Cyclopropenes Diels-Alder reactions

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... 

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. 

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. 

CYCLOPROPENE A NEW SIMPLE SYNTHESIS AND ITS DIELS-ALDER REACTION WITH CYCLOPENTADIENE (Tricyclo[3.2.1.02>4]oct-6-ene, (1a,2a,4a,5a)-)... 

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). 

CYCLOPROPENE A NEW SYNTHESIS AND ITS DIELS-ALDER REACTION WITH CYCLOPENTADIENE. 

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). 

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. 

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-... 

DIELS-ALDER REACTIONS Bis(triphen-ylphosphine)dicyanonickel(O). 2-Chloro-acrylonitrile. 2-Chloroacrylyl chloride. Cyclopropene. Dichloromaleic anhydride. Dicyanoacetylene. Indanocyclone. Oxygen, singlet. 4-Phenyl-l,2,4-triazoline-3,5-dione. 

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. 

Thus photolysis of the tosylhydrazone sodium salt of 5//-dibenzo[a,c]cyclohepten-5-one (14) at — 60 °C in the presence of cyclopentadiene or furan with tetrahydrofuran as cosolvent gave the cyclopropene Diels-Alder adducts 17 and 18 in 73 and 47% yields, respectively. If the photolysis was stopped shortly after all the tosylhydrazone salt had decomposed, adduct 17 was the only isomer found in the cyclopentadiene reaction. In the formation of the furan adduct 18, the reaction was not so clean and a number of unidentified products were also formed. Unfortunately, adduct 18 is thermally unstable at the temperature necessary for thermal formation of carbene 15. No trace of adduct 18 was detected when the arylcarbene 19 was generated directly from its tosylhydrazone salt in the presence of furan. ... 

Treatment of vicinal-substituted dibromocyclopropanes with /ert-butyllithium gives cyclopropenes, which, if strained, can react with the lithium reagent, undergo dimerization, or participate in Diels-Alder reactions if a dienophile is present. 22,737 three types of reactivity are exhibited by l,6-dibromobicyclo[4.1.0]heptane when treated with er/-butyllithium. If the reaction is carried out in the presence of 1,3-diphenylisobenzofuran, cycloaddition predominates giving 6 in 71% yield. If the same reaction is carried out in the absence of the diene, however, l-tert-butylbicyclo[4.1.0]heptane (8) and the dimer of bicyclo[4.1.0]hept-l(6)-ene 7 are the most abundant products. 

The formation of cyclopropene (1) by a retro-Diels-Alder reaction occurred with low efficiency, and the product polymerized under the reaction conditions. -... 

The endo selectivity in many Diels-Alder reactions has been attributed to attractive secondary orbital interactions. In addition to the primary stabilizing HOMO-LUMO interactions, additional stabilizing interactions between the remaining parts of the diene and the dienophile are possible in the endo transition state (Figure 3). This secondary orbital interaction was originally proposed for substituents having tt orbitals, e.g. CN and CHO, but was later extended to substituents with 7r(CH2) type of orbitals, as encountered in cyclopropene- . ... 

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