The Uranyl Cation as a Visible-Light Photocatalyst for C(sp(3) )-H Fluorination

Abstract

The fluorination of unactivated C(sp(3) )-H bonds remains a desirable and challenging transformation for pharmaceutical, agricultural, and materials scientists. Previous methods for this transformation have used bench-stable fluorine atom sources; however, many still rely on the use of UV-active photocatalysts for the requisite high-energy hydrogen atom abstraction event. Uranyl nitrate hexahydrate is described as a convenient, hydrogen atom abstraction catalyst that can mediate fluorinations of certain alkanes upon activation with visible light.

Keywords: C−H activation; fluorination; hydrogen atom transfer; photocatalysis; uranium.

Figure 1

Several near-UV light HAT catalysts and electrophilic fluorine sources used for fluorinations of unactivated C–H bonds and a general mechanism illustrating their function

Figure 2

The catalytic aerobic oxidation of alkanes using uranyl cation has been reported

Figure 3

The uranyl excited state [UO₂]²⁺* reacts with alkanes primarily through hydrogen atom transfer (HAT) and with arenes through unproductive exciplex decay

Figure 4

Reactions containing both cyclooctane and toluene (top) or cyclopentanone (bottom) behave differently with respect to reagent conversion. The toluene result suggests that it is an effective quencher of the uranyl excited state [UO₂]²⁺*.