Steric Crowding Makes Challenging C_sp3–F Reductive Eliminations Feasible

Abstract

A high-yielding fluorination of (triphos)Pt-R+ has been achieved using an array of F+ sources, with XeF2 yielding R–F in minutes. The C–F coupling proved to be a stereoretentive process that proceeds via a concerted reductive elimination from a putative dicationic Pt(IV) center. The larger the steric congestion of the (triphos)Pt–Csp3+ complexes, the more efficient the fluorination, seemingly a result of sterically accelerated C–F reductive elimination along with simultaneous deceleration of its competing processes (β-H elimination).

Controlling Pd(iv) reductive elimination pathways enables Pd(ii)-catalysed enantioselective C(sp3)−H fluorination

“…However, using an amino acid-type transient directing group ( TDG1 - 3 ) will lead to a neutral, five-coordinate Pd(IV) intermediate (Figure 3, I–III ) and render the desired C(sp 3 )–F reductive elimination slower than competing pathways, which is the S N 2 C(sp 3 )–O formation in our case. Among the amino acid-type transient directing groups, we clearly observed a steric effect (Figure 3, I–III ) which complies with the study of Gagné and coworkers on C(sp 3 )–F reductive elimination from high-valent Pt and Pd species 46 . When the carboxylic acid moiety of the transient directing group is replaced with a neutral diethyl amide group, a cationic, five-coordinate Pd(IV) intermediate suitable for C(sp 3 )–F reductive elimination can be formed (Figure 3, IV ), which shifts the selectivity towards C(sp 3 )–F formation without affecting the overall yield.…”

Electrophilic fluorination of cationic Pt-aryl complexes

“…Nevertheless, several notable Pd 0/II catalyzed nucleophilic fluorinations have been recently reported. 3 More fruitful have been recent metal-catalyzed electrophilic fluorination reactions, 5–8 wherein high-valent metal fluoro intermediates ( e.g. Pd(IV), 4 Ag(II)···Ag(II), 5 Au(III), 6 etc. )…”

Transition Metal-Mediated and Metal-Catalyzed Carbon–Fluorine Bond Formation

“…Gold(III) intermediates were observable by NMR in several cases. Interesting parallels exist with the reactivity of platinum(II) alkyls reported by Gagné [46]: a sterically congested Au-menthyl complex underwent C-F bond formation in higher yield than the less bulky alkylgold complexes, and with retention of configuration; β-hydride elimination was a significant side reaction for many substrates; and the gold(I) methyl complex did not afford Me-F product. Among Toste's wide substrate survey, several of the alkyl fragments underwent rearrangements characteristic of carbocations.…”

“…Gagné reported a study on the relevance of steric congestion for facile C(sp 3 )-F reductive elimination [46]. A series of alkylplatinum(II) complexes was described that have β -Pt C-H bonds, yet are stable toward β -hydride elimination at room temperature.…”