Simple Silver Salts and Palladium Bis(N-heterocyclic carbene) Complexes As Complementary Catalysts for the Nazarov Cyclization

Abstract

Simple silver salts and dicationic palladium bis(N-heterocyclic carbene) complexes were found to be effective and complementary catalysts for Nazarov cyclizations of divinyl β-keto esters. A sterically hindered palladium bis(NHC) catalyst provided remarkable endo:exo selectivity in cyclizations of cyclohexenyl β-keto esters, whereas AgSbF6 gave improved yields with electron-deficient substrates. The high activities observed with silver salts are significant, given their widespread use as additives in catalytic Nazarov reactions.

ortho-Phenylene bridged palladium bis-N-heterocyclic carbene complexes: synthesis, structure and catalysis

“…Slaughter reported deviating elemental analyses for similar ortho-phenylene bridged bis-NHC palladium complexes and imidazolium salts which are most probably due to insufficient combustion and which might as well be responsible for small deviations of the elemental analyses reported herein for compound 8b. 66 Positive mode ESI-MS spectra for the synthesised compounds were recorded on a Bruker Esquire MS with an Ion Trap Detector on samples dissolved in NH 4 OAc buffered methanol, which has been reported to exchange the bromide ligand by acetate.…”

“…59,60 In the literature there are so far only few reports about the synthesis and especially catalytic activity of phenylene bridged bis-NHC complexes and to our knowledge no transition metal complexes with additional functionality at the aryl moiety of the ligand have been prepared. [61][62][63][64][65][66][67][68][69][70]…”

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

“…As shown in Scheme a, the Nazarov cyclization of divinyl β‐ketoesters was achieved by Slaughter and co‐workers with high endo / exo selectivity by using 1 b (Figure ) and sterically demanding Pd–bis(NHC) complexes 111 a , b (Figure ) as catalysts . The addition of silver salts, such as AgSbF 6 , significantly boosted the catalytic efficiency.…”

“…As showni nS cheme10a,t he Nazarov cyclization of divinyl bketoesters was achieved by Slaughter and co-workers with high endo/exo selectivity by using 1b (Figure 1) and sterically demanding Pd-bis(NHC) complexes 111 a,b (Figure 23) as catalysts. [83] The addition of silver salts, such as AgSbF 6 ,significantly boosted the catalytic efficiency.S imilar efficaciesw erea lso obtained for dicationic complexes.S creening of the substrate scope was carried out by using complex 111b,w hereupon consistent endo/exo selectivity was observed. Essentially,a counterion effect was observed for the combinationo fP dbis(NHC) complexes and silver salts.…”

The search for formal electrostatic effects on molecular conformation and crystal packing: crystal structure of 2,2′′-disubstituted (HversusPPh2) 1,1′-(1,2-phenylene)bis(3-methyl-1H-imidazol-3-ium) bis(trifluoromethanesulfonate)

“…The behaviour of 1,2-bis(1H-imidazol-1-yl)benzene, (1), and its analogues as ligands of transition metal centres has been extensively explored, especially for catalytic purposes (Albrecht et al, 2002;Rentzsch et al, 2009;Subramanium et al, 2011;Munz et al, 2013;Howell et al, 2014). However, despite the report of its symmetric salt [one step on from (1)], the structure of the 3,3 0 -dimethylated 1,1 0 -(1,2-phenylene)bis(3-methyl-1H-imidazol-3-ium) dication seen in (2) has not been reported in the CSD.…”