Selective Activation And Functionalization Of C-F/C-H Bond In Fluorinated Schiff Base

Because of the presence of an incomplete d sub-shell, transition metals easily form coordination complexes with ligands and are widely used in organic synthesis, stoichiometry and industrial catalytic processes. Of all the transition metals, cobalt has low cost, low toxicity and so on. Cobalt attracts more and more attentions of chemists in the modern synthetic chemistry. Trimethylphosphine is a kind of excellent ligands, which has strong coordination ability, convenient preparation and simple spectroscopic characteristics. Cobalt complexes supported with trimethylphosphine are so activated that they can selectively activate the C-F bonds and C-H bonds.In this paper, fluorinated Schiff bases with imine anchoring groups were selected as raw materials. The explore of C-F/C-H bond activation was developed according to reactions of Schiff bases and Co(PMe3)4/Co(PMe3)4Me supported with trimethylphosphine. What’s more, we reported kinds of C-F bond functionalization reactions without transition-metal catalyst.1. The ortho-C-F bond activation in the fluorinated Schiff bases were investigated using Co(PMe3)4supported with trimethylphosphine. We researched two factors at ortho-C-F bond activations including fluorine atoms’position and number in benzene ring. Three kinds of product were discovered, π-coordinated cobalt(0) complexes, cobalt(I) complex with ortho-C-F bond activation and cobalt(II) complex containing a [C-Co-F] fragment. A proposed mechanism was put forward and we obtained further progress in catalysis. With Co(PMe3)4as catalyst, a new C-C coupling reaction was researched between fluorinated Schiff base and organic zinc which used as nucleophilic reagent. A large number of ortho-disubstitutional fluorinated Schiff bases were synthetized. According to all the work, we made certain the reaction condition and mechanism of C-F ortho-bond activation. This work is of importance for cobalt-catalyzed C-F bond activation and functionalization of organic fluorides.2. The selective ortho-C-F/C-H bond activation in the fluorinated Schiff bases were investigated using CoMe(PMe3)4supported with trimethylphosphine. We prepared three kinds of fluorinated Schiff bases, which had both an ortho-C-F bond and an ortho-C-H bond in the same chemical environment. CoMe(PMe3)4could activate ortho-C-H bond first. With the releasing of CH4, we got penta-coordinate cobalt(Ⅰ) complexes. What’s more, we discovered that hexa-coordinate cobalt(III) intermediate was not stable because of steric hindrance and weak coordination ability. It would change to penta-coordinate cobalt(Ⅰ) complex with the the releasing of CH3F. The cobalt(Ⅰ) complex could react with CO, which replaced the position of PMe3. The reactions of cobalt(Ⅰ) complex with MeI and EtBr afforded organicfluorides as C-C coupling products.3. We prepared five kinds of fluorinated Schiff bases, which had ortho-C-F/C-H bonds and another C-H bond in another aromatic nucleus. Both the ortho-C-F/C-H bond of fluorophenyl group and C-H bond of naphthyl ring in the fluorinated Schiff bases20,22,23,24could be activated by CoMe(PMe3)4supported with trimethylphosphine. In this process, CoMe(PMe3)4activated ortho-C-F/C-H bond first to form a cobalt(Ⅲ) intermediate. However it was changed to a cobalt(Ⅰ) intermediate because of instability. The cobalt(Ⅰ) intermediate could activate the C-H bond of naphthyl ring. After insertion reaction, the hexa-coordinate cobalt(Ⅲ) complex containing a Co-H fragment was observed. This kind of cobalt-metal hydride could react with CO and became the cobalt(Ⅰ) intermediate again. This discovery proved that the mechanism we put forward was right. We made certain the di-activation process of C-F/C-H bond and found a new method to synthesize metal hydride.4. We reported an ortho-selective nucleophilic substitution of polyfluoroarenes with Grignard reagents via C-F bond activation to generate fluorinated benzaldehyde under mild conditions without transition metal catalyst. The functionalization of the ortho-(C-F) bond of polyfluoroaryl imines with Grignard reagents competed with the addition of imine group. We considered that several factors, such as the number and the position of the F atoms, the property (aromatic or aliphatic of Grignard reagents), the steric effect of both substrates, played important role in this competition. For the excellent ortho-selectivity, the imine group must play a decisive role in the substitution. A possible mechanism is proposed. In general, the reactions have high yields, and the process is simply operated. This work is of great significance for C-F bond activation and functionalization of organic fluorides.