Organometalloid compositions of silicon and boron permit rapid, high-yielding, one-step radiolabeling of a covalently linked protein ligand (biotin) under aqueous conditions to give the corresponding alkyltetrafluorosilicates and aryltrifluoroborate salts. Biotin was chosen as a test ligand for protein targeting because of its quantitative interaction with avidin, which in turn allowed us to calculate fluoridation yields that approach 80-100%. The silicate was found to be moderately stable to hydrolysis, whereas the borate appears to be so stable that its hydrolytic decomposition was not readily measured. With the stability of both compounds ascertained, this work describes a novel and robust radiolabeling method that may find use in the development of positron emission tomography radiopharmaceuticals.

Whereas electron withdrawing substituents retard the rate of aryltrifluoroborate solvolysis, electron-donating groups enhance it. Herein is presented a Hammett analysis of the solvolytic lability of aryltrifluoroborates where log(k(solv)) values correlate to sigma values with a rho value of approximately -1. This work provides a predictable rubric for tuning the reactivity of boron for several uses including (18)F-labeled PET reagents and has mechanistic implications for ArBF(3)-enhanced ligandless metal-mediated cross coupling reactions with aryltrifluoroborates.

The use of a boronic ester as a captor of aqueous [(18)F]-fluoride has been previously suggested as a means of labeling biomolecules in one step for positron emission tomography (PET) imaging. For this approach to be seriously considered, the [(18)F]-labeled trifluoroborate should be humorally stable such that it neither leaches free [(18)F]-fluoride to the bone nor accumulates therein. Herein, we have synthesized a biotinylated boronic ester that is converted to the corresponding trifluoroborate salt in the presence of aqueous [(18)F]-fluoride. In keeping with its in vitro aqueous kinetic stability at pH 7.5, the trifluoroborate appears to clear in vivo quite rapidly to the bladder as the stable trifluoroborate salt with no detectable leaching of free [(18)F]-fluoride to the bone. When this labeled biotin is preincubated with avidin, the pharmacokinetic clearance of the resulting complex is visibly altered. This work validates initial claims that boronic esters are potentially useful as readily labeled precursors to [(18)F]-PET reagents.