Cyclopropenyl system

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. 

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 ... 

It should be noted that functional groups, e.g. C=0 at the exocyclic carbon (C4) of triafulvenes often show characteristic shifts to lower frequency compared to the corresponding bands in the non-cyclopropenylated systems. Thus, in tria-... 

Analogously, cyclopropenyl systems 34, 35 were also studied (Scheme 7.9 Table 7.3, Fig. 7.4). Here, further heating of 35 afforded substituted pyridines 36. ... 

Show that if the overlap between torsional-vibration wave functions corresponding to oscillation about different equilibrium configurations is neglected, the perturbation-theory secular equation (1.207) for internal rotation in ethane has the same form as the secular equation for the Hiickel MOs of the cyclopropenyl system, thereby justifying (5.96)-(5.98). Write down an expression (in terms of the Hamiltonian and the wave functions) for the energy splitting between sublevels of each torsional level. 

Comparable stabilization is not possible in structure C because neither a cyclopropenyl system nor a cycloheptatrienyl system is aromatic in its anionic form. Both are aromatic as cations. 

Hiickel s rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts An species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl system two n electrons, the cyclopropenyl cation, corresponds to n 0. and is strongly aromatic. Other aromatic species are the cyclopentadienyl anion (six n electrons, n = 1 Hiickel predicted the enhanced acidity of cyclopentadiene) and the cyclohep-tatrienyl cation. Only reasonably planar species can be expected to provide the AO overlap need for cyclic electron delocalization and aromaticity, and care is needed in applying the rule. Electron delocalization and aromaticity within the SHM have recently been revisited [43]. 

This difference in reduction potentials is accounted for by the loss in resonance energy of the stable two Ti-electron cyclopropenyl cations, whereas the difference in the first reduction process is 1 V ( 24 kcalmoP ) it is only 0.7 V in the second reduction process. This reflects a decreased electron repulsion in the larger cyclopropenyl system. 

Although the reactions of cyclopropylcarbinyl cations have been investigated in considerable detail, corresponding reactions of the cyclopropenyl system have received much less attention, but they appear to be similar . Solvolysis of tosylate 291 proceeds only three times more rapidly than its saturated analogue a,j5-unsaturated carbonyl compounds ensue, presumably from ring expansion of cation 292 (equation 90). Analogous processes account for the first steps of the dehydration of 293 to naphthalene 294 and azulene 295 under various conditions as shown opposite. 

Figure 29.11. Cyclopropenyl system. Configuration of n electrons in cation, free radical, and anion.

Cyclopropenyl systems can be formed from metallacyclobutadiene complexes. Here no decomplexation of unsaturated cyclopropanes is observed. The tungstacyclobutadiene complex 12 is converted into a cyclopropenyl complex 13 upon addition of a nitrogen nucleophile. ... 

Three olefinic fatty acids containing a cyclopropenyl system [9]-[ll] were synthesized starting from 2-bromoalk-l-ene by dibromocyclopropanation to give the corresponding l,l,2-tribromo-2-alkylcyclopropane. Treatment of the latter interme-... 

The allyl system results when three Jt AOs internet in a linear arrangement wherein ffi2 = H23 = P, but H 2, = 0. We ean also treat the situation where the three tt AOs approach each other on vertices of an ever-shrinking equilateral triangle. In this case, each AO interacts equally with the other two. This triangular system is the cyclopropenyl system C3H3 shown in Fig. 8-6. 

Thus far, we have used the allyl and cyclopropenyl systems as examples. We will now describe the results of HMO calculations on some other simple but important systems. 

The cyclopropenyl system is perturbed so that the Huckel matrix element H22 = a- -cP, where c is positive. 

The cyclopropenyl anion has been the subject of many theoretic studies, - and the main conclusion is that it adopts a nonplanar configuration which minimizes the interaction of the anionic lone-pair electrons with the C—C jr-bond. Glukhovtsev, Laiter, and Pross carried out a careful study of the cyclopropenyl system at the G2 theoretical level. They showed that these calculations reproduced the available experimental data, such as the enthalpy of formation of the cyclopropenium ion, and then made use of a series of isodesmic reactions to explore the stability of the species ... 

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