Trapping dienes, unstable

Selective trapping of unstable dienes. This complex has the useful property of trapping the unstable diene tautomers of cycloocta,triene (2) and derivatives such as 5. The kinetics of complex formation with unstable dienes has been studied and arguments have been advanced to account for the selectivity observed. 

Benzo[c]furans (isobenzofurans) are very reactive but generally unstable dienes which are prepared in situ and trapped. The in ihu-generated isobenzo-furan 33 was trapped by cycloaddition reaction with bis(methyl (S)-lactyl) ester 34 to afford [32] optically active naphthols (Equation 2.12). The cycloaddition was carried out in the presence of a catalytic amount of glacial acetic acid and represents a facile one-pot procedure to synthesize substituted naphthols. 

Sulphines may react as dienophiles with 1,3-dienes with the formation of cyclic sulphoxides. Unstable 2,2-dichloro-5,6-dihydro-2ff-thiin-l-oxide 191 was formed in an exothermic reaction between 173aandcyclopentadieneat — 40 (equation 101). The simplest, parent sulphine, CH2 = S = O, prepared in situ by treatment of a-trimethylsilylmethanesulphinyl chloride with cesium fluoride, reacts with cyclopentadiene to give bicyclic, unsaturated sulphoxide 192 as a mixture of two diastereoisomers in a 9 1 ratio (equation 102). On the other hand, a,j8-unsaturated sulphine 193 (generated by thermolysis of 2-benzylidene-l-thiotetralone dimer S-oxide) in boiling toluene behaves as a 1,3-diene and was trapped by norborene forming sulphoxide 194 in 78% yield ° (equation 103). 

Since the corresponding endoperoxide precursors are all too unstable for isolation, the diimide reduction constitutes an important chemical structure confirmation of these elusive intermediates that are obtained in the singlet oxygenation of the respective 1,3-dienes. However, the aza-derivative 14 and the keto-derivative 15 could not be prepared,17> because the respective endoperoxides of the pyrroles 18) and cyclopentadienones suffered complex transformations even at —50 °C, so that the trapping by the diimide reagent was ineffective. 

Because of its great reactivity PTAD has found wide use in the interception of reactive, unstable dienes. For example, unstable isoindenes,226 3a//-indenes,146 1,3-divinylallenes,227 and benzene oxides228 have all been successfully trapped with PTAD. 4-(4-Bromophenyl)-l,2,4-triazole-3,5-dione (5, R = 4-Br—C6H4) is often used if the derivatives ai e required for X-ray structure determination.229 Azodicarboxylic esters have been used to trap tetra(trifluoromethyl)cyclobutadiene,230 and spiro[4.4]nonatetraene.231... 

Treatment of the azetidinone 139 with zinc chloride generates the highly unstable azetinone 140, which is trapped as the carbacephem 142 in the presence of the diene... 

Treatment of alkyl nitrites with arylsulfinic acids 194 generates the unstable nitroso compounds 195, which, in the presence of dienes, are trapped as cycloadducts 196 (equation 109)102. 

Pentafluorophenyllithium is thermally unstable and decomposes to generate tetrafluorobenzyne by / -elimination of lithium fluoride. The benzyne can be trapped by a variety of dienes to give the corresponding Diels-Alder adducts [193,196,197,206] (Scheme 71). 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Other selenocarbonyl cycloaddition products react similarly. Thermal cleavage of the dimeric 163 leads to unstable a,/3-unsaturated selones which can be trapped by a diene affording the selenin 164 (Equation 67) <1994CL2283>. 

Diels-Alder reactions have been used to a great extent to trap unstable cycloalkynes and to characterize angle strained cycloalkynes, the most popular diene being tetra-... 

In order to work out the mechanism of the reaction you might like to take these additional facts into account. Compound A is unstable and even at 20 °C isomerizes to B. If the diazo compound is decomposed in methanol containing a diene, compound A is trapped as an adduct. Account for all of these reactions. 

The unstable l,3-diazahepta-l,2-diene was trapped with diphenylcarbodiimide to give the [2+2] cyeloadduct in 74 % yield. The larger ring cyelic carbodiimides also afford the [2+2] cycloadducts 43 with diphenylcarbodiimide. 

Nakayama and co-workers have shown that 3, 4-di-/er/-butylthiophene 1-oxide can serve as trapping agents for unstable thioaldehydes and thioketones. Benzothiophenium salts 118 have been shown to undergo [4+2] cycloaddition with a number of dienes to give compounds of type 119<03JOC731>. 

Oxazoles are anodier class of heteroaromatic dienes which readily undergo Diels-Alder reactions with benzynes. For example, slow, simultaneous injection of solutions of triazole (483) and lead tetraacetate to a solution of oxazole (515) in CH2CI2 at 0 C afforded cycloadduct (516) in essentially quantitative yield. The latter is a convenient source for the unstable isobenzofiiran (517), which can be trapped by [4 + 2] cycloaddition to a variety of dienophiles, e.g. by -methylmaleimide (Scheme 121). 

The rDA cycloieversitHi of a protected butadiene adduct has been applied to the synthesis of unsaturated oxygen heterocycles. The very unstable 2,3-dihydro-2,3-bis(methylene)furan and its 4-methyl derivative have been generated by rDA reaction of 4,5,6,7-tetrahydrobenzofuran and its dimethylated derivative (equation 82). Characterization of these dienes was accomplished by trapping with various dienophiles. This method was extended to a two-step rDA synthesis of tetramethylenetetrahydrofuran as shown in equatirai (83). ... 

Thermolysis of acetates has been used to synthesize unstable bridgehead alkenes and dienes, which in some cases have been isolated by trapping at low temperatures. For example, the bicyclic acetate (129 equation 60) gave a mixture of bicyclo[3.3.1]non-l-ene (130) and its retro-Diels-Alder product (131) on pyrolysis at 375-4(X) and thermolysis of the 3-homoadamantyl acetate (132) gave a mixture of products whose formation may have been initiated by acetate elimination.Bridgehead dienes are believed to be involved in the formation of products firom the diacetate (133), and novel dienes were isolated from pyrolysis of acetate (134). ... 

Dichlorotricyclo[4.2.1.0 ]non-3-ene (20) is even more strained and rearranges on warming to room temperature. The main products were derived from reactions of the unstable bridgehead alkene, 6,9-dichlorobicyclo[4.2.1]nona-l(9),3-diene (21), which was formed as a transient intermediate. This species dimerized to give 22, or could be trapped as a cycloadduct 23 with furan. ... 

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