Heptatrienyl

FIGURE 11 14 Their molecular orbitals of cycio heptatrienyl cation... 

In contrast to pentadienyl anions, heptatrienyl anions have been found to cyclize readily to cycloheptadienyl anions. The transformation of heptatrienyl anion to cyclo-heptadienyl anion proceeds with a half-life of 13 min at — 13°C. The Woodward-Hoffinann rules predict that this would be a conrotatory closure. ... 

Figure 11.15 shows the Fliickel MOs of cyclopentadienyl anion. Like benzene and cyclo-heptatrienyl cation, cyclopentadienyl anion has six tt electrons and a closed-shell electron configuration. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

The same method was used to determine the electron affinities of pentadienyl radical (0.91 eV) and heptatrienyl radical (1.27 eV)35. The corresponding bond dissociation... 

Two species in Figure 6-7 are not molecules but ions. Aromaticity is not restricted to molecules. The cyclopentadienyl ion and tropylium ion (cyclo-heptatrienyl ion) are aromatic species where n = 1. 

Dipoles with a,/3 y,8-unsaturation are isoelectronic with the hepta-trienyl anion 1 and, as such, would be expected to undergo a 1,7-electrocy-clization (8tt electron process) analogous to the heptatrienyl 1-cyclohepta-dienyl 2 rearrangement (Scheme 2). Examination of the HOMO of the heptatrienyl anion, F4,3 shows that this ring closure must proceed in a con-... 

Winstein and coworkers measured the UV spectrum of the homotropenylium ion and showed that its long-wavelength absorption band (/.max 313 nm) was intermediate between that of the tropylium (2max 273.5 nm) and heptatrienyl (/. rdx 470 nm) cations70. Using a Hiickel molecular orbital approach the C(l)—C(7) bond order was estimated to be 0.56. 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

Structure 13 below represents 8 as combination of a one-carbon radical and a hep-tatrienyl radical. The nonbonding orbital coefficients in the heptatrienyl radical are 0.5, so the resonance energy of 8, R(8), is given by... 

The transition state can be disconnected into the allyl and the heptatrienyl radicals. An analysis of the interaction of their nonbonding orbitals reveals two favorable contributions and one which is unfavorable. Consequently, the transition state is nonaro-matic and the reaction is not favored. 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

Add the appropriate number of electrons to the MO energy level diagram for the cy-cloheptatrienyl cation in Figure 16.3. Is this ion aromatic or antiaromatic The cyclo-heptatrienyl anion has two more electrons than the cation. Do you expect this anion to be a stable species Explain. 

ClsH16)t Diphenylpropane, 1-cyclo-heptatrienyl-2-phenyl ether, 1-tolyl-2-phenylethane 17-21 g. P 502... 

Apparently, the pentadienyl anion portion moved along the carbon atom backbone until a heptatrienyl anion was formed, which then underwent the expected cyclization (see Sect. C.I) to a heptadienyl anion, which is of greater stability due to its extra carborn carbon single bond. Finally, while the 3-methylcycloheptadienyI anion was found to be thermally stable, it could be completely converted to the 2-methylcycloheptadienyl anion photochemically. 

In a number of cases, pentadienyl species are subject to cyclization reactions89-92 (cf., heptatrienyl, Sect. C.I). Thus, cyclooctadienyllithium undergoes a first-order conversion to a bicyclo(3.3.0)octenyl anion with a half-life of 80 min at 35°93. The 1,3,5-triphenyl-... 

Using Equation 1.12, or its geometrical equivalent, calculate the coefficients for the frontier orbitals of the heptatrienyl cation. 

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