Sterically inhibited amines

The sterically inhibited amines (HALS) were the most important new development in the field of photoprotectants of the past 10 years. The low molecular types have now been supplemented by highly effective polymeric types. They are particularly effective with very oxidation-sensitive PPs and other polyolefins - even in products with minimum wall thicknesses (foils, fibers). The polymeric HALS types have proven highly resistant to extraction and migration. 

That this difference is not due to differing electron availability at the nitrogen atom in the two cases is confirmed by the fact that the two amines differ very little in their strengths as bases (cf p. 72) the uptake of a proton constituting very much less of a steric obstacle than the uptake of the relatively bulky BMe3. Esterification and ester hydrolysis are other reactions particularly susceptible to steric inhibition (cf. p. 241). 

One of the simplest demonstrations of the effect incarceration has on a guest s reactivity is the measurement of the basicity of included amine ligands. Solutions of pyridine in CDC13 may be shown by H NMR spectroscopy to be readily protonated by CF3C02D. An analogous reaction of the pyridine hemicarceplex of the open portal hemicarcerand 6.101 results in the pyridine remaining unprotonated. This means that incarcerated pyridine is a much weaker base than the free molecule. This difference is explained most reasonably by the fact that the host has only a very limited ability to solvate the pyridinium ion and will sterically inhibit the formation of pyridinium-trifluoroacetate contact ion pairs. 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

As surprising as it sounds, this aniline is about as basic as a tertiary aliphatic amine, except that the aromatic ring substituent is electron-withdrawing by induction, decreasing the basicity slightly. This phenomenon is called "steric inhibition of resonance". We will see more examples in future chapters. Also, it is the last topic in Appendix 2. (See another example in the solution to 19-37(0.)... 

Initially, H2 binding in M(CO)3(PR3)2(H2) seemed unique because the bulky phosphines sterically inhibited formation of a seven-coordinate dihydride via oxidative addition. However, complexes with very simple ligands such as amine, CO, and even water have now been identified, indicating electronic considerations are also important. The hundreds of H2 complexes synthesized after the initial discovery could not initially have been imagined, and it was difficult at first to know where to search for new ones. It would take over a year before others were identified, notably by Morris,... 

Sommelet-Hauser product 678. Small amounts of the para rearrangement product (679) and the direct displacement product (680) are also present. The ammonium salt precursor to the ylid (674) can generate either ylid (675 or 681), hut formation of 681 is sterically inhibited relative to 675. The Stevens product of 681 (amine 682) is more sterically crowded than 676 or 677. In polar aprotic solvents the major product is the Sommelet-Hauser product 678, hut in nonpolar solvents (hexane) the Stevens product 676 predominates. In DMSO, the enhanced nucleophilicity of the base causes the displacement product 680 (base attacks the methyl carbon and displaces the amine) to be formed in high yield. Alkoxide bases are not strong enough to generate the ylid from 674, and the displacement reaction (to give 680) dominates in those cases. 

With a catalytic amount of the palladium complex PdCl2(CH3CN)2 the presence of benzoquinone, anilines 37 react with methyl vinyl ketone or methyl acrylate to afford the enaminones 38 in moderate to good yields, Scheme 11. The reaction is sensitive to the amine substitution for example, benzylamine and A-unsubstituted anilines except o-bromoaniline failed to give the desired enaminones. Alkyl-substituted enones (methyl crotonate, cyclopentenone, cyclohexenone) presumably were sterically inhibited from coordination with the palladium because they also failed to undergo the reaction (81JOC2561). 

In so-called thermooxidative degradation, primary radicals are produced in the presence of oxygen due to the effects of heat, possibly coupled with mechanical stressing of the plastic. Certain substances like sterically inhibited phenols or amines can trap (scavenge) these radicals. For example, di-tert-butylphenols act as radical scavengers (antioxidants). The reaction scheme described below makes it clear that the oxidation inhibitor is consumed in the process. Thermooxidation can therefore be delayed until the radical scavenger is completely used up. 

Typical representatives of primary antioxidants are sterically inhibited phenols and aromatic amines. The reduction of hydroperoxide to alcohol... 

It is certain that the effect does not arise simply through a modification of the resonance in the amine itself caused by the presence of an alkyl group. Such steric inhibition of resonance would in fact tend to increase the base strength, as it would increase the energy of the base relative to that of the ion. The decrease in pK, would suggest, therefore, a steric promotion of resonance rather than its inhibition, but there is no evidence of this. Other physical properties such as the ultraviolet absorption spectra and molecular refractions of the compounds concerned indicate little or no change in the meso-meric effect of the amino group produced by the presence of ortho substituents. 

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