Tellurium twist on hypervalent electrophilic fluorination chemistry

Hypervalent iodine compounds are versatile reagents for transferring functional groups to organic molecules. For example, these compounds have become popular as electrophilic trifluoromethylating reagents. The fluorinating reagents, first reported in 2006 by Antonio Togni and his group at the Swiss Federal Institute of Technology (ETH), Zurich, take advantage of iodine(III)’s ability to maintain two lone pairs of electrons while bonding with a CF₃ group and a phenyl ring and forming a stabilizing side ring through interactions with an adjacent oxygen or nitrogen phenyl substituent (one example shown). Since 2006, Togni and coworkers have made several modifications to the original compounds by way of changing the fluorinated and phenyl substituent groups. In the latest effort, Togni and Ewa Pietrasiak in his group have gone further to replace iodine itself with its periodic table neighbor tellurium (Organometallics 2017, DOI: 10.1021/acs.organomet.7b00535). Hypervalent compounds are those that contain a group 13 to 18 element bearing more than eight valence electrons. Because iodine(III) with 10 valence electrons fits that bill, Togni and Pietrasiak thought tellurium(II) compounds with a similar electronic structure might work as fluorinating reagents as well. The researchers made a series of fluorinated tellurium(II) compounds (shown), including ones with CF₂H and C₆F₅ groups that aren’t known for the iodine reagents, and studied their hypervalent nature via structural and spectroscopic data. The team is now testing the tellurium derivatives for their ability to function as perfluoroalkyl transfer reagents.