Precatalysts preparation

Scheme 7.1 Examples of precatalysts prepared using the modified procedure.

Fig. 9.6 Novel thiazolium salt precatalysts prepared by Bach and Miller.

Research is active to produce precatalysts that are stable in air, highly active, well-defined, and possess a broad spectrum of activity. One such precatalyst is complex 68, which consists of Pd complexed with a sterically hindered NHC carbene (Section 10-1-1). Also present is a substituted pyridine ligand (a so-called throw-away ligand). Complex 68 is a member of the PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and mtitiation) family... 

The PEPPSI (pyridine-enhanced, precatalyst, preparation, stabilization and initiation) precatalyst 20 is a very versatile catalyst for Kumada couphngs of highly hindered substrates and heterocycles at ambient temperature (Equation 2.64) [106]. 

B,4 )-isomer 1 was shown to be at least 10 by using 0.001 mol% of PEPPSI (Pyridine-Enhanced Precatalyst Preparation Stabilization and Initiation) [105]. 

Much recent attention has been paid to the development of new ligand systems providing enhanced reactivity. As for nickel, catalytic systems of interest can be obtained with bulky NHC ligands. The family of PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization, and initiation) precatalysts is well suited for crowded or poorly reactive electrophiles such as aromatic chlorides 67 (Scheme 5.12) [53]. As illustrated with the formation of 69 from biaryhnagnesium bromide 68, in the presence of PEPPSI-IPr (70) (2 mol%), reactions take place with high yields at room temperature in THF. It has been proposed that the pyridine attaches to and detaches from the Pd(0) complex in solution and therefore slows down the rate of palladium black formation and deactivation of the catalyst... 

Further invention of pyridine-enhanced precatalyst preparation stabilization and initiation (PEPPSI) ligands has led to a remarkable improvement in palladium-catalyzed C—S coupling [159, 160]. This catalyst effectively couples the less reactive aryl chlorides even at room tempauture [161]. Recently, NHC-based catalyst, [Pd(IPr )(cin)Cl] 18, has been developed for the coupling of aryl halides with aliphatic or aromatic thiols (Scheme 20.53) [162]. 

PEPPSI Pyridine-enhanced precatalyst preparation stabilization and initiation PPAPM Pyrrolidine-2-phosphonic acid phenyl monoester... 

A slight improvement was observed for 17b formed with EGDMA a polar erosslinker (conversion 10%, ee 88%). Compared to them 17e present a better conversion (89%) with >99% of de and ee. The precatalysts prepared from 17c, 17d, 17f, 17g from better hydrophilic polymer support have exhibited exeellent catalytie performance for this dynamic kinetic resolution of racemie a-amide ketone via hydrogenation to yield the corresponding syn P-amide alcohol (100% conversion, de >99% and ee ranging from 90 to >99%). In addition, the polymer-supported catalyst could be reused several times without loss of catalytic activity. 

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