1.2.4- Triazoline-3,5-diones Diels-Alder reactions

The surfactant dienes 90 and 91 (Figure 4.3), analogs to commercially available sodium dodecyl sulfate (SDS) and dodecyl maltoside, react rapidly with highly hydrophilic and reactive triazoline dione 92 in water at 25 °C forming, quantitatively, the corresponding adducts. The Diels-Alder reactions with less potent dienophile 93 gave, similarly, quantitative yields in 0.5 h and 3 h with 90 and 91, respectively. 

An example of a /zctcro-Diels-Alder reaction in SC-CO2 is the cycloaddition of anthracene with 4-phenyl-1,2,4-triazoline-3,5-dione, carried out at 40 °C and at a pressures between 75 and 216 bar [86]. The rate constant increases with decreasing pressure and the highest reactivity was observed at the critical pressure. The value of the rate constant at the critical pressure was higher than that observed in liquid CHCI3 and MeCN at the same temperature. At higher pressures, the rate is slower than that in the polar solvents, which reflects the apolar nature of SC-CO2 as a solvent. 

Scheme 18 Separation of zingiberene from ginger oil using a selective and reversible Diels-Alder reaction with phenyl l,2,4-triazoline-3,5-dione (PTAD) [105]...

In common with other azodicarboxylic acid derivatives, the uses of 4-phenyl-l,2,4-triazoline-3,5-dione are many. It undergoes a Diels-Alder reaction with most dienes11-14 and is, in fact, the most reactive dienophile so far reported.15 16 As with the formation of all Diels-Alder adducts the reaction is reversible, and in the case of the adduct with 3-j3-acetoxy-17-cyano-5,14,16-androstatriene, the reverse reaction can be made to proceed under especially mild conditions.14 An instance has also been reported of the dione photochemically catalyzing other retro Diels-Alder reactions.17 Along with the proven use of azodicarboxylic ester,18-18 the dione should be potentially important in the preparation of strained ring compounds. 

Where the carbon-carbon double bond is a part of an aromatic system, in general, cyclopropanation of diazoketones results in the formation of unstable cyclopropane adducts. For example, Saba140 has shown that in the intramolecular cyclopropanation of diazoketone 57 the norcaradiene ketone 58 can be detected by low-temperature NMR and can be trapped in a Diels Alder reaction with 4-phenyl-l,2,4-triazoline-3,5-dione (equation 69). In addition, Wenkert and Liu have isolated the stable norcaradiene 60 from the rhodium catalysed decomposition of diazoketone 59 (equation 70)105. Cyclopropyl ketones derived from intramolecular cyclopropanation of hetereoaromatic diazoketones are also known and two representative examples are shown in equations 71 and 72106. Rhodium(II) compounds are the most suitable catalysts for the cyclopropanation of aromatic diazoketones. 

The aziridinofullerenes and azafulleroids obtained by Mattay et al. were subjected to further reactions to yield fullerene adducts of the second generation. As a model compound 4-methyl-l,2,4-triazoline-3,5-dione (NMTAD) 187, which is one of the most reactive dienophiles for Diels-Alder reactions, was selected. Sheridan et al. studied the photoinduced [4 + 2] cycloadditions of NMTAD to arenes [295], With alkenes, however, NMTAD reacts in photoinduced cycloadditions in a [2 + 2] fashion under formation of diazetidines [296], Both the sulfonyl-azafulleroids (185) and aziridinofullerene (186) were irradiated at 420 nm to give both the same [2 + 2] cycloadduct 186 (Scheme 71) [291], This indicates that at a certain stage rearrangements must be involved. 

Diels-Alder reaction of 93 with dimethyl acetylenedicarboxylate (DMAD), maleic anhydride, or 4-phenyl-l,2,4-triazoline-3,5-dione gives the tri- and polycyclic products 94-97 in 57-95% yield (Scheme 3) <2004S2665>. 

Ten years later, the synthesis of alkyl-substituted thiirene sulfoxide 140 was reported (Scheme 69), making use of a Diels-Alder reaction between thiirano-radialene sulfoxide 138 and the highly reactive dienophilic 4-substituted 1,2,4-triazoline-3,5-diones 139 (TAD) [132]. The fact that other classical reactive di-enophiles, including maleic anhydride and ethyl azodicarboxylate, do not react with sulfinyldiene 138, reveals its low reactivity. 

