Palladium catalysts Larock indole synthesis

To make tryptophan analogs, Gronowitz and coworkers conducted a pyrrole annulation from an aminoiodopyrimidine utilizing the Larock indole synthesis conditions (see Section 1.10.) [80]. They prepared heterocondensed pyrrole 115 by treating 4-amino-5-iodopyrimidine 114 with trimethylsilyl propargyl alcohol under the influence of a palladium catalyst The regiochemical outcome was governed by steric effects. 

In 1991, Larock and Yum reported the heteroannulation of internal al-l nes with 2-iodoanilines, which is now known as Larock indole synthesis.In the presence of a palladium catalyst, the desired indoles were isolated in moderate to good yields under mild reaction conditions (Scheme 2.90). However, a large excess of all nes was necessary in some cases (5 equiv.). 

Heterogeneous palladium catalysts have been prepared by covalent immobilization of palladium (11) complexes onto SBA-15 sdica. The heteroannu-lation of 2-iodoaniline with triethyl(phenylethynyl)silane using these preformed palladium complexes gives excellent yields in Larock synthesis of indoles. These palladium catalysts have been demonstrated to be recyclable through multiple recycling experiments (2010MI179). 

Ring Closure Reactions Palladium(II) complexes (PdCl2L2, L=P, CN), covalently immobilized onto SBA-15 silica by a postsynthetic method, were used as catalysts in the Larock synthesis of indoles [86]. The reaction consisted of the heteroannulation of 2-iodoaniline and 2-bromoaniline with triethyl(phenylethynyl)silane showing high activities and selectivities to 3-phenyl-lH-indole 26 (Scheme 17). 

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