Ethynyl pi-extended 2,5-diphenyl-1,3,4-oxadiazoles and 2-phenyl 5-(2-thienyl)-1,3,4-oxadiazoles: synthesis, X-ray crystal structures and optical properties

Hughes, G.; Kreher, D.; Wang, C.S.; Batsanov, A.S.; Bryce, M.R.

Abstract

2-(4-tert-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 reacts with a series of heteroaryl iodides under standard Sonogashira cross-coupling conditions (Pd[PPh3](2)Cl-2, CuI, triethylamine, THF) to yield products 2a-g in 40-79% yields (heteroaryl = 2- pyridyl, 3-pyridyl, 4-pyridyl, 2-pyrazyl, 5-bromo-2-pyrimidyl, 2-thienyl and 3-thienyl, respectively). Compound 2f was lithiated followed by electrophilic iodination ( BuLi, perfluorohexyl iodide) to give 3, which by a two-step sequence gave the terminal ethynylthienyl derivative 5. Conversion of 5 into the terminal ethynylaldehyde derivative 7, via acetal derivative 6, proceeded in high yield. Starting from 2-iodo-5-methoxy-carbonylthiophene, a five-step sequence afforded 2-(4-tert-butylphenyl)-5-(4-ethynylthienyl)-1,3,4-oxadiazole 13 (13% overall yield). Reactions of 13 gave terminal pyridyl, pyrazyl, pyrimidyl and thienyl derivatives, analogous to those obtained from 1. Two-fold reaction of 13 with 2,5-diiodothiophene gave the bis(ethynylthienyl)thiophene derivative 15 (30% yield). Solution UV-Vis absorption and photoluminescence spectra establish that replacement of the phenyl ring in the 2,5-diphenyl-1,3,4-oxadiazole series 2a-g by a thienyl ring [i.e. the 2-phenyl-5-(2-thienyl)-1,3,4-oxadiazole series 14a-g] leads to a red shift in the lowest energy band in both the absorption spectra and emission spectra. The X-ray crystal structures of compounds 2d, 2g, 5 and 14d.CHCl3 reveal that the molecular structures are approximately planar although there are substantial differences in the conformations.

Citation

Hughes, G., Kreher, D., Wang, C., Batsanov, A., & Bryce, M. (2004). Ethynyl pi-extended 2,5-diphenyl-1,3,4-oxadiazoles and 2-phenyl 5-(2-thienyl)-1,3,4-oxadiazoles: synthesis, X-ray crystal structures and optical properties. Organic and Biomolecular Chemistry, 2(22), 3363-3367

• Journal Article Type: Article
• Publication Date: 2004
• Journal: Organic and Biomolecular Chemistry
• Print ISSN: 1477-0520
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 2
• Issue: 22
• Pages: 3363-3367