Cyclopropenes reactions with electrophiles

The similarity between the reactions of alkenes and cyclopropanes is further demonstrated by the reactions of electrophilic cyclopropanes and cyclopropenes with enamines. Cyclopropylcyanoester74, when treated with the pyrrolidine enamine of cyclohexanone, undergoes what would be a 1,2 cycloaddition in the analogous alkene case, but is actually a 1,3 cycloaddition here, to form adduct 75 (90). A similar reaction between the... 

Alkynes can be converted into cyclopropenes by inter- [587,1022,1052,1060-1062] or intramolecular [1070] cyclopropanation with electrophilic carbene complexes, Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result from rearrangement or other transformations of the cyclopropenes initially formed (cf. Section 4,1,3),... 

In the same manner, 3,3-ethylenedioxycyclopropcne was prepared from l-bromo-3-chloro-2,2-ethylenedioxypropanc, while 2-chloromethyl-2-halomethyl-l,3-dioxancs 2 w ere converted into the cyclopropenes 3 by reaction with potassium amide.Alternatively 2 (X = Cl) was converted directly into 1-sodiocyclopropene 4 by reaction with an excess of sodium amide in liquid ammonia.Trapping of 1-sodiocyclopropene 4 by other electrophiles is discussed in Section 4.2.1.1. 

The protons of the C5 methyl group are quite acidic and deprotonation gives an anion which can be reacted with electrophiles. Reaction with acetone, for example, gave the modified 3//-pyrazole 2. On photolysis this gave the cyclopropene 3, despite the fact that a )S-hydroxy carbene is involved, and this cyclopropene was used to make cA-chrysanthemic acid. °... 

The normal mode of reaction of cycloproparenes with electrophiles involves opening of the three-membered ring and leads to benzylic derivatives (see Section 3.B.3.). The bis(triiso-propylsilyl) group offers efficient protection of the cyclopropene moiety, so that electrophilic attack of the protected benzocyclopropene occurs at the aromatic ring. Reaction of 1,1-bis(triisopropylsilyl)benzocyclopropene with 67% nitric acid gave the 3-nitro derivative 1 in 58% yield. Electrophilic attack on the cycloproparene occurs at C3 and this is consistent with theoretical calculations. A variety of substituted derivatives of benzocyclopropene are available from the nitro compound (see Section 3.5.). 

Reactions with other Electrophiles. Reaction with trifluoroacetic acid leads to the slow formation of a ring-opened product trifluoroacetic anhydride provides a his(trifluoroacetoxy)cyclopropene [110], Triethyloxonium tetrafluoroborate ethylates the oxygen atom, giving an ethoxycyclopropenium salt [110] ... 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Since cobalt carbonyl anions react readily with a large variety of cyclopropenylcarbonyl chlorides to give selectively / -oxocyclobutenyl complexes (equation 198), it was suggested that reactions of cyclopropenylium cations and [Co(CO)4] involve direct electrophilic attack at the CO ligand rather than at cobalt . However, observation of ring expansion products in the -cyclopropen-3-yl rhenium complex (vide suggests that an... 

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) °. 

Cyclopropene and its deuterium-labelled derivatives can be obtained by the photo-decarbonylation of the corresponding furan at 254 nm but the method is of strictly limited value because of the photolability of many cyclopropenes (Section IV.B.2). West and his coworkers have shown that aryltrihalo- and diaryldihalocyclopropenes are available from classical Friedel-Crafts aromatic substitution reactions employing the cyclopropenyl cation as electrophile. Thus tetrachlorocyclopropene is converted to its derived cation which is then allowed to react with an aromatic compound. The exothermic reaction provides monoarylcyclopropene at low ( 0°C) temperature and the diaryl derivative at higher ( > 50° C) temperature (equation 26). In this way 2-phenyl-1,3,3-trichlorocyclopropene can be obtained in 58 % yield and the p-fluorophenyl analogue in... 

A number of cyclopropanes can be obtained by replacing one or several groups attached to a cyclopropane ring with other atoms or substituents. Many of these substitution reactions are two-step or multistep processes, which may involve base-induced generation of cyclopropyl anions that are quenched by an electrophile, formation under basic conditions of cyclopropenes which are trapped by a nucleophile, or generation of a cyclopropyl radical that undergoes subsequent reactions under neutral conditions. Formal substitution also takes place when cyclopropanes are converted to cyclopropylidenes which suffer l,n insertion, n > 3. Substitution reactions therefore cover a variety of compounds and a wide range of reaction conditions. 

The reaction of l-chloro-3,3-dimethyl-2-phenylcyclopropene with lithium metal resulted in lithium-halogen exchange and the derived cyclopropene 1 was trapped by a range of electrophiles giving substituted cyclopropenes 2. 

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