Methylenecyclopropene, structure

Derivatives of the cyclopropenylium ion have also been studied intensively. A few examples may serve as illustrations. For the open shell system C4H4, which is one of the major primary fragment ions produced in the unimolecular dissociation of many excited precursors, it is now well established that there exist at least two stable species, the most stable one may have the methylenecyclopropene structure (213) and the less stable one may be an acyclic species, perhaps vinylacetylene (214). 

Methylenecyclopropene (structure b in Problem 4.4) is a nonaltemant hydrocarbon. What did the results of the HMO calculation for methylenecycloprojjene indicate about the charge density on each atom Do those results agree with the statement on page 195 about the nature of alternant and nonaltemant systems How could you have predicted the polarity of methylenecyclopropene on the basis of resonance theory ... 

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. 

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. ... 

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... 

Methylenecyclopropene (triafulvene) is the simplest member of the cross-conjugated, non-alternant, hydrocarbon family known as the fulvenes. Strong polarization of the tc-system might be expected for such a structure and extensive efforts have been made... 

The propensity of methylenecyclopropenes to react with electrophiles and nucleophiles is determined by their electronic structure. Thus, methylenecyclopropenes with electron-withdrawing substituents such as 78 fail to protonate in trifluoroacetic acid , whereas... 

The structures of several substituted methylenecyclopropenes and calicenes have been determined and reviewed . Of particular interest, however, is structural data of the parent hydrocarbon. The electric dipole moment (/i = 1.88 D) and the heavy atom molecular structure of methylenecyclopropene have been determined by microwave spectroscopy C=C, 1.323 A C-C, 1.441 A C=0, 1.212 A. The experimental and calculated structure... 

The reactions of bis(alkynyl)metallocenes Cp2M(C=CR)2 (M = Zr, Hf R = Me, Pr11, Bu11, Gy) with B(C6F5)3 afford the corresponding metallocene borate betaines of structure type 760.586 An assumed intramolecular alkyne insertion reaction leads to their less stable methylenecyclopropene-derived isomers, which are effectively trapped by the added / /7-butylisocyanide to yield complexes 764 which contain a methylenecyclopropene derived [Pg.906]

Triafulvene (methylenecyclopropene, 1) is the smallest member of the fulvenoid series and is, therefore, generally expected to gain some 7t-delocalization energy by resonance contribution of the dipolar structure IB. The first known system of this type was quinocyclopropene 3 reported in 1963. The parent compound 1 was synthesized in 1984 as a highly reactive and unstable substance, whose spectra could only be recorded at low temperature (< — 75 °C in solution). - Compound 1 is much less stable than cyclopropenone (2), which is fairly stable in solution at room temperature and can be stored in its crystalline state for a long period at below... 

For methylenecyclopropene, a microwave structure determination has established bond lengths that show the strong alternation anticipated for a localized structure. The molecule does have a significant (1.90 D) dipole moment, implying a contribution from the dipolar resonance structure. The net stabilization calculated at the MP/6-31G level is small and comparable to the stabilization of 1,3-butadiene. The molecular geometry... 

Figure 3. Structure formulas for methylenecyclopropene (1), biphenyl (2), bithiophene (3), indole (4), imidazole (5), naphthalene (6), and anthracene (7).

We have shown [13,79, 80] that the electronic spectra of c -l,3-buta-diene [81], cyclopentadiene, aromatic five-membered heterocycles [48,79], and norbornadiene [80] can be understood on the basis of a model with two interacting double bonds. Cyclopentadiene (CP) is the prime example of a ring-shaped molecule with a conjugated rr-electron system, and its structure can be related to that of short polyenes such as cM-1,3-butadiene (CB) and the simplest heterocycles, such as pyrrole (PY), furan (FU), and thiophene (TP). In the series, ci5-l,3-butadiene, cyclopentadiene, norbornadiene (NB), the latter is the most complex system, with the two ethylenic units coupled through indirect conjugation and 77, a interaction. One more system will be added here to the set of molecules with two interacting double bonds methylenecyclopropene (MCP) [(1) in Fig 3] the simplest cross-conjugated 77-electron system. 

The substrates of the present investigation are derivatives of triafulvene ([3] fulvene, alias methylenecyclopropene) (IV), the simplest nontrivial [n] fulvene [8-12]. Interest in these microcyclic three-membered ring systems stems from a possible contribution of the aromatic , dipolar structure (IVa) in the ground-state as well as in the excited state. Aromatic" - in inverted commas - in the sense that the three-membered ring acquires a non-negligible cyclopropenium ion character. It is emphasized from the outset that this attribute should be treated as a matter of gradation [13]. The question to be asked is whether a certain deviation from a complete bond-alternation picture towards aromaticity is indicated. 

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