Cyclopropene

An obvious test reaction is the dimerization of cyclopropene to tricyclo[3.1.0.0 ] -hexane (TCH) and its reversal, the fragmentation of TCH to cyclopropene. Formally, the analysis is easily seen to hold here as well, but it does not follow that the energetic advantages of this perturbation are sufficient for concerted cycloaddition to syn-TCR to overcome the normal preference for formation of a rans-biradicaloid species, illustrated profusely in the preceding chapter. 

The experimental evidence is suggestive, but inconclusive. Although cyclopropene is a highly strained molecule, with an estimated strain energy of 26 kcal/mol,[28] it does not dimerize spontaneously. The an e-isomer of TCH, the 6z5- em-dimethyl derivative of which has been formed by catalytic dimerization of em-dimethyl cyclopropene,[29] does not revert thermally to two monomer molecules. Instead it isomerizes to vibrationally excited cyclohexadiene [30]  

The four-membered ring evidently chooses to break in the alternative manner, producing a pair of endocyclic x bonds. The symmetry requirements are the same as those of the fragmentation. The fact that the product molecule is vibrationally excited confirms the conclusion reached above, that concerted bond rupture of the central ring of the anti isomer of a tricyclic molecule requires the gratuitous excitation of vibrational motion. We might expect the analogous isomerization of syn-TCR to proceed more readily to an unexcited product molecule, but the prediction cannot be checked experimentally syn-TCR has yet to be prepared. 

The course of the reverse reaction, thermal dimerization of cyclopropene, was followed computationally [31]. The molecules were mutually oriented in C2v] as they were brought together the symmetry was relaxed to C, so as to bias the reaction path in favor of concerted closure of the four-membered ring to syn-TCH, in analogy with the behaviour of CBD. As illustrated in Fig. 7.13, the dimerization follows the conventional stepwise path instead, bonding across a diagonal to form a transoid biradical that is more stable thermodynamically than the reactants and can close - if at all - only to the anti isomer. 

In a very recent experimental study [32], dimerization of cyclopropene was found to yield a dienic product with two three-membered rings, that could be 

If stored as a liquid, even at —78°, cyclopropane undergoes a fairly rapid polymerization reaction. However, in the gas phase, at temperatures above 325° (in a stream of helium), it isomerizes smoothly to yield methylacetylene. This is clearly analogous to the isomerization of cyclopropane to propylene. 

The pyrolysis of this compound at around 490° in a stream of nitrogen yields 1,2,3-trimethylbuta-l,3-diene. Unlike the parent compound, it does not yield a substituted acetylene (Stechl, 1964). It is thus apparent that the movement of a hydrogen atom is much easier than that of a methyl group. It is possible that the isomerization involves a five-membered ring, viz.  

Since the diene readily forms an adduct with maleic anhydride it probably has the stereochemistry demanded by the reaction path shown in equation (13). 

Systems Containing Foijr-membered Rings A. Cyclobutane 

The thermal decomposition of cyclobutane to yield ethylene has been very extensively investigated (Genaux and Walters, 1951 Kem and Walters, 1952, 1953). The reaction is homogeneous and kinetically first order. Addition of inhibitors to the reactant does not affect the rate, and 

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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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