Alkenals carbon chain cleavage

The reactions often proceed in high yield and predominantly with retention of configuration at the vinyl—aluminum bond. Many experimental variations of this substitution reaction remain to be studied systematically and the method appears to have great promise for the synthesis of unsaturated carbon chains. Because only one C—Al bond in R2AIR responds to cleavage the method has little appeal for application to the R2AIH adducts of alkenes (Scheme 3). Little preference for cleavage of the three C—Al bonds would be expected, so conversion to the R E would be low. 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

Condensation. Generation of stabilized free radicals from dithiocarbonate esters via C—S bond cleavage is promoted by BusSnSnBus. By providing an alkene and a trapping agent, homologation of a carbon chain while performing functionalization, for example,... 

The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed CO2 selectivity as well in the alkene-added reaction, as CO2 is the byproduct from CO in the water-gas shift reaction. 

Radical addition to alkenes (Sections 10.9 and 10.10) A process by which an atom with an unshared electron, such as a bromine atom, adds to an alkene with homolytic cleavage of the IT-bond and formation of a cr-bond from the radical to the carbon the resulting carbon radical then continues the chain reaction to product the final product plus another species with an unshared electron. 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

The photochemical addition of pofyhalomethanes to alkenes has been a known reaction for a long time. Efficient cleavage occurs preferentially at the weakest carbon-halogen bond, and the formation of the adduct f5.64) may well be a chain process. 

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