Electrocyclic reactions 6-electron processes conrotatory

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

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,... 

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. 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

The cyclobutene ring first opens in an electrocyclic reaction 152. This must be conrotatory as it is a four electron process but there is no stereochemistry at this stage. Then an intramolecular Diels-Alder cycloaddition 153 closes the new six-membered ring. This is a particularly favourable reaction as the formation of the alkene completes a benzene ring. It would not be possible to prepare such an unstable diene so a tandem process is necessary. 

The Nazarov reaction, in which the key electrocyclic step is the conrotatory process 6.505, has one more atom in the ring but the same number of electrons. The question with respect to torquoselectivity now, since this reaction is taking place in the opposite direction, namely ring-closing, is which reacts faster, a dienone... 

A common type of electrocyclic reaction is the ring-opening of a cyclobutene to a butadiene. The stereochemistry of the new alkene(s) in the diene can be interpreted on the basis of the Woodward-Hoffmann rules. For a four electron component, thermal ring-opening occurs by a conrotatory process (both terminal p-orbitals moving clockwise or anticlockwise), whereas the photochemical reaction... 

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. 

The conversion of a pentadienyl to a cyclopentenyl cation is an electrocyclic reaction of a system containing loury-electrons delocalized over five atomic orbitals, and should therefore be a csorotatory process. In the example just described, the stereochemistry of the cyclization reaction is masked by subsequent rearrangement of (alkj l groups, which ultimately affords the most stable carbonium ion. The stereochemistry of the cyclization of divinyl ketones under acidic conditions, however, is conrotatory, as predicted ... 

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. 

Vitamin D2 is produced by two pericyclic reactions. One of them is photochemicaUy initiated the second thermally initiated. The first step is a photochemical electrocyclic reaction in which a cyclohexadiene of the B ring is isomerized to a triene. The reaction involves six k electrons and is the reverse of the photochemical cyclization reaction discussed in Section 28.4. Thus, by the principle of microscopic reversibility, this photochemicaUy allowed ring opening involving a 4 +2 71 system must occur by a conrotatory process. 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

The Nazarov reaction [196] is a conrotatory electrocyclization involving four electrons over a five-carbon span. Usually, a more highly substituted cyclopentenone is obtained. However, contrathermodynamic products may be generated by placing a silyl group at the p-position of a bare vinyl moiety in the cross-conjugated dienone [197]. The acceptor facilitates and controls the regiochemistry of the cyclization process. 

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 second reaction involves a reverse Diels-Alder reaction and an electrocyclic opening of the dobutene product. This is a four-electron conrotatory process. The two substituents may both. otate out to give the , -diene or both in to give the 2,Z-diene. 

A common type of six electron electrocycUc reaction occurs in the photochemical reaction of 1,2-diaryl alkenes. ° The parent substrate, stilbene can be converted to phenanthrene, a process that involves conrotatory electrocyclization under photochemical conditions and subsequent oxidation of the product to the polycyclic aromatic structure (3.218). 

Both the reactions are four electron conrotatory electrocyclic processes. 

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