Methylenecyclopropenes

A, Calculate the dipole moment of methylenecyclopropene using the geometry given in Chapter 7. 

Obtain the dipole moment of methylenecyclopropene by a MNDO calculation and compare your answer with the result obtained from Hnckel molecular orbital calculations. 

Methylenecyclopropene was first synthesized by Staley and Norden in the mid-1980 s. They observed three peaks in the UV spectrum of this system, which they identified as follows ... 

Wittig product (57). The methylenecyclobutanes (60) were formed from the same phosphoranes and the methylenecyclopropene (59). The formation of the pyran-2-ones may involve the intermediate cyclobutenones (56) as shown. 

Nakata, M., H. Takeo, C. Matsumura, K. Yamanouchi, K. Kuchitsu, andT. Fukuyama. 1981. Structures of 1,2-Dimethylhydrazine Conformers as Determined by Microwave Spectroscopy and Gas Electron Diffraction, Chem. Phys. Letters 83, 246-249. Norden, T. D., S. W. Staley, W. H. Taylor, and M. D. Harmony. 1986. On the Electronic Character of Methylenecyclopropene Microwave Spectrum, Structure, and Dipole Moment, J. Am. Chem. Soc. 108, 7912-7918. 

Pentacylic cage compounds 202 were synthesized through a double cycloaddition reaction to methylenecyclopropene 199, which involves an intermol-ecular 1,3-dipolar cycloaddition of 198 on the endo double bond to give 200 followed by an intramolecular Diels-Alder reaction between a benzylidenecyclo-propane moiety and an ene function (Table 19) [49],... 

It will be seen that our results are in good agreement with experiment whereas the ab initio values are clearly much too large. Note the low predicted values for methylenecyclopropene, cyclopropylidenecyclopropane, and cyclo-propenylidenecyclopropene it will be interesting to see if these predictions can be confirmed experimentally. 

The discovery of alkylidenecycloproparenes as a class of isolable compounds is the most significant event in recent cycloproparene chemistry. Methylidenecyclo-propabenzene (234) combines the structural features of benzocyclopropene (1), methylenecyclopropene (233), and trimethylenecyclopropane (235) in one and the same structure. Although the parent 234 has so far not been isolated, many substituted and annulated derivatives have been synthesized and charaeterized. Their unexpected stability contrasts sharply with that of the benzocyclopropenones 219, which are only observable as transient reaction intermediates. Reviews on the chemistry of alkylidenecycloproparenes are available. ... 

Some years after the successful isolation and characterization of cyclobutadiene, another C4H4 isomer, methylenecyclopropene (3), was prepared by Billups and... 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

The structure and electronic character of methylenecyclopropene or triafulvene (171) have been studied by ab initio calculations and MW spectroscopy27. Experimental and calculated structural parameters are compared in Table 18. The rs C3=C4 bond length is... 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

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