Reactive oxirane functions

Many authors elucidated functionalization of polymers containing reactive oxirane moieties. Epoxidized NR, BR, IR and/or the respective model hydrocarbons, poly (butadiene-co-isoprene, various epoxy resins, poly (2,3-epoxypro-pyl methacrylate) and its copolymers or grafted systems were mostly exploited. Stabilizers based on epoxidized unsaturated rubbers are of the top interest. The mechanism of the functionalization process was studied in details by means of 3,4-epoxy-4-methylheptane and 1,2-epoxy-3-ethyl-2-methylpentane as model compounds [289]. The ring opening of the asymmetric oxirane is regiospecific. Aliphatic primary amines attack the least substituted carbon atom and can be involved in crosslink formation. Aromatic primary and secondary amines are less reactive than aliphatic ones because of their lower basicity the attack on the least substituted carbon atom is however preferred too. 

All cationically polymerizable monomers can be potentially used in this process however, the main study has been focused so far on the most reactive oxirane and vinyl ethers [4], Alkoxysilane derivatives - the most common acid-sensitive monomers for the synthesis of siloxane materials through the use of sol-gel methods - were not used extensively. Only a few examples of their application in photo-activated cross-linking can be noted, mainly in co-reaction with oxirane sites [5]. Typically, alkoxysilanes are subjected to an acid- or base-catalyzed process involving hydrolysis of an =SiOR group and then condensation of the formed silanol with another molecule bearing an =SiOH or =SiOR function to give a siloxane linkage [6]. It was of interest to combine the properties of cross-linked silicone materials with the ones provided by sterically overloaded... 

The epoxide (oxirane) functional group has special features that allow it to play very important roles in synthesis and as an electrophilic species in biologically reactive intermediates. The epoxide group has an estimated strain energy of 27kcalmol-1,19 and its reactions with both acidic and nucleophilic reagents result in epoxide ring... 

Conversion of internal unsaturations in triglycerides and their fatty acids into epoxide moieties has been optimised to enable their industrial implementation an economically viable operation with some oils. This strategy has opened the way to elaboration of macromolecular materials that could become real commodities. This rationale implies that the performance of these polymers should adequately match the respective applications they are conceived to fulfill. Apart from such common epoxidised oils, this section deals with other more elaborate monomers in which the oxirane function is present as an end group and hence more reactive. 

Preformed polymers containing reactive functionalities other than vinyl unsaturation (oxirane, hydroxyl, carboxyl, etc.)... 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

Furthermore, marked substrate selectivity was noted, such that the rate of reaction decreased the series in the order 14,15-epoxy > 11,12-epoxy > 8,9-epoxy > 5,6-epoxy. In other words, the more removed the oxirane ring was from the carboxy group, the faster its enzymatic hydration. It was suggested that the low reactivity of 5,6-EET is related to its possible physiological functions. 

Nonstabiiized lithiooxiranes can be prepared by the reaction of strong bases such as alkylithium reagents or lithium amides. However, as already discussed in a preceding section (Section II), the competition between a- and /3-deprotonation has to be adressed, and the issue of this competition is highly dependent on the structure of the starting oxirane as well as on the nature of the base used. These lithiooxiranes are very reactive species. In order to prevent their decomposition, they can be stabilized by a diamine ligand. Further stabilization can be obtained by a remote functionality. 

Functionality adjacent to the epoxide can modify its reactivity. For example, 2,3-epoxy sulfides can be converted to a thiiranium species upon treatment with TMS triflate. This intermediate reacts with 0-silyl amides regiospeci-fically to form l-substituted-3-hydroxy-2-thioethers. Simple primary amines undergo polyalkylation, but imines can be used as an indirect amine equivalent <1996T3609>. Nitriles react with functionalized oxiranes in a regioselective manner in a tandem epoxide opening-Ritter reaction (Equation 17) <2005JOC7447>. 

Vinyl epoxides can be cross-coupled with vinylstannanes <2001JOC589>. This reaction proceeds through an (if-allyOpalladium complex (Equation 57) <2001JOC589>. Similar reactivity can be observed using bismuth reagents (Equation 58) <2001SC2365>. Vinyl oxiranes react with substituted allenes to form functionalized allyl alcohols <2004JOC4686>. 

Functionalized polymers of the type B and C (Scheme 1) can be formed via polyaddition processes of bifunctional reactants, without splitting off of low molecular weight compounds. Most syntheses of stabilizers have been based on reactions of bis-isocyanates with H-acid nucleophiles. Some reactions of oxiranes may be listed here too (syntheses involving oxiranes are listed in Sect. 3.1.1.2 if polymerization aspects are more evident syntheses of stabilizers formed via reactivity of oxirane moieties attached to an oligomeric or polymeric chain are classified as reactions on polymers. Sect. 3.2.2.1... 

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