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Al(Salen) Metal Complexes in Stereoselective Catalysis.
Molecules
May 2019
ALMA MATER STUDIORUM Università di Bologna, Dipartimento di Chimica "G. Ciamician", Via Selmi 2, 40126 Bologna, Italy.
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 ,'-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.
View Article and Find Full Text PDFComplexities in the ring-opening polymerization of lactide by chiral salen aluminum initiators.
Inorg Chem
April 2008
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185, USA.
The preparation of (R, R)- and (S, S)-salen Al(OR) complexes, where R = Et, CH2(i)Pr, CH2(t)Bu, and CH2CH(S)MeCl, are reported, along with their reactions with rac-lactide (salen = N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino). Rapid, reversible coordination of LA to the salen metal complex is observed, and it is shown that the relative rates of alcohol/alkoxide exchange are comparable to the NMR time scale while the rate of chain transfer involving (R, R)-salenAl(O-R-R) and (S, S)-salenAl(O-S-R) is much faster than the initial rate of ring opening of the LA monomer. For a primary [Al-OR] moiety, the ring opening of rac-LA is much faster than the ring-opening polymerization/enchainment of LA, and in the initial ring-opening event, the diastereoselectivity is dependent on the solvent, the chirality of the salen ligand, and the OR group.
View Article and Find Full Text PDFChiral salen-aluminum complex as a catalyst for enantioselective alpha-addition of isocyanides to aldehydes: asymmetric synthesis of 2-(1-hydroxyalkyl)-5-aminooxazoles.
Org Lett
August 2007
National Laboratory for Molecular Sciences, Laboratory of Chemical Biology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
In the presence of a catalytic amount of (salen)Al(III)Cl complex (4e), reaction between alpha-isocyanoacetamides (1) and aldehydes (2) afforded the corresponding 5-aminooxazoles (3) in good yields and enantioselectivity.
View Article and Find Full Text PDFPhotochemistry of a chiral salen aluminum complex in nonconventional solvents: use of imidazolium ionic liquids and chiral alcohols.
J Phys Chem A
July 2007
Instituto de Tecnologia Quimica CSIC-UPV and Departamento de Quimica, Universidad Politecnica de Valencia, Avda de los Naranjos s/n, 46022 Valencia, Spain.
The photochemistry of a chiral (salen)aluminum(III) chloride complex has been studied in nonconventional solvents, namely, two imidazolium ionic liquids differing on the hydrophobicity (hydrophilic BF(4)(-) or hydrophobic PF(6)(-) counter anions) and in chiral 2-butanols (R and S). Upon 355 nm laser excitation, the same transient absorption spectrum (with some solvatochromic shift in lambda(max)) was recorded in all cases and assigned to the (salen)Al(II) complex with radicaloid character at the metal atom. This intermediate arises from the photoinduced homolytic cleavage of the apical Al-Cl bond.
View Article and Find Full Text PDFEnantioselective cyanation/Brook rearrangement/C-acylation reactions of acylsilanes catalyzed by chiral metal alkoxides.
J Org Chem
October 2004
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA.
New catalytic enantioselective cyanation/1,2-Brook rearrangement/C-acylation reactions of acylsilanes (4) with cyanoformate esters (7) are described. The products of the reaction are fully substituted malonic acid derivatives (8). Catalysts for this transformation were discovered via a directed candidate screen of 96 metal-ligand complexes.
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