Conrotatory and disrotatory

Conrotatory and disrotatory eoncerted reaetions ean often be distinguished by chemical means. For example, using the results of the previous calculation, predict whether the cyclizations of hexa-2,4-diene will lead to cii or traa.i dimethyl cyclo-butene... 

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). 

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

Problem 30.2 Draw the products you would expect from conrotatory and disrotatory cyclizations of (2Z,4Z.6/)-2,4,6-octatriene. Which of the two paths would you expect the thermal reaction to follow ... 

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

Examples of this type of process are the conrotatory and disrotatory reactions of butadiene ... 

Cyclopropyl ions and radicals (23) can undergo conversion to allyl (24) by typical electrocyclic ring opening processes we have carried out calculations for ring opening by both conrotatory and disrotatory paths. Table 5 shows calculated activation energies for the various processes. 

Spin-coupled theory has been used to smdy the changes that occur in the electronic wavefunction as a system moves along the intrinsic reaction coordinate for the case of the conrotatory and disrotatory pathways in the electrocyclization of cyclobutene to c/x-butadiene. Against intuitive expectations, conrotatory opening of cyclobutenes was found to be promoted by pressure. Ab initio MO and density functional calculations have indicated that the ring opening of the cyclobutene... 

Dynamics calculations have also provided new approaches to the stereochemical modes through which cyclopropanes and trimethylene intermediates may be related. Full quantum dynamics calculations for the trimethylene diradical based on a reduced dimensionality model that followed wave packet densities and time constants for formation of products led to the conclusion that conrotatory and disrotatory double rotations of both terminal methylene groups are favored over a single rotation of just one by a 2.2 1 ratio." °... 

Figure 22.11. 1,3 -Diradicals (15a and 15b) formed by conrotatory and disrotatory ring opening of cis- and franj-l,2-dimethylspiropentanes (14a and 14b).

This picture is from Woodward Hoffmann J. Am. Chan. Soc. 1965,87,395. who coined the terms, conrotatory and disrotatory. 

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

Figure 7.17 Orbital correlation diagram for conrotatory and disrotatory ring openings of cyclobutenes.

Figure 4.41 (a) Orbital overlaps in conrotatory and disrotatory o bond formation in a ring-closure reaction, (b) Stereochemistry of preferred products in photochemical and thermal (dark) processes... 

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. 

Construct correlation diagrams for conrotatory and disrotatory electrocyclic opening of each of the following ... 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

Since the detailed calculation of these matrices is sufficiently described in the original literature [33, 58], it is possible to present directly the final results first for the case of concerted reactions for which there are two alternative reaction mechanisms, conrotatory and disrotatory. The first of these mechanisms is allowed by the Woodward-Hoffmann rules while the second one is forbidden. 

Fig. 1. The conrotatory and disrotatory pathways of an electrocyclic isomerization lead to stereoisomers with different activation barriers...

Both the conrotatory and disrotatory modes are indicated in the following reaction sequence which involves a tetraene, a triene, and a diene in ring opening or closing (Marvell and Seubert, 1967). 

It should be pointed out that our description of electrocyclic reactions thus far has been qualitative. Woodward and Hoffmann (1965a) do refer to unpublished HMO calculations which back up the almost intuitive symmetry arguments. Nevertheless, Fukui (1965,1966) and Zimmerman (1966) outlined HMO treatments in which they obtained changes in energy for conrotatory and disrotatory processes. On the basis that paths involving minimum energy between reactants and transition states were favored, their predictions were in essential agreement with those of Woodward and Hoffmann. 

Brulet J, Schaefer HF III, J Am Chem Soc, Conrotatory and disrotatory stationary points for die... 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

Figure 7-21 demonstrates the nuclear movements involved in the conrotatory and disrotatory ring opening. These movements define the reaction coordinate, and they belong to the A2 and B representation of the C2v point group, respectively. 

Figure 7-21. The symmetry of the reaction coordinate in the conrotatory and disrotatory ring opening of cyclobutene.

After studying the correlation diagrams for both conrotatory and disrotatory processes, it is concluded that the interconversion of butadiene to cyclobutene thermally proceeds in a conrotatory fashion while photochemically it proceeds in a disrotatory fashion. 

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