Diels-Alder reactions of cyclopropene

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

Diek-AUer reactions. The Diels-Alder reaction of cyclopropene with cyclopcnt-adicne has already been mentioned (I, 1040). In fact, cyclopropene is one of the most reactive dienophiles. It reacts with tropone (1) in methylene chloride at 0° to give the 1,4-addition product (2). Similarly, reaction with tropolone (3) affords the adduct (4). 

Diels-Alder Reaction of Cyclopropene with Butadiene... 

Sodupe, M., Rios, R., Branchadell, V., Nicholas, T., Oliva, A., Dannenberg, J. J., 1997b, A Theoretical Study of the Endo/Exo Selectivity of the Diels-Alder Reaction between Cyclopropene and Butadiene , J. Am. Chem. Soc., 119, 4232. 

The pressure-promoted [4 + 2]-cycloaddition of 2-pyrone 318 with cyclo-propenone ketal 377 (25 C, 6.2 kbar) affords a mixture of reaction products exo-adduct 378a, cycloheptatrienone ketal 379, and cycloheptatrienone 380 (resulting from Si02 hydrolysis of 379), each representing the product derived from the Diels-Alder reaction of 2-pyrone 318 with cyclopropen-one ketal 377 (86JA6695). The mdo-adduct 378b loses carbon dioxide upon depressurization, while the exo-adduct 378a is thermally stable. 3-... 

The Diels-Alder reaction of a cyclopropene with a diene has been extensively studied, and indeed the isolation of such products from the trapping of cyclopropenes which are too reactive to isolate has frequently been used to establish their presence. The mechanism with tetrahalo-cyclopropenes has been discussed nevertheless, the stereochemistry of such products has, until recently, been poorly defined. 

Wiberg reported the Diels-Alder reaction of butadiene and cyclopropene [53] and Baldwin estimated from the reaction between cyclopropene and 1-deuteriobutadiene at 0°C that 99.4% of the formed cycloadduct was the endo isomer [54], There are many suggestions which attempt to explain endo selectivity in Diels-Alder reactions (Alder s rule [55]), but none are firmly established. According to Woodward and Hoffmann [56], the preference is the result of favorable Secondary Orbital Interactions (SOI) or secondary orbital overlap [57-59] between the diene and dienophile in the corresponding transition state structure. One can also find an explanation for the reaction preference in the difference between primary overlap [60], volumes of activation [61], and the polarity of the transition states [62]. Secondary orbital overlap between the diene and the dienophile does not lead to bonds in the adduct, but primary orbital overlaps do. 

Diels-Alder reaction of the diene (668) and the cyclopropene (669), both generated in situ, affords compound (670), whose dehydrogenation and dehydrobromination produces the thermally labile benzo[c]thiophene (672) as a yellow solid (Scheme 135) <87TL4217>. The singlet biradical (673) also... 

Cyclopropene shows an endo selectivity in its Diels-Alder reactions with the tropylium ion, cyclopentene, and acenaphthene because of Ti-alkyl interactions. " Hoffmann calculations show that in the Diels-Alder reaction of butadiene and cyclo-propenone the two reactants do not approach each other in parallel planes. ... 

Cycloheptatrienes were isolated from reactions of 1,2,3-triphenylcyclopropane with cyclopentadienes. Thermolysis of the Diels-Alder adduct of cyclopropene... 

Simple Diels-Alder Additions.—The [4 + 2] and reverse-[4 + 2] cycloaddition reactions of species containing C=X bonds (X = P, As, Sb, Bi, Si, Ge, etc.) have been discussed and the evidence reviewed.The general chemical reactivities of cyclo-propenones and triafulvenes, including their behaviour in Diels-Alder reactions, have been broadly summarized. Kagi and Johnson have reported extensive studies on the Diels-Alder reactions between cyclopropene and various chloro-cyclopentadienes in which endo-adducts are obtained, selective dechlorination, hydrolysis and other reactions of these adducts, and H n.m.r. spectral correlations. 

Homoquinones are also prepared by a variety of methods that involve the free-radical reactions of propellanes with benzoquinones, ° the Michael addition of suUbxonium methyhde" and ethyl cyanoac-etate anion to naphthoquinone and of sulfonium methyhdes to benzoquinones, the photooxygenation of norcaradienes, the reaction of diethoxyphthalans with quinones, and the Diels-Alder reactions of siloxy- and alkoxy-substituted furans with cyclopropenes. ... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

According to recent quantum mechanical calculations, die importance of secondary orbital interactions, which have also been frequently used to explain die endo diastereoselectivity of Diels-Alder reactions, seems to be questionable and to be reserved for special cases like the addition of cyclopropene to various dienes. T. Karcher, W. Sicking, J. Sauer and R. Sustmann, Tetrahedron Lett., 33, 8027 (1992) R. Sustmann and W. Sicking, Tetrahedron, 48, 10293 (1992) Y. Apeloig and E. Matzner,./. Am. Chem. Soc., 117, 5375 (1995). 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

Sustmann and Binsch132 described a method which started from the same point, but invoked the zero-differential-overlap approximation on the other hand, it was not confined to jr-electrons and the perturbation energy was refined iteratively. Using a MINDO parameterization they then applied the method to Diels-Alder reactions, and were able to account for effects which cannot be explained in terms of simple jr-electron theory, such as the preference for endo addition of cyclopropene to cyclo-pentadiene. 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

This reaction is the first example of a Diels-Alder reaction with tropolone. The emh selectivity seems to be a general property of cyclopropene. ... 

Cyclopropenes, even unactivated ones, exhibit extraordinarily high reactivity in both electrophilic and nucleophilic addition reactions. They are also good dienophiles and react with a variety of conjugated dienes including acyclic 1,3-dienes, alicyclic 1,3-dienes, anthracenes and furans. An endo selectivity is usually observed. An alkyl or aryl substituent at the 3-position of cyclopropene sterically hinders the Diels-Alder addition and thus 3,3-dialkyl- and 3,3-diarylcyclopropenes exhibit a reduced dienophilicity (equation 131) . On the other hand, numerous Diels-Alder reactions have been reported for 3,3-dicyano- and 3,3-dihalocyclopropenes. The reactions of 3-monosubsti-tuted cyclopropenes with the diene take place stereoselectively from the less crowded side of the substrates (equation 132) °. 

Whilst the use of cyclopropenes as dienophiles in the Diels-Alder reaction is best known, the ring participates in a wide range of cycloaddition processes. 

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