Conrotatory process

Figure 11.3 illustrates the classification of the MOs of butadiene and cyclobutene. There are two elements of symmetry that are common to both s-cw-butadiene and cyclobutene. These are a plane of symmetry and a twofold axis of rotation. The plane of symmetry is maintained during a disrotatory transformation of butadiene to cyclobutene. In the conrotatory transformation, the axis of rotation is maintained throughout the process. Therefore, to analyze the disrotatory process, the orbitals must be classified with respect to the plane of symmetry, and to analyze the conrotatory process, they must be classified with respect to the axis of rotation. 

There are two stereochemically distinct possibilities for the conrotatory process. A substituent group might move toward or away from the breaking bond ... 

Theoretical treatment of the reaction as a conrotatory process proceeding through the very unstable Z,Z, -isomer of benzene satisfactorily accounts for the observed activation barrier. ... 

An example of preferred conrotatory cyclization of four-7c-electron pentadienyl cation systems can be found in the acid-catalyzed cyclization of the dienone 12, which proceeds through the 3-hydroxypentadienyl cation 13. The stereochemistry is that expected for a conrotatory process. 

In the thermal conrotatory process the molecule maintains a Ca axis of symmetry throughout the entire reaction, while the photochemical disrotatory process maintains a plane of symmetry as shown in Figure 9.13 for butadiene. 

Cyclobutenes are thermally converted into butadienes and the ring opening corresponds to a conrotatory process. 

The same arguments can be applied to construct the correlation diagram for the conrotatory process in the transformation of cyclobutene v " butadiene system. In such a case, as we have seen, a C2 symmetry is maintained throughout. 

There is a correlation between the ground state orbitals of cyclobutene and butadiene i.e. r m2 Therefore a thermal conrotatory process would be a symmetry allowed... 

Similarly the first excited state of butadiene V1V2V3 is correlated with a high energy upper excited state G27tc of cyclobutene. Thus a photochemical conrotatory process in either direction would be a symmetry forbidden reaction. 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

For an open chain system with (4rH- 2) 7T-electrons, the reverse is true and hence cyclization and ring rupture are in this case disrotatory. This description of the process shows why the conrotatory process is favoured in the cyclobutene isomerization. It does not rule out the reverse process where the conrotatory process is energetically very unfavourable. 

Theoretical analyses of the reaction path of photocyclization point to the same conclusion. Thus the qualitative state correlation procedure clearly indicates that photocyclization takes place by a conrotatory process in the Orbital Symmetry Conservation sense requiring a C2 molecular symmetry in 7 and in its symmetric congeners. The same conclusion were reached in the subsequent numerical analysis of the photocyclization of 7 and of 44 The detailed molecular structures of these two molecules and of 61 have been calculated by semi-empirical energy minimization procedures (cf also Ref. ). 

In this context one should also consider the formation of cis-4a,4b-dihydro-phenanthrenes (e.g., 4a and 4b hydrogens in cis conformation). While forbidden as an excited state concerted conrotatory process (see Sect. VI D) it could possibly take place by another route. Such cis-conformers would be less stable than the normal trans-conformers. ... 

Considerable effort has been expended on elucidating the mechanism of the photocyclizations. Di-p-tolylamine can be cyclized to 3,6-dimethylcar-bazole either photolytically in petrol or thermally at 880°C. Each process was viewed as electrocyclic, proceeding via cis and trans versions of 312 (R = Me, = H) produced by dis- or conrotatory processes, respectively. ... 

A similar kinetic effect was found for the perfluoro(3,4-diethylcyclobutene)/perfluoroocta-3,5-diene system.6 For the perfluoro(3-methylcyclobutene) (3)/perfluoropenta-1,3-diene (4) system,6-7 the kinetic preference lor the outward rotation of fluorine at position 3 is reflected in a A Ea of 12.9 kcal mol 1 between the conrotatory processes resulting in the formation of the Z- and E-isomers of 4. The equilibrium lies far on the side of perfluoro(3-methylcyclobutene) (3). with only 0.5% of (Z)-4 and 0.3% of ( )-4 present at 200 C. As is the case for per-iluorohexa-2.4-diene (2), the Z-isomer of 4 is more stable than ( >4, in this case by 2.2 kcal mol ... 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

Figure 11.8 reproduces the important molecular orbitals and classifies them according to their symmetry with respect to the C2 axis, the element that defines the local symmetry during the conrotatory process. Orbital nlr antisymmetric under C2, must change continuously into an orbital of the product in such a way as to remain at all stages antisymmetric under the C2 operation. 

Figure 11.8 Classification of the reacting molecular orbitals of butadiene and cyclobutene for the conrotatory process. Symmetry classifications are with respect to the C2 axis, S indicating symmetric and A antisymmetric orbitals. The correlation lines are obtained by connecting orbitals of the same symmetry.

The hexatriene reaction is slow, because the unstrained transition structure, a graceful spiral bringing the p-orbitals easily within bonding distance, corresponds to the forbidden [rt6a] conrotatory process ... 

The reaction of the octatetraene is faster, because the easily achieved spiral transition structure corresponds to the allowed [K8a] conrotatory process ... 

The Nazarov2 is probably the most important of reactions like 3. The cation 6 is formed from a dienone 5 by protonation and cyclises to the allylic cation 7. Though this is presumably a conrotatory process, the stereochemistry is usually lost in the formation of the cyclopentenone 9. 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

These selection rules can also be obtained from an analysis of polyene cyclizations. If the interaction between the terminal atoms Ca and Cn is bonding (antibonding), it will favor (disfavor) the cyclization. Figure 4.5 shows how the contribution of any given MO changes as a function of the reaction stereochemistry. When p is odd (even) the conrotatory process is disfavored (favored) and the disrotatory process is favored (disfavored). Obviously, the preferred pathway can be deduced by summing the contributions of all of the occupied MOs, up to and including the HOMO ... 

Can we extend this approach The analysis shown in Figure 6.1 remains valid for all conrotatory processes donors should rotate preferentially outward and attractors... 

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