Reducing agent, hindered

The basis to the chain breaking donor (CB—D) mechanism, which was the first antioxidant mechanism to be investigated, was laid down by the late 1940s [10-12]. Many reducing agents, e.g., hindered phenols and aromatic amines, which reduce the ROO to hydroperoxide in a CB—D step have already been empirically selected and used for rubbers and by this time also for the newer plastics industry (e.g., Table la, AO 1-8 and 9-12). The major mechanistic landmarks of the antioxi-... 

If we reduce the ester after completing the ffittig reaction, no protection of the aldehyde will be necessary. in practice, the hindered reducing agent (42) gives good results. 

The copper-catalyzed chiral reduction of -substituted ,/Tunsaturated lactones with PMHS and (S)-/ -Tol-BINAP in the presence of a hindered alcohol can be carried out in moderate to good yields with moderate ee values.599 The reaction is useful for both butenolides and pentenolides. Inferior results are realized with diphenylsilane as the reducing agent. Excellent results employing PMHS and the DTBM-SEGPHOS ligand are possible (Eq. 354).598... 

Stable hindered selenoketones222 are easily reduced to give selenolate ions with various reducing agents and oxidized by air to form diselenides (Scheme 39). 

A good diastereocontrol is obtained for the debromination of Reaction (4.14) and it is attributed to the bulky reducing agent, which approaches the radical intermediate from the less hindered face anti to the two vicinal substituents [35]. 

In ketones having a chiral cluster next to the carbonyl carbon reduction with lithium aluminum hydride gave one of the two possible diastereomers, erythro or threo, in larger proportions. The outcome of the reduction is determined by the approach of the reducing agent from the least hindered side (steric control of asymmetric induction) [828]. With lithium aluminum hydride as much as 80% of one diasteromer was obtained. This ratio is higher with more bulky reducing hydrides [837]. 

In the presence of excess monoalkylamine, carbonyl compounds in aqueous solution are in equilibrium with the corresponding imine. In most cases these imines cannot be isolated but they are reduced at a less negative potential than the carbonyl compound. Selective reduction of such equilibrium mixtures is a useful route to alkylamines from ketones in yields of 70-90%. The process fails with hindered ketones such as camphor and with bulky amines such as fert.-butyl amine. Overall the reaction has advantages of lower costs and simpler work-up compared to the use of cyanoborohydride reducing agents. In the electrochemical reaction, protonation of carbanion intermediates occurs from the more hindered side and where two isomeric products are fomied, the least hindered amine predominates [193]. 

Sterically Hindered Metalloporphyrins Capable of Direct Aerobic Oxygenation. The catalytic aerobic olefin epoxidation system of Quinn and Groves, (tetramesitylporphyrinato)Ru/02/olefin substrate, effects equations 3-6, that is, the direct oxygenation of substrate using O2 as the oxidant without consumption of reducing agent 14), The (tetramesitylporphyrinato)Ru complex sterically... 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

In the second step, achiral 9-borabicyclo[3.3.1]nonane (9-BBN) adds to the less hindered diastereotopic face of a-pinene to yield the chiral reducing agent Alpine-Borane. Aldehydes are rapidly reduced to alcohols. The reaction with deuterio-Alpine-Borane, which yields (R)-a-d-henzy alcohol in 98% enantiomeric excess ( ) reveals a very high degree of selectivity of the enantiotopic faces of the aldehyde group in a crowded transition state ... 

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