Nitrobenzene cycloaddition

Following a report by Biichi and Ayer > on the photolysis of nitrobenzene in cyclohexene and 2-methyl-2-butene, a thorough study on nitrobenzene cycloaddition to olefines has been undertaken by de Mayo and co-workers 3 . ). Evidence was presented for the previously postulated ) formation of 1,3,2-dioxazolidines ... 

Polyimides have been synthesized by Diels-Alder cycloaddition of bismaleimides and substituted biscydopentadienones (81,82). The iatermediate tricychc ketone stmcture spontaneously expeU carbon monoxide to form dihydrophthalimide rings, which are readily oxidized to imides ia the presence of nitrobenzene. 

Whereas in all previously mentioned inverse cycloaddition reactions [h]-fused pyrido annelated systems are formed, some reactions are described which lead to [c]-pyridine annelated bicyclic systems. 5-(Butynylthio)pyrimidines (R = Ph, NHCOCH3) give on heating at 180°C in nitrobenzene 5-R-2,3-dihydrothieno[2,3-c]pyridines (89T803). 5-Propynyloxymethylpyrimidines also readily undergo cycloaddition into l,3-dihydrofuro[3,4-c]pyridines (89T5151) (Scheme 39). Considerable rate enhancements were observed with quaternized pyrimidinium salts. Whereas... 

Full details of the [3 + 2] photoaddition of nitrobenzene (296) to cyclohexene (297) to give the 1,3,2-dioxazole (298), stable at -70°C, have now been published.244 An analogous intramolecular cycloaddition has been proposed to account for the novel photocyclization of the nitrouracil 299 to the triazole 300 and the pathway is outlined in Scheme 9.245 A 2-azaallyt radical has been shown to be an intermediate in the photoaddition of benzo-... 

The reaction of 9 with nitrobenzene is probably initiated by a [2 + 3] cycloaddition to give the adduct 113, which can be identified at low temperature by NMR spectroscopy. On increasing the temperature, the Si—Si bond is cleaved to afford the rearranged five-membered ring product 114 (equation 24).116 The similarity to the formation of the ring system 100 is noteworthy. 

The dipolar cycloaddition of an alkyl azide with an alkene to form an aziridine has been exploited in the total synthesis of the alkaloid ( )-aspidospermidine <20050BC213>. Enone 353 was prepared in 11 steps from 3-ethoxycyclohexenone and coupled to 2-iodo nitrobenzene under Ullman cross-coupling conditions. The acetate group of 354 was hydrolyzed and the resulting alcohol converted to an azide using standard conditions in 75% overall yield. The cycloaddition of the azide with the enone was conducted in refluxing benzene for 3 days. The fused-ring aziridine 355 was the only product isolated. None of the initial dipolar cycloadduct triazoline was observed. The... 

This unique addition-transformation process has also been found in cycloaddition of acetylenic esters to dienes and azadienes containing the 5,6-double bond of uracil as part of the chromophore. Activated and polarized dienes of this type afford, with olefines, pyridol[2,3-Michael adducts, which are thermally converted into 8-(di-methylamino)theoph-ylline. 

Alkynes react with the excited states of aromatic nitro compounds (equation 98) , perhaps in part by way of cycloaddition to give an unstable dioxazole by analogy with the photoaddition of nitrobenzene to alkenes. However, [2 + 2] cycloaddition is an attractive alternative, since photochemical transformation of nitrones (33) to amides via oxaziridines is well documented. 

Arylideneanilines are converted into fused pyridines by heating with an alkyne and an oxidizing agent such as di-isopropyl peroxydicarbonate (DP) when the aniline is unsymmetrically substituted, a mixture of isomers may be formed [3173]. a-Halo ketones (and, less efficiently, simple aliphatic ketones [2639]) react with imines to give fused pyridines in variable yields [2715]. An imine formed by reaction of a 6-aminopyrimidinedione with DMFDMA undergoes cycloaddition with an electron-deficient alkene the first product is dehydrogenated in hot nitrobenzene [3620]. 

In the presence of nitrobenzene, a [3 + 2] cycloaddition takes place, forming stereoisomers 5,5-bis(trifluoromethyl)-2,3-diaryl-l,2,4-oxadiazol-3-in 12 and 3,3-bis(trifluoromethyl)-1,5-diaryl-1,2,4-oxadiazol-4-in 13 (77CB605) (Scheme 16). 

