Heterocycles tricyclohexylphosphine

The combined features of charge and catalyst stabilisation find their expression in markedly enhanced recyclability. Using catalyst loadings of 2.5 - 5 mol%, the activity decreased by only a few percent after 10 cycles and remained above 90% of the initial value. Furthermore, the catalyst solution could be kept for months without measurable loss in activity. By substituting the tricyclohexylphosphine ligand for a A -heterocyclic carbene, catalyst stability was further improved which finds its expression in very low residual ruthenium levels in the isolated product.[44] As these are only the... 

Silver(I) carbonate is responsible for generating an aryl silver species, which can subsequently undergo transmetalation with Pd. To prevent protodecarboxylation and decarboxylative homocoupling, tricyclohexylphosphine was required to accelerate transmetalation and reductive elimination. Electron-rich, electron-deficient, and heterocyclic benzoic acids were compatible coupling partners and a wide range of functional groups were tolerated for both thiophene and carboxylic acid moieties. Additional heterocycles such as benzothiophenes and 2-methylfuran could also be arylated. 

Rh-catalyzed Arylation of Heterocycles. Arylation of benzimidazole was achieved using aryl iodides in the presence of a Rh catalyst and 40 mol % of PCys. Other ligands, such as alkyl and aryl phosphines, phosphites, and phosphoramidites, produced inferior conversions compared with tricyclohexylphosphine. Excess phosphine inhibited the reaction, suggesting that phosphine dissociation occurs prior to oxidative addition of the aryl iodide. 

Silver(I) carbonate functioned as an cooxidant with TEMPO. Tricyclohexylphosphine was employed to suppress homocoupling between heteroarenes. Substituted thiophenes, furans, and indoles could be selectively olefinated (C5-alkenylation for thiophenes and furans, C3-alkenylation for indoles, E/Z > 99 1). Unsubstituted thiophenes produced poor yields (24%) however, formyl, acetyl, and ketyl substituents were well tolerated. For electron-deficient substrates, tricyclohexylphosphine was reduced to 10 mol % to achieve good conversions. A variety of ketones could be employed using 2-methyl thiophene as a coupling partner. A related methodology employing saturated ketones and heterocyclic carboxylic acids via a Pd-catalyzed decarboxylative process was also reported (eq 44). ... 

Replacement of a phosphine ligand with an A7-heterocycUc carbene (NHC) increases the stability and efficiency of the catalyst precursors, thus affording a second generation of monometallic ruthenium initiators. Several vinylidene complexes [RuCl2(=C=CHR)(PCy3)(NHC)] bearing mixed ligand sets made up of tricyclohexylphosphine (PCys) and Ai-heterocyclic carbenes promoted several RCM and ROMP reactions (Scheme 48) [170, 171]. 

---