Cyclopropenes—

With Nudeophiles.—Cyclopropanes and Cyclopropenes. The use of ethoxycarbonyl-cyclopropyltriphenylphosphonium tetrafluoroborate in synthesis has continued, and its reaction with carboxylate anion now provides a new route to 2,3-dihydrofurans. The reaction proceeds with ring cleavage, giving (205), which subsequently under- 

Attempted dehydrochlorination of (206) by potassium t-butoxide does not generate the expected spirene, but affords (209) instead. Cyclopropenes, a-elimination, and simple substitution have been excluded from the process, and the route is believed to involve (207), reacting from the preferred conformation by anti- and syn-elimination (see also p. 25), or (208). The cyclopentadienyl anion has also been implicated in the nucleophilic addition of 1,2,4-triazines to dimethyl 4,5,6,7-tetrachlorospiro[2,4]-hepta-1,4,6-triene-1,2-dicarboxylate.  

Danishefsky and co-workers have continued their investigations on the nucleophilic cleavage of activated cyclopropanes. Alkyl-substituted cyclopropane diesters (210) show a preferance for cleavage of the C-1—C-2 bond, with the alkyl 

The cleavage of (212 R = COMe or COPh) by lithium dialkylcuprates has been developed into a preparative route for the synthesis of y6-unsaturated ketones in yields of 60— 98 %. The cleavage of cyclopropyl ketones by lithium dialkylcuprates can also be effected by aluminium trialkyls, with nickel acetylacetonate as catalyst.  

Ohkata, Y. Kubo, A. Tamura, and T. Hanafusa, Chem. Letters, 1975, 859. 

The base-catalysed cis-trans isomerization of l-aryl-2-phenylcyclopropanes has been subjected to kinetic analysis and a complexed anion, e.g. (190), is thought to be produced initially. Configurational inversion of the cyclopropyl sulphoxide (173) can also be effected by base. A kinetic analysis of the hydrolysis of 2- and 2,2-di-substituted bromocyclopropanes suggests that for 2-vinyl-substituted compounds considerable progress towards an allyl cation has been been made at the transition state. Metallation studies show that, for nortricyclanes at least, the cyclopropyl hydrogen atoms are most readily attacked by pentylsodium in the presence of potas- 

Mihailovi6, J. BoSnjak, and 2. CekoviC, Helv. Chim. Acta, 1976, 59, 475. 

Alkylation at the bridge positions of a bicyclobutane has been well documented. However, with the fused bicyclobutane (191) bridgehead-substitution products are obtained only at low temperature. At temperatures approaching 0°C the initially produced bridgehead carbanion suffers skeletal rearrangement and products with the structure (192) are obtained.  

A strictly concerted mechanism for the Sj 2 substitutionof a halogenocyclopropane in which the configuration is retained is supported by EH and ab initio MO calculations. However, Dreiding and co-workers believe that the retentive acetolysis of the syn-bromotricycloundecenone (193) is better explained by 1,4-dehydro-bromination to give the Z,Z-diene (194) which undergoes addition (HOAc) from the 

The half reduction of gfem-dihalogenocyclopropanes has been developed to provide a stereospecific synthesis of (rans-bromocyclopropylcarbinols. Treatment of the dibromo-ketones (208) with an excess of methyl-lithium at precisely — 15°C causes replacement of the cis-bromo-substituent only, owing to oxygen-lithium co-ordination. Subsequent (or concomitant) nucleophilic addition to the carbonyl group, and work-up, affords the alcohols (209) in yields of 75—85%. The major by-product is the corresponding trans-bromo-ketone (210). 

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Compute the eigenvalues and draw the energy level diagram for methylene-cyclopropene. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Double bonds are accommodated by rings of all sizes The smallest cycloalkene cyclo propene was first synthesized m 1922 A cyclopropene nng is present m sterculic acid a substance derived from one of the components of the oil present m the seeds of a tree (Sterculia foelida) that grows m the Philippines and Indonesia... 

MgATP. The numbers in parenthesis represent the number of electrons required for the reaction shown. is cyclopropene A cyclopropane. 

Loss of nitrogen occurs to give the cyclopropene (16), which loses a proton to yield the charged species Loss of HCN leads to the unsaturated... 

Pyridazinium dicyanomethylide gives a mixture of the pyrazole (38) and the substituted cyclopropene (39 Scheme 13). 

This synthetic appproach has been used in a few cases for the preparation of pyridazines from diazo compounds and cyclopropenes. In general, cycloadducts (176) are formed first and these rearrange in the presence of acid or alkali to pyridazines (Scheme 98) (69TL2659, 76H(5)40l). Tetrachlorocyclopropene reacts similarly and it was found that the stability of the bicyclic intermediates is mainly dependent on substitution (78JCR(S)40, 78JCR(M)0582>. 

With unsymmetrically substituted cyclopropenes, isomeric cycloadducts (177) and (178) and pyridazines are formed (Scheme 99) (80LA590). 

Considerable effort has been devoted to studying the photolysis of pyrazolenines since it still constitutes the best way to obtain cyclopropenes. An important publication by Gloss... 

Oxirene (2) is one of a number of heterocycles in which the CH2 group of cyclopropene has been replaced by a group or element associated with Groups V or VI of the periodic table. Replacement of the CHj group of cyclopropene by an NH group gives l//-azirine... 

The same dichotomy was observed with hexafluorodimethylcarbene (228), formed by thermolysis of diazirine (227) at 150 °C. The carbene (228) can stabilize itself either intramolecularly to perfluoropropene (229), or intermolecularly by addition to multiple bonds. Oxirane (230) is formed with hexafluoroacetone, cyclopropene (231) with 2-butyne (66MI50800). 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

Cyclopropenone was flrst synthesized " by the hydrolysis of an equilibrating mixture of 3,3-dichlorocyclopropene and 1,3-dichloro-cyclopropene (prepared by reduction of tetrachlorocyclopropene with tributyltin hydride). This procedure has been adapted - to prepare... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

Cyclopropenes and eyelopentadienes with exoeyelie double bonds provide the possibility of dipolar resonanee struetures that suggest aromatie eharacter in the eyelic strueture ... 

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