N-Heterocyclic Carbenes Functioning as Monoligating Clamps



Benzimidazolium salts N,N′-disubstituted with 9-alkylfluorenyl groups (3ae, 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 2ae were then obtained in yields superior or equal to 77 % by reacting diimine 1 with the appropriate organolithium reagent. In the final step, diamines 2ae were treated with ethylorthoformate under acidic conditions to afford benzimidazolium salts 3ae. These were readily converted into the PEPPSI palladium complexes 4ae (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 4ae are highly efficient in Suzuki–Miyaura cross-coupling reactions; their activity is equal or superior to the best PEPPSI catalysts reported to date.