Mixed Phosphite/N-heterocyclic Carbene Ni(?) Complexes: Synthesis,characterization And Catalysis

A series of novel mixed phosphite/N-heterocyclic carbene?NHC?Ni???complexes have been synthesized and fully characterized.The catalytic behaviors of these Ni???complexes in the Miyaura borylation of aryl chlorides,aryl bromides and benzyl chlorides with B₂?OR?₄ have been studied.1.Mixed phosphite/NHC Ni???complexes Ni?NHC?[P?OR?₃]X₂{NHC=1,3-bis[2,4,6-?trimethyl?-phenyl]Imidazolin-2-ylidene,SIMes;1,3-bis[?2,6-diisopropyl?phenyl]imidazolin-2-ylidene,SIPr;1,3-bis?tertbutyl?imidazolin-2-ylidene,SI^tBu;R=Et,ⁱPr;X=Br,Cl}1–6 and 8–10 were easily prepared by the reaction of Ni[P?OR?₃]₂X₂ with one equivalent of NHC in yields among 50%and 71%.All of them are red?SIMes or SIPr?or brown?SI^tBu?solid powders and stable in open air.These Ni???complexes have been characterized by element analysis,NMR spectra and X-ray crystallography for complex 5,6 and 9.2.Catalytic performances of complexes 1–6 and 8–10 were studied in the Miyaura borylation of aryl chlorides or bromides with B₂?OR?₄,wherein the significant synergic effect between phosphite ligands and NHC ligands were observed.Moreover,the structural variation of these Ni???complexes,i.e.the variation of phosphite or NHC ligand,exerts great influence on their catalytic activities.Among them,complex 1exhibited the highest catalytic activity in the Miyaura borylation of aryl chlorides with neopentylglycolboronates?B₂nep₂?,meanwhile complex 2 showed the best catalytic performance in the Miyaura borylation of aryl bromides with pinacolboronates?B₂pin₂?.The difference in catalytic activity between 1 and 2 has been successfully employed to construct the aromatic ring with different boric acid esters by selective Miyaura borylation.3.It was found that mixed Ni???complexes 1–6 and 8–10 could be used as the catalysts for Miyaura borylation of benzyl chlorides with B₂?OR?₄.Complex 1 exhibited best catalytic performance in the Miyaura borylation of benzyl chlorides with B₂nep₂,and in the Miyaura borylation of benzyl chlorides with B₂pin₂,complex 6 exhibited best catalytic performance.Correspondingly,the difference in catalytic activity between 1 and6 has been successfully employed to synthesize benzyl compounds with different boric acid esters.