Cyclopropene, structure

The argument was still going on in the 1950s when the first NMR spectrometers appeared. Though infrared appeared to support the cyclopropene structure, one of the first problems resolved by the primitive 40 MHz instruments available was that of Feist s acid, which had no methyl group signal but did have two protons on a double bond and so had to be the exomethylene isomer after all. 

Activated Acetylenes. The unstable 1 2 adduct formed from the reaction of triphenylphosphine and dimethyl acetylenedicarboxylate at -50°C is now thought to have the cyclopropene structure (22) the... 

Sterol from the Mediterranean sponge Calyx nicaen-sis containing the unusually reactive cyclopropene structure. C29H46O, Mr 410.68, mp. 114-116°C, [aJo ... 

In addition to unsaturated fatty acids, several other modified fatty acids are found in nature. Microorganisms, for example, often contain branched-chain fatty acids, such as tuberculostearic acid (Figure 8.2). When these fatty acids are incorporated in membranes, the methyl group constitutes a local structural perturbation in a manner similar to the double bonds in unsaturated fatty acids (see Chapter 9). Some bacteria also synthesize fatty acids containing cyclic structures such as cyclopropane, cyclopropene, and even cyclopentane rings. 

Prolonged heating (1 -14 d) of equimolecular amounts of 1,2,4,5-tetrazines with cyclopropenes in refluxing benzene gives 4//-1,2-diazepines directly, which, however, were assigned the 5//-structure.106... 

Benzocyclopropene is an intriguing example in which the electronic structure of benzene is greatly perturbed by the fusion of the smallest alicyclic ring, cyclopropene, to the aromatic system. Benzocyclopropene thus arouses theoretical interest and the high strain energy (approximately 68 kcal./mole)3 associated with the compound suggests unusual chemical reactivity. A review article has recently appeared.4... 

Self-Test 18.3B (a) Name the alkene (Cl I,CH2)2CI ICI F=CHCH, and (b) write the structural formula for cyclopropene. 

Trimethylenemethane is a special type of alkene that does not exist as the free compound. Various synthetic equivalents to the synthon 43 shown below have been reported. Trost, in particular, has exploited these compounds in 1,3-dipolar cycloaddition reactions.138 139 A metal-bound, isolated trimethylenemethane species was recently reported by Ando (Scheme 6). It resulted from the complexation of an ero-methylenesila-cyclopropene with group 8 carbonyls (Fe, Ru).140,140a The structure was proved by X-ray crystal structure analysis.29Si NMR data were consistent with the -structure shown. 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

Baird, M. S. Functionalised Cyclopropenes as Synthetic Intermediates, 144, 137-209 (1987). Balaban, A. T., Motoc, I., Bonchev, D., and Mekenyan, O. Topilogical Indices for Structure-Activity... 

The Wittig reaction of diphenyl cyclopropenone with the phosphorane 68 failed to give the methylene cyclopropene instead a hydrocarbon of probable structure 67 was obtained71. ... 

Type 61b of the intensely colored quinocyclopropenes is represented by the di-cyanomethylene species 115 and 118 of p- and o-quinonoid structure. In addition to the systems 115,119, and 122 reported by Gompper1001 a series of o- and p-quino-cyclopropenes in the benzene, naphthalene, anthracene, phenanthrene, and fluorene series (718-125) were prepared75) carrying the bis-(p-anisyl)-cyclopropenyl residue, which brings about a better stabilization of the cyclopropenium moiety101 ... 

Table 5. Structural data of cyclopropenone compared to cyclopropene (obtained from microwave spectra)...

Fohlisch99 reported a remarkable dependence of the electron spectra of quino-cyclopropenes on their structure. As shown in Table 9, the merocyanine-like quino-cyclopropenes show positive solvatochromy when they contain an anthraquinonoid chromophore (198), but negative solvatochromy when they contain a benzoquino-noid system (199). This can be interpreted in terms of a markedly increased participation of dipolar resonance forms in the ground state of the benzoquinonoid 199 compared to the anthraquinonoid 198. From the dipole moment of 198 (9.4 D99 ) the dipolar contribution was estimated to be in the range of 23%. 

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

A further proof for the structure of 3a is the independent synthesis of this cyclopropene via intramolecular addition of a carbene carbon atom to the triple... 

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