Activity of Silver-X-PHOS complex catalyst

X-PHOS was first used at the beginning of the 21st century for the general amination and amidation of arenesulfonates and aryl halides and it has been used in the Pd catalyzed borylation of aryl and heteroaryl chlorides very popularly [1]. The interest in silver in catalytic systems stems from its possible coordination modes and affinity for hard donor atoms such as nitrogen or oxygen. 
Firstly, let see X-PHOS silver-catalyzed for hydrogenation of aldehyde. The hydrogenation of organic compounds (Scheme 1) is one of the most important classes of reactions for the synthesis of bio-renewable chemicals and fuels, commodity chemicals, fine chemicals, and pharmaceuticals. Cheaper metal is a trend for catalyst development for the reaction except rhodium, ruthenium, palladium, and iridium precious metal catalysts, so silver-catalyzed was studied by researchers [2].

Scheme 1 Hydrogenation reaction

Zhenhua Jia used benzaldehyde as the model substrate and tested a series of monodentate and bidentate ligands by using AgPF6 as the catalyst (Scheme 2). Among these, X-PHOS was more effective than the others. 

Scheme 2 Silver-catalyzed hydrogenation in water

Abdessamad Grirrane and his colleagues found that silver-ligand provided high catalytic activity in Mannich and Aza-Diels–Alder coupling reactions (Scheme 3)[3]. Silver is not popular as gold and copper for activating and promoting a variety of organic reactions, however, silver has recently also emerged as an efficient metal catalyst in Mannich and Aza-Diel-Alder reactions. A series of cationic and neutral silver(I) complexes bearing Buchwald‐type phosphane ligands have been synthesized and characterized, and these cationic and neutral silver(I) complexes exhibit higher activity than analogous copper(I) and gold(I) complexes as catalysts for the single and double Mannich three-component and two‐ and three‐component aza‐Diels–Alder coupling reactions. 

Scheme 3 Diagram of reactions with Silver -L complex catalyst

Reference

[1] https://en.wikipedia.org/wiki/Dialkylbiaryl_phosphine_ligands#XPhos
[2] Zhenhua Jia, etc., Silver‐Catalyzed Hydrogenation of Aldehydes in Water, Angew. Chem. Int. Ed2013, 52, 11871 –11874
[3] Abdessamad Grirrane, Eleuterio Álvarez, Hermenegildo García, Avelino Corma, Catalytic Activity of Cationic and Neutral Silver(I)–XPhos Complexes with Nitrogen Ligands or Tolylsulfonate for Mannich and Aza-Diels–Alder Coupling Reactions, Chemistry - A European Journal, 2016, 22, 340-354

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