Sterically Hindered Amines HAS

Sterically hindered amines, used originally as light stabilizers, are being 

Schematic of the loss of mechanical properties of polypropylene stabilized with phenolic antioxidant and sterically hindered amine [543] 

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For color-critical applications requiring phenol-free stabilization synergistic mixtures of sterically hindered amines (HA (L)S) and a hydroxylamine-based stabilizer, used during melt processing, with or without an organophosphate or organo-phosphonite, can be used to avoid the undesired discoloration typically associated with the oxidation of the phenolic antioxidants into products with quinoid stmctures. ... 

The use of stabilizers based on the heavy metal nickel is imder discussion due to industry awareness, initiatives such as Responsible Care, and the availability of non-nicfcel-containing alternatives such as HA(L)S (seebelow). Furthermore, these materials are generally much less efficient than sterically hindered amines (HA(L)S) in terms of light stability and espedally of thermal stability. Effident photostabilization of polymers requires effident peroxide decomposition and radical scavenging. 

The reductive alkylation of DAP with acetone led to high conversions and selectivity to the dialkylated product over Al, Bl, and BS2 catalysts. The ASl catalyst, which typically has lower activity than the Al or Pt-based catalysts showed greater formation of heterocycles. These results indicate that a more active catalyst, a shorter reaction time, a higher operating temperature, or sterically hindered amines/ketones will help minimize the formation of the heterocycles. Similar high selectivities were obtained with DAP-MIBK reaction over BSl and BS2 catalysts with no heterocycles being formed. However, over Al, the undesired heterocyclic compound was over 15%. This indicates that the reaction between diamines and ketones has a significant potential to form heterocyclic compounds unless the interaction between these is kept to a minimum by the use of a continuous flow reactor as proposed by Speranza et al. (16) or by other methods. 

Three factors determine the effectiveness of antioxidants in polymers, namely (i) intrinsic molar activity (ii) substantivity in the polymer and (iii) solubility in the polymer. Multifunctional AOs combine multiple functions in one molecule. Sterically hindered amine stabilisers (HAS), such as Chimassorb 944, Tinuvin 622 and Tinuvin 783 are prime examples. 

A further effect of sterically hindered amines that could contribute to polymer stabilization has been described by Allan / McKellar31i-lft-41. They reported a conversion of a,3 to 3,Y unsaturated ketones in polyolefins and assumed that this played a role in the photooxidation of the polymers. The conversion... 

The group of Caddick has described the synthesis of functionalized sulfonamides by microwave-assisted displacement of pentafluorophenyl (PFP) sulfonate esters with amines (Scheme 6.159) [306]. Their ease of handling due to their higher crystallinity, along with their long shelf-life and their ability to react under aqueous reaction conditions, makes these an attractive alternative to sulfonyl chlorides. The microwave-assisted reaction of alkyl PFP esters with amines is a facile process which proceeds cleanly and in good yields with a number of different amines, including primary, secondary, and sterically hindered amines, and anilines. Optimum condi-... 

A variety of amides has been formed in moderate to high yields with aryl bromides or iodides as aryl precursors and single-mode microwave heating for 15 min at 150 °C (Eq. 11.15) [29]. Under these conditions, aliphatic, unhindered primary, and secondary amines reacted readily, whereas sterically hindered amines or amines of low nucleophilicity, e.g. anilines, afforded low yields and incomplete conversions. Among the homogeneous catalytic systems tested the most suitable for the use with aryl bromides was a 2 1 mixture of BINAP and Herrmann s palladacycle. 

The lithium perchlorate-catalysed aminolysis of styrene oxide has been investigated. Amines of low nucleophilicity, such as aromatic amines, give almost exclusively products of type 30, sterically hindered amines (diisopropylamine, dicyclohexylamine etc.) give... 

DR. STEPHEN NEUMANN (Eastman Kodak Co.) In your observation of the proton transfer rates between the metal base and the amine base, do you have any feeling for whether the slower transfer is unique to the metal bases or whether it has to do more with the bulk of the overall base participating in the proton transfer For instance, would a sterically hindered amine show rates similar to the metal bases. 

