2.5- Cyclohexadien-l-ones

Physical Properties. The stmctures and the boiling and melting points of several diaLkyl peroxides are Hsted in Table 2 a comprehensive Hst is given in the Hterature (66). The melting point of 4,4 -dioxybis[2,4,6-tris(/ i -butyl)-2,5-cyclohexadien-l-one] [1975-14-0] is 148—149°C. 

Fig. 4. Starting materials and iatermediates ia the TMHQ syatheses 2,3,6-trimethylpheaol [2416-94-6] (16), 2,3,5-trimethylquiaoae [935-92-2] (17), 2,4,6-trimethylpheaol [527-60-6] (18), and 4-hydroxy-2,4,6-trimethyl-2,5-cyclohexadien-l-one [2875052-9] (19).

Tribromophenol can be further brominated in buffered acetic acid to give 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one [20244-61-5] a useful hrominating agent (26). 

Great differences m product structures and distnbuaons are obtained dunng oxidation with lead dioxide or tetraacetate in different solvents and media [63, 64,65J Oxidation of pentafluorophenol with lead tetraacetate gives perfluoro-2,5-cyclohexadien-l-one in good yield [6 ] (equation 57)... 

Intramolecular dipolar azide-olefin cycloaddition of 723 took place upon heating in benzene to afford 724 (83JA3273). An alternative rearrangement process can take place upon photolysis of 724 to give 725. Mesylation of 4-(3-hydroxypropyl)-2,4,6-trimethyl-2,5-cyclohexadiene-l-one (78JA4618) and subsequent treatment with sodium azide in DMF afforded the respective azide 726 which underwent intramolecular cycloaddition to afford the triazoline 727 (83JOC2432). Irradiation of 727 gave the triazole derivative 728 (Scheme 126). 

In aromatic diazonium compounds containing an ionized hydroxyl group ( —O-) in the 2- or 4-position, it is necessary to consider delocalization of electrons and, therefore, two mesomeric structures (1.7a-1.7b) (see Sec. 4.2). This fact has implications for nomenclature compounds of this type are considered as quinone derivatives following IUPAC Rule C-815.3 (Exception) compounds of this class are called quinone diazides. As a specific compound 1.7a-1.7b is indexed in Chemical Abstracts as 4-diazo-2,5-cyclohexadien-l-one. If reference is made specifically to mesomeric structure 1.7b, however, it is called 4-diazoniophenolate. 

A. 2,i,l,()-Tetrabromo-2,5-cyclohexadien-l-one. A mixture of 66.2 g. (0.2 mole) of 2,4,6-tribromophenol (Note 1), 27.2 g. (0.2 mole) of sodium acetate trihydrate, and 400 ml. of glacial acetic acid is placed in a 1-1. Erlenmeyer flask and warmed until a clear solution is obtained. The temperature of the solution is approximately 70°. The solution is magnetically stirred and cooled to room temperature to produce a finely divided suspension of the phenol. A solution of 32 g. (0.2 mole) of bromine in 200 ml. of glacial acetic acid is added dropwise over 1 hour (Note 2). The resulting mixture is kept at room temperature for 30 minutes and is then poured onto 2 kg. of crushed ice. The yellow solid which separates is removed by suction filtration after the ice has melted, and the damp crystals are dissolved in the minimum quantity of warm chloroform (Note 3). The upper aqueous layer is removed by means of a pipet fitted with a suction bulb. The dienone crystallizes from the... 

Rabin, O. Vigalok, A. Milstein, D. A novel approach towards intermolecular stabilization of para-quinone methides. First complexation of the elusive, simplest quinone methide, 4-methylene-2,5-cyclohexadien-l-one. Chem. Eur. J. 2000, 6, 454-462. 

Li, S. M. Lundquist, K. Soubbotin, N. Stomberg, R. 2-Bromo-4-[2-bromo-( )-propy-lidene]-6-methoxy-2,5-cyclohexadien-l-one. Acta Crystallogr. 1995, 51, 2366-2369. 

DiazopbenoL See Benzenediazooxide, Vol 2, p B58-L and 4-Diazo-2,5-Cyclohexadien-l-one in this Vol Motion itroderi vs... 

Os(NH3)5(/n-cresol)]2+ exist primarily as the 2,5-cyclohexadien-l-one tautomers under equilibrium conditions in methanol (178). 

Further to its ability to perform allylic and benzylic oxidations,149 /-butylpcroxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields.150 o,o-Coupling dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

These a-silylamino- and a-silylamido-2,5-cyclohexadien-l-one have been submitted to SET photochemistry (see Sections IV.C.7 and VI.B.7) to synthesize functional hydroisoquinolines.301... 

Cyclohexadien-l-ones. This system (1 1) is effective for oxidation of 3,3-disubstituted-l,4-cyclohexadienes to 2,5-cyclohexadien-l-ones.1... 

Pentafluorophenol reacted with xenon difluoride in hydrogen fluoride or acetonitrile at -30 to +20 °C (Scheme 35) and the following products were observed perfluoro-2, 5-cyclohexadiene-l-one, 2-pentafluorophenoxy-perfluoro-3,5-cyclohexadiene-l-one, 4-pentafluorophenoxy-perfluoro-2,5-cyclohexadiene- 1-one and 2-pentafluorophenoxy-perfluoro-2,5-cyclohexadiene-l-one, the product distribution depending on the reaction conditions100,10. ... 

The 4-(3 -butenyl)-2,5-cyclohexadien-l-one derivative 8 functionalized with a chiral substituent group produced two types of polymorphic crystals, the a- and p-forms. Solid-state irradiation of the a-crystal caused [2 + 2] photocyclization to give 10 in 80% diastereomeric excess at 60% conversion (Scheme 1) [38]. In contrast, photolysis of the (3-crystals afforded almost complete reversal of the sense of diastereoselection resulting in the formation of 9 in 90% de at very low conversion. Photolysis of 8 in benzene solution resulted in low diastereodifferentiation. 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

The C-2 proton of benzo[/ ]furan 82 underwent regioselective metallation by treatment with -butyllithium to form 2-lithiated benzo[/ ]furan, which directly reacted with electrophiles, such as 1,4-cyclohexadienone to form 4-(ben-zo[/ ]furan-2-yl) -hydroxy-2,5-cyclohexadien-l-one in high yield, as shown in Equation (71) <2005TL7511>. 

Reactivity with Diazo Oxides and Azides. Diazo oxides, e.g. 2,6-di-f0rf-butyl-4-diazo-2,5-cyclohexadiene-l-one (184, R = terr-butyl) or 2,6-diphenyl-4-diazo-2,5-cyclohexadiene-l-one (184, R = phenyl) are thermolabile and photolabile. Car-benes 185 are formed from 184 as reactive intermediates. The process proceeds in a one-step mechanism in which the carbene species reacts in the singlet state [238], The attachement of 185 to saturated hydrocarbon chains proceeds via insertion in C—H bond during processing. LDPE and PP were modified in this way [239, 240]. Poly(oxymethylene) was modified similarly. 

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