Epoxidising agents

Peracids would attack the other, more electron-rich, double bond. We need a nucleophilic epoxidising agent... 

Lithium ferf-butylperoxide has been used effectively as an epoxidising agent with electron deficient alkenes (Scheme 40) [91, 92]. However, application of this methodology to systems containing fluorine has only recently been explored, and it is now established that this can be a very successful procedure with fluoroalkenes. Indeed, the Lithium tert-butylperoxide system worked in some cases where the calcium hypochlorite reaction was ineffective [93,94]. 

Bleaching powder has proved to be a very effective reagent for various epoxidations [158, 182-184], and lithium t-butyl peroxide has been used as an epoxidising agent [185] with electron-deficient alkenes (Figure 8.55). 

Many epoxidising agents, notably mCPBA and f-BuOOH/VO(acac)2, are capable of bonding to the OH group of an allylic alcohol. Epoxidation then occurs more readily on allylic alcohols and on the same side of the alkene as the OH groups, rather in the style of the Simmons-Smith reaction. The stereochemical outcome is easiest to see in cyclic allylic alcohols. Thus the cyclohexenol 120 gives the syn epoxide 121 cleanly with mCPBA via the conformation 122 in which the axial OH delivers the reagent to the same face of the alkene. 

The message is that you should not treat a-hydroxy carbonyl compounds with base if they can enolise on the hydroxyl side. Because MoOPH is not an epoxidising agent, there is good chemo-selectivity if the molecule also contains alkenes. This steroid example 256 shows that good yields and high stereoselectivity can be obtained. 

Aziridines can also be prepared by addition of nitrenes to alkenes, or by the use of nitrogen-transfer agents analogons to epoxidising agents. ... 

The linear diblocks are then coupled by a polyfunctional coupling agent such as epoxidised linseed oil to give a star-shaped polymer. As already mentioned, commercial materials of this type have a tetramodal distribution. 

On the other hand, in the areas of Deep etch plates, the emphasis seems to be on the stability and etch resistance of the images (to withstand chemical agents) necessitate the use of chemically stable compositions (such as cyclized isoprene, epoxidised esters). Additional factors such as adhesion of the photoresist... 

The amoimt of plasticiser required can sometimes be reduced by replacing part of it with a different, cheaper softening agent, sometimes known as an extender. Extenders have similar effects to plasticisers, but are not capable of completely replacing them because of their inadequate compatibility. Examples include the chlorinated hydrocarbons, epoxidised soya bean oil and epoxidised linseed oil. These last two substances have already been described as plasticisers the boimdaiy between plasticisers and extenders is not sharp, becanse compatibiUty depends on the formulation. Note that the term extender is sometimes used for other low cost additives designed to bulk out the volume of a mix, such as cheap fillers. 

The company also makes PVC lubricants and modifiers (hquid and sohd ester lubricants, hydrocarbon waxes, polyacrylate elastomer impact modifiers, metal soaps, fatty acids and their derivatives) and plasticisers (epoxidised monoester and epoxidised soybean oil), together with acrylic process aids and amide wax antiblocking agents. 

Epoxides can be simply difunctional or polyfunctional. The most widely used version is the difunctional epoxy type diglycidyl ether of bisphenol A, with (n) from 0.2 to 12 (formula 1 in Figure 6.17) which can be used with different types of curing agents, (i.e., various amines). Epoxidised novolaks (formula 2 in Figure 6.17) have multi-... 

When substituted by an electron-withdrawing substituent on nitrogen, oxaziridines become quite powerful oxidising agents, capable of epoxidising alkenes. Furthermore, when incorporated into an... 

Among the additives used to modify the properties of polymeric packaging materials, plasticisers have raised much concern from the hygienic point of view. Butyl stearate, acetyltributyl citrate, alkyl sebacates and adipates are important because they are types of plasticisers that typically have low toxicities. Materials such as epoxidised soybean oil are widely used in polyvinyl chloride, polyvinylidene chloride and polystyrene as thermal stabilisers and lubricants at a level of 0.1-27%. Toxicity of epoxidised soybean oil is affected by the presence of oxirane, also known as ethylene oxide, which was upgraded to Group 1 as a carcinogenic agent to humans, based on mechanistic and other relevant data. 

---