Huckel-type transition states

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Huckel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry rules were formulated. We will discuss a few representative examples in the following paragraphs. 

This argument can obviously be extended to concerted pericyclic reactions of all kinds. The transition state for any such reaction will be isoconjugate with a normal Hiickel-type cyclic polyene or an anti-Hiickel analog of one. If the transition state is aromatic, the resulting stabilization will lower its energy and so accelerate the reaction. If it is antiaromatic, the converse will be true. Since, moreover, the rules for aromaticity in Huckel-type and anti-Huckel-type systems are diametrically opposite, in each case one will be aromatic and the other antiaromatic. If, then, a reaction can follow one of two alternative pericyclic paths, one involving a Hiickel-type transition state and the other an anti-Hiickel-type transition, the reaction will prefer to follow the path in which the transition state is aromatic. If, on the other hand, only one of the two alternatives is sterically possible, the reaction will take place relatively easily if the corresponding transition state is aromatic and with relative difficulty if it is antiaromatic. In the latter case, the antiaromatic transition state will, if possible, be bypassed by a two-step mechanism in which the transition state is linear instead of cyclic [e.g., equation (5.291)]. 

The transition state in suprafacial attack is designated as of Huckel type in which no sign inversion of the cycle has taken place. The other type of migration involves one sign inversion. This is called mobius type inversion. The Huckel type of inversion occurs when the total number of electrons is 2, 6,. .., (4n + 2). This is also called aromatic transition state. In mobius type the participating electrons is 4, 8,. .. i.e. An. 

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. 

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

A number of studies have compared the use of the multiple regression technique using semiempirical parameters such as tt and o-, and parameters calculated for the particular molecules from molecular orbital theory. Hermann, Culp, McMahon, and Marsh (23) studied the relationship between the maximum velocity of acetophenone substrates for a rabbit kidney reductase. These workers were interested in the reaction mechanism, and two types of quantum chemical calculations were made (1) extended Huckel treatment, and (2) complete neglect of differential overlap (CNDO/2). Hydride interaction energy and approaching transition-state energies were calculated from the CNDO/2 treatment. All these parameters plus ir and a values were then subjected to regression analysis. The best results are presented in Table II. 

The theory of solutions of flexible uncharged polymers with excluded volume is at present well developed, but the properties of polyelectrolytes and especially polyampholytes have been considered much less from the theoretical point of view. It is well known that polyampholytes exhibit a change in phase from the extended random flight configuration to a condensed microphase. The polyampholyte theory of Edwards et al. [6] considers the isoelectric state of polyampholytes as a microelectrolyte satisfying a Debye-Huckel-type of structure. The criterion of transition from the collapsed conformation to the extended one is described as follows ... 

In the PMO method, we analyze an electrocyclic reaction through the following steps (1) Define a basis set of 2p-atomic orbitals for all atoms involved (li for hydrogen atoms). (2) Then connect the orbital lobes that interact in the starting materials. (3) Now let the reaction start and then we identify the new interactions that are occurring at the transition state. (4) Depending upon the number of electrons in the cyclic array of orbitals and whether the orbital interaction topology corresponds to a Huckel-type system or Mobius-type system, we conclude about the feasibility of the reaction under thermal and photochemical conditions. 

Consider now the n MOs in allyl. These are formed by Ti-type interactions of three 2p AOs, one from each carbon atom. The 2p AO of the central carbon atoms overlaps with the 2p AOs of the terminal atoms but the latter do not overlap significantly with one another (Fig. 5.2b). There is clearly an exact parallel with the three-center MOs of the transition state of Fig. 5.2(a). As can be seen from Fig. 5.2, both systems are moreover of Huckel type. The topology of the orbital overlap involved in forming the three-center (T-type MOs in the S 2 transition state of Fig. 5.2(a) is therefore the same as that of the Ti-type orbital overlap of p AOs in allyl. 

We are dealing here with four-atom conjugated systems containing four electrons, i.e., the two pairs of electrons that form the C=C n bond and the CH2—CH2 (7 bond in (144). The disrotatory transition state (145), being of Huckel type, will then be isoconjugate with normal cyclobutadiene and so will be antiaromatic, whereas the conrotatory transition state will be isoconjugate with an anti-Hiickel analog of cyclobutadiene and so will be aromatic (see Table 4.2). 

The transition state is precisely analogous to that in a ti cycloaddition, the only difference being that the reactants in the n cycloaddition are both even, while here R and S are both odd. We can therefore see at once that if the 71 systems in (176) both interact in cis fashion or both in trans fashion, the transition state will be of Huckel type, while a cis-trans interaction will lead to an anti-Hiickel transition state. 

The photocyclization of (108) to (110) is another ten-electron process, this time involving a system isoconjugate with an odd hydrocarbon anion. The first step is probably a photochemical conrotatory closure to (109). The transition state, being of anti-Huckel type in a fused 6-5 system with ten delocalized electrons, should be antiaromatic. The intermediate (109), isoconjugate with a linear C7 carbanion, was not isolated but underwent rearrangement, probably by a 1,5 sigmatropic shift via a transition state isoconjugate with the indenyl anion (111). 

The transition state for this reaction is a superantiaromatic (anti-Hiickel-dibenzo)-normal-Huckel-cyclooctatetraene. If the phase relationships in these reactions are examined, it is clear that there will be a phase inversion at both ring junctions. By proper choice of the basis set of AOs, these phase inversions can be eliminated, so the transition state is of Hiickel type. 

Cycloaddition reactions can also be predicted through PMO-method. If a given process is allowed can be determined through transition state, jc s-t-n s cycloaddition, i.e., Diels-Alder reaction is allowed because the transition state is Huckel s type and is isoconjugate with benzene (a Hiickel t e system). 

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