More recently, Polish chemists have reported a synthesis of both aryl and aliphatic secondary nitramines by treating amine substrates with ethyl magnesium bromide followed by reaction with n-butyl nitrate (Equation 5.8). This method, which uses nonpolar solvents like hexane or benzene, has been used to synthesize aliphatic secondary nitramines, and At-nitro-A-methylanilines which otherwise undergo facile Bamberger rearrangement in the presence of acid. The direct nitration of At-unsubstituted arylamines usually requires the presence of an electron-withdrawing group. Reactions are retarded and yields are low for sterically hindered amines. 

The simple alkylation of an amine with an alkyl halide can occasionally be used if the product is less nucleophilic than the starting material. This may be for electronic reasons glycine 6 can be made by alkylation of ammonia with 5 as it exists mostly as the zwitterion 7 which no longer has a nucleophilic nitrogen. It may be for steric reasons the alkylation of the a-bromoketone 8, mentioned at the end of chapter 7, with the sterically hindered amine 9 gives a good yield of the even more sterically hindered amine 10 and no quaternary salt is formed. If the reaction is a cyclisation (chapter 7) it may also work well. 

Here we report the use of a readily prepared polymer immobilised TEMPO as a catalyst for alcohol oxidations.15 It was derived from a commercially available oligomeric, sterically hindered amine, poly[[6-[(l,l,3,3-tetramethylbutyl)amino]-l,3,5-triazine-2,4-diyl] [2,2,6,6-teramethyl-4-piperidinyl)-imino]-1,6-hexane-diyl[(2,2,6,6-tetramethyl-4-piperidinylimino]], better known as Chimassorb 944 (MW 3000 see figure 3 for structure). This compound is used as an antioxidant and a light stabiliser for plastics. It contributes significantly to the long-term heat stability of polyolefins and has broad approval for use in polyolefin food packaging.16... 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

Exxon s Flexsorb process [669], [670] uses a sterically hindered amine as an effective promoter of a hot potash system. A lower energy requirement and less solution makeup than in other processes are claimed. So far it has been successfully used in several large plants. 

The use of less basic and sterically hindered amines [e.g., collidine or 2,6-di-tert-butyl-4-(dimethylamino)pyridine ], the avoidance of preactivation, and the use of less polar solvent systems are additional appropriate strategies for the suppression of racemization. Except for fragment condensations in solution and couplings involving racemization-prone derivatives, the addition of HOXt to the reaction mixture has no pronounced effect on a uronium salt mediated racemization/epimerization, in contrast to the striking effects of HOAt on epimerization caused by PfPyU and haloamidinium salts such as CIP or TFFH (Section 3.8.1.2.1). 

The hindered-amine stabilizers readily form nitroxyl compounds. Denisov (10,11) has found facile formation in polypropylene. Rozantsev (12,13) indicates that the mechanisms by which these sterically hindered amines act is via the generation of stable piperidinoxyl radicals, and he proposes their formation. 

The latest development in the field of light stabilizers for plastics is represented by sterically hindered amine-type light stabilizers (HALS). A typical such compound is bis-(2,2,6,6-tetramethyl-4-piperidyl)-sebacate (XXV). Since it does not absorb any light above 250 nm, it cannot be considered a UV absorber or a quencher of excited states. This has been confirmed in polypropylene through luminescence measurements. 

Surprisingly, high photo-stabilizing efficiency towards polyalkenes and polydienes is displayed by sterically hindered amines (HALS). It has been shown that the effect of HALS cannot be interpreted by mechanisms of quenching or absorption but by some chemical process of scavenging peroxyl or alkyl radicals. In this reaction, semistable nitroxyl radicals are regenerated periodically as seen in the following schemes (Schemes 12.12 and 12.13). 

The protection of polyamides against light has been an important objective. Clariant markets a sterically hindered amine, Nylostab S-EED, as a UV stabiliser and antioxidant for polyamides. 

This group has further developed this methodology in a sequential catalytic process for sterically hindered amine products (Figure 4.12) [27]. The reaction gave good yields, and showed reasonable substrate scope. 

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