Cyclopropene nucleophile addition/electrophile

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

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 double bond of cyclopropenes is sufficiently reactive that in many cases attack of either electrophiles or nucleophiles can occur. 1,2-Addition to cyclopropenes can create up to three new chiral centres ... 

The addition of an electrophile to the cyclopropene double bond formally leads to a cyclopropyl cation this may be expected to undergo ring opening to an allyl ion unless it is rapidly trapped by a nucleophile. In some cases, however, electrophilic attack may occur at one of the a-bonds, leading directly to an allylic cation. 

As noted earlier (Section I V.A.5), the addition of nucleophiles to cyclopropenes relieves ring strain. Electrocyclic opening of the anion generated in this manner competes with capture by an ambient electrophile (E" ) (equation 85). Cyclopropene 278 reacts with... 

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