Environmentally friendly synthetic methods based on the use of polymers.
Modern synthetic chemistry needs to become more efficient and clean and, for this reason, the minimization of waste and cumbersome purification protocols is a very important goal.
We are interested in the synthesis of new functionalized polymers and their application in sustainable synthetic chemistry. These macromolecular scaffolds can be used to anchor reagents or catalysts so they can be more easily recovered and separated and, ideally, reused multiple times.
For that purpose we have developed polymerization and post-polymerization functionalization methods to synthesize robust polymers based on norbornene derivatives. Some norbornene polymerization routes afford an all-aliphatic polymer backbone, thermally and chemically resistant, well suited for a support material.
New polynorbornenes with aliphatic backbone:
Our work is centered on vinylic addition polynorbornenes (VA-PNBs), a type of polymers that keep the aliphatic bicyclic structure of norbornene in the polymer and are very resistant both thermally and chemically.
The direct polymerization of functionalized norbornenes is not easy, so we have developed brominated VA-polynorbornenes that can be further functionalized by a variety of postpolymerization reactions to introduce the desired supported moiety.
S. Martínez-Arranz, A. C. Albéniz, P. Espinet. Macromolecules 2010, 43, 7482.
S. Martínez-Arranz, E. Sánchez-Pérez, J. A. Molina de la Torre, I. Pérez-Ortega, A. C. Albéniz. RSC Advances, 2016, 6, 105878-105887. DOI:0.1039/c6ra23123c
All-aliphatic polynorbornenes (VA-PNBs), a good support for reagents or catalysts:
A clean Stille reaction:
We have synthesized stannylated polymers to be used as recyclable reagents in the Stille reaction and other processes that use organotin reagents and face the problem of the generation of organotin toxic byproducts. Our work is centered on vinylic addition polynorbornenes but we have also used other polymeric scaffolds (polyfluorenes, polyphosphazenes) for this purpose.
N. Carrera, E. Gutiérrez, R. Benavente, M. M. Villavieja, A. C. Albéniz, P. Espinet. ”Stannylated Polynorbornenes as New Reagents for a Clean Stille Reaction” Chem. Eur. J. 2008, 14, 10141.
S. Martínez-Arranz, D. Presa-Soto, G. A. Carriedo, A. Presa Soto, A. C. Albéniz. “Polyphosphazenes for the Stille reaction: A new type of recyclable stannyl reagents” Dalton Trans. 2016, 45, 2227-2236. DOI: 10.1039/C5DT02670A.
The Stille reaction can be carried out in a batch way using immobilized tin reagents with a very simple workup and very low tin contamination in the products (15 ppm). The reagent can be regenerated and reused multiple times.
S. Martínez-Arranz, N. Carrera, A. C. Albéniz, P. Espinet, A. Vidal Moya. “Batch Stille Coupling with Insoluble and Recyclable Stannylated Polynorbornenes” Adv. Synth. & Cat. 2012, 354, 3551-3560. DOI: 10.1002/adsc.201200624
N-Heterocyclic carbenes supported on VA-PNBs are good catalysts for umpolung organic transformations, recyclable and can be recovered as imidazolium salts and stored indefinitely for further use:
J. A. Molina de la Torre, A. C. Albéniz, ChemCatChem, 2014, 6, 3547-3552. DOI:10.1002/cctc.201402767
Asymmetric aldol reactions in water/solvent mixtures were performed using proline-derivatives supported on VA-PNBs as catalysts, with excellent results as far as selectivity and recyclability:
A palladium reservoir on polymer for use in catalysis.
Polymer-supported metal complexes have been used in catalysis with the aim of bringing together the advantages of both homo- and heterogenous catalysis. This has met some success, but the mechanisms of the actual catalysis are far from obvious in many cases.
A vinylic addition polynorbornene (VA-PNB) supported NHC palladium complex can used as precatalyst in C-C cross coupling reactions and it is reusable. We have studied the system in some detail and found out that after the first use, the polymer contains Pd aggregates (about 65 nm). A small amount of palladium (a few ppms) is released from those aggregates and it is enough to catalyze a Suzuki or a Negishi reaction in a homogeneous way. The polymer is a reservoir of palladium that can be used multiple times.
J. A. Molina de la Torre, A. C. Albéniz. ChemCatChem, 2016, 8, 2241-2248. DOI: 10.1002/cctc.201600194.