Pd-PEPPSI complexes in water: Synthesis of (E)-2-(styryl)pyridines via Heck coupling reaction and screening of the optoelectronicproperties

Abstract

A series of Pd-N-heterocyclic carbenes, i.e., palladium-pyridine enhanced pre-catalyst preparation stabilization and initiation (Pd-PEPPSI) complexes, were synthesized and tested their catalytic efficiency in our previous works towards C2-arylation, C-C and C-heteroatom bond formation reactions. Herein, we wish to extend our research studies to synthesizing styryl pyridines via Heck coupling reactions (HCR) and oxidative Heck coupling reactions (OHCR). In the current report, we have developed a method in two different routes; method i represents HCR; it involves terminal alkenes and aryl halides in the presence of a base and Pd-PEPPSI complexes in H2O. In method ii, the reaction undergoes between aryl boronic acids, terminal alkenes, oxidant and Pd-PEPPSI catalyst, i.e., OHCR. Our results revealed that the boronic acids gave superior yields over aryl halides for the formation of styryl pyridines without adding any extraneous base. Further, the catalyst can be resumed and reused for three cycles and there is no loss of catalytic activity. It can also be noted that these Pd-PEPPSI complexes might help to reduce the organic waste and avoid hazardous reaction conditions. The prepared styryl pyridine derivatives possess an E-configuration and are structurally confirmed by 1H NMR spectra. We examined the photophysical properties of the prepared compounds of styryl pyridines. The absorption led to identifying the absorption maxima in between 200 and 450 nm, whereas emission spectra demonstrated less intensity shoulder peaks and more intense vibrationally resolved peaks starting from 300 nm to 600 nm.