Diels-Alder reaction with 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... 

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)-)... 

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. ... 

Cyclopropene A New Simple Synthesis and its Diels-Alder Reaction with Cyclopentadiene. 

Diels-Alder Reaction of Cyclopropene with Butadiene... 

This bond order deviation from an ideal transition state structure to an example of cyclopropene added to a benzo-fused heterocycle may now be applied. Before we exEunine bond order deviation from an ideal transition state, we can take a look at the sums of rings bond order in the transition state structures. To simplify this picture, we will focus only on exo transition state structures between cyclopropene and benzo[c]heterocycles. Previously, we mentioned that the Diels-Alder reaction with benzo[c]heterocycles as dienes is a HOMO controlled diene reaction, therefore, an electron rich (higher sum of bond order) transition state structure should be energetically preferred. If this is the case, the order of reactivity should be benzo[c]furan, benzo[c]thiophene, and then benzo[c]pyrrole, which is exactly the same as determined on the basis FMO energy change (Table 23). 

Elimination of the ring s x-orbital delocalization in five-membered heterocycles is a most efficient chemical manipulation used to synthesize highly reactive dienes for Diels-Alder reactions from heterocycles that would otherwise not readily participate in the Diels-Alder reaction. One way to accomplish this goal using heterocycles that contain sulfur atoms is through their oxidation to sulfone derivatives. We will demonstrate the usefulness of this approach by studying the reactivity of 1,3-thiazole 1,1-dioxide and 1,3,4-thiadiazole 1,1,-dioxide as dienes for Diels-Alder reactions with acetylene, ethylene, and cyclopropene. 

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. ... 

Further evidence for the intermediacy of cyclopropenes in the cycloheptatrienyli-dene-arylcarbene interconversion comes from the trapping of the cyclopropene by Diels-Alder reactions with cyclopentadiene, furan, butadiene, and tetracyclone (see also p. 97). 

Diphenylisobenzofuran (174) undergoes stereospecific Diels-Alder reactions with the cyclopropenes (175 R, R = H or Me) to form compounds (176)... 

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. 

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) °. 

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. 

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