1-octene liberation from

Ligand dissociation is the reverse reaction of ligand assodation. Consequently, the CN of the complex reduces by one, the VE by two, and the ON stays constant. As ligand assodation is usually a reversible step, all reverse readions of the examples given above for the assodation step represent suitable examples for ligand dissociation. In ethylene oligomerization, the dissociation step plays a role in the release of the finally formed 1-alkene from the catalyst (after f-H-elimination). An example for the case of 1-octene liberation from a Cr(I) complex is given below in Scheme 6.16.8. 

Finally, to sum up this topic on catalytic elementary steps, we have a closer look at the mechanism of 1-alkene liberation from the Cr-metallacyclic intermediate. This process requires the interplay of three elementary steps, namely, extrusion to form an intermediate Cr-hydride species, reductive elemination to form the alkene complex of Cr(I), and, finally, dissociation of the alkene product. Scheme 6.16.8 illustrates these three steps for the example of 1-octene liberation from the nine-membered Cr-metallacycle. 

The resulting complex can be separated into diastereoisomers [6-48] and [6-49]. Trans-cyclo-octene was liberated from the separated complexes by the addition of an aqueous potassium cyanide solution, equations (6-92) and... 

The strategy denoted as A leads to release of the cyclized product. At the same time, the Ru complex is immobilized on the support. Thus, ethene or octene is added to re-liberate the Ru complex from the support in a metathesis reaction. This strategy has been successfully applied to the synthesis of a seven-membered lactam [124] (Scheme 48). 

Scheme 6.16.8 Liberation of 1-octene from the nine-membered Cr-metallacycle by combination of the elementa7 steps extrusion, reductive elimination, and ligand dissociation.

Warrener reported that when 3,6-disubstituted-s-tetrazine 147 was employed as a heterodiene in Diels-Alder cycloadditions with alkenes such as 146, tandem [4 + 2]/[4 + 2] reaction takes place under high pressure (8-14 kbar, 16h) (Scheme 36) [55]. Due to its high reactivity, initial [4 + 2] cycloaddition of tetrazine 147 and nor-bornene 146 is carried out at atmospheric pressure. Spontaneous elimination of nitrogen from primary formed Diels-Alder adduct liberates the second diene, 1,2-dihydropyridazine. By addition of another equivalent of alkene 148 and application of high pressure, the second [4 + 2] cycloaddition generates the central diazabicy-clo[2.2.2]octene skeleton in high yield. A number of functionalized polynorbornane systems were prepared by this synthetic protocol [56]. 

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