(Diethylamino)difluorosulfonium tetrafluoroborate is a highly reactive electrophilic fluorinating reagent. It is a crystalline solid, often referred to as XtalFluor-E, and is known for its stability and ease of handling compared to some other fluorinating agents like DAST (diethylaminosulfur trifluoride). Its primary application is in organic synthesis for the conversion of alcohols to alkyl fluorides and the deoxofluorination of carbonyl compounds (aldehydes and ketones) to gem-difluorides. The reagent operates through an SN2-type mechanism, where the sulfur center acts as a strong electrophile to activate the hydroxyl or carbonyl oxygen, facilitating fluoride ion delivery from the tetrafluoroborate counterion. It is particularly valued for its selectivity and reduced tendency to cause rearrangements or eliminations that are common with other fluorinating reagents. XtalFluor-E is a commercially available, bench-stable reagent widely used in medicinal chemistry and materials science for the selective introduction of fluorine atoms into organic molecules, a key modification for altering physicochemical properties, metabolic stability, and bioavailability.