Disrotatory transition state, electrocyclic

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. 

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

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. 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

It should be pointed out that our description of electrocyclic reactions thus far has been qualitative. Woodward and Hoffmann (1965a) do refer to unpublished HMO calculations which back up the almost intuitive symmetry arguments. Nevertheless, Fukui (1965,1966) and Zimmerman (1966) outlined HMO treatments in which they obtained changes in energy for conrotatory and disrotatory processes. On the basis that paths involving minimum energy between reactants and transition states were favored, their predictions were in essential agreement with those of Woodward and Hoffmann. 

The electrocyclization of 5-oxo-2,4-pentadienal (89) to pyran-2-one (90) is an example of a psendopericyclic reaction. As shown in Fignre 4.21, the disrotatory electrocyclization of 88 occurs with a closed loop of the p-orbitals in the transition state. On the other hand, the electrocyclization of 89 has no such closed loop. Rather, two orbital disconnections interchange the role of bonding and nonbonding orbitals. 

An important consequence of the pseudopericyclic mechanism is that the planar (or nearly planar) transition states preclude orbital overlap between the a- and Jt-orbitals. This implies that all pseudopericyclic reactions are allowed. Therefore, Jt-electron count, which dictates whether a pericyclic reaction will be allowed or forbidden, disrotatory, or conrotatory, is inconsequential when it comes to pseudopericyclic reactions. Bimey demonstrated this allowedness for all pseudopericyclic reactions in the study of the electrocyclic reactions 4.11-4.14. 

These electrocyclic reactions and their conrotatory or disrotatory nature can be readily understood on the basis of aromaticity in the transition state (Figure 7.16). For the conrotatory mode, the rotations of the breaking cr orbitals bring about a phase change in the cyclic transition state continuous red-to-red overlap cannot be maintained. Thus the... 

Electrocyclic reactions can also be analyzed on the basis of the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground state molecules. A stabilized aromatic TS results in a low activation energy, i.e., an allowed reaction. An antiaromatic TS has a high energy barrier and corresponds to a forbidden process. The analysis of electrocyclizations by this process consists of examining the array of basis set orbitals that is present in the transition structure and classifying the system as aromatic or antiaromatic. For the butadiene-cyclobutene interconversion, the TSs for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, that is, the complete set of 2p orbitals involved in the reaction process, not the individual molecular orbitals. The tilt at C(l) and C(4) as the butadiene system rotates toward the TS is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

Finally, the preferred direction of disrotatory ring closure has been addressed just as in the electrocyclic ring opening of cyclobutene. In the six-electron case, tt acceptor substituents control the direction of rotation in the same sense as in cyclobutene but the torquoselectivity effect is attenuated. Further, steric effects tend to play a bigger role, and both effects were attributed to the disrotatory nature of the transition state. 

Generalization of either the frontier orbital, the orbital symmetry, or the transition-state aromaticity analysis leads to the same conclusion about the preferred stereochemistry for concerted thermal electrocyclic reactions The stereochemistry is a function of the number of electrons involved. Processes involving 4n + 2 electrons will be disrotatory those involving 4n electrons will be conrotatory for Hiickel transition states. The converse holds for Mobius transition states. 

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.

The first step is a disrotatory chelotropic reaction, the transition state being isoconjugate with naphthalene (218). The second step is a conrotatory electrocyclization, the transition state being isoconjugate with an anti-Hiickel analog of (219). 

Here, with six electrons involved, it is the disrotatory mode (Hiickel system) in which the transition state is stabilized. There are numerous examples of interconversion of 1,3,5-trienes and 1,3-cyclohexadiene systems by the electrocyclic mechanism.The chart which follows gives a general summary of the relationship between transition-state topology, the number of electrons, and the stability of the transition state. 

The key concept for the formulation of the rules in question is the concept of the transition vector (see Sect. 1.3.3.1). This vector may be regarded as a quite short, albeit finite, portion of the part of the reaction path whose beginning lies at the transition state point. A displacement of the system in the direction defined by the transition vector lowers its potential energy. Figure 1.10 shows as an example the form of the transition vector for the electrocyclic reaction of disrotatory rearrangement of the cis-Dewar benzene into benzene [49]. 

Consider now the electrocyclic processes depicted in Fig. 5.1. Case (a) clearly involves a — H type transition state, whereas in cases (b) and (c) the interactions are of the + A-H type. Accordingly, the electrocyclic interconversion of butadiene and cyclobutene should occur by the lower energy conro-tatory pathway if the process is concerted. In the first excited state processes, because of the reversal of Evans Rule, the disrotatory mode should be strongly preferred,... 

Comparison of the disrotatory and conrotatory paths for electrocyclization indicates that the disrotatory path can proceed without a barrier. Figure 12.27 is a cross section of the S2 and Sq surfaces and depicts the formation of the observed products via transition from S2 to the ground state surface. Figure 12.28 is a reaction cube in which each of the edges represents rotation about one of the bonds. The shaded central area is near 90-90-90 and is the region in which the CIs are located. Figure 12.29 shows the structures of three CIs found in this area. 

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