C−F Bond Activation in Organic Synthesis

Ultrasound promoted facile synthesis of some pentafluoropyridine derivatives at ambient conditions

Some pentafluoropyridine derivatives have been synthesized by the reaction of pentafluoropyridine with appropriate mono and bidentate nucleophile under ultrasonic irradiation. This new methodology provides good to excellent yields in short reaction times (25-120 min) at room temperature.

Exploration of new C-O electrophiles in cross-coupling reactions

Since their development in the 1970s, cross-coupling reactions catalyzed by transition metals have become one of the most important tools for constructing both carbon-carbon and carbon-heteroatom bonds. Traditionally, organohalides were widely studied and broadly used as the electrophile, both in the laboratory and in industry. Unfortunately, the high cost, environmental toxicity, and sluggish preparation often associated with aryl halides can make them undesirable for the large-scale syntheses of industrial applications. However, with the further development of catalytic systems, and particularly of the ligands contained therein, a variety of electrophiles have now been successfully applied to cross-coupling reactions. Oxygen-based electrophiles have attracted much attention due to their ready availability from phenol and carbonyl compounds. Initially, aryl and alkenyl triflates were used in cross-coupling reactions due to their high reactivity; however, low moisture stability and high cost hampered their application. Later, with the development of highly efficient catalytic systems, the less reactive sulfonates and phosphates were successfully employed in cross-coupling reactions. Although they have higher stability and can be easily prepared, low atom economy remains an obstacle to their broader utility. Our group has worked to directly apply the abundant and readily available oxygen-containing compounds, such as phenols, alcohols, ethers, and carbonyl compounds, to cross-coupling reactions. In this Account, we describe our recent efforts in transition-metal-catalyzed cross-coupling reactions of new O-based electrophiles via C-O bond activation. We began by developing the methylation of aryl methyl ethers and benzyl methyl ethers via Ni-catalyzed selective C-O bond cleavage. With the refined Ni-based catalytic system, we further applied aryl/alkenyl carboxylates and carbamates to Suzuki-Miyaura, Negishi, and Kumada-Tamao-Corriu reactions to construct various biaryl scaffolds and highly substituted alkenes. To further improve the carbon atom economy, we developed the diaryl sulfates as one-by-one electrophiles (that is, both aryl groups are used in the reaction). Most recently, we have achieved the first successful cross-coupling reaction of magnesium naphtholates with aryl Grignard reagents. These results extend aryl and benzyl ethers, aryl and alkenyl carboxylates/carbamates, and magnesium naphtholates as novel electrophiles in cross-coupling reactions. More importantly, these studies contribute to our better understanding the intrinsic nature of C-O bonds, which were traditionally considered "inert" but clearly show enormous synthetic potential with the proper catalysts.

Reagent-dependent formation of C-C and C-F bonds in Pt complexes: an unexpected twist in the electrophilic fluorination chemistry

Cyclometalated platinum(II) complexes (C-P)Pt(Aryl)py undergo oxidative addition reaction upon treatment with electrophilic fluorination reagents, XeF(2) or N-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate, giving eventually products of C-C coupling. However, when aryl = mesityl, the two reagents give completely different products: the N-F salt still favors the C-C coupling reaction while XeF(2) gives the unprecedented benzylic fluorination of one of the ortho-methyl groups of the mesityl ligand.

Selective aromatic C-F and C-H bond activation with silylenes of different coordinate silicon

Herein we report on the reaction of stable two-coordinate silylene, L(1)Si [L(1) = CH{(C=CH(2))(CMe)(2,6-iPr(2)C(6)H(3)N)(2)}] (1) and three-coordinate silylene (Lewis base stabilized silylene), L(2)SiCl [L(2) = PhC(NtBu)(2)] (2) with aromatic compounds containing C-F and C-H bonds. The reaction of 1 and 2 with hexafluorobenzene (C(6)F(6)) affords the silicon(IV) fluorides, L(1)SiF(C(6)F(5)) (3) and L(2)SiFCl(C(6)F(5)) (4), respectively. The reaction proceeds through the unprecedented oxidative addition of one of the C-F bonds to the silicon(II) center without any additional catalyst. When 1 and 2 are treated with octafluorotoluene (C(6)F(5)CF(3)), pentafluoropyridine (C(5)F(5)N) regioselective C-F bond activation occurs leading to the formation of L(1)SiF(4-C(6)F(4)CF(3)) (5), L(1)SiF(4-C(5)F(4)N) (6), L(2)SiFCl(4-C(6)F(4)CF(3)) (7), and L(2)SiFCl(4-C(5)F(4)N) (8), respectively. More interestingly, compounds 1 and 2 react with pentafluorobenzene (C(6)F(5)H) under formation of silicon(IV) hydride L(1)SiH(C(6)F(5)) (9) by chemoselective C-H bond activation, in the latter case producing silicon(IV) fluoride L(2)SiFCl(4-C(6)F(4)H) (10) by chemo- as well as regioselective C-F bond activation. Furthermore, the reaction of 1 with 1,3,5-trifluorobenzene (1,3,5-C(6)F(3)H(3)) leads to the chemoselective formation of silicon(IV) hydride L(1)SiH(1,3,5-C(6)F(3)H(2)) (11). The formation of compounds 9 and 11 occurs via oxidative addition of the aromatic C-H bond to the silicon(II) center instead of C-F bond activation. All reported reactions proceed without any additional catalyst. Compounds 3, 4, 5, 6, 7, 8, 9, 10, and 11 were investigated by microanalysis and multinuclear NMR spectroscopy and compounds 3, 7, 8, and 9 additionally by single crystal X-ray structural analyses.

TBAF-Catalysed Facile Synthesis of Unsymmetrical Diaryl Thioethers via Mild SNAr Reactions

By using tetrabutylammonium fluoride as the catalyst, the synthesis of unsymmetrical diaryl thioethers could be easily achieved in high yields via a mild nucleopilic aromatic substitution reaction of aryl fluorides and phenylthiotrimethylsilane.