Tert ortho-arylation

As described above regarding the behavior of aromatic carboxyhc acids, tert-benzyl alcohols are also unique substrates, which undergo not only hydroxy-directed ortho-arylation via C—H bond cleavage, but also ipso-arylation via C—C bond cleavage involving P-carbon elimination with the hberation of a ketone molecule (Scheme 4.84) [88]. 

There are some known unsuccessful attempts to carry out alkylation (Mel, Me2S04), halogenation (tert-butyl hypochloride) and nitration of aromatic dihydrobenzodiazepines [7, 105]. Such attempts only resulted in the destruction of the seven-membered heterocycle. As a rule, these destructive processes are typical of dihydrodiazepine systems and often manifest themselves during the synthesis and study of these compounds. Therefore, the results of the destruction of a seven-membered heterocycle are most widespread and include its decomposition into ortho-diamine and carbonyl compounds (Scheme 4.43, reactions A and B) [105, 106] and benzimidazole rearrangement accompanied by splitting out of a methyl aryl ketone molecule (Scheme 4.43, reaction C) [117]. 

Fluorinated aromatic substrates have been used in the synthesis of fluorinated biaryl derivatives via S l reactions650. Substrates YC6H4Br (Y = F, CF3 or OCF3) were treated with the anions from 2,4-di-ter/-butylphenol, 2,6-di-tert-butylphenol, / ara-methoxyphenol, / 0ra-(trifluoromethoxy)phenol and 2-naphthol leading to the biaryls YC6H4—ArOH by C-arylation at the carbon atom ortho to the deprotonated hydroxyl group (C-l in 2-naphthol), but at the para carbon atom in 2,6-di-terr-butylphenol. 

Most recently, Monteiro et al. have reported that cyclopalladated compounds derived from the ortho-metalation of benzylic tert-butyl thioethers are excellent catalyst precursors for the Suzuki cross-coupling reaction of aryl bromides and chlorides with phenylboronic acid under mild reaction conditions. A broad range of substrates and functional groups are tolerated in this protocol, and high catalytic activity is attained (Eq. (58)) [93]. 

Curran [12] showed in 1994 that maleimides bearing ortho-substituted aryl groups could react diastereoselectively, favouring a single atropi-somer of the product (Curran termed these reactions atroposelective ). Racemic maleimide 8 underwent radical reactions and cycloadditions from the face unshielded by the tert-butyl group, as shown in Scheme 4. 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

A number of di- or trimerization reactions of aryl and vinyl halides, which are mechanistically related to those in Eqs. 37 and 38 [7], have been reported. Interestingly, in the reaction of o-tert-butyliodobenzene,one of the aliphatic C-H bonds in the ortho substituent is intramolecularly activated and the successive reaction with another iodide molecule leads to a benzocyclobutene derivative (Eq. 41) [112]. A monomeric benzocyclobutene is produced selectively in the case of ethyl 2-(2-bromophenyl)-2-methylpropionate (Eq. 42) [113]. 2-(2-Bro-mophenyl)-2-ethylbutanoate undergoes an intramolecular redox reaction, that is, dehydrobromination-unsaturation (Eq. 43). The reaction has been proposed to proceed via C-H bond activation at one of the terminal methyl groups and P-hydrogen elimination. 

When a monosubstituted benzene is attacked by aryl radical, a mixture of all three isomeric biphenyls is produced, as well as in the GBH reaction [105-107]. The nature of substituents apparently does not play any important role except those of bulky ort/jo-groups. The ortAo-substituted biaryl is always the main product followed by para- and meto-arylation product. The presence of a bulky ortho-tert-huXy group significantly decreases the amount of ort/io-arylation product, indicating the strong ort/io-steric effect. The typical isomer distribution and the yields of biaryls in the arylation of four mono-substituted benzenes with dibenzoyl peroxide are given in the Table 4. 

Stabilizers UVA 2-(2H-benzotriazol-2-yl)-p-cresol phenol, 2-(5-chloro-2H-benzotriazole-2-yl)-6-(1,1 -dimethylethyl)-4-methyl- 2-(2H-benzotriazol-2-yl)-4,6-bis(1 -methyl-1 -phenylethyl)phenol isopropenyl ethinyl trimethyl piperidol (cellulose diacetate), biphenyl cellulose (UV absorber fro paper), phenylbenzimid-azole (reactive stabilizer for application in cellulosic textiles) Optical brighteners 2,2 -(2,5-thiophenediyl)bis(5-tert-butyl-benzoxazole) Mixtures an ortho-hydroxy tris-aryl-s-triazine compound+hindered hydroxybenzoate compound+hindered amine compound containing a 2,2,6,6-tetraalkylpiperidine or 2,2,6,6-tetraalkylpiperazinone radical ... 

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