Cyclopropenyl

Other aromatic ions include cyclopropenyl cation (two rr electrons) and cycloocta tetraene dianion (ten tt electrons)... 

Some naturally occurring fatty acids have ahcycHc substituents such as the cyclopentenyl-containing chauJmoogra acids (1), notable for thek use ki treatkig leprosy (see Antiparasitic agents, antimycotics), and the cyclopropenyl (2) or stercuhc acids (Table 6). 

Cyclopropenyl azides (350), the obvious precursors to cyclopropenylnitrene and hence possibly azetes by ring expansion of the latter, give 1,2,3-triazines on heating or irradiation (78HC(33)5). 

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

Alkylation of enamines with epoxides or acetoxybromoalkanes provided intermediates for cyclic enol ethers (668) and branched chain sugars were obtained by enamine alkylation (669). Sodium enolates of vinylogous amides underwent carbon and nitrogen methylation (570), while vicinal endiamines formed bis-quaternary amonium salts (647). Reactions of enamines with a cyclopropenyl cation gave alkylated imonium products (57/), and 2-benzylidene-3-methylbenzothiazoline was shown to undergo enamine alkylation and acylation (572). A cyclic enamine was alkylated with methylbromoacetate and the product reduced with sodium borohydride to the key intermediate in a synthesis of the quebrachamine skeleton (57i). 

When a molecule is symmetric, it is often convenient to start the numbering with atoms lying on a rotation axis or in a symmetry plane. If there are no real atoms on a rotation axis or in a mirror plane, dummy atoms can be useful for defining the symmetry element. Consider for example the cyclopropenyl system which has symmetry. Without dummy atoms one of the C-C bond lengths will be given in terms of the two other C-C distances and the C-C-C angle, and it will be complicated to force the three C-C bonds to be identical. By introducing two dummy atoms to define the C3 axis, this becomes easy. 

Semiempirical (PM3) and ab initio (6-3IG basis set) calculations are in agreement with the hypothesis described in Section I (99MI233 OOOJOC2494). In the case of the sensitized reaction, when the excited triplet state is populated, only the formation of the radical intermediate is allowed. This intermediate can evolve to the corresponding cyclopropenyl derivative or to the decomposition products. In a previously reported mechanism the decomposition products resulted from the excited cyclopropenyl derivative. In our hypothesis the formation of both the decomposition products and the cyclopropenyl derivatives can be considered as competitive reactions. 

The irradiation of 2,5-dimethylfuran in the presence of mercury vapor gave a complex mixture of products. Carbon monoxide and propene were removed as gaseous products. Then, cis- and rran.s-l,3-pentadiene, isoprene, 1,3-dimethylcyclopropene, 2-pentyne, 2-ethyl-5-methylfuran, hexa-3,4-dien-2-one, 1-methyl-3-acetylcyclopropene, and 4-methylcyclopent-2-enone were obtained (Scheme 8) (68JA2720 70JA1824). The most abundantproduct was the cyclopentenone 19, the second was the 1,3-pentadiene 12, while the third product was the cyclopropenyl derivative 18. 

The formation of 26 can be explained on the basis of the mechanism depicted in the Scheme 11, where the irradiation of the cyclopropenyl derivatives 28 induces a radical reaction to give 26. 

Also in this case theoretical calculations are in agreement with experimental results. In fact, the triplet state of 20 can be converted into the corresponding biradical to give the cyclopropenyl derivative (Fig. 3) (OOOJOC2494). 

The authors, assuming the intervention of a cyclopropenyl intermediate, explained the formation of this type of product. 

Also in this case calculation results fit the experimental data (Fig. 7) [99H(50)1115]. In fact, the singlet excited state can evolve, giving the Dewar thiophene (and then isomeric thiophenes) or the corresponding excited triplet state. This triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. 

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system (C l I3). How ate these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical Which of the three substances is aromatic according to Hiickel s rule ... 

Das relativ stabile Triphenyl-cyclopropenyl-Kation wird mit Guanidinium-perchlorat als Leitsalz zum 1,2,3-Triphenyl-cyclopropen (12% d.Th.) reduziert4. 

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

The di-n-propyl cyclopropenyl cation failed to photolyze either in aqueous acid or organic solvents, with or without sensitizers. A possible explanation in the discrepancy between the triphenyl system and this one lies in the calculated energy differences between the cations and their corresponding radicals. In the triphenyl system this energy difference is 0-5 3 or 16 kcal/mol, while in the di-n-propyl case it is 1 00)3 or 32 kcal/ mol, based on calculated delocalization energies for the two species. 

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

It is strong evidence for Hiickel s rule that 59 and 60 are not aromatic while the cyclopropenyl cation (55) and the cyclopentadienyl anion (39) are, since simple resonance theory predicts no difference between 59 and 55 or 60 and 39 (the same number of equivalent canonical forms can be drawn for 59 as for 55 and for 60 as for 39). 

We now report the synthesis of new antibacterial 3H-pyrazoles by regioselective 1,3-dipolar cycloaddition of the versatile 2-diazopropane to nonprotected disubstituted propargyl alcohols and that the unsubstituted propar-gyl alcohol allows the double addition of 2-diazopropane and gives a 3H-pyrazole with formal insertion of the dimethylcarbene into a carbon-carbon bond. We also show that the photolysis of the 3H-pyrazoles leads to new alcohols containing the cyclopropenyl unit. 

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