Al(Salen) Metal Complexes in Stereoselective Catalysis

Abstract

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N′-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.

Aluminum(III) Salen Complexes as Active Photoredox Catalysts

“…Although Al(III) complexes with Salophen ligand, complexes 5 and 6, were also conceivable catalysts (Table 1 entry The best conditions for performing the reaction were: 5 mol-% of the photocatalyst 2, 14 mol-% of the imidazolidinone catalyst 9 in DMF with 2,6-lutidine as the organic base and compact fluorescent lamp (CFL, 23 W) as light source. Chiral (Salen)metal complexes catalyze an array of asymmetric nucleophileelectrophile reactions [16] including TMSN 3 and carboxylic acid additions to meso epoxides, [27] hetero Diels-Alder reaction, [28] Scheme and TMSCN addition to aldehydes. [29] In addition, 2 was described to promote the enantioselective addition of HCN to imines.…”

“…Some years ago, we have reported the photophysical properties of Salen compounds, discovering that when ligand (1 R ,2 R )‐(–)‐[1,2‐cyclohexanediamino‐N,N'‐bis(3,5‐di‐ tert ‐butylsalicylidene)] 1 (Figure ) is bound to Al(III), Zn(II), Sn(II), Si(IV), and Mg(II) an unstructured fluorescence band appears in the 400–700 nm region with a lifetime in the nanosecond range . The (1 R ,2 R )‐[Al(Salen)Cl] complex 2 also exhibits electrogenerated chemiluminescence in acetonitrile solutions, although less efficiently than [Ru(bpy) 3 ] 2+ in the same conditions . Furthermore, Garcia has reported an interesting photochemical investigation of [Al(Salen)Cl] complex 2 , in a benchmark reaction, reported by MacMillan in 2008 using [Ru(bpy) 3 ] 2+ photocatalyst .…”

Structural analysis of five-coordinate aluminium(salen) complexes and its relationship to their catalytic activity

“…Pure styrene carbonate 13 was obtained by column chromatography using hexane:EtOAc mixtures (1:0, 1:6, 1:4, 1:2, 1:1 and 0:1) as eluent. max (CH 2 Cl 2 ) 3078, 3038, 2926, 1784, 1459 and 1064 cm -1 ; 1 H NMR ] H (400 MHz, CDCl 3 ): 7.5-7.4 (3H, m, ArH), 7.4-7.3 (2H, m, ArH), 5.67 (1H, dd 3 J 8.1, 7.9 Hz, PhCHO), 4.80 (1H, dd 3…”

“…Metal complexed salen ligands are not planar, but rather adopt stepped and bowl conformations, giving four potentially interconverting conformations as shown in Figure 2A. 3 Any substituents on the diamine derived part of the salen ligand (R 1 in Figure 1) can then adopt axial or equatorial positions within the 5-membered chelate ring ( Figure 2B). A metal ion coordinated to a salen ligand is most commonly six-coordinate with two additional ligands (X and Y) also coordinated to the metal to give an octahedral geometry.…”

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

“…Catalysis is another field of application of SB ligand and their complexes. Many SB complexes show excellent catalytic activity and can be used in organic synthesis [12,54,[65][66][67][68][69][70]. SBs can be applied for solvent extraction of metal ions [71][72][73].…”