Cationic Iridium Complexes with Chiral Dithioether Ligands: Synthesis, Characterisation and Reactivity under Hydrogenation Conditions

Abstract

A series of cationic Ir I complexes containing chiral dithioether ligands have been prepared in order to study the influence of the sulfur substituents and the metallacycle size on the acetamidoacrylate hydrogenation reaction. In the case of complexes 6, 7 and 10, a mixture of diastereomers is observed in solution due to the sulfur inversion processes. In contrast, this fluxional behaviour is efficiently controlled by using bicyclic ligands which inhibit the S-inversion in complexes 8 and 9. The solid-state structure of complex 10b shows only one diastereomer with the sulfur substituents in a relative anti disposition and in an overall configuration of S C S C S S S S at the coordinated dithioether ligand. Iridium com-

Allylic Alkylations Catalyzed by Palladium Systems Containing Modular Chiral Dithioethers. A Structural Study of the Allylic Intermediates

“…Pd allylic complexes 13 and 14 , coordinated to DEGUS ligands 8 and 9 , respectively, show the presence of four diastereomeric species (at 223 K, relative ratio for 13 , 10/10/10/1; at 243 K, for 14 , 4/3/2/1), which exhibited a fluxional behavior in solution. Results were similar for the Pd 1,3-diphenyl allyl derivative containing ligand 7 8a. This isomeric distribution was in agreement with the two possible absolute configurations at the sulfur atom and the allylic arrangements relative to the ligand backbone ( exo , endo ), assuming free rotation for the N−CH 2 Ph bond.…”

“…The Pd-chiral dithioether 1 − 12 systems were tested as catalysts in the allylic alkylation of rac - I (Table ). Our preliminary work in the allylic alkylation of rac - I with Pd/ 1 − 12 catalyst precursors showed that the metallacycle size and the nature of the sulfur atom substituent (electronic induction and steric hindrance) influence the activity, but also the selectivity of the process 8a. Pd/type-DIOS systems afforded low conversions (entries 1−3, Table ).…”

“…Pd/type-DIOS systems afforded low conversions (entries 1−3, Table ). The Pd/ 1 system was not active (entry 1, Table ) probably because 1 did not coordinate strongly enough to the metal 8b. When six- (ligands 4 − 6 ) and five-membered (ligands 7 − 10 ) metallacycles are involved, both aryl and alkyl S-substituted ligands were active.…”

“…In fact, palladium complexes containing bis(thioglycoside) ligands provided excellent enantioselectivities . On the other hand, non- C 2 -symmetric dithioether derived from pyrrolidine also gave excellent enantioselectivities (up to 90%), , but C 2 -symmetric S,S-homodonor dithioethers derived from 2,3-diisopropylidene treitol did not (enantioselectivities up to 42%) 8a…”

“…In this study we prepared chiral dithioethers type-DIOS ( 1 − 3 ), type-DMPS ( 4 − 6 ), and type-DEGUS ( 7 − 10 ) (Figure ) and the corresponding palladium complexes with seven- to five-membered chelates. Sulfur inversion, which is often regarded as a cause of enantioselectivity erosion, can be effectively eradicated using bicyclic ligands such as type-DIOS-S ( 11 , 12 , Figure d), which forms butterfly metallabicycles with 7/5- and 7/6-members . Whenever possible, we studied the behavior of the solution and the solid-state structure in relation to the results obtained in catalytic studies in the allylic alkylation of rac -3-acetoxy-1,3-diphenyl-1-propene ( rac - I ) and rac -3-acetoxy-1-cyclohexene ( rac - III ) using dimethyl malonate as nucleophile.…”