Abstract
A facile, catalyst-free synthesis of a norbornylated cellulosic material (NC) with a high degree of substitution (2.9) is presented by direct reaction of trimethylsilyl cellulose with norbornene acid chloride. The resulting NC is highly soluble in organic solvents and its reactive double bonds were exploited for the copper-free inverse-electron demand Diels–Alder (iEDDA) “click” reaction with 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine. Reaction kinetics are comparable to the well-known Huisgen type 1,3-dipolar cycloaddition of azide with alkynes, while avoiding toxic catalysts.
Highlights
In recent years, “click” chemistry entered into the focus of research in many disciplines, since it allows for a selective coupling of two or more moieties via dedicated functional groups [1,2]
It should be mentioned that there are some examples where Huisgen type “click” reactions proceed without any copper catalyst even at room temperature albeit reaction kinetics are rather slow and regioselectivity is often not realized in a satisfactory manner [13]
We identified norbornenes as suitable reaction partners to be immobilized on the cellulose backbone since they are sufficiently reactive dienophiles for inverse-electron demand Diels–Alder (iEDDA) reactions due to their strain-activated double bond while avoiding the problems typically associated with more reactive dienophiles like trans-cyclooctenes such as storage stability due to isomerization [24]
Summary
“click” chemistry entered into the focus of research in many disciplines, since it allows for a selective coupling of two or more moieties via dedicated functional groups [1,2]. In order to prevent undesired side reactions and by-products, the effort for working up the reaction product is minimized. A wellknown “click” reaction is the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes (CuAAC) to give 1,2,3-triazoles [1,3]. Intense efforts have been made to establish “click” chemistries which avoid copper catalysts. In this context, it should be mentioned that there are some examples where Huisgen type “click” reactions proceed without any copper catalyst even at room temperature (e.g., cycloadditions of cyclooctynes) albeit reaction kinetics are rather slow and regioselectivity is often not realized in a satisfactory manner [13]
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