Benzoates as photosensitization catalysts and auxiliaries in efficient, practical, light-powered direct C(sp³)–H fluorinations

Abstract

Benzoates serve as catalysts or auxiliaries for photochemical EnT radical C(sp3)–H fluorinations. The auxiliary markedly increases scope and efficiency, enabling reactions of free alcohols, amines, and allowing rapid gram-scale fluorinations in air.

Facilitating [2+2] Photocycloadditions by Promoting Oxygen Tolerance and Substrate Activation in Water

“…To our surprise, the [2+2] photocycloaddition proceeds to some extent without (Table 2, entry 2) and drastically improves upon removal of oxygen in weak hydrogen bonding solvent methanol (entry 4). These results allow us to demonstrate experimentally that the reaction as expected is oxygen sensitive in organic solvents [39] . Although methanol normally contains less oxygen than many other solvents, [40] the drop in yield without a freeze – pump – thaw procedure is significant.…”

“…These results allow us to demonstrate experimentally that the reaction as expected is oxygen sensitive in organic solvents. [39] Although methanol normally contains less oxygen than many other solvents, [40] the drop in yield without a freeze -pumpthaw procedure is significant. Furthermore, the results indicate that surprisingly weakly hydrogen bonding solvents like methanol are able to reduce the triplet energy of this type of α,β-unsaturated ketones.…”

Self-Sensitized Photooxidation of Naphthols to Naphthoquinones and the Use of Naphthoquinones as Visible Light Photocatalysts in Batch and Continuous Flow Reactors

“…An additional perspective would be the covalent or noncovalent tethering of juglone to induce stereoselective or other useful visible light transformations. 81,102 Finally, as juglone is an important natural product and dying agent, we also believe this study could stimulate new research works in areas beyond green chemistry.…”

Photocatalytic Late-Stage C–H Functionalization

“…The reaction was found to override the directing ability of α,β-unsaturated ketones, such in the case of 17α-hydroxyprogesterone, which was selectively fluorinated at the C-ring with complete diastereoselectivity. Barham and co-workers recognized that photoexcited fluorobenzoates promote exciplex formation with Selectfluor to produce hydrogen-atom-abstracting quinuclidine radical-cations (Scheme C) . Quinuclidine radical-cation can abstract a hydrogen atom from 127.5 , which subsequently undergo fluorination to afford 127.6 .…”

“…Barham and co-workers recognized that photoexcited fluorobenzoates promote exciplex formation with Selectfluor to produce hydrogen-atom-abstracting quinuclidine radical-cations (Scheme 137C). 550 Quinuclidine radical-cation can abstract a hydrogen atom from 127.5, which subsequently undergo fluorination to afford 127.6. Cholest-4-en-3-one and methyl dehydrocholate underwent late-stage fluorination, albeit with moderate levels of site-selectivity.…”