Metallacycles geometry

The purpose of this section is to apply symmetry arguments to the important reaction sequences in which metallacycles are involved, while refraining from postulating novel bond types or coordination geometries. Such things do no doubt remain to be discovered, but should not be introduced into the discussion without good reason. 

The spirocyclic structures of 8b, 9b and 10 consist of two zincacyclopentane (8b and 10) or two zincacyclohexane (9b) rings, having the zinc atom in common. Each of the lithium atoms is bonded to the a-carbon atoms of the two metallacycles, while a tetrahedral coordination geometry at lithium is reached by bidentate N—Ei coordination... 

Compound 63 has been structurally characterised by single crystal X-ray diffraction (Fig. 21a).52 53 The geometry of the five-membered metallacyclic ring is puckered with notably different Co C( ) distances. The C-C bonds are also observed to be significantly shorter than the standard (sp3)C-(sp3)C bond length. Compound 63 was found to be stable at room temperature under an N2 atmosphere and could be stored for months at —30 °C.67 However, in benzene solution, 63 decomposes at room temperature giving a mixture of 1-butene (11%), trans 2-butene (64%) and cis 2-butene (25%).68 Reaction of 63 with CO at 44 atm pressure afforded cyclopentanone as the product in 33% yield, while reaction with ethylene at 1 atm gave 1-hexene (65%) and 1-butene (11%) as the products.68... 

The stereochemical outcome was rationalized by a Zimmerman-Traxler type transition state 45.64 Assuming the titanium enolate of 42 has a Z-geometry and forms a 7-membered metallacycle with a chairlike conformation, a model can be proposed where a second titanium metal coordinates to the indanol and aldehyde oxygens in a 6-membered chairlike conformation. The involvement of two titanium centers was supported by the fact that aldehydes that were not precomplexed with titanium tetrachloride did not react (Scheme 24.7).63 Ghosh and co-workers further hypothesized that a chelating substituent on the aldehyde would alter the transition state 46 and consequently the stereochemical outcome of the condensation, leading to. vyn-aldol products 47.64 Indeed, reaction of the titanium enolate of 42 with bidentate oxyaldehydes proceeded with excellent. s v -diastereo-selectivity (Scheme 24.8).65... 

Figure 50 Trigonal bipyramidal geometry possibly accommodated by the strain due to steric bulk of the highly oxygenated seven-membered metallacycle.

The crystal structure of yellow metallacyclic-osmium(iv) complex 9 was also reported <20030M414>. The distribution of ligands around the osmium atom can be described as a four-legged piano-stool geometry with the carbon atom C-5 of the metallated phenyl group disposed transoid to the hydride ligand. The bidentate carbon donor... 

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