Aproximaciones a las aplicaciones de complejos salen y salan-metal en las reacciones de epoxi dación de olefinas, polimerización y síntesis asimétrica
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Las bases de Schiff se constituyen en una clase importante de compuestos que han recibido mucha atención en una amplia variedad de campos de investigación debido a las diferentes aplicaciones y propiedades características tales como: su preparación, accesibilidad, variedad estructural, habilidad de coordinación, estabilidad térmica y propiedades catalítica. Los complejos de base de Schiff con centro de coordinación ON-NO son conocidos como ligandos salenos (salen: N,N-bis(salicilideno)-etilendiimina). Sus aplicaciones han crecido rápidamente y un amplio rango de reacciones catalíticas asimétricas han sido descritas tales como la epoxidación de olefinas no funcionalizadas, polimerización, entre otros tipos de reacciones donde se requiere inducción asimétrica. Los ligandos salenos son sintéticamente accesibles, económicos y no tóxicos y pueden conducir a altos excesos enantioméricos, consecuentemente muchos ligandos salenos han sido diseñados para llevar a cabo procesos catalíticos asimétricos. Su síntesis se lleva a cabo por la combinación de una diamina y 2 equivalentes de salicilaldehidos o derivados de este. En contraste, los complejos salen reducidos también llamados “salanos” son especies confiables con actividad redox sobre el ion metálico. Estos compuestos pueden ser sintetizados por reducción de los grupos azometinos de los ligandos salen con NaBH4. Los ligandos salalenos es una clase de hibrido [ON-NO] entre los ligandos salenos y salanos. Una forma directa de síntesis se logra partiendo de precursores salenos, en dos pasos de condensación, utilizando 1 equivalente de salicilaldehido o utilizando grupos protectores para evitar la doble condensación. Luego de la reducción se desprotege para posterior condensación. Los salanos y salalenos han atraído significativamente la atención como ligando efectivos para la síntesis asimétrica debido a que poseen mayor flexibilidad y pueden ser fácilmente modificados en comparación con los ligandos salenos. Además los complejos metal salano y metal salalenos tienen una gran tendencia a adoptar distintas configuraciones esteroquímicas que podrían producir inducciones asimétricas.
ABSTRACT As important class of compounds, Schiff bases have received much attention in the wide variety of fields due to their differents applications and characteristics properties such as preparative, accessibility, structural variety, varied coordinating ability, thermal stability, catalysis properties. The complexes of the Schiff bases with ON-NO coordination sphere are known as salen ligands (salen- N,N-bis(salicylidene)-ethylendiamine. Their applications have grown rapidly and a broad range of as Asymmetric catalytic reactions have been described such as epoxidations of olefins, polymerizations and other asymmetric reactions. The salen ligands are synthetically accessible, inexpensive and nontoxic and could afford high enantiomérico excess, consequently many salen ligands have been designed to carry out asymmetric catalysis. The salen ligands can be synthesized with one diamine and 2 equivalent salicylaldehyde moieties. In contrast to this, reduced salen complexes also called “salan” reliably form species with redox active metal ion and a ligand-centers radical. These compounds can be synthetized by reduction of the azometines groups with NaBH4 The salalen ligand is a kind of hybrid between salens and salans tetradentate [ONNO] 2- type ligands. A straightforward approach for the syntheses of its parents ligands, namely. A two-step condensation sequence of a mono-N-substituted salicyladehyde to form the “half salen” part, followed by a Mannich condensation of the secondary amine group. Chiral salalen and salans have attracted significant recent interest as effective ligands for metal catalyzed asymmetric syntheses because they are more flexible and can be readily modified compared to salen ligands. In addition, metal- salan and metal-salalen complexes have a greater tendency to adopt the cis-β-configuration that might produce strong asymmetric induction in some reactions.
ABSTRACT As important class of compounds, Schiff bases have received much attention in the wide variety of fields due to their differents applications and characteristics properties such as preparative, accessibility, structural variety, varied coordinating ability, thermal stability, catalysis properties. The complexes of the Schiff bases with ON-NO coordination sphere are known as salen ligands (salen- N,N-bis(salicylidene)-ethylendiamine. Their applications have grown rapidly and a broad range of as Asymmetric catalytic reactions have been described such as epoxidations of olefins, polymerizations and other asymmetric reactions. The salen ligands are synthetically accessible, inexpensive and nontoxic and could afford high enantiomérico excess, consequently many salen ligands have been designed to carry out asymmetric catalysis. The salen ligands can be synthesized with one diamine and 2 equivalent salicylaldehyde moieties. In contrast to this, reduced salen complexes also called “salan” reliably form species with redox active metal ion and a ligand-centers radical. These compounds can be synthetized by reduction of the azometines groups with NaBH4 The salalen ligand is a kind of hybrid between salens and salans tetradentate [ONNO] 2- type ligands. A straightforward approach for the syntheses of its parents ligands, namely. A two-step condensation sequence of a mono-N-substituted salicyladehyde to form the “half salen” part, followed by a Mannich condensation of the secondary amine group. Chiral salalen and salans have attracted significant recent interest as effective ligands for metal catalyzed asymmetric syntheses because they are more flexible and can be readily modified compared to salen ligands. In addition, metal- salan and metal-salalen complexes have a greater tendency to adopt the cis-β-configuration that might produce strong asymmetric induction in some reactions.