Conrotatory reaction

A complete mechanistic description of these reactions must explain not only their high degree of stereospecificity, but also why four-ir-electron systems undergo conrotatory reactions whereas six-Ji-electron systems undergo disrotatory reactions. Woodward and Hoifinann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system. The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory, which holds that it is the electrons of highest energy, i.e., those in the HOMO, that are of prime importance. The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1. 

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. 

Fig. 11.5. Correlation diagram for cyclobutene and butadiene orbitals (symmetry-allowed conrotatory reaction).

With a smaller ring in the bicycloheptene 4.60, a conrotatory reaction is virtually impossible, since it would put a trans double bond into a 7-mem-bered ring. No visible reaction occurs until the forbidden disrotatory reaction 4.62 gives the cis,cis-diene 4.61 at 400°, and even then only in low yield. 

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

SCRF methods [92-94] (Scheme 27). It was found that the first step of the reaction takes place with a concomitant N-O displacement of the MH3 moiety, thus yielding neutral azadienic intermediates (109), whose electrocyclic conrotatory reaction yields cycloadducts (110). The N-substituted-p-lactams (111) are formed via another O-N pseudopericyclic displacement of the MH3 unit. 

If we now look at the values of the above indices, it is possible to see that the prediction of the Woodward-Hoffmann rules is indeed confirmed since the greater values of the similarity index for the conrotatory reaction clearly imply, in keeping with the expectations of the least-motion principle, the lower electron reorganisation. If now the same formalism is applied to a stepwise reaction mechanism, the following values of the similarity indices result (Eq. 21). 

If now the same analysis is applied to allowed conrotatory reaction then the corresponding More O Ferrall diagram (Fig. 7) is in this case compatible with both concerted and nonconcerted mechanisms and the discrimination between them requires a knowledge of the corresponding reaction path. 

A priori, we would expect disrotatory reactions to show poorer torquoselectivity than conrotatory reactions for two reasons. Consider, for example, the hexatriene cyclohexadiene interconversion. On the one hand, the overlap between R and the distal carbon C6 is similar for the in and out pathways, as in the in mode, the major lobe at C6 is oriented away from R ... 

In conrotatory reactions the two groups rotate in the same way both clockwise or both anticlockwise... 

The side reactions of l,8a-DHNs were due to the 8a-hydrogen. Replacement of the 8a-hydrogen by a methyl group can eliminate the //-shift reactions and prevent the thermal disrotatory ring opening reaction. However, the photocyclization and photo-opening reaction are little affected, based on their conrotatory reaction... 

The Woodward-Hoffmann rules for electrocyclic reactions can also be formulated using the terms suprafacial and antarafacial (Table 4.3). A it system is said to react suprafacially in a pericyclic reaction when the bonds being made to the two termini of the it system are made to the same face of the 77 system. It reacts an-tarafacially when the bonds are made to opposite faces of the 7r system. In electrocyclic reactions, disrotatory reactions are suprafacial, and conrotatory reactions are antarafacial. 

Figure 6.4 The butadiene cyclobutene ring closure reaction, (a) For the conrotatory reaction, the loop constructed from butadiene, cyclobutene, and bicyclobutane is p (or pi ), and no conical intersection is enclosed, (b) For the disrotatory reaction, the loop is ip (or i ) and encloses a conical intersection.

Analysis of the conrotatory process is carried out in exactly the same way. In this case the element of symmetry that is maintained throughout the reaction process is the twofold rotation axis. The resulting correlation diagram is shown in Figure 10.24. The conrotatory reaction is symmetry allowed, since the bonding orbitals of butadiene correlate with the bonding orbitals of cyclobutene and vice versa. Figure 10.25 is a pictorial representation of the orbital in the reactant, transition structure, and product. 

Fig. 10.24. Correlation diagrams for interconversion of cyclobutene and 1,3-butadiene (left) symmetry forbidden disrotatory reaction (right) symmetry allowed conrotatory reaction.

Formation of a tr bond by ( + )-( +) overlap of p orbital lobes at the termini of i/>2 of 1,3-butadiene through a conrotatory reaction pathway (thermal reaction). 

Not only must we consider the symmetry properties of 1,3-butadiene orbitals, but we must also consider the s)mtunetry properties of both cyclobutene and the transition structure expected for the conversion of the reactant to product. The two pathways for the closure of 13-butadiene to cyclobutene are illustrated in Figure 11.16. The Cl—C2, C2—C3, and C3—C4 a bonds are shown as solid lines. The p orbitals of 1,3-butadiene and cyclobutene, as well as the sp orbitals of the C3—C4 cr bond of cyclobutene, are represented by the shapes of the atomic p or sp orbitals. This is therefore only a basis set representation, not an illustration of a particular molecular orbital. Although there are many symmetry elements present in the representations of both 1,3-butadiene and cyclobutene, in the conrotatory reaction the only symmetry element that is present continuously from reactant through transition structure to product is the C2 rotation. Similarly, only the a reflection is present from reactant through transition structure to product for the disrotatory pathway. 

More interesting than this first qualitative result is, however, the comparison of the relative ease of concerted and stepwise reaction mechanisms. Here it is possible to see that whereas in the case of allowed conrotatory reaction the concerted... 

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