title: Calix[4]arenes with one and two N-linked imidazolium units as precursors of N-heterocyclic carbene complexes. Coordination chemistry and use in Suzuki-Miyaura cross-coupling.
abstract: The calix[4]arene-imidazolium salts 5-(3-butyl-1-imidazolylium)-25,26,27,28-tetrabenzyloxy-calix[4]arene bromide (cone) (2), and 5,11-bis(3-alkyl-1-imidazolylium)-25,26,27,28-tetrabenzyloxycalix[4]arene diiodide (cone) (R = methyl, 3a; R = n-butyl, 3b) have been synthesised. Reaction of 2 in dioxane with PdCl(2) in the presence of CsCO(3) and KBr (80 °C, 24 h) gives the carbene complex trans-[PdBr(2)(calix-monocarbene)(2)] (14), containing two N-heterocyclic carbene ligands derived from 2 (yield: 63%). Repeating the reaction in pyridine instead of dioxane gives the mixed pyridine-carbene complex trans-[PdBr(2)(calix-carbene)(pyridine)] (15) in 75% yield. Treatment of the bis-imidazolium salt 3a with [Pd(OAc)(2)] affords a chelate complex, trans-[PdI(2){calix-bis(carbene)}] (16), in which a metallo-(bis-carbene) fragment caps the upper rim of the calixarene basket. Complex 16, as well as its analogue 17, obtained from 3b, display apparent C(s)-symmetry in solution. This is not the case in the solid state, a single X-ray diffraction study carried out for 16 revealing a pinced cone structure for the calixarene skeleton, which reduces the symmetry to C(1). The chelate complex 17 shows poor activity in Suzuki-Miyaura cross-coupling of phenyl boronic acid and p-tolyl halides, an observation that suggests the presence of a strained metallocyclic unit preventing easy stereochemical rearrangement to an active species. Unlike 17, complexes 14 and 15 show good activities in cross-coupling. A comparative study using the carbene precursor 1-butyl-3-(2,6-diisopropylphenyl)imidazolium bromide (18), which is devoid of the receptor fragment, strongly suggests that the carbene ligands of 14 and 15 operate typically as bulky NHC-ligands.
authors: Laboratoire de Chimie Inorganique Moléculaire et Catalyse, Institut de Chimie, UMR CNRS Université de Strasbourg, France. brenner@unistra.fr
authors: Eric
authors: E
authors: Brenner
authors: Dominique
authors: D
authors: Matt
authors: Mickaël
authors: M
authors: Henrion
authors: Matthieu
authors: M
authors: Teci
authors: Loïc
authors: L
authors: Toupet
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D056831:Coordination Complexes / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D010165:Palladium / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D056831:Coordination Complexes; D006571:Heterocyclic Compounds; D007093:Imidazoles; D010636:Phenols; C121325:calix(4)arene; D047250:Calixarenes; C030011:carbene; D010165:Palladium; D008697:Methane