Directional properties of fluorenylidene moieties in unsymmetrically substituted N-heterocyclic carbenes. Unexpected CH activation of a methylfluorenyl group with palladium. Use in palladium catalysed Suzuki- Miyaura cross coupling of aryl chlorides

International audienceBenzimidazolium salts having their two nitrogen atoms substituted by different 9-alkylfluorenyl groups (4a-e and 4g, alkyl(1)/alkyl(2) = Me/Et, Me/Pr, Me/n-Bu, Me/i-Pr, Me/Bn, Me/CH2SMe have been synthesised in high yields in two or three steps from N,N'-bis(9H-fluoren-9-ylidene)benzene-1,2-diamine (1). The imidazolium salts 4a-e were converted readily into the corresponding PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilisation and initiation), while reaction of the methylthioether-substituted salt 4g with PdCl2/K2CO3/pyridine afforded the palladacycle 5g resulting from metallation of the methyl group attached to the fluorenylidene moiety. NMR and X-ray diffraction studies revealed that the carbene ligands in 5a-5e behave as clamp-like ligands, the resulting metal confinement arising from a combination of the orientational properties of the fluorenylidene moieties that push the alkyl groups towards the metal centre and attractive anagostic interactions involving CH2(fluorenyl) groups. Complexes 5a-e were assessed in Suzuki-Miyaura cross-coupling reactions. Like their symmetrical analogues they displayed high activity in the coupling of phenyl boronic acid with p-tolyichloride but their performance remained slightly inferior to that of the related, symmetrical Et/Et complex 5h