Conrotatory mode of reaction

For the thermal electrocyclic reaction of dienes, the HOMO for the diene is n2, since there are four electrons to accommodate in the n-orbitals (two paired electrons per orbital). Thus, for hexa-2,4-diene the conrotatory mode of reaction gives the trans isomer (Scheme 8.3). 

The electrocyclic ring-closure reaction proceeds exclusively in the S state and yields solely the trans product by a conrotatory mode of reaction, as is to be expected from Table 7.3 for a 6 r-electron system. Competing reactions are cis-trans isomerization (cf. Section 7.1.4) and intersystem crossing to T,. From the T, state, generally only cis-trans isomerization is observed. Stilbenes with substituents that enhance spin inversion, such as Br, RCO, and NO2, do not undergo the cyclization reaction efficiently. 

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... 

The conrotatory mode of ring opening of the P-lithiocyclopropyloxirane 17 is suggested first by the isolation of the cw-fused cyclobutene 20 conclusive evidence for the intermediate 18 (the precursor of 19 and 20) is provided by trapping the diene 18 in a Diels-Alder reaction with 21 to give the tarn-fused adduct 22. 

Similarly, we can analyze the hexatriene-cyclohexadiene system having (4n + 2) 7T-electrons (Figure 2.10). In this case, a disrotatory mode of ring closure leads to a Hiickel array, which is aromatic with (4n + 2) rr-electrons. Therefore, the disrotatory mode of reaction now becomes thermally allowed. However, a conrotatory mode of ring closure uses a Mobius array, which is antiaromatic with (4n + 2) 7r-electrons. Therefore, the reaction is thermally forbidden in this mode. 

Fig. 2.3. Minimal energy reaction paths of the electrocy-clic butadiene-cyclobutene reaction in the HF and HF + CI approximations [26]. The reaction coordinate is represented by the angle of synchronous rotation of the methylene groups. The value of 154.0 hartrees is taken to be the zero energy DIS denotes the disrotatory and CON—the conrotatory mode of the cycloreversion. (Reproduced with permission from the American Chemical Society)...

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. 

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

The fact that the reactions take place in the direction of ring-opening is determined by thermodynamics, but the stereochemistry is most certainly not, for the cyclobutene 4.36 gives the thermodynamically more strained product 4.37 with one of the double bonds cis. Thermodynamics affects the stereochemistry only with the opening of the cyclobutene 4.38, which shows a preference for one of the conrotatory modes, that giving the trans,trans diene 4.39, where the rules could have led to the cis,cis diene equally well. This type of selectivity is called torqueoselectivity. 

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. 

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