Hydrohalogenation of Ethynylpyridines Involving Nucleophilic Attack of a Halide Ion

Abstract

Efficient hydrochlorination of 2-ethynylpyridines was achieved without the use of special reagents. Ethynylpyridine readily reacts with hydrochloric acid to form a pyridinium salt. The salt formation considerably enhances the electrophilicity of the ethynyl group and attracts a chloride ion as the counteranion. The spatial proximity facilitates the nucleophilic addition of the halide anion to the ethynyl group, producing 2-(2-chloroethenyl)pyridine in high yields. This protocol could also be applied for hydrobromination and hydroiodination using hydrobromic and hydroiodic acids, respectively. In the case of acetic acid, the reaction did not proceed because of the low acidity and lack of salt formation. This problem was overcome by exchanging the counteranion using silver acetate; the resultant pyridinium acetate underwent hydroacetoxylation.

Reactivity of 5-alkynyl-3,4-dihydro-2H-pyrroles with Au(III): Route to vinylgold(III) complexes, aurocycles by cyclisation of these complexes and ML complexes

“…Then, imine-enamine tautomerism enabled formation of the postulated intermediate 7 acting as HCl source. This mechanism could also explain the trans addition which was also recently reported with ethynylpyridines and HX (11). As AuCl3 was both acting for alkyne activation (2b, 2c>2a) and Clsource, the yields were always <50% and the conversion of the starting material was complete.…”

Metal‐Free and syn‐Selective Hydrohalogenation of Alkynes through a Pseudo‐Intramolecular Process

“…In the 1 H NMR spectrum of 2 a, a singlet signal appeared at 6.99 ppm. In addition, two signals of 1 a at 86.0 and 94.8 ppm assigned to sp carbons disappeared in the 13 C NMR spectrum. In the MS spectrum, two peaks of [M + ] and [M + + 2] were observed in 3 : 1 intensity.…”

“…The three plausible mechanisms are illustrated in Scheme 2. After forming salt 1 a • HCl, the proximity is considered to enable 1) the attack of the less nucleophilic chloride ion to an alkyne, [13] 2) the protonation of electronegative the sp carbon, leading to cation, and 3) the concerted addition. To obtain insights into the reaction mechanism, the density-functional theory calculations were performed under the RB3LYP/6-311 + G(d,p) level, as in acetonitrile (Figure 2).…”

“…Dry acetonitrile was also purchased from commercial source and used as received. 1 H and 13 C NMR spectra were recorded on Bruker DPX-400 spectrometer (400 MHz and 100 MHz, respectively) in CDCl 3 using TMS as an internal standard. The assignments of the 13 C NMR were performed by DEPT experiments.…”

“…1 H and 13 C NMR spectra were recorded on Bruker DPX-400 spectrometer (400 MHz and 100 MHz, respectively) in CDCl 3 using TMS as an internal standard. The assignments of the 13 C NMR were performed by DEPT experiments. IR spectra were recorded on a JASCO FT/IR-4200 spectrometer equipped with an ATM detector.…”