A facile regioselective
switch for nickel-catalyzed hydroarylation
of vinylarenes with benzothiazoles has been developed, which relies
on the simple structural variation of novel Ni(II) complexes of the
type Ni(NHC)[P(OR)3]Br2. Using magnesium turnings
as the reductant, Ni(IMes)[P(OEt)3]Br2 afforded
branched products, while Ni(IPr*OMe)[P(OEt)3]Br2 created steric demand to afford linear products.
This work also provides a rare example of the rational design of heteroleptic
Ni(II) complexes that display the required air stability, reactivity,
and regioselectivity via synergism between NHC and phosphite ligands.
A bisoxazoline-derived NHC known as IBiox-6 reacted smoothly with Ni[P(OEt) 3 ] 2 Br 2 and Ni(PPh 3 ) 2 Br 2 to give the respective heteroleptic Ni(II) complexes Ni(IBiox-6)[P(OEt) 3 ]Br 2 (1) and Ni(IBiox-6)(PPh 3 )Br 2 (2) in yields of 60% and 71%. Their crystal structures were characterized to reveal a rare cis disposition of the IBiox-6 ligand to the phosphite ligand in 1, while 2 possessed the more common trans configuration. Both complexes catalyzed the cross-electrophile coupling of benzyl chlorides with aryl chlorides and fluorides in the presence of Mg turnings at 50 °C via a "real one-pot" procedure, featuring no requirement for temperature variation or portionwise addition of any coupling partner. In particular, complex 1 showed a better balance between the catalytic activity and selectivity. The scope of the procedure catalyzed by 1 and Mg turnings was investigated, providing a highly selective, simple, and practical approach to the synthesis of diarylmethanes with high steric hindrance and various functional groups, including oligo-diarylmethane with asymmetric structures.
Novel heteroleptic Ni (II) complexes bearing a highly hindered yet flexible IPr* ligand, Ni (IPr*)(PPh3)Br2 (1) and Ni (IPr*)(PCy3)Br2 (2) (IPr* = 1,3‐bis(2,6‐bis (diphenylmethyl)‐4‐methylphenyl)imidazol‐2‐ylidene), were easily prepared in 78% and 89% yield, respectively. Both were characterized by elemental analysis and NMR spectroscopy, and 1 was subjected to X‐ray crystallography. Compared with 2 and its analogue bearing a less sterically demanding IPr ligand (IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene), complex 1 exhibited superior catalytic activity in the magnesium‐mediated reductive coupling of benzyl chlorides with aryl chlorides, featuring outstanding tolerance of both coupling partners with steric demand. This study discloses a ligand‐facilitated reductive coupling of benzyl chlorides with aryl chlorides, which provides a new and practical synthetic tool for the synthesis of diarylmethanes.
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