Electrocyclic reactions 4-electron processes disrotatory

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. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the 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. 

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. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The first anion A is formed by removal of the only possible proton one from the NCH2 group. This anion might be considered aromatic (six electrons from the three alkenes, two from N and two from the anion) but it is clearly unstable as it closes in an electrocyclic reaction at > -35 °C. This is a six-electron process and must therefore be disrotatory. The rotating hydrogens are shown on the structure of A. It is essential that the 5,5 ring closure must be cis and that demands a disrotatory reaction. Both anions A and B are extensively delocalized and it is a matter of choice where you draw the anion. 

Although the orbitals crossing shown above are not real, this diagram gives us a convenient way to treat electrocyclic reactions. We can determine the nature of the process conrotatory or disrotatory) by assuming that the ti-HOMO must be converted to the new a-MO. This was the hypothesis that was originally published by Woodward and Hoffmann. It means that, because the symmetry of the HOMO alternates as one more double bond (or two electrons) is added, the conrotatory or disrotatory nature of electrocyclic reactions also alternates. This can then be formulated as a set of rules, as shown in Table 4.1. 

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. 

Woodward—Hoffmann orbital symmetry rules can be applied to the charged systems as well. The conversion of a cyclopropyl cation to an allylic cation is the simplest one, which involves only 27r-electrons (Figure 2.13). This is an electrocyclic reaction of (4n + 2) type (n = 0) and should, therefore, be a disrotatory process. 

The next four items (examples 2-5, inclusive) in Table 6.6 are all electrocyclic reactions, clearly related to the cycloadditions and others already discussed earlier in this chapter and the symmetry controlled processes of Chapter 4. Example 2, a conrotatory four-electron 2% + 27t = 27t + 2d) process relating trans or ( )-3,4-dimethylcyclobutene to trans, trans or (2 ,4 )-hexadiene conserves C2 symmetry as shown in Figure 4.41 and again here in Equation 6.59. Examples 3,4, and 5 are six-electron disrotatory processes. 

Figure 15.18 shows several examples of electrocyclic processes. Since the reactions are always allowed in either a conrotatory or disrotatory manner, the key issue is the control of stereochemistry. Electrocyclic reactions provide a good example of the power of pericyclic reactions in this regard. In all cases, the reaction proceeds as predicted from the various theoretical approaches. The restrictions placed by the orbital analysis on the reaction pathway are nicely demonstrated by examples D and E in Figure 15.18 only the stereochemistry given is found. An instructive example of the fact that it is the number of electrons that controls the process, not the number of atoms or orbitals, is the conrotatory ring closure of the four-electron pentadienyl cation prepared by protonation of a divinyl ketone (example G). 

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

Orbital symmetry considerations dictate that in 4n-electron reactions the thermal process must use a conrotatory motion, whereas the photochemical reaction must be disrotatory.Just the opposite rules apply for reactions involving 4re + 2 electrons. The key to analyzing electrocyclic reactions is to look at the way the p orbitals at the end of the open-chain K system must move in order to generate a bonding interaction in the developing G bond. 

If we now consider the HOMO of hexatriene (Figure 18.30), where the closure reaction is a six-electron process, to bring the orbitals together to make the new o-bond, the two orbitals must now rotate in opposite directions, one clockwise and one counterclockwise. This is a disrotatory process, and this rotation applies to all 4 + 2 electrocyclic reactions. As always, it is easier to see what is happening, when there are substituents (Figure 18.31). 

The essential aspects of reaction (25) are depicted in Fig. 6. Huisgen et al. (1967) have provided a beautiful example of an odd electrocyclic change in (26). The aziridine opens up to a dipolar four-electron allylic species. Since the HOMO is b (Fig. 1), the thermal change is conrotatory and the excited state process is disrotatory. To avoid equilibration of the dipolar ions, these workers trap them with an acetylenic ester in a stereospecific cycloaddition, which we shall discuss presently. 

The state-symmetry correlation also indicates that electrocyclic radical interconversion favors a conrotatory path from the first excited state and a disrotatory path from the second excited state. Because of the proximity of the energy levels and the violations of the noncrossing rule, it is probable that the excited state process will not be highly stereoselective. The same detailed considerations must be applied to the five-atom five-electron system and yield the results given in Table 1. Differences between the stereochemical predictions of Table 1 and those of others (Woodward and Hoffmann, 1965a Fukui and Fujimoto, 1966b Zimmerman, 1966) tend to be limited to the excited-state reactions of odd-atom radicals. 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

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