Palladium Complexes Based on Ylide‐Functionalized Phosphines (YPhos): Broadly Applicable High‐Performance Precatalysts for the Amination of Aryl Halides at Room Temperature

Abstract

Palladium allyl, cinnamyl, and indenyl complexes with the ylide‐substituted phosphines Cy3P+−C−(R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3P+−C−(Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C−N coupling reactions. The complexes are highly active in the amination of 4‐chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β‐hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C−N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram‐scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h−1.

Selective Pd-Catalyzed Monoarylation of Small Primary Alkyl Amines through Backbone-Modification in Ylide-Functionalized Phosphines (YPhos)

“…The synthesis of the ligands L2 and L3 was attempted via the same protocol as used for the synthesis of ligand L1 . 14 For the ortho- tolyl ligand Cy Y oTol PCy 2 , the formation of the phosphonium salt 2a and subsequent deprotonation to L2 was revealed to be facile and allowed the isolation of the ligand as a colorless solid in 84% yield from 1a ( Scheme 1 ). In contrast, the preparation of the mesityl ligand Cy Y Mes PCy 2 ( L3 ) failed under the same reaction conditions.…”

“… 12 In palladium catalysis, the analogous PCy 3 -substituted YPhos ligand joYPhos ( L1 ) with a phenyl group in the ylide-backbone proved to be highly effective in Buchwald–Hartwig aminations of aryl chlorides at room temperature, allowing for turnover frequencies greater than 10.000 h –1 with improved selectivities in comparison to its methyl-substituted analogue. 14 However, diarylation was observed as a side product with small primary amines. To address this limitation of L1 , we became interested in the impact of the steric demand of the backbone substituent on the selectivity in mono vs diarylation reactions.…”

The Pincer Platform Beyond Classical Coordination Patterns

“…For instance, it is well‐known that for certain catalytic reactions, such as cross‐coupling reactions of inert substrates or olefin metathesis, strongly σ‐donating ligands in transition metal catalysts can accelerate these reactions. [ 85–93 ] In this context, we have demonstrated for a series of iridium(III) carbonyl pincer complexes with the general formula [(PXP)IrCl(CO)(H)] q that ligands of the general type (R 3 P) 2 BH are very strong neutral donors (Figure 6). It was possible to extrapolate the Tolman electronic parameter for monodentate ligands of this (R 3 P) 2 BH type, using the wavenumber of the C–O‐stretching vibration in iridium pincer complexes [(PXP)IrCl(CO)(H)] q (the group X is in trans ‐position to CO) that can be correlated with known TEP values of the corresponding monodentate derivatives of X.…”

Phosphorus‐Containing Superbases: Recent Progress in the Chemistry of Electron‐Abundant Phosphines and Phosphazenes

“…[19,42] During research on ylide and yldiide compounds as ligands in transition metal complexes and the stabilization of reactive low-valent main group elements, Gessner and co-workers connected the strongly π-donating ylidyl groups as substituents to a phosphorus atom and obtained electron-abundant ylidylphosphines (YPhos). [108,109,110,111,112] Three simple reaction paths A-C in Scheme 14 were presented to synthesize ylidylphosphines starting from alkylphosphonium salts, which can be easily prepared via reaction of the respective phosphine with alkylhalides (Scheme 14). [113,115] Depending on the nature of Z, double deprotonation with metal bases and subsequent nucleophilic substitution at halophosphines deliver the desired YPhos ligands 34a-e in moderate to high yields of 69-92 % (route A).…”

“…[106] The latest findings in palladium catalysis gave rise to rather labile PÀ C bonds of triphenylphosphonium functionalities in common YPhos ligands, which results in inactive catalytic systems on course of the reaction, Gessner et al replaced the phenyl by cyclohexyl groups. [108,109,114] The resulting ligands in Cy Y Me PCy 2 (34k-m) show elevated stability and a remarkable performance in palladium-catalyzed amination reactions of aryl chlorides at room temperature (Scheme 14). [108,109] YPhos ligands in general represent valuable electron-rich phosphines, which was demonstrated by their readily formation of transition metal complexes, such as gold(I) [106,111,115] or palladium [112][113][114] complexes, which have been successfully employed as catalysts in amination and hydroamination reactions, as well as alpha arylation with arylchlorides among others.…”