Thermally disrotatory process

To depict these symmetry requirements leading to the stereospecificity of the interconversion of (1) to (2), consider the orbital correlation diagram involving the six rr-orbitals of the hexatriene and the four tt-orbitals and two cr-orbitals of cyclohexadiene (Figure 3). In the thermal disrotatory process, which maintains a mirror plane of symmetry (or o-symmetry), ground state reactant (1) orbitals (TTa TTb TTc ) correlate with the corresponding product (2) orbitals This suggests that the disrotatory ther-... 

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. 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

In the thermal conrotatory process the molecule maintains a Ca axis of symmetry throughout the entire reaction, while the photochemical disrotatory process maintains a plane of symmetry as shown in Figure 9.13 for butadiene. 

In the above example although the ground state orbital of cyclobutene, o, correlates with the ground state orbitals of butadiene /b the n orbital of the former does not correlate the /2 orbital of the latter, but rather it correlates with the /3 which is an excited state. So in such a case the thermal transformation by disrotatory processes will be a symmetry forbidden reaction. 

The thermal retro-reaction of 3-sulfolenes to dienes and sulfur dioxide occurs under mild conditions (about 120-200°C), and is, as predicted from the Woodward-Hoffmann rules, a disrotatory process so for 2,5-dimethyl-3-sulfolenes [541,542] ... 

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. 

Figure 4.41 (a) Orbital overlaps in conrotatory and disrotatory o bond formation in a ring-closure reaction, (b) Stereochemistry of preferred products in photochemical and thermal (dark) processes... 

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... 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

Two electrocyclic reactions, involving three electron pairs each, occur in this isomerization. The thermal reaction is a disrotatory process that yields two cis-fused six-membered rings. The photochemical reaction yields the rrans-fused isomer. The two pairs of n electrons in the eight-membered ring do not take part in the electrocyclic reaction. 

After studying the correlation diagrams for both conrotatory and disrotatory processes, it is concluded that the interconversion of butadiene to cyclobutene thermally proceeds in a conrotatory fashion while photochemically it proceeds in a disrotatory fashion. 

The correlation diagram for the disrotatory process (m symmetry is maintained) clearly indicates that the bonding orbitals of the reactant correlate with the bonding orbitals of the product. Thus, this process is thermally allowed (Fig. 8.43). 

For thermal reactions, with 4n electrons in the transition state the conrotatory process is allowed, and with [4n -1- 2] electrons in the transition state the disrotatory process is allowed. For photochemical reactions, rules are usually reversed. 

The anmdenes from [10]armulene to [16]armulene react quite selectively by 6e disrotatory processes. Thus the low-teirqierature photochemical generation of cyclodecapentaene (143) from rranr-9,10-dihydroiiaphthalene (144) or from (145) was followed by its thermal isomerization to cis-9,10-dihydronaphthalene (146). The E JZJZ -cyclodecapsaUiene (147) has been postulated as inter-... 

Thermal isomerization of all-(Z)-cyclononatetraene at 35 °C gave c/5-bicyclo[4.3.0]nona-2,4,7-triene (1) in a disrotatory process in which only three double bonds are involved, whereas photochemical isomerization involving the four double bonds gave c -bicyclo[6.1.0]nona-2,4,6-triene (2). ... 

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