Hindered amine base

These hindered amine bases have pKa 41 -44 and thus are strong enough to deprotonate all carbonyl compounds quantitatively (see following table). 

We have considered the addition of three radicals, Cl, H- and CH3, as models for bases. In the Ziegler-Natta reaction, Lewis bases involved are esters of aromatic acids and hindered amines. Bases are supposed either to interact with the support (MgCl2) or to coordinate with Ti [40, 41]. 

However, steric factors appear to play an important role in displacement reactions. Thus, for example, a sterically hindered amine base will not displace an unhindered potentially weaker base such as sulfide. 

No other adducts are formed, and the endo/exo diastereomeric ratio is essentially the same for all of these methods. Further, the existence of an acid catalyzed mechanism for cycloaddition can be explicitly excluded by using an excess of a hindered amine base (2,6-di-tert-butylpyridine, DTBP) in the aminium salt induced reaction and by examining the results of an authentic acid catalyzed reaction (using, for example, triflic acid). In the former case, the same endo and exo adducts are formed in virtually the same relative amounts, but in the latter case neither of these adducts is formed. It is worth noting that acid catalyzed reactions have indeed sometimes been observed under typical aminium salt conditions [70], but these have never been observed, nor would they be expected, under PET conditions. Finally, in the instance where cation radicals are generated by the aminium salt method, the intervention of substrate cation radicals can usually be verified by the addition of the reduced form of the catalyst, i.e., the neutral triarylamine, to the reaction mixture. 

In an attempt to further extend the foregoing methodology to include the preparation of o,p-epoxy sulfides, the condensation of chloromethyl p-tolyl sulfide and benzaldehyde using a protocol similar to that employed successfully in the sulfone case (Bu OK) gave only poor conversion to the desired epoxide. However, addition of the hindered amine base 1,4-diazabicyclo[2.2.2]octane (DABCO) results in the formation of the isomeric expoxides (29) in good yield (equation 9). In the case of benzaldehyde, the cis-epoxide predominates over the trans-epoxide (29b 82% versus 18%), while in the corresponding condensation with pivalaldehyde, the only epoxy sulfide obtained is the cis isomer (29c). Several attempts to effect an analogous reaction with ketones were unsuccessful formation of (30) (displacement-elimination) becomes the favored process (equation 10). 

Tridentate BINOL catalysts can be derived from titanium tetra(isopropoxide), BINOL ligands, and hindered amine bases. These catalysts have also been shown to provide good yields and ee s for the aldol reactions at low temperatures of 2-methoxypropene with several aldehydes to the P-hydroxyketones, but an acid workup of the products is required. 

For polyolefins, hindered amine-based on 2,2,6,6-tetramethylpiperidine derivatives are gaining importance as light stabilisers [14,17]. They are effective as antioxidants at low and moderate temperatures [18]. Hindered amine light stabilisers have a synergistic effect in combination with aliphatic phosphites under photo-oxidative conditions [19]. 

In the early 1970s, T. Mizoroki and R. F. Heck independently discovered that aryl, benzyl and styryl halides react with olefinic compounds and elevated temperature in the presence of a hindered amine base and a catalytic amount... 

Although peroxides have dominated as free radical initiators for CRPP production, a PP-based nitroxide (NOR), with the trade name Ciba Irgatec CR76, has recently been introduced by Ciba Specialty Chemicals. The use of these NOR free radical generators has been briefly described recently [18-19] and it has been pointed out that hindered amine-based NORs ean have a crucial... 

---