Fragmentation reactions metal assisted

An interesting case of metal-assisted ring opening of (109) has been described by Wolczanski using the low-valent (silox)3Nb fragment that binds (109) in the //2(N,C) mode (Scheme 50). Derivative (113) undergoes C—N insertion by thermolysis in benzene at 70 °C only. The reaction gives 0.5 equiv. of (109) and 0.5 equiv. of (114) as a thermodynamic mixture of cis,cis-, trans,cis-, trans,trans- and cis,trans-isomers.255-257... 

There seems to be no compelling reason to discuss silicon- and metal-assisted fragmentations in a special chapter, since they are mostly ordinary Orob types leading to medium-sized ring systems. However, justification is given by the fact that these reactions use unusual reagents under mild conditions or involve subsequent reactions. 

Transition metals such as It were used to control polyborane formation. Metal-assisted borane condensation is demonstrated by the formation of [ara A) o-2,5- Cp ltH]2B4Hs] 791 by the reaction of two BHs THF moieties with the metal centers of [ Cp IrH 2(/r-H)B2Hs] 792. Mild thermolysis of 791, which contains separated B2H4 fragments, results in H2 elimination and the formation of the [ /i ( i-l,2- Cp"lr 2(/t-H)B4H7] 793. 

Chloroarene compounds can be activated for nucleophilic substitution by complexation to transition metals. [CpRu] fragments have been particularly useful for this SjjAr reaction from a synthetic point of view because they can be used for the formation of arylethers as shown in Scheme 5.39 [175]. There is no catalytic variant of this reaction yet. The [CpRu(arene)] complexes are stable in air and water and can be isolated, purified and characterized. Indeed, metal decom-plexation is achieved by photolysis of the compounds in CHjCN. It is worth noting that this reaction is the exact reverse of the arene complexation reaction mentioned in Section 5.3.2.5. The Ru-assisted arylether formation is the key step in recent total syntheses of several natural products, for example the protease inhibitors K-13 (76) and OF-4949-III (77, Scheme 5.40). Model compounds for several antibiotics such as Vancomycin have also been synthesized via this route. 

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