Photo-initiated thiol-ene click reactions as a potential strategy for incorporation of [M^I(CO)₃]⁺ (M = Re, ^99mTc) complexes

Click reactions offer a rapid technique to covalently assemble two molecules. In radiopharmaceutical construction, these reactions can be utilized to combine a radioactive metal complex with a biological targeting molecule to yield a potent tool for imaging or therapy applications. The photo-initiated radical thiol-ene click reaction between a thiol and an alkene was examined for the incorporation of [M^I(CO)₃]⁺ (M = Re, ^99mTc) systems for conjugating biologically active targeting molecules containing a thiol. In this strategy, a potent chelate system, 2,2′-dipicolylamine (DPA), for [M^I(CO)₃]⁺ was functionalized at the central amine with a terminal alkene linker that was explored with two synthetic approaches, click then chelate and chelate then click, to determine the flexibility and applicability of the thiol-ene click reaction to specifically incorporate ligand systems and metal complexes with a thiol containing molecule. In the click then chelate approach, the thiol-ene click reaction was carried out with the DPA chelate followed by complexation with [M^I(CO)₃]⁺. In the chelate then click approach, the alkene functionalized DPA chelate was first complexed with [M ^I(CO)₃]⁺ followed by the conduction of the thiol-ene click reaction. Initial studies utilized benzyl mercaptan as a model thiol for both strategies to generate the identical product from either route to provide information on reactivity and product formation. DPA ligands functionalized with two unique linker systems (allyl and propyl allyl ether) were prepared to examine the effect of the proximity of the chelate or complex on the thiol-ene click reaction. Both the thiol-ene click and coordination reactions with Re, ^99mTc were performed in moderate to high yields demonstrating the potential of the thiol-ene click reaction for [M ^I(CO)₃]⁺ incorporation into thiol containing biomolecules.