An alternative to the use of ligands and homogeneous catalysis to promote the synthesis of 2,3-disubstituted indoles starting from 2-bromoanilines is described for the first time. This reaction could be achieved through a Larock heteroannulation via a ligand free heterogeneous catalysis.
A new ligand- and salt-free procedure using heterogeneous palladium catalysts for the Larockindole and benzofuran synthesis is reported. After optimisation of the reaction conditions, good to high isolated yields have been achieved for a variety of structures. Recycling studies have shown that the palladium catalysts can be readily recovered and reused. Reactions and recovery of the palladium catalysts
Heterogeneous palladium catalysts were prepared by covalent immobilization of palladium (II) complexes of the general formula PdCl2L2 (L = P, CN) onto SBA-15 silica using a post-synthetic method. The state of the hybrid materials was characterized using a wide variety of molecular and solid-state techniques. In general, all the palladium modified solids exhibited highly ordered mesostructures while keeping the integrity of the parent molecular precursors. The catalytic performances of the materials were evaluated in the heteroannulation of 2-iodoaniline and 2-bromoaniline with triethyl(phenylethynyl)silane which showed high activities and selectivities with isolated yields in the range of 80-85%. Despite a decrease in the initial activity, quantitative conversion of iodoaniline to the expected product was observed over multiple recyclings, making these materials particularly efficient for such applications. The catalytic process was shown to be homogeneous in nature, the solid apparently serving as a "reservoir" for the metal between cycles. (C) 2010 Elsevier B.V. All rights reserved.