Self‐Assembly Approach toward Chiral Bimetallic Catalysts: Bis‐Urea‐Functionalized (Salen)Cobalt Complexes for the Hydrolytic Kinetic Resolution of Epoxides

Abstract

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.

Hydrogen‐Bond‐Promoted Palladium Catalysis: Allylic Alkylation of Indoles with Unsymmetrical 1,3‐Disubstituted Allyl Acetates Using Chiral Bis(sulfoxide) Phosphine Ligands

“…[9] Jacobsen and co-workers demonstrated that the hydrogen-bond interaction between the tert-butyl sulfinyl moiety of chiral urea catalysts and a protonated imine substrate effectively promoted an asymmetric [4+2] cycloaddition. [10] We envisioned that incorporation of an additional sulfinyl group into a bidentate ligand, like our previous sulfoxide phosphine ligand L1 (Figure 1), [8a-c] might generate a new type of ligand which could perform cooperatively in catalysis, [11] that is, serve 1) as a Lewis base to bind a metal, 2) as a hydrogen-bond acceptor to activate a substrate, and 3) to provide more of a chiral environment. Herein, we report a new class of chiral bis(sulfoxide) phosphine(BiSO-P) ligands, and employ them in an unprecedented palladiumcatalyzed DYKAT of racemic unsymmetrical 1,3-disubstituted allylic acetates with indoles [Eq.…”

“…Particularly, some of 1,3dialkyl allylic acetates were also suitable substrates for this transformation. For instance, when R was nPr (1 m), iPr (1 n), and cyclohexyl (1 o), the corresponding 3-allylindoles 6 k, 6 l, and 6 m were obtained with good to excellent regioselectivities, as well as high enantioselectivities ( Table 2, entries [11][12][13]. [13] We next investigated the scope of indole nucleophiles ( Table 2, entries [14][15][16][17][18][19][20][21][22][23][24].…”

Calix[8]arene as New Platform for Cobalt‐Salen Complexes Immobilization and Use in Hydrolytic Kinetic Resolution of Epoxides

“…For instance, different routes have been described, which involved the formation of linear oligomers and polymers by different methods, either by polymerization of complexed monomers or post‐functionalization . The association of two salen ligands by non‐covalent interactions also resulted in enhanced efficiency . The same applied to the formation of macrocyclic structures containing salen derivatives .…”

Organocatalytic Asymmetric Reactions of Epoxides: Recent Progress

“…[3a] Besides water, [17a] azide, [16] anilines, [37] alkyl amines, [38] N-Boc protected sulfonamides [39] and carbamates [17b] have been employed in the catalytic kinetic resolution. In the field of organocatalysis, cornerstone research showed that achiral ureas [40] and thioureas [41] are able to promote the regioselective ringopeningr eaction of epoxides. In 2008, Schreinera nd co-authors developed ar egioselective alcoholysis of styrene oxides assisted by an organocatalytic system composed of two Brønsteda cids, N,N-bis-[3,5-bis-(trifluoromethyl)phenyl]-thiourea 24 and mandelic acid 25 (Scheme 11).…”