Piperidine hindered

Light Stabilizers Pigments (carbon black, iron oxides), UV absorbers (hydroxyphenones, benzotriazoles), excited-state quenchers (organic Ni complexes), free-radical scavengers Hindered amine light stabilizers [piperidines, hindered amine light stabilizers (HALS)]... 

Photostabilization processes have not received much attention in the last year other than a multitude of review and specialist industrial articles. These include stabilization of polypropylene fibres, new stabilizer developments, sorting of stabilizers for polyethylene, trends in UV absorbers, hindered piperidines (hindered amine stabilizers (HAS)) for polyacrylics and urethanes, new stabilizers for polyolefins, enhanced service life applications, stabilizer mas-terbatches for polyester, stabilizers for UV cured acrylic overcoats for PVC, ° stabilization of clearcoats and various commercial packages. ... 

There is only one report in the literature of a [3-1-3] cycloaddition involving TMM and activated aziridines to give the corresponding piperidine (124) [44]. The formation of the six-membered ring adduct is presumed to proceed via the ringopening of the aziridine by the attack of TMM complex (2) on the least hindered carbon, which is then followed by an intramolecular cyclization (Scheme 2.34). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

Irradiation of diisopropyl ketone under oxygen in the presence of the hindered piperidine II likewise results in formation of isobutyric acid, acetone and small amounts of isopropanol. At the same time the amine is quantitatively oxidized to the corresponding nitroxide I (Fig. 7, reaction (17)) ... 

In the literature the possibility of oxidation of hindered piperidines by hydroperoxides has occasionally been discussed-. In the light of our findings at RT, however, this could at the most occur with particular hydroperoxides such as hydroperoxide sequences or ot-keto, a-hydroxy and a-unsaturated hydroperoxides i 63 All these are quite likely to be present in oxidized polymers. 

Recent developments for reactive C02 sorbents include sterically hindered amines such as 2-amino-2-methyl-l-propanol (AMP) and 1,8-p-methanediamine (MDA) and 2-piperidine ethanol (PE), which are claimed to have good reversible C02 capacity (Veawab et al., 1998) and low-temperature molten salts called ionic liquids (Bates et al., 2002). Ionic liquids are attractive due to their negligible vapor pressure up to their decomposition at... 

There is good evidence that the a-forms (X, R = R = Me) have a cis and the 3 a trans 5,9-dimethyl configuration with respect to the hydroaromatic ring B (with the piperidine ring as reference, (a- is the trans and the cis isomer). The rates of quaternization of a- are faster than those of the -isomers, reaction in the latter being hindered by the 9-methyl group (cf. (XI) and (XII) in addition,... 

Imine intermediates can be trapped by an added nucleophile. However the only reactions of general preparative value are those in which a carbon-carbon bond is formed. In most other cases the product is unstable under the reaction conditions, reverting to the imine which reacts further. Reactions are best carried out in the flow through cell devised by Moinet and Raoult, illustrated in Figure 8.1 [87]. This cell permits total oxidation of tlie substrate in one pass through the porous anode, tlius exposing the product to further oxidation for only a short time, a-aminonitriles are obtained when cyanide ion is added to the electrolyte [88, 89]. In the case of piperidine ring oxidation, addition to the imine is from the less hindered... 

Some improvements occurred by changing to nitroxides with a hydrogen on at least one of the a-carbons of the piperidine ring, in contrast to TEMPO, which has no hydrogens on a-carbons. However, the major breakthrough came by using sterically hindered alicyclic nitroxides with a hydrogen on one of the a-carbons. f-Butyl 2-methyl-1-phenylpropyl nitroxide (LVIII) and f-butyl l-diethylphosphono-2,2-dimethylpropyl nitroxide (LIX) are examples of... 

NCA polymerization by secondary amines may involve the amine or activated monomer mechanisms or both mechanisms simultaneously. Unhindered secondary amines such as dimethylamine and piperidine react like primary amines, and polymerization occurs by the amine mechanism. Polymerization by slightly hindered amines such as diethylamine, N-methylbenzylamine, and di-n-propylamine involves a combination of the amine and activated monomer mechanisms. More hindered secondary amines, such as di-n-isopropylamine and dicyclohexylamine, react almost exclusively via the activated monomer mechanism. 

M. Dagonneau, E.S. Kagan, VI. Mikhailov, E.G. Rozantsev u. V.D. Sholle, Synthesis 1984, 895-916 . .Chemistry of Hindered Amines from the Piperidine Series". 

Chemical shifts indicate that N-methylsulfonyl groups in piperidines exist mainly in the equatorial orientation.179 In N-alkylpiperidine N-oxides, the N-oxide group shows a strong preference for the axial position.180 The planar or perpendicular orientation of the N—X=Y group in hindered piperidylamides and -amidines has been studied by 13C-NMR spectroscopy.180 ... 

Steric effects can alter the reactivity of a heterocyclic compound as compared to its aliphatic analogue. For example, piperidine is less sterically hindered and more strongly nucleophilic than diethylamine. Conformations of these compounds are discussed elsewhere (Section 2.2.4.3). 

If suitable substituents as sterically hindered piperidine groups are located on the grafted molecules, the synthesis of new polymeric stabilizers can be achieved. Such stabilizers can be expected to have a good efficiency in the protection of the parent polymer, especially in the case of thermal ageing. 

Polymers containing sterically hindered piperidine groups... 

Figure 8. ESR signal observed during peracetic acid trealment of synthetic polymer bearing hindered piperidine group.

The second additive acts most probably as a scavenger of free radicals. A class of compounds known as hindered piperidines is one of the most widely used for this purpose. Their chemical structure is shown in Figure 6.17. 

The hindered-amine light stabilizers for polymers are piperidine derivatives. An example of these products is bis-(2.2,6,6-tetramethyl-4-pipendinyl)sebaeale, tradenained Tinuviii 770, useful as a light stabilizer for polyolefins and styrenics. 

The reaction proceeds well with unhindered secondary amines as both nucleophiles and bases. The yield of allylic amine formed depends upon how easily palladium hydride elimination occurs from the intermediate. In cases such as the phenylation of 2,4-pentadienoic acid, elimination is very facile and no allylic amines are formed with secondary amine nucleophiles, while phenylation of isoprene in the presence of piperidine gives 29% phenylated diene and 69% phenylated allylic amine (equation 30).84 Arylation occurs at the least-substituted and least-hindered terminal diene carbon and the amine attacks the least-hindered terminal ir-allyl carbon. If one of the terminal ir-allyl carbons is substituted with two methyl groups, however, then amine substitution takes place at this carbon. The reasons for this unexpected result are not clear but perhaps the intermediate reacts in a a- rather than a ir-form and the tertiary center is more accessible to the nucleophile. Primary amines have been used in this reaction also, but yields are only low to moderate.85 A cyclic version occurs with o-iodoaniline and isoprene.85... 

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