Benzene, additional reactions ring isomerization

Substitution on benzene raises no regiochemical issues because every hydrogen is equivalent, but as soon as there is one substituent on the ring, isomeric products can result— namely, ortho, meta, and para. The reactivities of the different sites on substituted aromatic rings are quantified by what are known as partial rate factors (where n = o, m, or p for ortho, meta, or para, respectively, and R = substituent). These numbers reflect the rate constants (k ) for reaction of the individual ortho, meta, or para sites with an electrophile compared to the rate constant (k) for addition to benzene itself (Eqs. 10.113 A, B, and C). The rate constants for ortho and meta are divided by two because there are two ortho and meta hydrogens, and the rate constant for benzene is divided by six due to the six hydrogens. 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

Chlorination of 2,1,3-benzothiadiazoles can take place either by addition or substitution, depending on the reaction conditions, the nature of the substituents in the benzene ring, and the catalyst employed. The uncatalyzed reaction of (1) with chlorine is exothermic and produces an isomeric mixture of tetrachloro addition products, which form 4,7-dichloro-2,l,3-benzothiadiazole on treatment with base <70RCR923>. In the presence of an iron catalyst, chlorine substitution in the 4,7-positions predominates. 

The ozonide 287 resulting from a singlet oxygen reaction with 288 in methanol is converted to 289 by transannular epoxidation of the benzene ring. This oxide undergoes cis-trans isomerization to 290, and subsequent intramolecular Diels-Alder addition finally yields the novel product 291.166... 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

The spectra of the intermediate transients formed in the reaction of OH with dichloro- and dibromo-benzenes and chloro- and bromo-toluenes exhibited absorption maxima around 325-330 nm with both ortho- and w to-isomers of dichlorobenzenes, dibromobenzenes, and bromotoluenes (Fig. 6). The transients were assigned to the isomeric OH adducts formed from the addition of OH to the benzene ring. A blue shift was observed in the absorption maximum of -bromotoluene (315 nm) when compared to its ortho- and meta-isomers which have maxima at 330 nm. Such a behavior was also seen in the absorption spectra of OH adducts of 0- and w-chlorotoluenes (325-330 nm) and w-xylenes (326-328 nm) as compared to their... 

Isomerization and the formation of addition products with alkenes are the most noted reactions of the benzene ring (16). Alkylbenzenes such as toluene form fairly stable epoxides upon the addition of hydroxyl radical, which are potential toxic and mutagenic compounds (17). The subsequent reactions of the epoxide could lead to the formation of epoxy carbonyls, which can react further with OH radicals or ozone until smaller molecules are formed. 

The main reaction in the conversion of toluene and ethylbenzene is hydrogenolysis of the side chain producing benzene from the former and a mixture of benzene and toluene from the latter. However, in addition hydrogenation of the benzene nucleus, isomerization of the six- to the five-membered ring, hydrogenolysis of the latter, and isomerization of the alkanes formed takes place. 

Among the reactions of 4i/-pyran, a few ring transformations are of synthetic interest. For instance, the triphenyl-4-benzyl-47/-pyran 1 isomerizes on irradiation to give 2-benzyl-2//-pyran 2 which, on addition of HCI, is converted quantitatively into 1,2,3,5-tetraphenylbenzene 4, The transformation into the benzene system probably occurs by an electrocyclic O/C-2 opening of compound 2 to give the dienone 3, followed by intramolecular aldol condensation ... 

---