Abstract:A new hybrid silica with urea-based bipyridyl bridging units has been synthesized by a solgel process. The copper-complexed hybrid silica is an efficient heterogeneous catalyst for the Ullmann coupling of aryl halides with phenols. It is easily withdrawn and recovered from the reaction media by filtration. It is also a versatile catalyst for subsequent reactions with several substrates yielding products with the same efficiency even after ten reuses and with minimal leaching. This is the first example of the use of a copper-complexed bridged silsesquioxane for the preparation of a recoverable catalyst in modern Ullmann chemistry. This may represent a promising route to the reduction of waste while maintaining economic viability.Keywords: C À O coupling; copper; heterogeneous catalysis; N ligands; sol-gel bridged silsesquioxane Diaryl ethers constitute an important class of structural motifs for life sciences and polymer industries.[1] In the mid 1990s Hartwigs and Buchwalds groups both developed Pd-catalyzed C À O coupling reactions of aryl halides and phenols.[2] Despite the high efficiency of Pd/ligand systems in terms of TON, TOF and recyclability for cross-coupling reactions, [3] an alternative and cheaper methodology to form biaryl ethers is the classical Ullmann reaction, [4] but the harsh conditions seriously restricted this methodology (Cu in stoichiometric amount at 150-250 8C). Recently, several groups focused on the revisited Ullmann-type C À O coupling catalyzed by copper/ligand (Cu/L) systems to overcome the high cost of Pd sources. [5,6] Since 2001, our group has developed attractive Cu/L (Schiff bases) systems for the mild coupling of aryl halides with phenols.[6] Due to the attractive cost of Cu sources for Ullmann-type reactions, very few systems have been proposed to recover and reuse the catalytic species.[7] The heterogenization of the Cu/L catalyst can avoid metal residue contamination in the desired coupling product and is of high importance for purity requirements in pharmaceutical chemistry. Moreover, simple filtration reduces the time-consuming processes and the solid catalyst which is easily separated from the products can be recycled. In this context the sol-gel process provides a suitable route to active heterogeneous catalysts. [8] Indeed metal-complexed hybrid silsesquioxanes have already been reported as efficient solid catalysts for asymmetric reactions, [9a,b] metathesis [9c] and Suzuki reactions.[3b]In the present study, we report the design of a Cu/L catalyst immobilized on silica A which was obtained by the sol-gel hydrolysis of a silylated bispyridinyl ligand L (Scheme 1) followed by complexation with Scheme 1. Immobilization of ligand L leading to the hybrid silica A by a sol-gel process.

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Direct synthesis of biaryl derivatives from aryl halides takes place under very mild temperature conditions by using a ligand-free iron catalytic system. The procedure, which proceeds via an in situ quantitative aryl halide exchange with alkyllithiums, allows for excellent control of the reactivity and is in line with the sustainable development. The method is also applicable to styryl and benzyl halides and to phenylacetylene.