C-Br activation of aryl bromides at Ni ~0(NHC) _2: Stoichiometric reactions, catalytic application in Suzuki-Miyaura cross-coupling, and catalyst degradation

摘要

Complex [Ni _2(~iPr _2Im) _4(COD)] (1) (~iPr _2Im = 1,3-diisopropylimidazolin-2-ylidene) is a very efficient catalyst for the Suzuki-Miyaura cross-coupling reaction of 4-bromotoluene with phenylboronic acid and also mediates the Ullmann-type homo-cross-coupling reaction of bromobenzene with a moderate efficiency. Stoichiometric reactions of complex 1 with aryl bromides (ArBr) at room temperature lead to mixtures of aryl bromo complexes of the type trans-[Ni(~iPr _2Im) _2(Br)(Ar)] and the bis(bromo) complex trans-[Ni(~iPr _2Im) _2(Br) _2] 2. The complexes trans-[Ni(~iPr _2Im) _2(Br)(Ar)] (for Ar = Ph 3, 4-MeC _6H _44, 4-Me(O)CC _6H _45, 4-MeOC _6H _46, 4-MeSC _6H _47, 4-Me _2NC _6H _48, 2-C _5NH _49) can be selectively synthesized by working at low temperatures and using a high dilution of the starting materials. A major deactivation pathway for trans-[Ni(~iPr _2Im) _2(Br)(Ar)] was identified in the presence of aryl bromides. This deactivation process includes (i) the formation of trans-[Ni(~iPr _2Im) _2(Br) _2] from trans-[Ni(~iPr _2Im) _2(Br)(Ar)] (2) and ArBr and (ii) the formation of an imidazolium salt of the type 2[~iPr _2Im-Ar] ~+[NiBr 4] 2- from trans-[Ni(~iPr _2Im) _2(Br) _2] (2) and ArBr. The reactions of complex 2 with a series of aryl halides at higher temperatures lead to the decomposition of the bis(carbene) nickel moiety with formation of the imidazolium salts 2[~iPr _2Im-Ar] ~+[NiBr 2X 2] ~(2-) (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-MeC _6H _412, 4-FC _6H _413, 4-OSi(CH _3) _3-C _6H _414) in high yields.