Chloranil, oxidative coupling

We reported the first total synthesis of l,T-bis(2-hydroxy-3-methylcarbazole) (213) using our molybdenum-mediated construction of the carbazole framework (560). The required monomer, 2-hydroxy-3-methylcarbazole (52), was obtained in three steps, and 22% overall yield, starting from dicarbonyl(ri -cyclohexa-l,3-diene)(ri -cyclopentadienyl)molybdenum hexafluorophosphate (663) and 3-meth-oxy-4-methylaniline (655) (560) (see Scheme 5.52). Finally, oxidative coupling of the monomer 52 using p-chloranil afforded l,T-bis(2-hydroxy-3-methylcarbazole) (213) in 38% yield (560) (Scheme 5.167). 

The reaction of tripyrranes with pyrrolecarboxaldehyde in the presence of TFA catalyst followed by oxidation with chloranil results in a simultaneous oxidative coupling and condensation to generate a meso-free corrole <02OL4233>. A fluorescence receptor based on triaza-18-crown-6 ether combined with two /V-guanidinium groups (and one A-anthracen-9-ylmethyl moiety) could bind several biologically important amino acids in aqueous methanol... 

Scheme 2-79. Preparation of tertiary amines via oxidative coupling of amidocuprales mediated by chloranil (201).

A totally different route based on dehydrogenation of a saturated polymer precursor was introduced by Francois et al. [49] (Scheme 2.9). The method is based on anionic copolymerization of cyclohexadiene with styrene, followed by oxidation with chloranil. Due to possible coupling of two styrene (or two cyclohexadiene) molecules, a block copolymer, containing oligo(phenylene vinylene) units separated by oligo(phenylacetylene) and oligo(phenylene) blocks, is obtained. To the best of our knowledge, it was, so far, used only in the synthesis of phenyl-substituted PPV 10. 

There have been some studies of the diazo coupling and carbonyl reactions with imi-dazo[2,1 -b ]thiazol-3 (2//)-ones (67CHE706), 2,3 -dihydrothiazolo[4,3-c ]-s-triazol-5-ones (76ZN(B)853) and 2,3-dihydroimidazo[2,l-6]thiazol-3(2//)-ones (81ZN(B)50l). Oxidation of 2,3-dihydrothieno[3,2-fc]pyrrole with chloranil to thieno[3,2-fc]pyrrole has been reported (72CHE1428). 

As shown in Scheme 23, solid-phase-bound Fmoc-protected amino acids 82 are deprotected and coupled with 4-fluoro-3-nitrobenzoic acid. The acid moiety of 83 is converted to the amide 84, and reduction of the nitro group with SnCU 2112O initiates cyclative cleavage to 85 after aqueous work-up. This synthesis gives two points of diversification, which can be increased by an alkylation step in solution to give 86. Additionally, the authors oxidized the heterocychc core 85 with p-chloranil to give the corresponding quinoxalinones (not shovm) [38]. 

Itami and coworkers established that pyrene can be selectively coupled with an arylboronic acid in an intermolecular fashion in the presence of a Pd-catalyst and stoichiometric amounts of chloranil as an oxidant [73]. With suitable coupling partners small graphene-type structures are accessible, which can be planarized using cyclodehydrogenation conditions (Scheme 28). 

In 2011, Itami and Studer [183] developed a palladium-catalyzed C4-selective arylation of thiophenes and thiazoles with arylboronic acids. Although they had already reported the C4(/J)-selective arylation of thiophenes with aryl iodides [88] (Scheme 17.18), this C-H/C-B coupling method [using a Pd"/hipy or phen/TEMPO ((2,2,6,6-tetramethylpiperidin-l-yl)oxyl) catalyst system] enabled the use of thiazoles as aryl nucleophiles. They also applied this coupling reaction to the synthesis of the core structure of SCH-785532, which is known as a BACE inhibitor. In the same year, Itami [184] reported a direct arylation of a PAH with arylboronic acids to generate a 7t-expanded PAH. Treatment of pyrene 142 with arylboroxine 143 in the presence of Pd(OAc)2 and o-chloranil as an oxidant, followed by cydiza-tion under stoichiometric FeClj, produced PAH 144. Key to the unprecedented C-H arylation was a notable combination of Pd" and o-chloranil. 

With Fe2(CO)9 as catalyst, the CDC reaction of saturated heterocycles with 1,3-diketones was accomplished using TBP as an eflhcient oxidant (Scheme 2.12) [48]. This protocol shows good compatibility to cyclic and acyclic ethers, thioe-thers, and tertiary amines. Gratifyingly, besides C(sp )-C(sp ) coupling, the oxidative C-N coupling of ethers with azoles also works well (Scheme 2.12) [49]. As a update, with 2-chloranil (tetrachloro-l,2-benzoquinone) as oxidant, benzyl thioethers can be employed as substrates under metal-free conditions [50]. Notably, 2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoro borate is also an effective oxidant for metal-free CDC reaction of isochromanes and carbonyl compounds [51, 52]. 

Riordan and Stammer (335, 336) employed o-chloranil to oxidize saturated A -oxazolinones, and obtained dioxinones. Treatment of the latter with nucleophiles such as CH3O", PhNH2 or PhCH2SH afforded a-substituted amino acid derivatives, possibly by addition to the acylimine in equilibrium with the quinone adduct. Use of a non-nucleophilic base coupled with scavenging of the phenolic hydroxy groups by etherification made possible the isolation of acylenamino acid derivatives. 

The arylation of carbon-hydrogen bonds in polycyclic aromatic hydrocarbons, using aryl boron compoimds or aryl silanes, may be achieved with a palladium acetate/o-chloranil catalyst. The Suzuki-Miyaura reaction involves palladium-catalysed coupling of an arylboronic acid with an aryl hahde in the presence of base. After oxidative addition of palladium to the hahde, reaction with base may form intermediates such as (105). Transmetalation with the boronic acid followed by reductive elimination yields... 

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