Double bonds, migration nature

Double-bond migration often passes unnoticed, for unless tracers are employed, there may be no direct evidence remaining that migration has occurred. Nonetheless, the fact that it does occur can have a number of important consequences. Selective removal of cis homoconjugated dienes and trienes in natural oils, used to make edible hydrogenated fats, depends mainly on prior isomerization of multiple unsaturation into conjugation under hydrogenation conditions (J9). 

The hopane series are the natural product biomarkers elucidated initially as attributable to bacteria.The 17a(H),2ip(H)-hopanes ranging from C27 to C35 (no C28) were encountered in numerous ancient sediments and petroleums, and diagenesis and maturation of the microbial precursors (e.g. bacteriohopanepolyol and diploptene, Fig. 4) were elucidated. The diagenesis of diploptene in contemporary sediments proceeds by double bond migration from via to... 

The electrochemical incorporation of CO2 into perfluoroalkyl derivatives has been explored in the case of (perfluoroalkyl)alkyl iodides and (perfluoroalkyl)alkenes, with an electrochemical system based on the use of consumable anodes combined with organometallic catalysis by nickel complexes. Iodide derivatives have been functionalized to the corresponding carboxylic acids by reductive carboxylation. Interesting and new results have been obtained from the fixation of CO2 into perfluoroalkyl olefins. Good yields of carboxylic acids could be reached by a carefull control of the reaction conditions and of the nature of the catalytic system. The main carboxylic acids are derived from the incorporation of carbon dioxide with a double bond migration and loss of one fluorine atom from the CF2 in a position of the double bond. 

It is interesting to note that this Rh-catalyzed isomerization mimics a similar double bond migration found in nature in the biosynthesis of cholesterol and terpenes in which isopen-tenyl diphosphate (92) is converted reversibly to dimethylallyl diphosphate (93), shown in equation 9.48.119The stereochemistry ofthis reaction is also suprafacial. 

Alumina can catalyse both inter- and intramolecular reactions, especially with compounds which are base sensitive due to its alkaline nature, e.g. dehydration, double bond migrations and ring expansions have all been observed. This disadvantage accounts largely for the preferred use of silica gel. 

The catalytic activity was found to depend on the nature of the metal catalyst. Higher efficiency was obtained with Mo catalysts as compared to Cr and W ones. The nature of the ligand attached to the metal has some control over the conversion. In addition to dehydrogenation double bond migration also takes place. 

The Michael-induced RBR exploits the electrophilic nature of vinyl-substituted sulfones. Upon nucleophilic attack, the alkene double bond migrates to form an episulfone intermediate by displacement of an a-halide fScheme R.12T The product alkene, resulting from subsequent SO2 extrusion, is transposed by one position from the original alkene. The a-halide may... 

The Nazarov cyclization was the key step in a synthesis of ( )-trichodiene (35) by Harding and co-workers. One significant challenge in the preparation of this natural product is the presence of two adjacent quaternary stereocenters, and Harding and co-workers selected the Nazarov cyclization to tackle this problem. Although the reaction was not efficient under protic acid catalysis, the presence of an excess of boron trifluoride etherate enabled the production of 34a/b in good yield, whereby double bond migration had occurred. These specific reaction conditions were key to the success of this transformation, since under milder conditions or shorter reaction times, the expected a,p-unsaturated ketone was observed. 

A few fatty acids with trans double bonds (trans fatty acids) occur naturally, but the major source of trans fats comes from partial hydrogenation of vegetable oils in, for example, the preparation of margarine. The same catalysts that catalyze the hydrogenation of the double bonds in a triacylglycerol also catalyze double-bond migration and stereoisomerization. 

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