Hiickel cyclobutadiene

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

I. Four-Electron Hiickel Systems. The literature abounds with examples of photochemical olefin addition reactions which proceed through transition states that are analogous to Hiickel cyclobutadiene e.g.. 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]annulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Hiickel rule can be tested by considering the properties of members of the annulene series. 

One of molecular- orbital theories early successes came in 1931 when Erich Hiickel discovered an interesting pattern in the tt orbital energy levels of benzene, cyclobutadiene, and cyclooctatetraene. By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar- geometries, Hiickel found that whether a hydrocar bon of this type was aromatic depended on its number of tt electrons. He set forth what we now call Hiickel s rule ... 

Benzene, cyclobutadiene, and cyclooctatetraene provide clear- examples of Hiickel s rule. Benzene, with six tt electrons is a (4n + 2) system and is predicted to be aromatic by the rule. Square cyclobutadiene and planar- cyclooctatetraene are 4n systems with four and eight tt electrons, respectively, and are antiarornatic. 

In the next section we ll explore Hiickel s rule for values of n greater than 1 to see how it can be extended beyond cyclobutadiene, benzene, and cyclooctatetraene. 

Section 11.19 An additional requiiement for aromaticity is that the number of tt electrons in conjugated, planar, monocyclic species must be equal to An + 2, where n is an integer. This is called Hiickel s rule. Benzene, with six TT electrons, satisfies Hiickel s rule for n =. Square cyclobutadiene (four TT electrons) and planar- cyclooctatetraene (eight tt electrons) do not. Both are examples of systems with An tt electrons and are antiaromatic. 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (52). Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4 + 2. There is a long history of attempts to prepare this... 

Problem 11-17. What is the resonance stabilization energy of cyclobutadiene, according to Hiickel theory Express your answer in terms of / . 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

Find the Hiickel energy levels and molecular orbitals for butadiene, cyclobutadiene, and pentatrienyl. 

Figure 5 Orbital correlation diagram for the suprafacial [1,3] shift of a proton in propene. The transition-state orbitals are based on the Hiickel orbitals for cyclobutadiene...

Fig. 4.17 The n molecular orbitals and n energy levels for a cyclic four-p-orbital system in the simple Hiickel method. The MOs are composed of the basis functions (four p AOs) and the eigenvectors, while the energies of the MOs follow from the eigenvalues (Eq. 4.69). This particular diagram is for the square cyclobutadiene molecule. The paired arrows represent a pair of electrons of opposite spin, in the fully-occupied lowest MO, i/q, and the single arrows represents unpaired electrons of the same spin, one in each of the two nonbonding MOs, ij/2 and 1//3 the highest n MO, 1I/4, is empty in the neutral molecule...

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]. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

The first attempts to prepare cyclobutadiene (CB) were made around 1870,33 but its antiaromaticity was only understood by Hiickel in 1932. The prediction that CB could be stabilized by complexation was published in 195634 and the first cyclobutadienes were made - as transition metal complexes - 3 years later.35... 

The 2-azirine ring system is of theoretical interest since it is a cyclic conjugated structure containing 4n electrons and is predicted by Hiickel s rule not to be stabilized by cyclic delocalization. Electronically it is analogous to cyclobutadiene. 

The theoretical models start with Kekule s [44] description of benzene, as having two structures. Later Hiickel [45,46] discovered his [4 +2] and [4n] rules, and was able to account for the stability of benzene ([4 +2]) and the instability of cyclobutadiene and cyclo-octatetraene (both [4 ]). The [4 +2] compounds were called aromatic after benzene, while the [4n compounds were given the designation anti-aromatic. 

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character-... 

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