Conrotatory defined

Figure 14. The absolute value of the average disrotatory angle as a function of time in femtoseconds. (The disrotatory angle is defined in the upper left inset.) Lower inset A onedimensional cut of the excited-state potential energy surface along the disrotatory and conrotatory coordinates. All other coordinates are kept at their ground-state equilibrium value, and the full and dashed lines correspond to two levels of electronic structure theory (see text for details). (Figure adapted from Ref. 216.)...

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 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. 

In order to describe the ring opening of the aziridine 6.55, we need to define what suprafacial and antarafacial mean when applied to a p orbital. This is shown in Fig. 6.9, and applied there to the conrotatory aziridine opening. When both lines are drawn into the same lobe it is suprafacial, and when there is one line drawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a n nor a a orbital, it is given the Greek letter uj. The same designations apply whether the orbital is filled (on the left) or unfilled (on the right), and whether it is a p orbital or any of the sp" hybrids. 

Fig. 6.9 Suprafacial and antarafacial defined for a p orbital, and the allowed conrotatory interconversion of an aziridine with an azomethine ylid...

The conservation of orbital symmetry dictates that electrocycUc reactions involving An electrons follow a conrotatory pathway while those involving 4 -l-2 electrons follow a disrotatory pathway. For each case, two different rotations are possible. For example, 3-substituted cyclobutenes can ring open via two allowed conrotatory but diastereomeric paths, leading to E- or Z-1,3-butadienes, as shown in Scheme 4.11. Little attention was paid to this fact until Houk and coworkers developed the theory of torquoselectivity in the mid-1980s. They defined torquoselec-tivity as the preference of one of these rotations over the other. 

Beyond the disrotatory or conrotatory stereochemical imperative which must accompany all Nazarov cyclizations there exists a secondary stereochemical feature. This feature arises because of the duality of allowed electrocyclization pathways. When the divinyl ketone is chiral the two pathways lead to dia-stereomers. The nature of the relationship between the newly created centers and preexisting centers depends upon the location of the cyclopentenone double bond. The placement of this double bond is established after the electrocyclization by proton loss from the cyclopentenyl cation (equation 5). Loss of H, H or in this instance generates three tautomeric products. The lack of control in this event is a drawback of the classical cyclization. Normally, the double bond occupies the most substituted position corresponding to a Saytzeff process. The issue of stereoselection with chiral divinyl ketones is iUustrated in Scheme 7. The sense of rotation is defined by clockwise (R) or counterclockwise (5) viewing down the C—O bond. Thus, depending on the placement of the double bond, the newly created center may be proximal or distal to the preexisting center. If = H the double bond must reside in a less substituted environment to establish stereoselectivity. 

In the projected synthesis of vitamin B12, the plan called for the construction of a key intermediate by the stereospecific cyclization of a stereochemically well-defined 1,3,5-triene to the corresponding 1,3-cyclohexadiene. From the inspection of molecular models, Woodward and his colleagues were confident that the minimization of angle strain coupled with appropriate orbital overlap would favor a conrotatory cyclization. While the reaction was indeed found to be highly stereospecific, it took the disrotatory path instead. To explain the observed contradiction, it was necessary to recognize a new control element that Woodward and Hoffmann christened conservation of orbital symmetry [2, 3]. 

The stereospecificity of the cyclobutene isomerization was rationalized in the first of the Woodward-Hoffmann papers in 1965 which outlined the principle of Conservation of Orbital Symmetry in Concerted Reactions. This isomerization was defined as a conrotatory electrocyclic process. Note should be made of the possibility that the conrotatory pathway could also result in a cw,cw-2,4-hexadiene from a ran -3,4-disubstitued cyclobutene, but unfavorable steric interactions apparently intervene when a sterically large group rotates inward on the molecular system. Thus ran.y-l,2,3,4-tetramethylcyclobutene undergoes rearrangement with log k = 13.85 — 33 600/2.3R7 while the cis isomer reacts with log A = 14.1 — 31000/2.3RT so at 177°C the trans material reacts 49 times faster than the cis isomer. 

Figure 15.9 defined suprafacial and antarafacial interactions of a lone pair in a p orbital (called an uj component). Using these definitions, predict if the ring-opening of the cyclopropyl anion shown below will occur in a conrotatory or disrotatory fashion. What will be the stereochemistry of the product ... 

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