The mechanism of this type of reaction has received some attention. ° It has been suggested that these reactions are pericyclic processes, and since N°=N dienophiles have lower lying LUMOs than C=—C compounds, diey are more reactive. Cyclic N—N species have an even lower LUMO energy. However, a recent investigation of the cycloadditions of 4-phenyl-1,2,4-triazoline-3,S-dione (vide infra) indicates that in at least some cases the Diels-Alder reactions involve discrete intermediates. It would appear that a range of mechanisms may be available to azo dienophiles. 

Since their introducticm by Cookson et al. in 1962, 4-substituted l,2,4-triazoline-3,5-diones have been used extensively as dienophiles (equation 62). These dienophiles are highly reactive towards most 1,3-dienes and undergo cycloadditions under very mild conditions. In general, these Diels-Alder reactions are syn selective with respect to the diene, although with some (Z)-dienes this selectivity is lost. As noted above, it has been found that these cycloadditions are stepwise reactions in (Z)-dienes which cannot achieve an s-cis conformation. However, it is not clear if tdl cycloadditions of triazolinediones with 1,3-dienes proceed this way. 

DIELS-ALDER REACTIONS Bis(triphen-ylphosphine)dicyanonickel(O). 2-Chloro-acrylonitrilc. 2-Chloroacrylyl chloride. Cyclopropcnc. Dichloromaleic anhydride. Dicyanoacctylenc. Indanocyclone. Oxygen, singlet. 4-Phcnyl-l, 2,4-triazoline-3,5-dione. 

There have been only a few reports of reactions of this type including cycloaddition of dienes 157 with the powerful dienophile 4-phenyl-l,2,4-triazoline-3,5-dione <1996J(P1)2297> and stereoselective cycloaddition of the chiral nitrone 158 with a variety of dipolarophiles <2000JOC7000>. A rare example of intramolecular hetero-Diels-Alder reaction involving a 4-methylene-l,3-oxathiolan-5-one 3 -oxide is provided by the cycloaddition reaction of 159 to give 160 (Equation 42) <1998EJ02733>. 

Formal [27t + 2ff + 2homo-homo-Diels-Alder) reactions include the cycloaddition of 4-phenyl-l,2-4-triazoline-3,5-dione to benzvalene (7), which served as the key step in the Katz prismane synthesis, and TCNE to 2-exo-methyl-endb-tricyclo[3.2.1.0 ]oct-6-ene (55). ... 

DIELS-ALDER REACTION Aluminum chloride. Cupric fluoroborate. 1,3-Di-phenylnaphtho[2.3-c]furane. 4-Phenyl-l,2,4-triazoline-3,5-dione. 3-Sulfolene. 

Keana and co-workers have recently developed some sulfonated 4-aryltriazolinediones which are sufficiently stable to hydrolysis that they can be used for aqueous Diels-Alder reactions.623 Thus, the triazoline-dione sodium sulfonate shown in Eq. (20)... 

Other Addition Reactions.—A review on the applications of retro-Diels-Alder reactions has appeared. Reformation of the steroidal 5,7-dienes from the 4-phenyl-l,2,4-triazoline-3,5-dione adducts was achieved by heating with K2CO3 in DMSO or DME. This provides a very useful alternative to the previously reported L1A1H4 cleavage. A cation-radical mechanism was proposed... 

Diels-Alder reactions. This butadiene dimer reacts with tetracyanoethyl-ene and maleic anhydride to give only 1 1 adducts, (2) and (3), even under forcing conditions. However, it does give the diadduct (4) with the highly reactive 4-phenyl-l, 2,4-triazoline-3,5-dione. 

DIELS-ALDER REACTIONS Benzocyclopropene. 1,2-BisC3-tosyIethoxycarbonyl)-diazene. Cyclobutadiene iron tricarbonyl. 1,2-Dicyanocyclobutene. Diethyl ketomalonate. 1,3,4,6-Heptatetraene. 2-Hydroxy-5-oxo-5,6-dihydro-2/f-pyrone. 3-Hydroxy-2-pyrone. Isopropylidene isopropylidenemalonate. Lithium tetra-methylpiperidide. 4-Methoxy-5-acetoxymethyl-o-benzoquinone. 4-Methoxy-5-methyl-o-benzoquinone. frans-l-Methoxy-3-trimethyl-silyloxy-l,3-butadiene. Per-fluorotetramethylcyclopentadiene. 4-Phenyl-l, 2,4-triazoline-3,5-dione. Potassium f-butoxide. 

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