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1,3,5-Cycloheptatriene orbitals

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). [Pg.1]

In sharp contrast to cyclopentadiene is cycloheptatriene (41), which has no unusual acidity. This would be hard to explain without the aromatic sextet theory, since, on the basis of resonance forms or a simple consideration of orbital overlaps. [Pg.52]

Early work (B-69MI51600) on 7V-substituted-l//-azepines revealed that they undergo photoinduced ring contraction to bicyclic valence tautomers as indicated in Scheme 1. Subsequently, it has been found that 3H- and 4H- azepines enter into analogous ring contractions, as do some of their oxo and benzo derivatives. These transformations, which parallel those undergone by cycloheptatriene, are often thermally reversible and occur by an orbital symmetry-controlled disrotatory electrocyclic process. [Pg.504]

Both 1,3-cyclohexadiene and 1,3,5-cycloheptatriene contain at least one carbon atom that does not have a p orbital, and so they are not completely conjugated and therefore not aromatic. [Pg.617]

In the cycloheptatriene molecule there are three double bonds and hence six p orbitals if a hydride anion is removed from the only methylene unit and that carbon is re-hybridised from sp to sp so as to create an empty p orbital, this completes the circuit of p orbitals around the cycloheptane ring. Then there is a continuous series of p orbitals that contain six n electrons, which gives rise to an aromatic system. [Pg.96]

Site selectivity in a number of other concerted cycloadditions which are not [4 + 2] cycloadditions is also explained by frontier orbital control. Thus diphenylketene (332) reacts with isoprene (333) mostly at the more substituted double bond, and with cis-butadiene-l-nitrile (334) at the terminal double bond.263 Dichlorocarbene reacts at the terminal double bond of cycloheptatriene (335),264 and the Simmons-Smith reaction (336 + 337)265 also takes place at the site with the higher coefficients in the HOMO. [Pg.169]

If the organic group is not cyclically conjugated, the spectra appear more complex 132) as the d-band shows structure and there is no longer degeneracy among the ligand 77-orbitals. For the 7 6-cycloheptatriene complexes two d-bands are observed,... [Pg.106]

More readily identifiable geometrical factors probably outweigh the contribution of the frontier orbitals in the remarkable reaction 6.47 between tetracyanoethylene and heptafulvalene giving the adduct 6.49 (see p. 261). The HOMO coefficients for heptafulvalene 6.420 (see p. 347) are highest at the central double bond, but a Diels-Alder reaction, with one bond forming at this site is impossible. The best reasonable possibility for a pericyclic cycloaddition, from the frontier orbital point of view, would be a Diels-Alder reaction across the 1,4-positions (HOMO coefficients of -0.199 and 0.253), but this evidently does not occur, probably because the carbon atoms are held too far apart. This is well-known to influence the rates of Diels-Alder reactions cyclopentadiene reacts much faster than cyclohexadiene, which reacts much faster than cycloheptatriene (see p. 302). The only remaining reaction is at the site which actually has the lowest frontier-orbital electron population, the antarafacial reaction across the 1, f-positions, which have HOMO coefficients of —0.199. [Pg.359]

Cycloheptatriene is not aromatic. Although it has the correct number of pairs of tt electrons (three) to be aromatic, it does not have an uninterrupted ring of p orbitalbearing atoms because one of the ring atoms is sp hybridized. Cyclopentadiene is also not aromatic It has an even number of pairs of tt electrons (two pairs), and it does not have an uninterrupted ring of p orbital-bearing atoms. Like cycloheptatriene, cyclopentadiene has an sp hybridized carbon. [Pg.596]

Fig. 8. Correlation between the highest occupied molecular orbitals of dihydrobullvalene, barbaralane, semibullvalene, homotropylidene and cycloheptatriene. The energy values are taken from experiment... Fig. 8. Correlation between the highest occupied molecular orbitals of dihydrobullvalene, barbaralane, semibullvalene, homotropylidene and cycloheptatriene. The energy values are taken from experiment...
Cycloheptatriene (Fig. 14.12) (a compound with the common name tropylidene) has six TT electrons. Flowever, the six tt electrons of cycloheptatriene cannot be fully delocalized because of the presence of the —CH2— group, a group that does not have an available p orbital (Fig. 14.12). [Pg.641]

FIGURE 14.12 Cycloheptatriene is not aromatic, even though it has six tt electrons, because it has an sp -hybridized carbon that prevents delocalization around the ring. Removal of a hydride (H ) produces the cycloheptatrienyl cation, which is aromatic because all of its carbon atoms now have a p orbital, and it still has 6 w electrons. [Pg.641]

As a hydride ion is removed from the —CH2— group of cycloheptatriene, a vacant p orbital is created, and the carbon atom becomes hybridized. The cation that results has seven overlapping p orbitals containing six delocalized is electrons. The cyclohepta trienyl cation is, therefore, an aromatic cation, and all of its hydrogen atoms should be equivalent again, this is exactly what we find experimentally. [Pg.642]

The experimental enthalpy of activation for disrotatory thermal isomerization of cis-l,3,5-hexatriene to 1,3-cyclohexadiene in the gas phase at 100 C is 29.2 kcal/mol [13]. The reaction is exothermic by 14.5 kcal/mol [14, p. 127], so of the reverse reaction is 43.7 kcal/mol, but - in spite of its high activation energy - it is characterized as allowed by all of the common orbital symmetry criteria. In norcaradiene ([4.1.0]hepta-2,4-diene), the cyclopropane ring bridging Cl and Ce of cyclohexadiene has built the disrotation into the molecule, desymmetrizing it - and its monocyclic isomer, cycloheptatriene - to C, in which the 61 and ai orbitals correlate directly (Fig. 5.3). The rate of isomerization is so much faster that it had to be measured at low temperature (ca. 100 K) in a hydrocarbon glass [15] is only 6.3 kcal/mol ... [Pg.116]

Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with this sp hybridized carbon having a vacant 2p atomic orbital. The cycloheptatrienyl (tropylium) cation is planar and has six tt electrons in seven 2p orbitals, one from each atom of the ring. It can be drawn as a resonance hybrid of seven equivalent contributing structures (Figure 21.13). [Pg.918]


See other pages where 1,3,5-Cycloheptatriene orbitals is mentioned: [Pg.772]    [Pg.234]    [Pg.277]    [Pg.29]    [Pg.353]    [Pg.389]    [Pg.654]    [Pg.411]    [Pg.136]    [Pg.353]    [Pg.389]    [Pg.1011]    [Pg.197]    [Pg.100]    [Pg.177]    [Pg.107]    [Pg.427]    [Pg.86]    [Pg.89]    [Pg.302]    [Pg.377]    [Pg.302]    [Pg.349]    [Pg.427]    [Pg.176]    [Pg.360]    [Pg.231]    [Pg.343]    [Pg.97]    [Pg.345]   
See also in sourсe #XX -- [ Pg.583 , Pg.584 , Pg.588 ]




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1.3.5- Cycloheptatrien

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