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

摘要

Treatment of [Ni(COD)_2](COD=1,5-cyclooctadiene)with 5-chloro-2,4,6-trifluoropyrimidine(1)in the presence of P~iPr_3 or PPh_3 effects the formation of the fluoro complexes trans-[NiF(4-C_4N_2ClF_2)(P~iPr_3)_2](3)and trans-[NiF(4-C_4N_2ClF_2)(PPh_3)_2](4).The chloro complex trans-[NiCl(4-C_4N_2ClF_2)(PPh_3)_2](5)can be prepared by reaction of 4 with Me_3SiCl.In contrast,a reaction of 1 with [Pd(PPh_3)_4] leads to the insertion of a {Pd(PPh_3)_2} unit into the C-CI bond yielding trans-[PdCl(5-C_4N_2F_3)(PPh_3)_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_4N_2TolClF)(PPh_3)_2](7)and subsequently 5-chloro-2-fluoro-4,6-ditolylpyrimidine(8).Quenching of a solution of 7 with Me_3SiCl leads to the chloro derivative trans-[NiCl(4-C_4N_2TolClF)(PPh_3)2](9).Treatment of 4 with PhB(OH)_2 followed by addition of Me_3SiCl gives the complex trans-[NiCl(4-C_4N_2PhClF)(PPh_3)_2](10).In catalytic experiments,1 is converted with the boronic acids TolB(OH)_2,PhB(OH)_2,and p-F_3CC_6H_4B(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.