Hindered aromatic

Further substitution rarely occurs between the two groups in a meta-disubstituted compound because this site is too hindered. Aromatic rings with three adjacent substituents must therefore be prepared by some other route, usually by substitution of an ortho-disubstihrted compound. 

The Ag-silica-alumina material is furthermore suited to assist in sterically hindered aromatic brominations. As an example we converted 1,3,5-tri-t-butylbenzene into 2,4,6-tri-t-butylbromobenzene. When using an acidic zeolite and Br2 de-alkylation prevails and 3,5-di-t-butylbromobenzene is formed (ref. 29). 

Nucleophilic Additions to Carbonyl Compounds Saponification of hindered aromatic esters... 

The homolysis of the C—NO bond and nitroxide formation147 have been studied using a series of sterically hindered aromatic nitroso compounds such as pentamethyl nitrosobenzene, 2,3,5,6-tetramethylnitrosobenzene, 2,4,6-trimethylnitrosobenzene and... 

The electroreductive coupling of the hindered aromatic ketones (367) has been achieved in a DMF-BU4NBr-(Hg) system by the aid of CrCh or MnCl2 as the electrocatalyst (Scheme 134) [484, 485]. The reductive coupling proceeds at a less negative potential [ 1/2 —1.44 —1.53 V (SCE)] than the reduction of the ketone (—1.63 —2.01 V). In some cases, Mn electrocatalysts favor the reduction to the carbinol (369), whereas a Cr catalyst promotes the formation of the pinacol (368). 

Several studies have been made of the effect of added metal ions on the pinacol/alcohol ratio. Addition of antimony(m) chloride in catalytic amounts changes the product of the electrochemical reduction of acetophenone in acidic alcohol at a lead electrode from the pinacol in the absence of added metal salt to the secondary alcohol in its presence53. Antimony metal was suspected to be an intermediate in the reduction. Conversely, addition of Sm(in) chloride to DMF solutions of aromatic aldehydes and ketones54 and manganese(II) chloride to DMF solutions of hindered aromatic ketones55 results in selective formation of pinacols in excellent yields. When considering these results one should keep in mind the fact that aromatic ketones tend to form pinacols in DMF even in the absence of added metal ions1,29,45. 

Diamines may be introduced as short chain extenders. As the amine reacts rapidly (Eq. 2.23), hindered aromatic amines must often be used to allow a controllable reaction. Polyurethane ureas are formed. 

For lower conversions levels, where asphaltenic sulfur removal is not deep, refractoriness appears to be largely influenced by a large proportion of sterically hindered aromatic sulfur compounds. Substituted thiophenes, benzothiophines and di-benzothiophenes are representative compounds. 

Phosphites are known to act by a preventive mechanism, i.e. preventing the formation of initiating radicals from hydroperoxides by reducing the latter to alcohols, see reaction 4 [20]. In addition to their peroxidolytic activity (PD), sterically hindered aromatic phosphites, e.g. Ultranox U626, act also by chain breaking (CB) mechanism. These phosphites react with the propagating alkylperoxyl (ROO ), reactions 5, and alkoxyl (RO) radicals, reactions... 

Barclay, L. R. C. Sonawane, H. R. MacDonald, M. C. Sterically hindered aromatic compounds. III. Add-catalyzed reactions of 2,4,6-Iri-I-bulyl and 2-methyl-4,6-di-t-butylbenzyl alcohols and chlorides. Can. J. Chem. 1972, 50, 281-290. 

These amides are a-lithiated because no other deprotonation can compete. With less hindered aromatic N,A-dimethylamides, a-lithiation still takes precedence over ortholithiation but the organolithium is immediately acylated by another molecule of amide. For example, LiTMP a-lithiates 46 even though ortholithiation could compete, but the product rapidly forms 47.7 With less readily lithiated groups on nitrogen (such as ethyl or isopropyl) ortholithiation (see section 2.3) takes over as the main reaction pathway. 

Hindered aromatic thioesters may be lithiated in a similar way to hindered aromatic esters 25.29 Thioimidate 117 is lithiated to give an organolithium 118 which reacts with aldehydes and ketones to form thiiranes 119.86>87... 

Available co-reactants-. The high reactivity of the MDI types also makes available a larger number of co-reactants. For example, where hindered aromatic amines yield a given level of reactivity, a variety of glycols can give equivalent reactivity thus allowing more formulation versatility. 

A series of 16-alkyl and 16-aryl derivatives of endo-ethenotetrahy-drothebaines has been prepared(363) by the reaction of Grignard reagents or lithium alkyls with 225. Nucleophilic attack was from the least hindered (aromatic ring) side of the molecule to give 16a-alkyl- and aryl- products (229), all of which were, at best, weak analgesics. Some had reasonable antitussive actions. 

The reaction of sterically hindered aromatic diisocyanates with bases or phospholene oxide catalysts afford oligomeric carbodiimides having terminal isocyanate groups. If the catalytic conversion of 4,4 -diphenylmethane diisocyanate (MDI) is conducted in the... 

Using highly hindered aromatic diisocyanates, such as 6, with a phospholene oxide catalyst produces oligomeric carbodiimides 7 ... 

Oligomeric carbodiimides derived from sterically hindered aromatic diisocyanates are used as stabilizers for polyester based polymers because the carbodiimide groups in the oligomers, or in sterically hindered diisocyanates, mop up HCl, which catalyzes the depolymerization of polyesters. The carbodiimide stabilizers are commercially available under the tradename Stabaxol from Rhein Chemie, a subsidiary of Bayer. 

A new microwave-assisted protocol for the generation of diversely substituted 3,4-dihydropyrimidine-5-carboxylic acid esters 40 has been developed by Kappe and co-workers [88, 89] using trimethylsilyl chloride (TMSCl) as a mediator for the Biginelli MCR. This involved the reaction of S-ethyl acetothioacetate or ethyl acetoacetate, an aromatic aldehyde and (monosubstituted) urea or thiourea as building blocks. Also sterically hindered aromatic and heterocyclic aldehydes... 

The electrochemical reversibility of the M(VI)/M(V) couple for the complexes with sterically hindered ligands contrasts with the reported behavior of the [MoO(SPh)4] complex, which exhibited electrochemical irreversibility for the Mo(V)/Mo(VI) step but a reversible Mo(IV)/ Mo(V) couple. The sterically hindered aromatic substituent groups stabilize the molybdenum(VI) complex and decrease relative to the thiophenol derivative. The molybdenum(VI) species can also be isolated by chemical oxidation. [MoO(PFTP)4] was prepared by chemical redu-tion of [MoO(PFTP)4] . The presence of the electron-withdrawing substituents on the aromatic thiolate increases E ei relative to the thio-phenolate derivative. Evidently the properties of these last complexes are influenced primarily by the electron-withdrawing characteristics of the fluorine substituents rather than by steric factors (33). 

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