N‐Heterocyclic Carbenes Functioning as Monoligating Clamps

Abstract

International audienceBenzimidazolium salts N,N′-disubstituted with 9-alkylfluorenyl groups (3a-e, alkyl = methyl, ethyl, propyl, butyl, benzyl) have been synthesised in high yields in three steps from o-phenylenediamine. This amine was treated with fluorenone in the presence of TiCl4 and tetramethylethylenediamine (TMEDA) to form N,N′-bis(9H-fluoren-9-ylidene)benzene-1,2-diamine (1) in 91 % yield. Diamines 2a-e were then obtained in yields superior or equal to 77 % by reacting diimine 1 with the appropriate organolithium reagent. In the final step, diamines 2a-e were treated with ethylorthoformate under acidic conditions to afford benzimidazolium salts 3a-e. These were readily converted into the PEPPSI palladium complexes 4a-e (PEPPSI = pyridine-enhanced precatalyst preparation stabilisation and initiation). NMR and X-ray diffraction studies revealed that the flat fluorenylidene moiety orientates the alkyl groups towards the metal centre and because of its restricted rotational freedom makes the ligand bulkiness time independent. Thus, the metal centre is permanently confined between the two alkyl groups, and thereby forms a monoligating clamp with the carbenic centre. The CH2 groups close to the palladium ion give rise to anagostic C-H***Pd interactions. Catalytic tests revealed that the palladium complexes 4a-e are highly efficient in Suzuki-Miyaura cross-coupling reactions; their activity is equal or superior to the best PEPPSI catalysts reported to date

Crystal structure of trans-dichlorido[1,3-bis(9-methyl-9H-fluoren-9-yl) benzimidazol-2-ylidene](pyridine)palladium(II) – a compound with anagostic CH–Pd interactions, C40H31Cl2N3Pd

“…In keeping with such electrostatic interactions, the 1 H NMR signal of the CH 3 groups appears at signi cantly lower eld than that seen for the same protons in the benzimidazolium precursor used for the preparation of the complex (δ = 4.02 vs. 2.96 ppm). Note that, as shown by NMR solution studies, the methyl uorenyl groups do not rotate about the corresponding N1-C4 bonds [1]. The PdCl (2.323(1) and 2.331(1) Å), Pd-C18 (1.966(5) Å), and Pd-N2 (2.144(4) Å) bond lengths are comparable to those found in related trans-[PdCl 2 (NHC)(pyridine)] complexes [1,3].…”

“…Note that, as shown by NMR solution studies, the methyl uorenyl groups do not rotate about the corresponding N1-C4 bonds [1]. The PdCl (2.323(1) and 2.331(1) Å), Pd-C18 (1.966(5) Å), and Pd-N2 (2.144(4) Å) bond lengths are comparable to those found in related trans-[PdCl 2 (NHC)(pyridine)] complexes [1,3].…”

“…The title compound was prepared according to a method reported elsewhere [1]. Crystals were obtained by slow evaporation of a CDCl 3 solution of the complex.…”

Crystal structure of trans-[1,3-bis(9-benzyl-9H-fluoren-9-yl) benzimidazol-2-ylidene] pyridine palladium(II) dichloride, C52H39Cl2N3Pd

“…The corresponding C 6 H 5 CH×××Pd separations (2.416-2.664 Å) lie in a rangewhich is characteristicofanagostic M×××H-Cinteractions [2]. In keeping with such interactions, whichare essentially electrostaticinnature, the 1 HNMR signalofthe CH 2 groupsappears at much lowerf ield thant hats een for the same protons in the imidazolium precursor used for the preparation of the complex (Dd =1 .75 ppm).T he Pd-Cl( 2.322(1) and 2.334(1) Å),P d-C (1.954(2) Å),and Pd-N (2.093(2) Å) bond lengths are comparable to those found in related trans-[PdCl 2 (NHC)(pyridine)] complexes [1,3] …”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…65 The utility of C28, based on a tetracyclic scaffold, in the Suzuki cross-coupling reaction is demonstrated with a low catalyst loading at room temperature (Table 4, entries 14-18). 66 The reaction times were reduced dramatically under microwave conditions.…”