Isomerisation of alkenes

In the above reaction the initially formed enols and enolethers are unstable and are instantaneously hydrolysed to give the corresponding carbonyl compounds. 

Carbonylation is a very important process for the preparation of carboxylic acids (and their derivatives), aldehydes and ketones. It was earlier carried out in presence of transition metal catalysts. 

The aryl halides can be converted into the corresponding carboxylic acid by carbonylation in presence of water. The use of PTC is a well established technique for carbonylation of organic halides.Carbonylation of organic halides using various types of phase transfer techniques has been extensively reviewed.  

Water soluble aryl iodides can be carbonylated in H O in presence of soluble palladium salt or complexes and K CO as base at 25-50 °C 

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The most common isomerisation reactions catalysed by transition metals are those involving the isomerisation of alkenes. Taller, Crabtree and co-workers have reported that the iridium bfy-NHC complex 55 is effective for the isomerisation of... 

Compare and contrast the photosensitised isomerisation of alkenes with the method of direct irradiation. 

If the products formed from the direct and sensitised reactions differ then the triplet state formed on sensitisation is not the reactive excited state formed by direct irradiation. This situation is illustrated in Sections 8.2 and 8.3, where direct and photosensitised cis-trans isomerisation of alkenes was considered. 

There are many instances of the isomerisation of alkenes or alkenilic compounds under the influence of transition metal compound catalyst. Some examples arepent-l-ene into cis- and trans-pent-2-ene (RhCl3... 

At low temperatures the intermediate /3-allylrhodium(III) hydride PF3 complex was detected by 1H and 19 F NMR spectroscopy and provided supportive evidence for the isomerisation of alkenes via an allyl-metal-hydride mechanism (285, 293). 

Double bond isomerisation of alkenes is promoted over basic catalysts, where the carbon skeleton is retained, in contrast to the action of acid catalysts which are important catalysts for skeletal isomerisation. The difference lies in the mechanism, which for basic catalysts involves carbanions, rather than the carbenium ions that form in the presence of acids (Scheme 9.20). 

Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-l-enes over Wilkinson s catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh ] in benzene is quite selective (i.e. was 45 times faster than the cis-aIk-2-ene [15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4 -diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. 

The highly reactive allyl-iron hydride [HFe(CO)3(T 3-CH2CHCH2)], first proposed over 30 years ago as a key intermediate in the [Fe(CO)s]-catalysed isomerisation of alkenes, has been characterised by NMR spectroscopy for the first time53. A procedure has been reported for the synthesis of tetrahydrofuran esters based on a formal [3 + 2] cycloaddition reaction of [CpFefii -C3H5)(CXD)2] with unactivated carbonyl compounds. 

The isomerisation of alkenes takes places via an excited state shown in fig. 4.3, in which two sp hybridised carbon atoms are twisted at 90 with respect to the other. The intermediate state formed is referred to as the perpendicular state. The geometry is believed to be of lowest energy geometry for both cis- and trans- excited states. 

Some examples of Geometrical photo isomerisation of alkenes are given below ... 

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