Disrotatory mode of reaction

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). 

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). 

From the stereochemistry at C-1 and C-5, it has been concluded that the ring closure occurs in a disrotatory way and anti with respect to the migrating vinyl group, as shown in Scheme 41. Only when the anti-disrotatory ring closure is not possible for steric reasons is a stereochemistry that corresponds to a syn-disrotatory mode of reaction observed. 

A variety of methods are available for preparing 2-substituted sulfolenes, as described in Section 6.3. The extrusion of sulfur dioxide is a concerted stereospecific process, and has been shown to follow a disrotatory pathway. Sulfur dioxide extrusion from mono-2-substituted sulfolenes could in principle lead to ( )- or (Z)-diene formation through alternative disrotatory modes of reaction. 

Orbital symmetry arguments imply that thermal sulfolene opening reactions should follow a disrotatory mode of reaction, and this prediction was confirmed experimentally [82,85-87]. When heated, cfs-2,5-dimethyl-3-sulfolene (168) was... 

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. 

We have seen that carbonium ions can undergo a variety of photoreactions, affording products which often vary considerably from those obtained in the photolysis of the corresponding uncharged compounds. The predominant mode of reaction encountered would seem to be isomerization to one or more valence bond isomers, which occurs via a symmetry-allowed disrotatory electrocyclic closure, rather than a [<72a-f 7r2a] cycloaddition in the case of alkylbenzenium ions and pro-... 

It is interesting to mention the entirely different situation in the cyclopropyl-allyl cation system the ring-opening reaction is very fast as compared to the isomerization of the allyl cation. In agreement with this situation the disrotatory mode of the cyclopropyl-allyl cation transformation has been much easier to verify ... 

The mechanism of this cycloaddition can be explained by FMO theory, in which one component acts as a HOMO and other as LUMO in a favorable low-energy TS to afford a stereoselective product. The Woodward-Hofifmann mles for electro-cyclic reactions are also applied to this cycloaddition reaction. The reaction of an alkene with a carbene is considered as a 4n electron process and of a conjugated diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for thermal reaction of 4n electron process, conrotatory motion of the substituents from the termini of the n system will favor a low-energy TS to afford the product and in photochemical process, the reverse disrotatory mode of motion will be the favored path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry allowed process in thermal reaction and conrotatory mode for its photochemical reaction. 

Similarly, photochemical reaction of 1,3-butadiene into cyclobutene and its reverse process are symmetry allowed processes in disrotatory mode of cyclization and opening of ring. 

Predictions for 1, 3, 5-Hexatriene cyclohexadiene interconversions These predictions can be made on the similar grounds as for 1, 3-butadiene cyclobutene interconversion. Photochemical reaction is feasible by conrotatory mode whereas thermal reaction follow disrotatory mode of ring closure as is explainable by Fig. 4.1. and Fig. 4.2, respectively. 

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 ... 

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. 

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. 

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