Synthesis of benzenes

A -Amino- and A-substituted amino-pyrroles readily undergo Diels-Alder additions and add to activated alkynes at room temperature. The resulting azanorbornadienes extrude A-aminonitrenes and this forms the basis of an unusual synthesis of benzene derivatives (81S753,81TL3347). It has been found that ethyl/3-phenylsulfonylpropiolate (135) is a superior dienophile to DMAD (Scheme 50). 

Even more than [6 + 4] and [8 + 2] cycloaddition reactions, the [2 + 2 + 2] cycloaddition reactions require a very well preorganized orientation of the three multiple bonds with respect to each other. In most cases, this kind of cycloaddition reaction is catalyzed by transition metal complexes which preorientate and activate the reacting multiple bonds111,324. The rarity of thermal [2 + 2 + 2] cycloadditions, which are symmetry allowed and usually strongly exothermic, is due to unfavorable entropic factors. High temperatures are required to induce a reaction, as was demonstrated by Berthelot, who described the synthesis of benzene from acetylene in 1866325, and Ullman, who described the reaction between nor-bomadiene and maleic anhydride in 1958326. As a consequence of the limiting scope of this chapter, this section only describes those reactions in which two of the participating multiple bonds are within the same molecule. 

Benzannulation reactions of Fischer carbene complexes have proved a useful route for the synthesis of benzene rings fused to one furan ring and one additional heteroaromatic ring (Equation 96) <2005TL2211>. Thus, reaction of chromium carbene complex 168 with the conjugated dienyne systems produces benzofuran derivatives (Table 7). 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

Mandolesi, S.D., Vaillard, S.E., Podesta, J.C. and Rossi, R.A. (2002) Synthesis of benzene- and pyridinediboronic acids via organotin compounds. Organometallics,... 

Based on earlier studies on the total synthesis of benzene (2), which he called the keystone of the total aromatic edifice , Berthelot in 1867 carried out a remarkable experiment heating acetylene (1) - which he had prepared from the elements - in a bent bell-jar at a temperature where the glass began to soften , he noticed the formation of polymeric substances . When these were subjected to fractional distillation, benzene, styrene, and other aromatic hydrocarbons could be isolated, with 2 constituting approximately half of the product mixture (Scheme 1) [1]. 

The reaction is reversible, and at equilibrium the final products reflect the D/H ratio of the solution. A large excess of deuterium gives a product with all six of the benzene hydrogens replaced by deuterium. This reaction serves as a synthesis of benzene-dg (CgDg), a common NMR solvent. 

Synthesis of benzene, cyclohexadiene, along with other related six-membered rings via circular trimerization of three C = C and/or C = C bonds has long attracted attention of the synthetic chemists. In most cases, it is thermo-... 

Selective synthesis of benzene derivatives via partially intramolecular cyclic carbopalladation is considerably more complex than the corresponding all-intramolecular processes. As it is generally difficult to specify the cascade-initiation point in the Type Ha cyclization process, it would generally be the least selective path. A priori, the most favorable might be the intra-inter cascade cyclization process (Type lib), since both the point of initiation and the queuing order between the two alkynes is sharply differentiated by the fact that one is intramolecular, while the other is intermolecular. Still, in-... 

Arene oxides 42 and 49 were obtained as minor products by an unusual thermal isomerization of the photooxide of 3-diphenylisobenzofuran. Although the thermal isomerization reaction of 3-oxaquadricyclane has been used in the synthesis of benzene oxide 1, this route to benzene oxides was initially limited to specifically substituted derivatives owing to the unavailability of the unsubstituted 7-oxanorbornadiene. Thus most of the substituted arene oxides prepared by this route contained electron-withdrawing substituents X [X = —CF3 (77), —COaMe (76, 86, 88-92, 94), and —C02Et (93)] which increased their thermal stability. 

Synthesis of Benzene and Homologues.—There are other methods of preparing benzene and its homologues which show their relation to the aliphatic series. We have already dwelt upon the relationship between benzene and cyclo-hexane, showing that benzene is dehydrogenated cyclo-hexane. 

Synthesis of Mesitylene from Allylene.—The second proof of the symmetrical structure of mesitylene is its synthesis from allylene and also from acetone. The synthesis from allylene has already been spoken of in connection with the proofs of the structure of benzene (p. 478), and is exactly analogous to the synthesis of benzene by the polymerization of acetylene. 

An alternate method of converting sulfonyl chlorides to sulfonyl fluorides has been reported by Sigler et al. (1966) in the synthesis of benzene sulfonylfluoride. Benzenesulfonyl chloride (18 g, 0.1 mole) was dissolved in 100 ml of acetone and then mixed vigorously at room temperature for 4 hr with 100 ml of aqueous 4 M potassium fluoride. On evaporation of the acetone, the pale yellow liquid product separates. It can be dried after dissolution in methylene chloride over anhydrous MgS04 and then distilled as a colorless oil (b.p. 63-67°C/6-7 mm). 

Measurement Techniques. Light isotopes Concentrations of were determined radiometrically after synthesis of propane (13) in a proportional counter after about twenty-fold electrolytical enrichment of (14), The C content of large samples (. 5 g C) were determined by liquid scintillation counting after synthesis of benzene (15) fi-om CO2 which was extracted fi-om the water sample after acidification. The C content of small samples (<0.5 g C) were measured by accelerator mass spectrometry (AMS) W the Isotrace Laboratory of the University of Toronto. The C, and isotopic... 

Synthesis of benzene derivatives. An acetoxy-1,3-diene (conveniently generated by healing an < ,/3-imsa urated aldehyde or ketone with isopropenyl acetate,... 

In the case of benzene, one possible species absorbing in the region of 275 m/Lt is phenol. By measuring the absorption of phenol vapor in one atm. of air at 24 °C. in a Cary spectrophotometer, we found the absorption coefficient to be 4-5 X 103M 1 cm."1 at 275.7 m/ however, the absorption peak at 275.7 was very sharp, the absorption reaching zero by about 280 m/. We do not know how much the 50 fold increase in pressure used in the pulsed-radiolysis experiment would affect the width of the absorption, but it is very unlikely that phenol is the only absorbing species. In a recent study (10) on the synthesis of benzene oxide (A) in isooctane solution, this compound was found to have a maximum absorption at 271 m/x, and to be in equilibrium with oxepin (B), with a maximum at 305 m/x. 

Synthesis of benzene oxides, which are in equilibrium with oxepines, is one of the most valuable methods for their preparation. As an example of its potential, a rather simple three-step synthesis of 2,7-diphenyloxepin can be given (Scheme 19) <86JOC2784>. The equilibrium of the latter with the respective benzene oxide is shifted to the side of oxepin, but treatment of the product with trifluoro-acetic acid quantitatively gives 2,6-diphenylphenol. 

B.iv.b. Regioselective Synthesis of Benzene Derivatives via Pd-Catalyzed Cross-Coupling of 4-Halo-2-cyclobuten-l-ones. The Pd-catalyzed reaction of 4-chloro-2-cyclobuten-l-ones with alkenylzirconium derivatives or alkenyltins and heteroaryltins followed by thermolysis at 100 °C has regioselectively (—95%) produced multiply substituted benzenet" (Scheme 17) and heteroarene-fused benzene derivatives (Scheme 18), respectively. 

---