Pd–Tetrahydrosalan‐Type Complexes as Catalysts for Sonogashira Couplings in Water: Efficient Greening of the Procedure

Abstract

New sulfonated tetrahydrosalen-type ligands and their water-soluble palladium(II) complexes have been synthesized. The palladium(II) complexes catalyze the Sonogashira coupling (23 examples) of various aryl halides (including chloroarenes) with terminal alkynes, with good to excellent conversions under mild conditions (80°C, air, no Cu(I) cocatalyst) in aqueous-organic mixtures and turnover frequencies of up to 2790 h(-1) . Under optimized reaction conditions to minimize environmental contamination, diphenylacetylenes can be isolated in 76-98% yield. The aqueous catalyst solution can be recycled four times with decreasing activity; however, yields between 93 and 98% can still be achieved with extended reaction times. Several water-insoluble products can be isolated in excellent yield by simple filtration and purification by washing with water; this method is used, for the first time, for this type of C-C coupling procedure.

Palladium (II)–Salan Complexes as Catalysts for Suzuki–Miyaura C–C Cross-Coupling in Water and Air. Effect of the Various Bridging Units within the Diamine Moieties on the Catalytic Performance

“…In the case of both complexes, the 2D structures are shaped by the electrostatic and van der Waals interactions between the K + ions and the O-atoms of the sulfonate groups of the ligand and water molecules, together with the hydrogen bonds within the lattice. Similar polymeric chains were detected by us in crystals of the n-K 4 [μ 8 -BuHSS][μ 2 -H 2 O] 4 [H 2 O] 6 sulfosalan [ 46 ] and in the cases of Ni(II) and Cu(II) complexes of bis (salicylidene)-1,2-diaminocyclohexane, CyS [ 52 ].…”

“…The new ligands, 4 , 5b and 5c , and the Pd (II) complexes 7 , 9 , 10b and 10c , were synthesized according to the procedure used by us earlier for the rest of the compounds, 1 – 3 , 6 , 8 and 10a [ 44 , 45 , 46 , 47 ]. Briefly, the starting salens were obtained by condensation of salicylaldehyde and the appropriate diamine, and the latter were reduced to the hydrogenated salens with four equivalents of NaBH 4 in methanol.…”

“…The starting compound for the synthesis of PrHSS ( 2 ), i.e., N , N ’- bis (2-hydroxybenzyl)-1,3-diaminopropane, PrHS, was previously crystallized with various aromatic polycarboxylates [ 48 ] and SC-XRD studies revealed the protonation of the secondary amine groups of PrHS, similar to the case of PrHSS ( 2 ) ( Figure 2 ). Comparison of the structure of n-K 4 [μ 8 -BuHSS][μ 2 -H 2 O] 4 [H 2 O] 6 published by us earlier [ 46 ] to the one of 3 in this study ( Figure 3 ), shows, that the N1–C7–C1 angles are almost the same (114.28° and 114.4°) in the two molecules, and only the positions of the aromatic groups are different ( Figure S15 ). Superposition of the structures of the salan ligand, meso (RS,SR)- N,N ’- bis (2-hydroxybenzyl)-1,2-diphenyl-1,2-diaminoethane [ 49 ] and its sulfonated product, dPhHSS ( 4 ) ( Figure 4 ) also shows high degree of similarity ( Figure S20 ) and proves that the starting salen underwent hydrogenation as well as sulfonation in the p- position relative to the phenolic oxygen.…”

“…HSS [ 44 ], PrHSS [ 45 ], BuHSS [ 46 ] and rac -CyHSS [ 46 ] were synthesized according to published methods. Synthetic procedures for dPhHSS as well as for cis - and trans -CyHSS are described below.…”

“…We have been interested in aqueous organometallic catalysis for several years [ 21 ] and employed as catalysts complexes of transition metals with water-soluble tertiary phosphine and/or N-heterocyclic carbene ligands. Recently, we launched a program to study in aqueous media the catalytic properties of sulfonated salan-based complexes in reactions such as hydrogenation of alkenes and ketones [ 43 ], redox isomerization of allylic alcohols [ 44 , 45 ] and carbon–carbon cross-coupling reactions [ 46 ]. In particular, some Pd (II)–salan complexes were found to be highly effective catalysts for the Sonogashira and the Suzuki–Miyaura cross-coupling reactions [ 46 , 47 ].…”