title: Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes.
abstract: The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.
authors.affiliation: Laboratori de Química Orgànica, Facultat de Farmàcia, Universitat de Barcelona, Avda. Joan XXIII s/n, E-08028, Barcelona, Spain. ealcalde@ub.edu
authors.forename: Ermitas
authors.initials: E
authors.lastname: Alcalde
authors.forename: Rosa M
authors.initials: RM
authors.lastname: Ceder
authors.forename: Cristina
authors.initials: C
authors.lastname: López
authors.forename: Neus
authors.initials: N
authors.lastname: Mesquida
authors.forename: Guillermo
authors.initials: G
authors.lastname: Muller
authors.forename: Sandra
authors.initials: S
authors.lastname: Rodríguez
mesh_terms: D000475:Alkenes / Q000138:chemical synthesis / Q000737:chemistry*; D001555:Benzene Derivatives / Q000138:chemical synthesis / Q000737:chemistry*; D006571:Heterocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D006838:Hydrocarbons / Q000737:chemistry; D007536:Isomerism; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010080:Oxazoles / Q000138:chemical synthesis / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*
chemical_list: D000475:Alkenes; D001555:Benzene Derivatives; D006571:Heterocyclic Compounds; D006838:Hydrocarbons; D008024:Ligands; D009942:Organometallic Compounds; D010080:Oxazoles; C030011:carbene; D012834:Silver; D010165:Palladium; D008697:Methane