Ligand Design for the Synthesis of Reactive Nickel(0) Complexes Capable of Inert Bond Activation: Carbon-Fluorine and Carbon-Hydrogen Bond Activation and Catalytic Functionalization

摘要

A strong neutral-donor, MeNC 6 H 4 N i Pr, with properties analogous to those of N -heterocyclic carbenes was developed to aid in the oxidative addition of challenging substrates to late-transition metals. Selective room-temperature C-F bond activation was observed with partially fluorinated aromatics using a nickel(0) source in the presence of this donor. Attempts to functionalize the C-F bond of fluorinated aromatics via a Stille coupling reaction with CH 2 =CHSnBu 3 and catalytic amounts of MeNC 6 H 4 N i Pr and Ni(COD) 2 , failed to produce the expected vinylated product. Rather this reaction provided new C-Sn bonds via C-H bond stannylation to form products of the type C 6 F n H 5-n SnBu 3 and ethylene, at room-temperature with MeNC 6 H 4 N i Pr and at 80 °C with i Pr 3 P. The scope of fluoroarenes has been examined. The complex ( i Pr 3 P)Ni(η 2 -CH 2 =CHSnBu 3 ) 2 was identified as the active species for catalytic C-H bond stannylation. The crystalline complex ( i Pr 3 P)Ni(η 2 -CH 2 =CHSnPh 3 ) 2 provided a more easily handled analogue, and was also capable of catalytic stannylation. Mechanistic studies involving deuterium labeling, concentration effects and competition reactions with various fluoroarenes were all consistent with the proposed mechanism. The reaction of CH 2 =CHSnR 3 (R = Ph, Bn) and C 6 F 5 H with MeNC 5 H 4 N i Pr and Ni(COD) 2 produced C 6 F 5 CH 2 CH 2 SnR 3 . The compound (MeNC 5 H 4 N i Pr)Ni(η 2 -CH 2 =CHSnPh 3 ) 2 , was shown to be a catalyst for C-H alkylation. The isolable complexes cis -(MeNC 5 H 4 N i Pr) 2 Ni(C 6 F 5 )(SnR 3 ) react with ethylene to give C 6 F 5 CH 2 CH 2 SnR 3 . Complexes cis -(MeNC 5 H 4 N i Pr) 2 Ni(C 6 F 5 )(SnR 3 ) are not directly in the catalytic cycle for C-H alkylation, however, they proved to be a resting state for both catalytic C-H stannylation and ethylene carbostannylation. Mechanistic studies involving concentration effects, ligand donor effects and R-group influence of CH 2 =CHSnR 3 (R = Ph, Bn, Bu) support the proposed mechanistic manifold. The scope for C-F activation and C-H stannylation with MeNC 5 H 4 N i Pr and Ni(COD) 2 was expanded to trifluoromethyl fluorinated benzene derivatives. The C-H stannylation products undergo further reactivity with MeNC 5 H 4 N i Pr and Ni(COD) 2 to form cis -(MeNC 5 H 4 N i Pr) 2 Ni(2,3,5,6-C 6 F 4 -4-CF 3 ) 2 , (2,4,5-trifluoro-6-(trifluoromethyl)-1,3-phenylene)bis(tributylstannane) and FSnBu 3 . The mechanism of this reactivity was studied and appears to be radical based. Support that meta -substituents have an even greater affect on the reaction rate of C-H activation than para, was gained from a competition study between various substrates with meta - and para -substituents.