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. 

Here, with six electrons involved, it is the disrotatory mode (Hiickel system) in which the transition state is stabilized. There are numerous examples of interconversion of 1,3,5-... 

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 ... 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

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... 

Use of substituted systems has shown that the reaction is stereospecific.300 The groups on C(2) and C(5) of the pyrroline ring rotate in the disrotatory mode on going to product. This stereochemistry is consistent with conservation of orbital symmetry. 

If A, B, C and D are all identical, then there will be only one possible product, but if they are all different, then there are four possible products. Basically the theory distinguishes only the disrotatory modes or conrotatory modes. But predictions can be made on the basis of steric effects. 

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). 

The molecular orbitals derived from overlap of six p atomic orbitals as found in trienes are shown in Figure 8.7. Since there are six electrons to accommodate, as two paired electrons per orbital, the HOMO is n3. Thus, for 2,4,6-octatriene the disrotatory mode of reaction gives the trans isomer (Scheme 8.5). 

On the other hand, it is conceivable that, if 7a undergoes disrotatory ring opening to 8a, energy remains in the disrotatory mode of methylene rotations for the short time that 8a takes to cross the TS for disrotatory closure to 7a. In this dynamical model, disrotatory ring opening to 8a, rather than resulting in preferential... 

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

For a second example I take the ring opening of butene to cis butadiene. Considerations of symmetry allow a distinction to be made between paths which are distinguished by the modes of rotation of the CHj groups conrotatory and dis-rotatory as shown in Fig. 9. In the conrotatory mode the molecule retains a 2-fold axis of symmetry so that orbitals or states can be characterized by the symmetry labels A or 5, and in the disrotatory mode there is a plane of symmetry so that the symmetry labels. <4 and A , can be used. 

Figure 8.7 Symmetry operations for conrotatory and disrotatory modes of cyclobutene butadiene transformation.

Figure 8.8 Reactant—product orbital correlation diagram for cyclobutene buta-diene transformation for (a) conrotatory and (b) disrotatory modes.

Figure 8.9, State correlation diagram for cyclobutene a butadiene transformation. (a) Disrotatory mode (b) Conrotatory mode.

It is instructive now to turn to the correlation diagrams in Figures 11.11 and 11.12 for conrotatory and disrotatory closure of hexatriene, a six tt electron system. The disrotatory mode is now allowed, the conrotatory forbidden. If correlation diagrams for larger systems are constructed, it will be found that with each addition of two carbons and an electron pair the predicted selectivity will reverse. 

Dienones, which because of the electron deficiency induced at the carbonyl carbon may be regarded as analogs of pentadienyl cations, are known to close photochemically.103 These closures occur readily in cyclic systems where the geometry requires the disrotatory mode, but the proposed intermediate product, formally a 1,3 diradical, is ordinarily stabilized through rearrangement.104 Woodward and co-workers have demonstrated the predicted photochemical... 

Mirror plane disrotatory mode symmetry operation o... 

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