Huckel transition states

Suprafacial or Huckel transition state in a pericyclic reaction is associated with a plane of symmetry and is particularly favourable if the number of cyclically conjugated Tr-electrons in the transition state equals [4n - - 2] (the Huckel rule, where = 0,1, 2,...). 

Huckel was also able to show that if a cyclic conjugated tt-system is irradiated with light so that it goes into the first excited singlet or triplet electronic state, it is especially stable if the number of cyclically conjugated electrons equals [4n]. Hence, photochemically activated pericyclic reactions will proceed suprafacially via a Huckel transition state if the electron count corresponds to [An],... 

Figure 2.10 The five-membered rings are arranged in an energetically favorable way by rearrangements via four

Figure 13.24 Huckel transition state (no sign inversion in the orbital basis) of the transition state of the 1,5-sigmatropic shift of penta-1,3-diene. The ring current includes the hydrogen atom that is transferred during...

These representations offer the advantage that one need not argue which of the reagents carries OH or Cl into the transition state. Since that is usually not known, this notation sidesteps the issue. From the Brpnsted-Debye-Huckel equation, we recognize that the concentration of each transition state (and therefore the reaction rate) will vary with ionic strength in proportion to the values of K for the given equation. For the first term we have... 

In this method, the orbital symmetry rules are related to the Hiickel aromaticity rule discussed in Chapter 2. Huckel s mle, which states that a cyclic system of electrons is aromatic (hence, stable) when it consists of 4n + 2 electrons, applies of course to molecules in their ground states. In applying the orbital symmetry principle, we are not concerned with ground states, but with transition states. In the present method, we do not examine the molecular orbitals themselves but rather the p orbitals before they overlap to form the MO. Such a set of p orbitals is called a basis set (Fig. 15.5). In investigating the possibility of a concerted reaction, we put the basis sets into the position they would occupy in the transition state. Figure 15.6 shows this for both the... 

FIGURE 15.6 Transition states illustrating Huckel-Mobius rules for cycloaddition reactions. 

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. 

Various geometries are possible for the transition state and they can be classified on whether each of the allyl systems interacts with lobes of the other system on the same side (suprafacially) or on opposite sides (antarafacially). Three transition states have been given. All have been classed on Huckels system, on the basis of aromatic transition state approach and so all are thermally allowed. The following picture gives the allowed transition state for thermal [3, 3] shifts. 

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. 

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

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. 

Use the procedure of Section 21-1 OF to set up transition-state orbitals and determine whether these lead to a favored Huckel or a favored Mobius transition state for the following processes ... 

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years the two concepts are inextricably linkedThis is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states and the Huckel/Mobius concept. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 1. ... 

Transition state aromaticity (Huckel and Mobius topologies)... 

Huckel aromatics The benzene molecule has the suprafacial topology this means that the TT - electron density in benzene is continuous along the top or bottom face of the molecule. If the transition state for the pericyclic reaction has the same topology, it is said to resemble Huckel topology (Fig. 8.17). 

Day has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/HUckel aromaticity, and has defined the terms supra-facial and antarafacial in terms of the nodal structure of the atomic basis functions. His approach makes quite explicit the assumption that the transition state involves a cyclic array of basis functions. Thus the interconversion of prismane (10) and benzene, apparently an allowed (B2g-t- 2s + 2s) process, is in fact forbidden because there are additional unfavourable overlaps across the ring. ... 

Moebius-Huckel Theory. The molecular orbital array of the transition state is analyzed in terms of aromaticity, which is determined by the number of TT electrons. 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

X is NR R-. The substituent is converted to a Z substituent via the low-lying cr orbital, and the ring is deactivated toward further electrophilic attack. The ortho and para channels lead to products. The interaction diagram for an X -substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels, are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with an amino substituent are shown in Figure 11.3. Notice that the LUMO of each is suitable to activate the C—H bond at the saturated site toward abstraction by the base. Curiously, the meta cation has the lowest LUMO and should most readily eliminate the proton. The stabilities of the transition states should be in the order of the Huckel n energies. These are 6a — 8.762 j, 6a — 8.499, and 6a — 8.718 j, respectively. Thus the ortho and para channels are favored over the meta channel, and the ortho route is slightly preferred over the para route. Experimentally, para substitution products are often the major ones in spite of there being two ortho pathways. The predominance of para products is usually attributed to steric effects. 

Note that although the use of activities is preferred for bulk species in kinetic expressions for electrochemical reactions, owing to the demonstrated ability of the Debye-Huckel limiting law to predict trends in reaction rate for ionic processes, the molarity of the transition-state complex is formally used in Eq. (6) since this species is ephemeral. It is usually neither convenient nor possible to measure this species concentration, but if it is assumed that the passage of the A B C complex over the activation barrier determines the rate of the reaction, the formation of the complex from the reactants (or products for the reverse reaction) can be considered to be a quasi -equilibrium process, for which a quasi-equiUbrium constant, can be defined. For a forward direction of reaction (3), this constant would be... 

For suprafacial pericyclic reactions, the cyclic transition state is stabilized if a Huckel number of electrons, 4n + 2, is involved. This will give faster reaction rates. 

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