## 7 Silicon Building Blocks Containing H, OH and Alkoxy moieties as Leaving Groups
interesting to compare the data from the hydrolysis experiments conducted by the investigators using macroscopic amounts of fluorosilanes with the \({}^{18}\)F-labeled analogs under true nca conditions. It could clearly be demonstrated that _iso_-propyl groups, when combined with a di-methyl substituted aryl system, result in a comparable hydrolytic stability of the Si-\({}^{18}\)F bond as two _tert_-butyl groups and an un-substituted aryl system. Figure 7 is an excerpt of the original published table of compounds, highlighting the substituents substantially impacting on hydrolytic stability. Interestingly, the substitution on the phenyl residue as one of three substituents connected to the silicon atom plays an important role for the hydrolytic stability, hinting at the possibility of electronic effects in addition to steric effects. **Figure 7**. **(A)** Silicon building blocks with very low hydrolytic stability. Compound **21**, bearing two _iso_-butyl groups exhibits a relatively high stability towards hydrolysis. **(B)** Silicon building blocks with highest hydrolytic stability. **C**: Influence of the substituent on the phenyl moiety on hydrolytic stability. (*t\({}_{1/2}\): hydrolytic half-life). Column A (Figure **7**) corroborates the results on hydrolytic stability found by [PERSON] _et al_. in 2006 [28]. Attaching three aryl substituents to Si results in a very low hydrolytic stability. Although the aryl moiety is considered to be a bulky substituent, water molecules seem to easily attack the Si atom in an S\({}_{N}\)2 kind of way. Replacing two of the three aryl residues by _iso_-butyl-groups stabilizes the Si-F bond significantly. The silicon building blocks displayed in column B show the highest stability towards hydrolysis. The _tert_-butyl groups undoubtedly impart the highest amount of hydrolytic stability which corroborates the findings made for the SiFA compounds. In order to be on par with two _tert_-butyl groups, two _iso_-propyl groups have to be accompanied by an aryl residue having a higher grade of substitution. As shown under B, two methyl groups _ortho_ to the Si atom combined with two _iso_-propyl residues result in a comparable hydrolytic stability. The substitution pattern of the aryl group also seems to have a profound influence on the hydrolytic stability of the Si-\({}^{18}\)F bond. Column C reveals that a longer alkyl chain positioned _para_ to the silicon atom leads to a higher stability, most probably due to increasing hyperconjugation and an increasing +I effect. The electron releasing effect of the alkyl chain stabilizes the highly polarized Si-F bond, affecting primarily the _ortho_ and _para_ position. In contrast, the 2-hydroxy ethyl chain in the _meta_ position (compound **27**) does not exhibit this stabilizing effect which is reflected by its shorter hydrolytic half-life in comparison to the _para_ substituted compound (**26**). The likely mechanism of hydrolysis and final loss of fluoride is shown in Figure 8.  
[PERSON] and co-workers furthermore found some DFT function on the basis of the already characterized 2-(phenylazo)phenyl-allyldifluorosilane which structurally resembles the situation in fluorosilanes. The most fitting functional to describe the bond length of the Si-F bond of their model compounds was found to be the TPSS/RI/TZVPP(P) functional. Besides the Si-F bond length, angles, partial charges as well as ligand exchange energies for the hydrolysis reaction were determined on that basis [33]. **Figure 8.** S\({}_{\text{N}}\)2 mechanism of Si-F bond hydrolysis with the common pentacoordinate transition state (_ii_) as described in the literature. According to [PERSON] and co-workers, water is a spectator in this mechanism. A model for the hydrolysis of Si-F compounds was derived from these data in combination with the experimentally acquired hydrolysis data from the aforementioned 15 model compounds. For the most stable compounds **22**, **23** and **24**, the bond lengths of the intermediate _ii_ were 1.831, 1.817 and 1.827 A respectively.