Conrotatory 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. 

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). 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

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 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). 

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. 

In the case of conrotatory mode, the symmetry is preserved with respeo to C2 axis of rotation. On 180° rotation along this axis, F goes to H. and H2 to H, and the new configuration is indistinguishable from the original. An orbital symmetric with respect to rotation is called a and antisymmetric as b. On the other hand, in the case of disrotatory moot-the elements of symmetry are described with respect to a mirror plane. Tilt symmetry and antisymmetry of an orbital with respect to a mirror plant of reflection is denoted by a and a" respectively (Section 2.9). The natun of each MO of cyclobutene with respect to these two operations is shov. n in the Table 8.4 for cyclobutene and butadiene. 

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

Molecular orbital calculations at the extended Hiickel level indicate that the two disro-tatory modes of ring cleavage (but not the symmetry-forbidden conrotatory mode) require similar activation energies for a model methylenecyclopropane-PdCl2(HCN) complex. The disrotatory motion of the carbon-carbon bond breaking away from the metal (dis-... 

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. 

Axis of rotation conrotatory mode symmetry operation C2... 

Fukui, 1965a). Depending on k, the highest occupied molecular orbital (HOMO) will be symmetric (a) or antisymmetric (6), with respect to a symmetry plane (or) perpendicular to the plane of the carbon chain of 1. Thus, la is a (in + 2)v and lb is a 4nir electron system (Fig. 6). Now, in order for the ring to close and form a cr-bond, the ends of the 7r-system of la must rotate in opposite directions or in a disrotatory mode, while the ends of lb must rotate in the same direction, or in a conrotatory mode. In either case, rotation occurs so as to maximize orbital overlap or bonding as measured by P in (14). 

The pyrolysis products of the cis- and trans-2,3-dimethylcyclobutenes are illustrative of the conrotatory mode (Criegee et al., 1965) ... 

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