted into alkyl dilhianes (equation II). 

The reaction of ozone and alkenes is sufficiently fast that it can compete with other removal processes and provide sinks for both ozone and alkenes in the troposphere. While kinetic data for a series of alkene/ozone reactions have been reported, not much is known about details of the reaction mechanisms, the role that carbonyl O oxides play, and the role that free radicals play in these processes. Our laboratory experiments provide the spectroscopic data (both infrared and UV/visible) that are important for the spectroscopic identification of Criegee intermediates in the troposphere. In addition, we were able to characterize secondary partially oxidized products (aldehydes, peroxides etc.) that are produced during the gas-phase ozonolysis. These products might lead to a net increase of ozone, if oxygen atoms are formed during their decomposition. 

The lipids that constitute the cell membrane, especially those lipids containing unsaturated double bonds, are susceptible to free radical attack, leading to the formation of lipid peroxides and aldehydes (Kako 1985). A number of short chain fragments produced from peroxidation of polyunsaturated fatty acids as 4-hydroxyperoxy nonenal and 4-hydroxy 2-alkenals react with sulphydryl groups of various enzymes modifying their activities. 

Substituted v-butyrolactones can be prepared by reaction of aldehydes or ketones with tbe dianion (28), and direct condensation of symmetrical ketones with diethyl 2-oxomalonate provides a useful synthetic route to the butenolides (29). A number of initiators have been used previously to promote the free-radical addition of ketones to alkenes now transition-metal oxides have been shown to be effective. Pent-4-enal is cyclized to cyclopentanone by chlorotris(triphenylphos-phine)rhodium(i) through a non-radical pathway. ... 

The reduction of alkyl hahdes has been important in many syntheses. Sodium cyanoborohydride in HMPA will reduce alkyl iodides, bromides, and tosylates selectively in the presence of ester, amide, nitro, chloro, cyano, alkene, epoxide, and aldehyde groups [118]. Tri-n-butyltin hydride will replace chloro, bromo, or iodo groups with hydrogen via a free radical chain reaction initiated by thermal decomposition of AIBN [119]. Other functionality such as ketones, esters, amides, ethers, and alcohols survive unchanged. The less toxic tris(trimethylsilyl) silane can be used similarly [120]. 

Cobalt complexes derived from Schiff bases 388 catalyzed the hydroxyacylation of electron-deficient alkenes (Fig. 90) [431, 432]. Thus, methyl acrylate 387 reacted with aliphatic aldehydes 386 in the presence of 5 mol% of the in situ generated catalyst, molecular oxygen, and acetic anhydride to 2-acyloxy-4-oxoesters 389 in 56-77% yield. When acetic anhydride was omitted, the yields of products were lower and mixtures of the free hydroxy compounds and acylated compounds resulting from Tishchenko reactions were obtained. Electron-rich alkenes did not undergo the transformation, since the addition of the acyl radical is much slower. The acylcobalt species inserts oxygen instead and acts as an epoxidation catalyst. 

Together, the two acids contribute between 16 and 35% of the free acidity in North American precipitation and between 25 and 98% of the free acidity in precipitation in remote areas. Photochemical production of organic acids occurs in the gas phase from ozone-alkene reactions and in cloud water by the hydrolysis of aldehydes followed by aqueous-phase reaction with OH radicals (see Chapter 6). These routes can explain, in part,... 

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