Electrocyclic reactions conrotation

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 selection rules for conrotatory electrocyclic reactions are the opposite of those just listed that is, for a molecule with an even number of electron pairs, conrotation is thermally allowed, and for a molecule with an odd number of electron pairs, conrotation is photochemically allowed. 

Explain whether each of these electrocyclic reactions proceeds by conrotation or disrotation. 

Conrotation (Section 22.1) Rotation of orbitals in the same direction in an electrocyclic reaction. 

In electrocyclic reactions, the suprafacial mode involves each component of the new CT-bond being formed from the same face of the reactant Tr-system (this is equivalent to disrotation). The antarafacial mode involves twisting of the orbitals so that the two components of the new a-bond form from the opposite face of the reactant TT-system (this is equivalent to conrotation) (Fig. 8.14). 

Figure 8.14 Suprafacial disrotation) and antarafacial conrotation) modes in an electrocyclic reaction.

Disrotatory and conrotatory rotation The concerted rotation around two bonds in the same direction, either clockwise or counterclockwise, is described as conrotatory. In the electrocyclic ring opening, the terminal p-orbitals rotate (or twist, roughly 90°) in the same direction known as conrotation (comparable to antarafacial) to form a new ct-bond. In disrotatory cyclization (comparable to suprafacial) the terminal p-orbitals rotate in opposite directions. These two modes of the electrocyclic reaction are shown in Fig. 8.37. 

A worked problem follows and several examples of electrocyclic reactions occur in the problems at the end of the chapter. The important principle is that for 4n electrons, conrotation will give the favoured Mobius transition state, whereas for An+ 2 electrons, disrotation will give the favoured Htickel transition state. 

For conjugated tetraenes, n = 8, conrotation should be preferred. The expectation that cyclization of eight TT-electron systems will be conrotatory has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature. The unsubstituted system, has an of 17.0 kcal/mol and A// of —11.2 kcal/mol. At slightly higher temperatures, the cyclooctatriene system that is formed undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene. ... 

The prediction of conrotation in the cyclization for eight-ir-electron systems has been confirmed by the study of a series of stereoisomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium favoring the cyclooctatriene product. At slightly more elevated temperatures, the... 

Bauid and co-workers have discussed the cyclobutene-butadiene anion radical electrocyclic reaction. The radicals of cis- and trans-3,4-diphenylbenzocyclobutene and 3,4-diphenylphenanthrocyclobutene have been found to undergo this transformation in the conrotatory mode, contrary to expectations from the symmetry properties of the HOMO. A more quantitative treatment, however, using correlation diagrams in conjunction with MO energies calculated by the INDO method, does predict conrotation as the preferred mode. 

An electrocyclic reaction is as easy to analyze as that. Identify the HOMO, and then see whether conrotatory or disrotatory motion is demanded of the end carbons by the lobes of that molecular orbital. All electrocyclic reactions can be understood in this same simple way. The theory tells us that the thermal interconversion of cyclobutene and 1,3-butadiene must take place in a conrotatory way. For the cyclobutene studied by Vogel, conrotation requires the stereochemical relationship that he observed. The cis 3,4-disubstituted cyclobutene can only open in conrotatory fashion, and conrotation forces the formation of the cis,trans diene. Note that there are always two possible conrotatory modes (Fig. 20.12), either one giving the same product in this case. 

Higher vinylogs continue the alternation. The closure (or opening) of 4n + 2 systems occurs in a disrotatory way, and for An systems, one finds that conrotation is the thermally allowed process. The rules reverse for the photochemical reactions. We can summarize the selection rules for allowed electrocyclic reactions as an alternating series in Table 5.3. 

Electrocyclization reactions are equilibria, and it is important to bear in mind that the selection rules do not allow a prediction to be made of the position of the equilibrium. Where the substrate is chiral, there will be two possibilities either for conrotation or for disrotation, each of which is allowed by the selection mles. Whether one mode of electrocyclization will be favored over the other will depend on the difference in activation barriers for the two processes. 

In the boat conformations (a)-(c), the sigma overlap of the shaded jr-orbitais at C4 and C5 can only occur if concerted rotations about the C3, C4 and C5, C6 axes take place as indicated. Such rotations, which are of particular significance in discussing electrocyclic reactions, either occur in the same relative direction as in (c), or are opposed as in (a) and (b). These types of concerted rotational motion have been called conrotation and disrotation by Woodward and Hoffmann (1965, 1968, 1969) see Section 3.3(b). 

The pericyclic process comes next and it is a Nazarov reaction (p. 962), a conrotator electrocyclic closure of a pentadienyl cation to give a cyclopentenyl cation. There is r stereochemistry and the only regiochemistry is the position of the double bond at the end of th -. reaction. Here it prefers the more substituted side of the ring. 

Pentadienyl cations can undergo electrocyclization to cyclopentenyl cations. As this is a four Tt-electron system, it should occur by conrotation. Based on gas phase ion stability data, the reaction is exothermic by 18 kcal/mol. ... 

We will now show that closed shell electrocyclization ractions are actual sigma-pi transfer reactions very much analogous to the Diels-Alder transfer reaction and that the conrotation and disrotation transition state complexes are bound electronically in a manner analogous to the antarafacial and suprafacial (or vice versa) transition states of 4N + 2 electron cycloadditions. 

The formation of j8-lactams from aliphatic acid chlorides and imines has been reviewed. The reaction is assumed to proceed through in situ formation of a ketene, followed by interaction with the imine to form a zwitterionic intermediate, which undergoes an electrocyclic conrotatory ring closure to give the jS-lactam ring (Scheme 8). The stereoselectivity of the reaction is sensitive to both the electronic and steric effects of the substituents on the ( )- or (Z)-imine, and the former manifest themselves in the electronic torquoselectivity that occurs during the conrotating cyclization of a zwitterionic intermediate [shown for ( )-imine in Scheme 8]. ... 

---