Amines hindered

Polynitroaromatic compounds are reduced more easily than mono-nitro compounds. Additional nitro groups facilitate the reduction however, the reduced functional groups, hydroxylamine and amine, hinder the reduction of other nitro groups. At pH < 3 the reduction of dinitroaromatic compounds follows two four-electron steps ... 

Aryl benzyl ethers are cleaved more easily than are alkyl benzyl ethers. The presence of an amine hinders the hydrogenolysis of alkyl benzyl ethers but has no effect on the cleavage of aryl benzyl ethers. - Because of this the benzyl ether is selectively hydrogenolyzed in compounds containing both a benzyl amine and an aryl benzyl ether (Eqn. 20.24)55 but the benzyl amine is selectively cleaved in compounds having both a benzyl amine and an alkyl benzyl ether (Eqn. 20.25).53,54,56... 

Milch of our present understanding of the mechanisms of NCA polymerization Is summarized In several recent reviews ( ) In addition, several papers ( - ) and references cited therein provide more current Information. Although there Is agreement about the mechanism of NCA polymerizations that are initiated by primary amines (weak base mechanism), there Is considerable controvert concerning the mechanisms of NCA polymerizations that are Initiated by tertiary amines, hindered secondary amines or basic salts. Three mechanisms have been invoked to explain these reactions. These are the mechanism of Wleland (10-11),... 

Hindered Amines. Hindered amines are extremely effective in protecting polyolefins and other polymeric materials against photodegradation. They usually are classified as light stabilizers rather than antioxidants. 

Sodium dichlorisocyanurate N-Chloramines from amines Hindered N-chloramines... 

Additives that are typically effective as antioxidants and UV stabilisers include hindered phenols, aromatic amines, hindered amines (only those that have a tendency to form N-oxides), hydroxylamines, benzofuranones, divalent sulfur compounds, phosphorus(III) compounds (phosphines and phosphites, with phosphites most common) and multidentate metal ligands (such as ethylenediaminetetracetic acid). Polymers to be processed above 300 C are exceptions as there are no commercial antioxidants thermally stable enough to be processed at that temperature. 

Radical scavenging Nickel phenolates Hindered amines Hindered benzoates... 

Hindered amine (hindered amine light stabilizers, HALS)... 

The less hindered f/ans-olefins may be obtained by reduction with lithium or sodium metal in liquid ammonia or amine solvents (Birch reduction). This reagent, however, attacks most polar functional groups (except for carboxylic acids R.E.A. Dear, 1963 J. Fried, 1968), and their protection is necessary (see section 2.6). 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

Triflates of phenols are carbonylated to form aromatic esters by using PhjP[328]. The reaction is 500 times faster if dppp is used[329]. This reaction is a good preparative method for benzoates from phenols and naphthoates (473) from naphthols. Carbonylation of the bis-triflate of axially chiral 1,1 -binaphthyl-2,2 -diol (474) using dppp was claimed to give the monocarboxy-late 475(330]. However, the optically pure dicarboxylate 476 is obtained under similar conditions[331]. The use of 4.4 equiv. of a hindered amine (ethyldiisopropylamine) is crucial for the dicarbonylation. The use of more or less than 4.4 equiv. of the amine gives the monoester 475. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Many organic syntheses requHe the use of stericaHy hindered and less nucleophilic bases than //-butyUithium. Lithium diisopropylamide (LDA) and lithium hexamethyldisilazide (LHS) are often used (140—142). Both compounds are soluble in a wide variety of aprotic solvents. Presence of a Lewis base, most commonly tetrahydrofuran, is requHed for LDA solubdity in hydrocarbons. A 30% solution of LHS can be prepared in hexane. Although these compounds may be prepared by reaction of the amine with //-butyUithium in the approprite medium just prior to use, they are also available commercially in hydrocarbon or mixed hydrocarbon—THF solvents as 1.0—2.0 M solutions. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

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