Synthesis, characterization and palladium complex formation of pyridinium- and quinolinium-3-acetylides: mesomeric betaines or betaine-stabilized carbenes?

Abstract

3-Ethynyl-1-methylpyridinium- and 3-ethynyl-1-methyl-quinolinium triflates were prepared by methylation of 3-ethynylpyridine and 3-ethynylquinoline with methyl triflate, respectively. Whereas deprotonation of the former mentioned salt gave no stable product, sodium hydroxide in methanol formed 1-methylquinolinium-3-acetylide which could be characterized NMR spectroscopically. Considerable upfield shifts of the resonance frequencies in combination with DFT calculations of bond lengths reveal that the structure has to be represented by resonance forms of a quinolinium-3-acetylide, i.e., as mesomeric betaine, as well as by allenylidene resonance forms. The Pd complex of this species was prepared by reaction of 3-ethynylquinoline with Pd(PPh3)2Cl2 in the presence of CuI and diethylamine to give bis(triphenylphosphine)bis(quinolin-3-yl-ethynyl)palladium(II) which was subsequently methylated at the nitrogen atoms. Insertion reactions of 3-(bromoethynyl)pyridine with Pd(PPh3)4 gave bis(triphenylphosphine)palladium(II)(pyridin-3-yl-ethynyl) bromide which was finally methylated to give bis(triphenylphosphine)[(1-methylpyridinium-3-yl)ethynyl]palladium(II)bromide triflate. Likewise, bis(triphenylphosphine)palladium(II) (quinolin-3-yl-ethynyl) bromide and bis(triphenylphosphine)((1-methylquinolinium-3-yl)ethynyl)palladium(II)bromide triflate were prepared. In contrast to the non-complexed forms, the spectroscopic data are in accordance with alkynyl palladium complexes rather than with allenylidene complexes.