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

Abstract

Benzimidazolium salts having their two nitrogen atoms substituted by different 9-alkylfluorenyl groups (4a–e and 4g, alkyl¹/alkyl² = 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-tolylchloride but their performance remained slightly inferior to that of the related, symmetrical Et/Et complex 5h.