4049-97-2Relevant articles and documents
Influence of N-Heterocyclic Carbene Steric Bulk on Selectivity in Nickel Catalyzed C-H Bond Silylation, Germylation, and Stannylation
Elsby, Matthew R.,Liu, Junyang,Zhu, Sha,Hu, Lingfei,Huang, Genping,Johnson, Samuel A.
, p. 436 - 450 (2019/01/04)
A series of Ni(0) compounds supported by electronically similar N-heterocyclic carbene (NHC) ancillary ligands with a range of %Vbur were used as catalysts for aryl C-H bond silylation, germylation, and stannylation. The NHC steric bulk strongly influenced the selectivity of C-H functionalization to give new carbon-heteroatom bonds versus alkene hydroarylation, despite little structural change in the resting state of the catalysts. Studies were performed by reacting C6F5H and H2C=CHER3 (ER3 = SnBu3, GePh3, SiMe3) using catalytic amounts of Ni(COD)2 and NHC ligands IPr, IMes, IBn, and iPr2Im. Catalytic C-H stannylation to give C6F5SnBu3 was facile with all ligands. The catalytic C-H germylation reaction was more difficult than stannylation but was demonstrated using H2C=CHGePh3 to give C6F5GePh3 for all but the largest NHC. The bulkiest NHC, IPr, gave a 96:4 ratio of the hydroarylation product C6F5CH2CH2GePh3 versus C6F5GePh3. The C-H silylation reactions required the highest temperatures and gave selective silylation product C6F5SiMe3 only for the smallest IBn and iPr2Im NHC ligands. Using the larger IMes carbene resulted in a 66:34 mixture of silylation and hydroarylation products, and the largest NHC, IPr, gave exclusive conversion to the hydroarylation product, C6F5CH2CH2SiMe3. DFT calculations are provided that give insight into the mechanism and key reaction steps, such as the relative difficulty of the critical β-Sn, Ge, and Si elimination steps.
