Cooperative Bimetallic Asymmetric Catalysis: Comparison of a Planar Chiral Ruthenocene Bis-Palladacycle to the Corresponding Ferrocene

Abstract

Cooperative asymmetric catalysts often offer advantages in terms of activity, stereoselectivity, and generality as compared to more traditional single point activation catalysts. In cooperative bimetallic catalysis, the intermetallic distance is a crucial parameter for the outcome of a reaction and an optimal synergy of both metal centers. We have recently developed a number of catalytic asymmetric reactions, which are efficiently catalyzed by a planar chiral ferrocene based bispalladacycle and for which the cooperativity of two Pd centers has already been demonstrated. To get more insight into the role of the Pd/Pd distance in such metallocene bismetallacycles, in the present study a corresponding ruthenocene based Pd2-complex has been prepared by the first direct diastereoselective biscyclopalladation of a chiral ruthenocene ligand. In addition, the first highly diastereoselective direct monocyclopalladation of a homochiral ruthenocene is reported. The effect of the increased Cp/Cp distance within the ruthenocene bispalladacycle has been examined in four catalytic asymmetric applications: the aza-Claisen rearrangement of Z-configured allylic N-aryltrifluoroacetimidates, the direct 1,4-addition of α-cyanoacetates to enones, a tandem azlactone formation/1,4-addition to enones and a tandem reaction to form quaternary α-aminosuccinimides by in situ azlactone formation, 1,4-addition to a nitroolefin, and a Nef-type nitro-to-carbonyl transformation as key steps. For each reaction studied, it was found that with some substrates the ferrocene based catalyst is superior, whereas for other substrates the ruthenocene backbone is more favorable. The ruthenocene based bispalladacycle can thus be considered to be a useful and complementary alternative for cooperative bimetallic catalysis.

Chiral Imidazoline Ligands and Their Applications in Metal‐Catalyzed Asymmetric Synthesis†

“…The Michael addition products were summarized and listed in Scheme 30. Three types of enolates were used: α ‐cyanoacetates [ 66‐67 ] ( 32a , 32b ), azlactones [ 68‐73 ] ( 33a — 33c ) and isoxazolinones [ 74 ] ( 33d ). In the case of α ‐cyanoacetates, FBIP or FIP could deliver different diastereoisomers as the major product, showing the stereodivergency of the two palladacycle catalysts.…”

“…Based on Overman rearrangement that has been studied in detail, the catalytic reactivity of the analogous ruthenocene palladacycles 35 was reported in 2014. [ 73 ] Ruthenocene has a longer distance betwen two Cp rings than ferrocene (3.58 Å vs. 3.29 Å), thus the distance of two palladium centers also increases (Figure 8). This is of great importance in bimetallic catalysis, especially for those substrates binding with both metal centers, as appropriate distance could eliminate potential strain and promote effective binding.…”

Recent Developments in Asymmetric Allylic Amination Reactions

“…194,195 This constitutes a significant practical advance, because unlike benzamides, the trihaloacetamide products are readily deprotected to afford the primary allylic amines in high yield. Peters and coworkers later reported that metallocenyl oxazoline and imidazoline catalysts permit a 100-fold reduction in catalyst loadings for both the Eand the less reactive Z-isomer courtesy of [PPFOP-Cl] 2 (318), 196 [RuBIP-Cl] 2 (319a) 197 and FIP-Cl (320) 198 (entries 8, 9 and 11). 199,200 Interestingly, the activated forms of [PPFOP-Cl] 2 (318) and FIP-Cl (320) feature a palladium(III) center and represent the first palladium(III) complexes to be employed in enantioselective catalysis.…”

Synthesis, characterization and antiparasitic activity of organometallic derivatives of the anthelmintic drug albendazole

“…More than one billion people, which amounts to onesixth of the world's population, are affected by NTDs within 149 countries. 1,2 Even though the morbidity exceeds the mortality, more than half a million deaths per year are caused by NTDs. 3 Among NTDs, worm infection, or helminthiasis, has been the target of global elimination programs.…”