Quinoxaline palladium-catalyzed reactions

Li and Yue also reported the intermolecular palladium catalyzed cross coupling reactions of bromo quinoxalines 214 and 216 with aryl boronic acids and heterocyclic stannanes, respectively <99TL4507>. The Suzuki couplings (i.e., 214 215) required the use of a... 

Palladium-catalyzed C—P bond formation on the benzene ring of quinoxaline has been reported. Phosphoric acid ester 223 was prepared from 7-bromoquinoxaline 222 and diethylphosphite via a Heck-type reaction [113]. 

Terminal alkenes can be used in a palladium-catalyzed alkenylation reaction of pyridine, pyrazine, quinoxaline, and pyridazine N-oxides 109 as reported by Chang (Scheme 53) (2008JA9254). This approach delivers regioselective C2-alkenylation products 110 and requires silver carbonate in a superstoichiometric amount as external oxidant. 

In 1984, Jeffery discovered that under ligand-free conditions, Pd-catalyzed vinylation of organic halides proceeds at or near room temperature, whereas normal Heck reactions require higher temperatuies [64]. Jeffery s hgand-free conditions have been broadly applied to a variety of Heck arylations that were not feasible using more standard reaction conditions. For example, efforts to use classical Heck-reaction conditions for the conversion of Al-allyl-Al-benzyl(3-bromoquinoxalin-2-yl)amine (64) to l-benzyl-3-methylpyrrolo[2,3-/ ]quinoxaline (65), resulted in slow reactions and low yields [65], which may be attributed to the poisoning of the palladium catalyst via complexation to the aminoquinoxalines. In contrast, the Jeffery conditions afforded the desired product in 83% yield. The enhanced reactivity and yield under Jeffery s ligand-free conditions may be due to the coordination/solvation of the palladium intermediates by bromide ions present in the reaction mixture, which presumably prevents the precipitation of Pd(0). 

In 1980, Ames et al. reported a related pyrrole formation catalyzed by mer-cury(II) [98], Pyrrole-fused quinoxaline 204 was obtained by treatment of a 2-amino-3-alkynylquinoxaline derivative 203 with a catalytic amount of Hg(OAc)2 (Scheme 19.53). On the other hand, dichloroquinoxaline 205, the precursor of 203, was converted directly to 204 under reaction conditions for Sonogashira coupling with phenylacetylene. A related palladium(II)-catalyzed pyrrole synthesis using 1-aminoalk-3-yn-2-ols 206 was reported by Utimoto, Nozaki and co-workers in 1981 (Scheme 19.54) [99]. It should be noted that PdCl2 showed excellent turnover to produce the corresponding pyrroles 207 in good yields with only 0.1 to 1 mol % catalyst loading. 

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