Microwave-accelerated Mizoroki-Heck and Sonogashira cross- coupling reactions in water using a heterogeneous palladium(II)-precatalyst

Abstract

The catalytic activity of a 2-pyridinealdoxime-based Pd(II)-complex covalently anchored via the oxime moiety to a glass/ polymer composite material was evaluated both under thermal as well as microwave (µw) irradiating conditions in water in Mizoroki-Heck as well as Sonogashira C-C cross-coupling reactions. Synthesis of benzo [b]furan derivatives via Sonogashira cross coupling reaction was achieved when ortho-halo-phenols were employed as aryl halides. The stability and reusability of this Pd-precatalyst was part of the present study.

Recent advances in Sonogashira reactions

“…196 In addition, a 2-pyridinealdoxime-based palladium(II) complex 126, anchored via the oxime moiety to a polystyrene phase which is part of a highly megaporous glass/polymer composite matrix shaped as Raschig rings, has been used as catalyst (0.7 mol%) under thermal (100 1C) and microwave irradiating conditions in water in the copper-free cross-coupling reaction of aryl and heteroaryl iodides and bromides with phenylacetylene, using diisopropylamine or sodium hydroxide as base and TBAB as additive. 197 Similarly, the 2-pyridinealdoxime motif has been incorporated into the polysaccharide chitosan to generate palladium(II) complex 127 after reaction with palladium(II) acetate, which has been assayed as supported catalyst in the cross-coupling of iodobenzene and phenylacetylene, in the presence of sodium acetate in DMF at 150 1C under microwave irradiation, although resulting poorly selective. 198 Scheme 31…”

Homogeneous and heterogeneous catalysts for multicomponent reactions

“…70,105 The most general methods to prepare benzo[b]furans involve reductive cyclization of ketoesters by low valent titanium, 106 photochemical rearrangement of phosphate esters, 107 Suzuki coupling of boronic acids with organic halides or triflates catalyzed by palladium, 108 Sonogashira cross coupling reaction of o-halophenols with terminal alkynes in the presence of palladium and/or copper as catalysts. 109 Recently, following the Mannich protocol, Kabalka et al 110 have reported the synthesis of a variety of propargylamines in good yields from different alkynes, primary or secondary amines and paraformaldehyde using cuprous iodide doped alumina as the catalyst under microwave irradiation. The reaction was extended to the synthesis of 2-substituted benzo[b]furan derivatives when ethynylphenol was condensed with secondary amines (such as piperidine, morpholine, 1-phenylpiperazine etc.)…”

A Photochemical Route to 2-Substituted Benzo[b]furans

“…Bromophenols have been likewise used in the synthesis of benzofurans, , but the experimental procedure is less straightforward than with the corresponding iodides, and in some cases a preliminary protection of the hydroxy group as acetate had to be adopted. As a matter of fact, iodophenols gave benzofurans twice as efficiently and as fast as bromophenols, while requiring a smaller amount of catalyst. Finally, we are aware of two examples of the use of 2-chlorophenol for the synthesis of benzofurans.…”