C(sp³)–H Alkenylation Catalyzed by Cationic Alkylhafnium Complexes: Stereoselective Synthesis of Trisubstituted Alkenes from 2,6-Dimethylpyridines and Internal Alkynes

Dibenzylhafnium complexes 3a–d, supported by dianionic bidentate or tridentate ligands, upon activation via abstraction by either [Ph3C]­[B­(C6F5)4] or B­(C6F5)3 served as catalysts for the C­(sp3)–H alkenylation of 2,6-dimethylpyridines with dialkylalkynes to give corresponding C­(sp3)–H alkenylated products 6. Complex 3c, containing a pyridine arm in the ligand skeleton, exhibited the highest catalytic activity among 3a–d; initial addition of 2,6-dimethylpyridine (4a) to the C6D5Br solution of 3c followed by [Ph3C]­[B­(C6F5)4] and 3-hexyne (5a) produced trisubstituted alkene 6aa in stereoselective manner in up to 50% yield without any byproducts, while the addition of 5a prior to 4a and [Ph3C]­[B­(C6F5)4] to the C6D5Br solution of 3c generated 6aa, together with the formation of byproduct (E)-(2-ethylpent-2-en-1-yl)­benzene (7). When an asymmetrical pyridine, 3-bromo-2,6-dimethylpyridine, was used as the coupling partner, the corresponding trisubstituted alkene was obtained selectively. Catalytically active cationic benzylhafnium complexes 8a–d, which were prepared by the reactions of 3a–d and B­(C6F5)3, respectively, were characterized by 1H, 13C, and 19F NMR spectroscopy. Kinetic studies of the catalytic reaction between 4a and 4-octyne (5b) using 3c and [Ph3C]­[B­(C6F5)4] in C6D5Br revealed that the catalytic reaction was zero-order for both 4a and 5b, indicating that the rate-determining step involved the C­(sp3)–H bond activation of 4a by vinylhafnium intermediate 11c.