Conrotatory transition state

In the butadiene example, the conrotatory transition state has a Mobius topology, and will be aromatic four electrons are present). Thus, the conrotatory process is allowed. The disrotatory transition state is a Hiickel system, and will require two or six electrons for aromaticity. Since in butadiene only four electrons are present, the disrotation pathway will be forbidden. 

Fig. 10.25. Correlation of orbitals of cyclobutene with the conrotatory transition state and the product, 1,3-butadiene. Energies (in eV) are from HF/6-31G(d) computations. Reproduced from J. Am. Chem. Soc., 125, 5072 (2003), by permission of the American Chemical Society.

SCF-MP2/6-3IG calculations on the conrotatory transition state structure roughly reproduced the isotope effects and suggested differential extents of rehybridization at the inner and outer C-H bonds - more at the outer carbon than the inner carbon. This would appear to contradict the usual notion that more sp to sp ... 

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 conrotatory transition state should therefore be the more stable, being stabilized by the cyclic conjugation instead of destabilized. The reaction is therefore predicted to take place by conrotatory opening rather than disrotatory, as indeed it does. For example, the 3,4-dimethylcyclobutenes open... 

The difference between the energy of the disrotatory transition state XXIIa forbidden by the orbital symmetry conservation rules and that of the allowed conrotatory transition state XXII does not exceed 15kcal/mol, as seen from Table 10.3. 

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

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 stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-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 mles were formulated. We will discuss a few representative examples in the following paragraphs. 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

This is evidence for a four-membered cyclic transition state and arises from conrotatory motion about the C-3—C-4 bond. It is called conrotatory because both movements are clockwise (or both counterclockwise). Because both rotate in the same direction, the cis isomer gives the cis-trans diene. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Figure 9.13. Transition states for (a) conrotatory and (b) disrotatory closure.

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

The retro-Diels-Alder reaction has been reviewed.A fully concerted cyclic transition state has been proposed for conrotatory opening of cyclobutenes, in order to account for the low activation entropy and unexpected activation volume of ca —2 to —3cm mol . ... 

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