Evidence in favour of a two-step mechanism has also been given in the case of the cycloaddition of perfluorocyclopentadiene to cyclopentadiene Two products are formed, the most abundant from the addition having cyclo-pentadiene as the diene and perfluorocyclopentadiene as the dienophile, the other from the opposite. The ratio of the two products maintains practically constant (81-84% to 19-16%) on changing the temperature from 20° to 120°C and the solvent from n-hexane to nitrobenzene, although they are kinetically stable products. A common intermediate whose formation is rate-determining and which is able to collapse quickly to either product, seems required to explain these facts. It could be represented as... 

Cycloaddition between electron-rich 6-[(dimethylamino)methylene]amino-1,3-dimethyl uracil 242 and various electron-deficient substrates such as quinones 243 and coumarins 244 was considered to take place by elimination of dimethyla-mine from the respective cycloadducts, followed by oxidative aromatization in a highly regiospecific manner to give pyrido[2,3-solvent-free conditions, the times were reduced to 6-7 min and the yields were increased from 70-80% to 85-94% (04SL1179). 

CYCLOADDITION Aluminum chloride. Ceric ammonium nitrate. N,N-Diethyl-l-propy-nylamine. Dimethyl aoetylenedicarboxylate. N,N-Dimethyl-0-ethylphenylpropioi-amidium tetrafluoroborate. Ethylaluminum dichloride. Nitrobenzene. Oxygen, singlet. Sulfur dioxide. Trichloroacetyl isocyanate. Trimethylsilylbromoketene. 

Cycloaddition. Nitrobenzene can undergo cycloaddition to a strained alkene such as (Z,E)-l,5-cyclooctadiene (1) to form a 1,3,2-dioxazolidine (2). The reaction proceeds in THF at room temperature over a period of 5 days. The adducts are formed more rapidly and in higher yield when the benzene ring... 

The chiral holmium(III)-complex-catalysed Diels-Alder cycloaddition of siloxyvinylindoles (70) with e-deficient olefins (71) formed exo-substituted hydro-carbazoles (72) in up to 99% yield and 94% ee. Alkylation of these cycloadducts gave tricyclic compounds (73) with four continuous chiral centres (Scheme 20). The thermal 4-i-2-cycloaddition reaction of 7-substituted 4-styrylcoumarins with A-phenylmaleimide and tetracyanoethylene in nitrobenzene yielded 3,4-annulated coumarins. The thermal Diels-Alder cycloaddition of ( )-l,3-dihydro-3-phenacylidene-2//-indol-2-ones (74) with l,2-dihydro-2-oxospiro[3//-indole-3,2 -[2H,9a//-pyrido[2,l-fe][l,3]oxazines]] (75) produced complex dispirooxindoline fused [l,3]oxazines (76) with high regio- and stereo-selectivity (Scheme 21). " ... 

Other computational studies involving NHC-Cu species considered the formation of phenylisocyanates from nitrobenzene, and the development of [3-1-2] cycloaddition reactions for the formation of 1,2,3-triazoles. In the latter case the use of NHCs allowed the direct use of copper(I) catalysts, whereas copper(II) precursors were predominant before. With [(NHQCuBrj the reaction could be run in water and was successful even for internal alkynes, an unusual observation as the intermediacy of Cu-acetylides had previously been assumed. Calculations showed that the [(SIMes)Cu] fragment was ideally set up to bind internal alkynes in an p -fashion and hence activate them towards cycloaddition. With terminal alkynes the acetylide route may still be operative. 

Other computational studies involving NHC-Cu species considered the formation of phenylisocyanates from nitrobenzene, and the development of [3+2] cycloaddition reactions for the formation of 1,2,3-triazoles. In the latter case the use of NHCs allowed the direct use of copper(i) catalysts, whereas copper(ii) precursors were predominant before. With [(NHC)CuBr] the reaction could be run on water and was successful even for internal alkynes, an unusual observation because the intermediacy of Cu-acetylides had previously been assumed. Calculations showed that the [(SIMes)Cu] fragment was ideally set up to bind internal alkynes in an i] -fashion and hence activate them towards cycloaddition. With terminal alkynes the acetylide route may still be operative. Other computational studies on the catalytic activity of [(NHC)Cu] complexes in which the NHC has no particular role but to stabilize the metal by strong o-donation and offer steric protection have been reported, including the activation of CO2 by [(NHC)Cu(EPh3)] (E = Si, Ge, Sn) and the carboxylation of the C-H bond of heteroarenes. The barriers of the two steps of the catalytic cycle of the [(NHC)Cu ]-catalyzed hydrosilylation of ketones have been computed, yet it was shown that the nature of the NHC was not a controlling factor. While the barrier of the hydrocupration step is determined by the nature of the ketone, that of the o-bond metathesis step occurs mainly under electronic control. 

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