Huckel ring

Propadiene is represented in Figure 13-4 as if it were two isolated Huckel ring systems. This molecule also may be represented as a stable Mobius system of Att electrons. Draw an orbital diagram of 1,2-propadiene to indicate this relationship. If 1,2-propadiene twisted so that the hydrogens on the ends all were in the same plane, 57, would it be a Huckel or a Mobius polyene, or neither ... 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

It is of interest to be able predict the stability of such fiised-ring compounds. Because Huckel s rule applies only to monocyclic systems, it cannot be applied to the fiised-ring compounds, and there have been many efforts to develop relationships which would predict their stabihty. The underlying concepts are the same as for monocyclic systems stabilization should result from a particularly stable arrangement of MOs whereas instability would be associated with unpaired electrons or electrons in high-energy orbitals. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

A thiepin is formally isoelectronic with the 8ic-electron 1,3,5,7-cyclooctatetraene and 1,3,5-cycloheptatrienide ion and, if planar, may actually be antiaromatic. Recently, the question of the antiaromaticity of thiepin has been the subject of interest for both synthetic and theoretical chemists. The apparent instability of the thiepin ring system is in good agreement with theoretical calculations. Dewar and Trinajstic 68) have reported that the thiepin is considered to be weakly antiaromatic (RE = — 1.45 kcal mol-1) based on PPP SCF MO calculations. On the other hand, Hess Jr. and Schaad 69) have found it to be substantially antiaromatic (RE = —0.232 J) by using the Huckel MO method. This result was also supported by a graph-theoretical treatment by Aihara 70). 

A suprafacial alkyl [1, 3] shift with retention of configuration and already discussed provides an example. The transition state contains four electrons and is of Huckel type and makes the reaction unfavourable in the ground state but many photo-chemical [1, 3] shifts do occur in the four numbered ring structure. 

Huckel s 4n+2 //-electron rule is a necessary but not a sufficient condition for aromaticity. Coplanarity and electronegativity restrictions of constituent atoms represent the most important restrictions. Phos-phole is a marginally aromatic five-membered heterocycle76 (see further examples and discussion). Mesoionic compounds, mesomeric betaines, and 2H-and 4i+-pyrone have all been considered to be weakly aromatic or non-aromatic, though their conjugated acids are aromatic. Spectroscopic data evidenced the aromaticity of dioxolium and oxathiolium cations 59 (Scheme 28) and mesoionic oxathioles not in the classical sense but by their ring currents and chemical stability.77... 

In the crystal the atoms of this molecule are found to be distributed in three planes Hie two tolyl rings are rotated by 24° (trans to Br) and 68° (cis to Br) with respect to the ethylene plane. Semiempirical Huckel molecular orbital (MO)-type calculations give essentially the same angles in the crystal, but predict 35° and 45° for these angles in the Bee molecule. 

HUckel s 4n -H 2 rule orgchem Aromatic (ring) compounds must have 4n + 2 pi-bonding electrons, where n is a whole number and generally limited to = 0 to 5. When M = 1, for example, there are six pi-electrons, as for benzene. Also known as Huckel s rule. hhk-alz jfor, en plos tu, rul ... 

For example, Horowitz (Revue Roumalne de Chlmle, 1978, 23, 603-611) performed Huckel molecular orbital calculations on a substituted, symmetrical trlazlne which demonstrated the alternating charge system In the ring. 

The photocyclodehydrogenation of thienyl ethylenes is well-defined when both thiophene rings are bound via a C(2) atom to the ethylenic bond as in (70). In other cases, however, more cyclization products are possible. To predict the photocyclization mode for heterohelicenes the F s rule fails in many cases, because correction factors for the hetero atoms in the Huckel MO calculation have to be introduced and the systems are not well comparable with carbocyclic diaryl ethylenes. A better reaction parameter in these cases is the Mulliken overlap population (nrs)51), introduced by Muszkat52) for these cases. The overlap populations of the atoms r and s in ground and excited state (nIiS and n s), are calculated using the extended Huckel method. Cyclizations should not occur when nr>s and An s (= nr>s — n s) have negative values. (This method can also be used for diaryl olefins, but in these cases calculation of F s is more simple.). 

An extensive homologous series is also known for unsaturated organic ring systems of the type (-HC=) , which are known as annulenes. The most important example is benzene or [6]annulene. The stability of annulenes is governed by Huckel s rule, which states that planar ring systems of this type... 

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