The coupling reaction of silyl hydrides with alkoxysilanes to produce siloxanes and
hydrocarbons catalyzed by tris(pentafluorophenyl)borane was studied by gas chromatography
and UV spectroscopy using model reagent systems: Ph2MeSiH + Ph2MeSiOn-Oct (I) and
Ph2MeSiH + Me3SiOn-Oct (II). Detailed kinetic studies performed for system I showed that
the reaction is first order in both substrates and the rate is proportional to the catalyst
concentration. A highly negative apparent entropy of activation points to a crowded transition
state structure, leading to a significant dependence of the rate on steric effects. Studies of
system II demonstrated that the exchange of the Si−H and Si−OR functionality accompanies
the coupling process and in many cases is the dominating reaction in this system. Ultraviolet
spectra recorded during the reaction show a distinct strong absorption band with λmax =
303−306 nm, which is due to an allowed electronic transition in the uncomplexed B(C6F5)3
molecule. This absorption also gives rise to intense fluorescence with a maximum of the
emission band at 460 nm. When the borane is complexed by oxygen nucleophiles, such as
water, alcohol, or silanol and is not active as a catalyst, it does not show the absorption in
the 303−306 nm region. This absorption may serve as a measure of the concentration of the
active uncomplexed catalyst in the reaction system. Since complexes of B(C6F5)3 with the
alkoxysilane substrates and the disiloxane products are relatively weak, the catalyst appears
in the reaction system mostly as an uncomplexed species and its concentration is not
significantly changed during the reaction. The mechanism proposed includes the transient
formation of a complex between hydrosilane, borane, and alkoxysilane in which H- is
transferred from silicon to boron and an oxonium ion moiety is generated by interaction of
alkoxysilane with positive silicon. The decomposition of the complex occurs by the H- transfer
to one of the three electrophilic centers of the oxonium structure, which explains the
competition between the siloxane formation and the Si−H/Si−OR exchange. In the case of
alkoxysilanes derived from primary alcohols, H- is preferably transferred to silicon. However,
for alkoxysilanes derived from a secondary alcohol, such as isopropyl alcohol, the secondary
carbon is more readily attacked than silicon by H-, which leads to a high yield of mixed
disiloxane.
The syntheses of different (18)F-labeled peptides using the highly effective labeling synthon p-(di- tert-butylfluorosilyl) benzaldehyde ([ (18)F]SiFA-A) for the development of (18)F-radiopharmaceuticals for oncological positron emission tomography (PET) is reported. The novel and mild labeling technique for the radiosynthesis of [ (18)F]SiFA-A, based on an unexpectedly efficient isotopic (19)F- (18)F exchange, yielded the (18)F-synthon [ (18)F]SiFA-A in almost quantitative yields in high specific activities between 225 and 680 GBq/micromol (6081-18 378 Ci/mmol) without applying HPLC purification. The [ (18)F]SiFA-A was finally used to label the N-terminal amino-oxy (N-AO) derivatized peptides AO-Tyr (3)-octreotate (AO-TATE), cyclo(fK(AO-N)RGD and N-AO-PEG 2-[D-Tyr-Gln-Trp-Ala-Val-betaAla-His-Thi-Nle-NH 2] (AO-BZH3, a bombesin derivative) in high radiochemical yields. Density functional theory (DFT) calculations confirmed high efficiency of the isotopic exchange, which is predicted to proceed via a pentacoordinate siliconate intermediate dissociating immediately to form the radiolabeled [ (18)F]SiFA-A.
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