Easily Prepared Air‐ and Moisture‐Stable Pd–NHC (NHC=N‐Heterocyclic Carbene) Complexes: A Reliable, User‐Friendly, Highly Active Palladium Precatalyst for the Suzuki–Miyaura Reaction

Abstract

The synthesis of NHC-PdCl(2)-3-chloropyridine (NHC=N-heterocyclic carbene) complexes from readily available starting materials in air is described. The 2,6-diisopropylphenyl derivative was found to be highly catalytically active in alkyl-alkyl Suzuki and Negishi cross-coupling reactions. The synthesis, ease-of-use, and activity of this complex are substantial improvements over in situ catalyst generation and all current Pd-NHC complexes. The utilization of complex 4 led to the development of a reliable, easily employed Suzuki-Miyama protocol. Employing various reaction conditions allowed a large array of hindered biaryl and drug-like heteroaromatic compounds to be synthesized without difficulty.

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…77,78 The catalyst is considerably more active than the analogous normally C2-bound imidazolylidene complex. In contrast, the palladium 2-pyridylidene catalyst 156b is not significantly faster in catalyzing Heck-type olefin arylation than Pd 2 (dba) 3 . 111 Similarly, the pyrazolylidene complex 73 has shown relatively low catalytic performance.…”

“…A rational route toward C4-bound carbenes therefore includes the selective protection of the most acidic C2-position, e.g., by incorporating alkyl or aryl substitutents. Thus, oxidative addition of the C4-H bond of the tetraalkylated C2-blocked imidazolium salt 14 to zerovalent Pt(norbornene) 3 , in the presence of equimolar amounts of the free carbene IMes, yields the platinum hydride complex 15 with the mixed C2-and C5-bound carbenes both attached to platinum (see Scheme 6). 27 The formation of this complex has been proposed to occur stepwise.…”

“…Studies of catalytic hydrosilylation were also undertaken by Cavell and co-workers using a catalyst generated in situ from Pt(nbe) 3 , KOtBu, and different imidazolium salts. The catalyst system based on the 2H-imidazolium salt 244 is the most active one and gives predominantly the hydrosilylated product 241 from the reaction of styrene with HSiEt 3 (see Scheme 91).…”

Polymer precursors from catalytic reactions of natural oils

“…and [PdCl 2 (IPr)(3-chloropyridine) 266 (PEPPSI-IPr). In order to assess the synergy resulting from the presence of the mixed NHC/Phosphine ligand, the dimer [Pd(µ-Cl)(Cl)(SPhos)] 2 (SPhos = 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl) was synthesised.…”

Synthesis of π-conjugated poly(thienylenearylene)s with nickel-catalyzed C–H functionalization polycondensation

“…When the catalyst was switched to NiCl 2 (PPh 3 )IPr (IPr: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), which showed excellent catalytic activity during the polymerization of 2-chloro-3-hexylthiophene, 24 polymer 5 was obtained with a 48% yield and low molecular weight (M n ¼ 4470). The similar reaction with Pd-PEPPSI-SIPr (pyridine-enhanced precatalyst preparation stabilization and initiation-1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) 33 afforded 5 with a 35% yield and M n ¼ 5190. Although the polymerization reaction of triflate 34 1b in the presence of NiCl 2 dppp proceeded at room temperature, only 56% of the oligomer was obtained.…”