Electrocyclic reactions disrotatory modes

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

A striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was... 

For the photochemical electrocyclic reaction of the diene, irradiation promotes one electron from n2 to n and the disrotatory mode of reaction gives the cis isomer (Scheme 8.4). 

It would be a good point here to remind you that, although all electrocyclic reactions are allowed both thermally and photochemically pro viding the rotation is right, the steric requirements for con- or disrotatory cyclization or ring opening may make one or both modes impossible. 

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. 

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... 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 29), we can further generalize the predictions based on the symmetry of the polyene HOMO. The HOMOs of the An systems are like those of 1,3-dienes in having opposite phases at the terminal atoms. The HOMOs of other An + 1 systems are like trienes and have the same phase at the terminal atoms. Systems with An tt electrons will undergo electrocyclic reactions by conrotatory motion, whereas systems with An+ 2 a electrons will react by the disrotatory mode. 

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). 

How do the symmetry properties of 1 2 influence electrocyclic reactions For convenience, let us examine the microscopic reverse of the ring-opening of a cyclobutene to a butadiene, realizing that any factors that appear on this reaction path also appear on the forward reaction path. For bonding to occur between the carbon atoms at the end of the ir-system, the positive lobe on C(l) must overlap with the positive lobe on C(4) (or negative with negative). This overlap can be accomplished only by conrotatory motion. Disrotatory motion causes overlap of orbitals of opposite sign, and precludes bond formation. Since similar symmetry properties of the HOMO exist for other 4n -systems, the conrotatory mode will also be preferred for all thermal electrocyclic reactions in these systems. 

Although it is more fruitfiil to constmct a correlation diagram for the detailed analysis of an electrocyclic reaction, there is, nevertheless, an alternative method that also enables us to reach similar conclusions. In this approach, which is extremely simple, our only guide is the symmetry of the highest occupied molecular orbital (HOMO) of the open-chain partner in an electrocyclic reaction. If this orbital has a C2 symmetry, then the reaction follows a conrotatory path, and if it has a mirror plane symmetry, a disrotatory mode is observed. The explanation for this alternative approach is based on the fact that overlapping of wave functions of the same sign is essential for bond formation. 

A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode of electrocyclic reaction is called disrotatory,... 

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. 

---