Catalytic C-C coupling reactions at nickel by C-F activation of a pyrimidine in the presence of a C-Cl bond: The crucial role of highly reactive fluoro complexes

Treatment of [Ni(COD)(2)] (COD = 1,5-cyclooctadiene) with 5-chloro-2,4,6-trifluoropyrimidine (1) in the presence of (PPr3)-Pr-i or PPh3 effects the formation of the fluoro complexes trans[NiF(4-C4N2ClF2)(PiPr3)2] (3) and trans- [NiF((4)-C4N2ClF2)(PPh3)(2)] (4). The chloro complex trans- [NiCl(4-C4N2ClF2)(PPh3)(2)] (5) can be prepared by reaction of 4 with Me3SiCl. In contrast, a reaction of 1 with [Pd(PPh3)(4)] leads to the insertion of a {Pd(PPh3)(2)}unit into the C-Cl bond yielding trans-[PdCl(5-C4N2F3)(PPh3)(2)] (6). Treatment of 4 with an excess of TolB(OH)2 at 273 K results in the slow formation of trans- [NiF(4-C(4)N(2)TolCIF)(PPh3)(2)] (7) and subsequently 5-chloro-2-fluoro-4,6-ditolylpyrimidine (8). (quenching of a solution of 7 with Me3SiCl leads to the chloro derivative trans- [NiCl(4-C(4)N(2)TolClF)(PPh3)(2)] (9). Treatment of 4 with PhB(OH)2 followed by addition of Me3SiCl gives the complex trans-[NiCl(4-C4N2PhClF)(PPh3)2] (10). In catalytic experiments, 1 is converted with the boronic acids TolB(OH)(2), PhB(OH)(2), and p-F3CC6H4B(OH)(2) into the 5-chloro-2-fluoro-4,6-diarylpyrimidines 8, 11, and 12 in 73%, 88%, and 37% yield, respectively, when 10% of 4 is employed as catalyst. The molecular structures of the complexes 5, 6, and 10 have been determined by X-ray crystallography. The studies reported in this paper represent the first catalytic C-C coupling reactions involving the activation of a C-F bond in the presence of a thermodynamically weaker C-Cl bond. They provide a route to access 5-chloro-2-fluoro-4,6-diarylpyrimidines, which have not been described before. There is considerable evidence that the presence of the fluoro ligand in 4 is crucial for the transmetalation step to occur and for the catalytic cycle to proceed.