Characterization of tetra-aryl benzene isomers by using preparative gas chromatography with mass spectrometry, nuclear magnetic resonance spectroscopy, and X-ray crystallographic methods.

The present study describes a preparative gas chromatographic (GC) approach employed to study a series of products arising from reaction of phenylacetylene with para-substituted aryl-iodides under Sonogashira catalysis conditions. GC analysis resolves the isomers from each reaction; however, it cannot provide structural details (their MS data are virtually identical). Since classical liquid chromatography cannot resolve the isomers, preparative-scale GC is the only practical approach to provide further spectroscopic characterization of the isomers. The products are well separated by GC so a single thick-film capillary column is adequate for this case, with operation up to approximately +300 degrees C. By collection of 50+ repeat injections, sufficient material could be isolated for (1)H NMR spectral analysis of the isomers, and for one isomer (isomer I) of a number of analogous related catalytic reaction mixtures, X-ray crystal structure determination enabled complete structural elucidation (absolute configuration) of the substitution pattern of the structure of this isomer. This confirmed isomer I to be the 2-para-aryl-substituted 1,3,5-triphenylbenzene product in all cases. (1)H NMR spectra of isomer I products generally had similar patterns but differed markedly from the second major isomer product (isomer II).

A new mechanistic pathway under Sonogashira reaction protocol involving multiple acetylene insertions.

An unreported product outcome from an intended Sonogashira coupling is presented. The generality of this finding has been demonstrated by screening of a range of pre-catalysts. Mechanistic studies are consistent with the tetra-aryl benzene product forming by interception of the aryl halide oxidative addition intermediate by repeated acetylene insertion.