An efficient approach to bis-benzoquinonylmethanes on water under catalysis of the bio-derived O-carboxymethyl chitosan

Synthesis of the dyes 3,3'-(arylmethylene)bis(2-hydroxynaphthalene-1,4-diones) was achieved efficiently in mild and metal-free conditions from a pseudo three-component reaction between 2-hydroxynaphthalene-1,4-dione and aldehydes under catalysis of O-carboxymethyl chitosan as a green, available, low-cost, and water-soluble bio-derived catalyst. This method combines the advantages of homogeneous catalysis in aqueous medium with convenient recovery and repeated recycling of the polymeric catalyst. The catalytic action of O-carboxymethyl chitosan is reminiscent of soft catalysis of enzymes and seems to be exerted by proton donating and accepting functions of the ammonium and amino groups arranged along the backbone of the polymeric bio-inspired catalyst.