Vinylic addition polynorbornenes (VA-PNB) result from the insertion polymerization of norbornene or specific norbornene derivatives catalyzed by transition metal complexes. The VA-PNB skeleton is completely aliphatic and keeps the bicyclic structure of norbornene. This saturated polymer is thermally and chemically very stable and it is a very robust scaffold to support catalysts or reagents for catalytic applications. Several VA-PNBs are now available with suitable functional groups (halogen, alkenyl, carbonates) that can be used as starting materials to introduce the functional-ization of choice by post-polymerization reactions. This has been applied to anchor organocatalysts on VA-PNB as well as ligands that can be used to synthesize supported metal catalysts. The reported examples of the use of VA-PNB-linked catalysts and their recyclability will be presented. VA-PNBs have also proved useful in the context of organotin chemistry to solve the problem of tin contamination by an efficient separation and the reuse of tin byproducts. Its uses in the Stille cross-coupling reaction and tin-hydride radical processes will be discussed. 2 3 4 5 6 7 8
ARTICLEThis journal is © The Royal Society of Chemistry 2013 J. Name., 2013 Rodrigo García-Loma and Ana C. Albéniz*The ring opening metathesis copolymerization of norbornene and ω-bromoalkylnorbornenes NB-(CH2)nBr (n = 1, 4) by Grubbs' 2nd generation catalyst, followed by hydrogenation, gives insoluble saturated polynorbornenes (ROMPH-PNBs) that have pendant ω-bromoalkyl chains (4a, b). These materials can be functionalized by nucleophilic substitution of bromide to give a variety of substituted polymers (ROMPH-PNB-(CH2)nNu where Nu = CN, SPh, OOCMe, N3, SnR3. The stannylated polymers were tested in a Pd-catalyzed reaction, the Stille coupling. The azido polynorbornenes ROMPH-PNB-(CH2)nN3easily undergo the click 1,3-dipolar cycloaddition with alkynes, which could be a useful strategy to anchor other functionalities of interest. The aliphatic nature of the ROMPH-PNB-(CH2)nBr backbone makes robust supports and the presence of the bromo substituent imparts versatility so they are good candidates to be used as a general starting material to anchor a group required for a specific synthetic purpose.
Vinylic addition polynorbornenes (VA‐PNB) with stannyl functional groups have been prepared and used in tin‐mediated radical dehalogenation reactions. The aliphatic and robust scaffold of VA‐PNB is well suited for a support in radical processes. VA‐PNB‐(CH2)nSnHBu2 can be used as a stoichiometric reagent and VA‐PNB‐(CH2)nSnBu2Cl as a catalyst in the presence of a hydride donor for the reduction of RBr. The mixture KF (aq.)/polymethylhydrosiloxane (PMHS) is the most convenient hydride source to generate VA‐PNB‐(CH2)nSnHBu2 in situ. Although quite popular in this context, boron hydrides, being a source of radicals themselves, are not adequate to correctly evaluate the performance of the anchored organotin group. VA‐PNB‐(CH2)4SnBu2Cl can be recycled and, even if it loses activity upon reuse, it is still useful after ten cycles. The stannylated VA‐PNB can be separated from the products by simple filtration, and it leads to very low tin contamination (at least 250 times lower than that with use of conventional separation methods).
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