Bridges, Styles

The use of a 7% fumed-silica gel with silica frit bridges of the style shown in Figure 5.6a has been recommended.14 Fumed silica forms stable gels with water and a number of organic solvents, but the aqueous gel is not stable above pH 8. 

In a similar style, epoxidation of the two alkenes is totally stereoselective, occurring exo in nor-born ene and endo when methyl groups are present on the bridge. These stereoselectivities would be remarkable in a simple monocyclic compound, but in a rigid bridged bicyclic structure they are almost to be expected. 

In type V, the ligand bridges two metal atoms through two sulfur atoms. Each sulfur atom coordinates to one of two metal atoms in this case, respectively. The two C-S bonds are completely delocalized. This kind of coordination style is often observed in transition metal complexes, especially in the so-called MMX complexes (see Sect. 2.6) [16]. 

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