Aluminium SALEN

Coates and co-workers reported polymerizations using both the enantiomerically pure (7) and the racemic (8) Ao-propoxide analogues [21], Complex 8 polymerized rac-lactide iso-selectively producing stereoblock PLA (P = 0.96). The isoselectivity was achieved due to the high selectivity of 7 for RR-lactide and corresponding S -enantiomer for S S -lactide. Further studies of the aluminium-salen system have led to the proposal that several factors influence the polymerization... 

North, M. Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide Using Bimetallic Aluminium (Salen) Complexes, ARKTVOC, no. i (2012) 610-628. 

In addition to amines and hydroxylamines, very high levels of enantioselection have been obtained during the conjugate addition of carbamates and hydrazoic acid using copper bis-oxazolines and aluminium(salen) complexes, respectively. ... 

Eric Jacobsen s first catalyst for the enantioselective Strecker reaction was an aluminium-salen complex (Tab. 4.2). [92]... 

Tab. 4.2 Enantioselective synthesis of amino-nitriles with an aluminium-Salen complex...

Aluminium(iii)-based salens seem, however, used with a cocatalyst, to be limited in most cases to cyclic carbonate formation, the higher Lewis acidity of aluminium(iii) being probably partly responsible for this. The first successful aluminium-salen, Al(salen)Cl (Scheme 18.37), used in the formation of cyclic carbonates was tested by the group of Ren He in 2002, with the rather uncommon substrate, ethylene oxide and was further optimised by Xiao Bing Lu in 2004. ... 

The role of the ammonium cocatalyst in the aluminium-salen-catalysed carbonate synthesis was precisely investigated a couple years earlier by the group of North. This group achieved in 2007 a major development in aluminium-salen chemistry and its related carbon-dioxide chemistry with a new class of dinuclear aluminium salens produced via a controlled hydrolysis of the related reactive monomeric aluminium-salens, as shown in Scheme 18.39. This dinuclear catalyst used with tetrabutylammonium bromide as cocatalyst reached a very high efficiency in the formation of cyclic carbonates in some cases under very mild operating conditions (cat range 1 to 2.5 mol% recyclable more than 60 times) as exemplified with styrene oxide that afforded styrene carbonate in 86% yield after 24 h. 

The group of North developed accordingly an efficient bifunctional bimetallic aluminium-salen with a grafted quaternary ammonium chloride obtained via reaction of the diethyl-amino-substituted salen with benzyl... 

Interestingly, Kureshy and coworkers reported recently on a similar approach performed with mononuclear bifunctional aluminium-salens bearing triphenylphosphonium chloride moieties immobilised on mont-morillonite The supported catalysts used with a supplementary... 

In 2009, the North group found that bimetallic aluminium(salen) complex 4 was a highly effective catalyst for the asymmetric addition of trimethylsilyl cyanide to aldehydes (Scheme 19.2). In the presence of a phosphine oxide cocatalyst and 2 mol% of the catalyst (l ,l )-[(salen)Al]20 4, the corresponding adducts were obtained in 53-96% enantiomeric excess, which were comparable to those obtained using mononuclear (salen)AlCl complexes. An analysis of the reaction kinetics showed that the reactions exhibited first-order kinetics, with the reaction rate being independent of the aldehyde... 

Subsequently, the authors used sodium azide, an inexpensive and easily handled hydrazoic acid precursor, in combination with concentrated aqueous hydrochloric acid, in the conjugate addition to a,p-unsaturated ketones. Chiral bimetallic aluminium(salen) complex (5, 5 )-[(salen)Al]20 ent-4 proved efficient (Scheme 19.32). 

In the catalytic enantioselective Strecker reaction, chiral aluminium complexes, especially aluminium-salen-based catalysts and aluminium-binaphthol-based catalysts have been widely used, and great achievements have been obtained. In 2010, the Li group reported a highly enantioselective Strecker reaction of achiral IV-phosphonyl imines by using primary free L-phenglycine 42 as the catalyst and diethylaluminium cyanide as the nucleophile. This work also presented the novel use of nonvolatile and inexpensive diethylaluminium cyanide in asymmetric catalysis (Scheme 19.51). ... 

Clegg W, Harrington RW, North M, Pasquale R (2010) Cyclic carbonate synthesis catalysed by bimetallic aluminium-salen complexes. Chem Eur J 16 6828-6843... 

In 1999, Jacobsen reported on a catalytic asymmetric conjugated addition of hydrazoic acid to unsaturated imide derivatives (Equation 4.1). This breakthrough was possible through the use of aluminium salen azide complex 1 as catalyst. The reaction proceeded in excellent yields and enantioselectivities for alkyl substituted acceptors. Two mechanisms were proposed for this reaction activation of the azide as an aluminium azide as shown by Chung and co-workers or Lewis acid activation of the imide. The first-order dependence of the rate law on catalyst 1 indicated that dual activation was improbable. In 2005, Jacobsen reported on the extension of this methodology to ,j8-unsaturated ketones. 

M. North, B. Wang, C. Yotmg, Influence of flue gas on the catalytic activity of an immobilized aluminium(salen) complex for cyclic carbonate synthesis. Energy Environ. Sci. 4 (2011) 4163-4170. 

Melendez, J. North, M. Pasquale, R. Synthesis of Cyclic Carbonates from Atmospheric Pressure Carbon Dioxide Using Exceptionally Active Aluminium(salen) Complexes as Catalysts. Eur. J. Inorg. Chem. 2007, 3323-3326. 

Jha SC, Joshi NN. Aluminium-SALEN complex a new catalyst for the enantioselective Michael reaction. Tetrahedron Asymm. 2001 12(17) 2463-2466. 

In 1998, Jacobsen reported an asymmetric metal-catalyzed Strecker reaction that employs aluminium salen complex 145 (Equation 20) [105], Interestingly, the salen(aluminum) complex 145 proved superior to a wide range of other salen(metal) complexes both in terms of conversion and of enantioselectivity. This constituted the first example of successful use of a main group salen(metal) catalyst for asymmetric synthesis. The N-allylated imine 144 was thus converted into the trifluoroacetyl derivative 146 in 91 % yield and 95 % ee. 

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