Isomerisation, alkene

There are important issues relating to chemoselectivity (aldehydes or alcohols may be the products and alkene isomerisation is a competing side reaction, which must be reduced to a minimum) and regioselectivity (linear aldehyde is much preferred over branched)... 

Preliminary mechanistic studies on the methoxycarbonylation of 1-octene showed that two pathways to methyl nonanoate occur, one involving the direct carbonylation of 1-octene to the linear ester, the other the alkene isomerisation in competition with the first one. Subsequently, the linear product forms by tandem isomerisation of the internal alkenes, with the terminal alkyl intermediate being trapped by migration to CO at a higher rate than any branched alkyl species. This has been confirmed by the analysis of products... 

As expected, many unsaturated transition metal hydride complexes catalyse isomerisation. Examples include monohydrides of Rh(I), Pd(II), Ni(II), Pt(II), Ti(IV), and Zr(IV). The general scheme for alkene isomerisation is very simple for instance it may read as follows (Figure 5.1) ... 

In general the activity of transition metal complexes for alkene isomerisation is low in the presence of carbon monoxide, but HCo(CO)4 is an exception to this rule. Depending on conditions, full equilibration of the alkene isomers is obtained. 

Again we see that an alkene isomerisation reaction has taken place. Another important, useful reagent applied in this field is also pictured in Figure 15.7, viz. the use of benzoquinone as the re-oxidant for palladium. Quinone takes the role of dioxygen as oxidising agent. It is very efficient and both quinone and hydroquinone are inert towards many substrates. Furthermore, no water is formed, as is the case when dioxygen is used. 

From these results it can be seen that in the hydrogenation of 1,2-dialkylcyclohexenes the expected cis-1,2-dialkylcyclohexane is not the sole product. Similarly, in the hydrogenation of the 1,4-dialky lcyclohexenes where both the cis- and trans-cyclohexanes are expected, the trans-isomer being the thermodynamically more stable, the stereoselectivity varies from metal to metal. Thus with palladium, the cis/trans ratio approaches the equilibrium composition, whereas with platinum and rhodium, the equilibrium composition is never approached. It is also instructive to note that in the palladium-catalysed reactions, hydrogenation is accompanied by extensive alkene isomerisation [220—223], whereas with rhodium and platinum, little or no isomerisation is observed [220,... 

Sequential one-pot RCM-cyclisation of dienes and alkene isomerisation have been achieved through conversion of the Ru-alkylidene RCM catalyst into a Ru-hydride catalyst, providing an efficient route to 3,4-dihydro-27/-pyrans from acyclic dienes <02JA13390>. 

Catalytic tributyltin hydride methodologies were also studied, with sodium borohydride used as a co-reductant <030BC4047>. In all cases examined, the yield for cyclisation of 157 to 159 was reduced substantially. This was due, in part, to the low solubility of sodium borohydride in toluene which made it necessary to employ either THF or ethanol as the reaction solvent. Unfortunately, their use promoted alkene isomerisation and substantial quantities of tra/jj-azastilbene 156 were formed as a by-product (Scheme 45). 

As with the pyridine series, competing alkene isomerisation proved to be a significant problem within the azastilbene series. It was especially prominent when aryl bromides were employed as radical precursors. For example, exposure of the bromide analogue of 193 to tributyltin hydride and AIBN returned the starting material as a mixture of cis- and trans-isomers. Alkene isomerisation was most pronounced when the alkene was conjoined at C4 of the quinoline. In this case, even carbon to iodide bond homolysis was slow by comparison (Scheme 54). 

By way of contrast, when attached at C3, no alkene isomerisation was observed with iodide 202 and it was only a minor side reaction with the corresponding aryl bromide. With two orr/zo-cyclisation pathways available to the radical intermediate, it is perhaps... 

The complexes [RuCl( 2.pjj pcH2PPh2)Cp] and [RuCl /2-HC(PPh2)3 -Cp] are extremely poor hydroformylation catalysts. They showed less than 0.5% conversion of alkene to aldehyde after 30 hours, linear to branched ratios of one or less, and undesirable amounts of alkene isomerisation and hydrogenation products. 

Easic Principles Practical Photochemistry General Considerations Carbonyl Compounds a-Cleavage Carbonyl Compounds Hydrogren Abstraction Steroids Carbonyl Compounds Cycloaddition Enone and Dienone Rearrangements Alkenes Isomerisation and Rearrangement Alkenes Cycloaddition Alkenes Photo-Cxidation Terpenoids Aromatic Compounds Isomerisation and Cycloaddition Practical Photochemistry Scale-up Aromatic Compounds Substitution and Cydisation Alkaloids Photoinitiated Free-radical Chain Reactions. 

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