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El entendimiento preciso del mecanismo de una reacci贸n permite, en general, mejorar sus condiciones de aplicaci贸n y la eficacia de la misma. En reacciones catal铆ticas las posibilidades de comprender su mecanismo aumentan considerablemente cuando pueden aislarse algunos intermedios y estudiarse separadamente algunas etapas de las que componen el ciclo catal铆tico. Eso lo hacemos mediante el seguimiento espectrosc贸pico de las reacciones y el estudio de sus cin茅ticas. En esta p谩gina te mostramos algunos de nuestros resultados m谩s recientes.

Recientemente hemos encontrado un sistema que nos permite estudiar lo que sucede cuando en la reacci贸n de Stille la transmetalaci贸n es m谩s r谩pida que las isomerizaciones de los complejos de paladio implicados.

ver Organometallics, 2011, 30, 611-617

La reacci贸n de Negishi transcurre de forma diferente cuando se emplean reactivos del tipo ZnRX o cuando se emplean reactivos ZnR2. La reacci贸n de transmetalaci贸n es reversible, y la estereoqu铆mica de la reacci贸n est谩 condicionada por la velocidad con la que se establecen los equilibrios de transmetalaci贸n-retrometalaci贸n.

FotoNegishi

De izquierda a derecha: Juan A. Casares, Ei-Ichi Negishi y Pablo Espinet en el 20 Congreso Euchems de Qu铆mica Organomet谩lica. (StAndrews, Escocia, julio de 2013)

[:en]The possibility of understanding the mechanism of聽 catalytic reactions depends very much on the chances of isolating intermediates and to study isolately each step in the catalytic cycle. We try to do this by monitoring the reactions and spectroscopic study of its kinetics. On this page we show you some of our latest results.

The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh2)2] and ZnMe2 in THF reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh2)2(THF)]+) opens ionic transmetalation pathways.

聽J. Am. Chem. Soc, 2011, 133, 13519鈥13526.

During the sutdy of the Negishi reaction we have also found that Pd takes a roundtrip before coupling: The large rate difference of the slow reductive elimination and the fast transmetalation-retrotransmetalation steps brings Pd to visit many times its complexes 3, 2, and 1, before coupling from 2 takes place. This fast recurrence of transformations offers many opportunities for the formation of undesired homocoupling products in Negishi reactions, via statistically accessible 鈥渕istaken鈥 retrotransmetalation events in the multitude of roundtrip processes preceding the slower irreversible reductive elimination of the coupling product.

 

Chem. Eur. J., 2010, 16, 8596-8599.

Chem. Eur. J., 2010, 16, 8596-8599.

 

We recently found a system that allows us to study what happens when during the Stille reaction聽 the transmetallation is faster than the isomerization reactions of the palladium complexes involved.