Ligandless palladium-catalyzed reaction

Molander, G. A., Biolatto, B. Efficient Ligandless Palladium-Catalyzed Suzuki Reactions of Potassium Aryltrifluoroborates. Org. Lett. 2002,... 

Bromo- and iodoimidazoles are useful intermediates for further functionalization. 4(5)-Aryl- I //-imidazoles 57 can be efficiently and selectively prepared by palladium-catalyzed Suzuki-Miyaura reaction of commercially available 4(5)-bromo-l//-imidazole 56 with arylboronic acids under phase-transfer conditions, which then underwent highly selective palladium-catalyzed and copper(I) iodide mediated direct C-2-arylation with a variety of aryl bromides and iodides under base-free and ligandless conditions to produce 2,4(5)-diaryl-l//-imidazoles 58 in modest to good yields <07JOC8543>. A new procedure for the synthesis of a series of substituted 2-phenylhistamines 60 utilizing a microwave-promoted Suzuki... 

This reaction was first reported by Larock et al. in 1991. It is a palladium-catalyzed chemoselective and ligandless heteroannulation between ort/io-iodoanilines and disub-stituted alkynes to form substituted indole derivatives. Therefore, this reaction is generally known as the Larock heteroannulation or Larock indole synthesis. ... 

The reaction is catalyzed by palladium complexes either pre-formed, as [Pd(TPPMS)3] [13], or prepared in situ from (usually) [Pd(OAc)2] and various phosphines [21,23-27], TPPTS being one of the most frequently used [14]. Other precursors, e.g. [ PdCl(T -C3H5) 2] and so-caUed ligandless (phosphine-free) Pd-catalysts can also be effective. In fact, in several cases a phosphine inhibition was observed [23]. The solvent can be only slightly aqueous (5 % water in CH3CN, [14]) or neat water [26]. In the latter case a biphasic reaction mixture (e.g. with toluene) facilitates catalyst separation albeit on the expense of the reaction rate. A short selection of the reactions studied in aqueous solvents is shown on Scheme 6.9. 

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