Disrotatory, 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 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. 

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 ring-chain tautomerism between cyclobutene and butadiene is perhaps the most famUiar example of an allowed Woodward-Hoffmann process. This transformation invariably is discussed in every attempt to rationalize or teach the Woodward-Hoffmann orbital symmetry concepts. This popularity is due in large part to the existence of a geometrically well defined (and easily visualized) alternate, forbidden, electrocyclic pathway. Thus it is exceedingly simple to set up a nonaUowed strawman, the disrotatory ring opening, and... 

In the second case (Figure 4.39b), the wipers move in opposite directions ( toward or away from each other). Such a motion is defined as disrotatory. As the tip of the first blade moves toward the second blade, the tip of the latter is moving toward the first. (It is as if your mirror image approached you as you approached the mirror, that is, the normal case for mirror images.) This process conserves symmetry about a mirror plane lying between the two wipers (and pointing over the hood of the car). 

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 ... 

The key concept for the formulation of the rules in question is the concept of the transition vector (see Sect. 1.3.3.1). This vector may be regarded as a quite short, albeit finite, portion of the part of the reaction path whose beginning lies at the transition state point. A displacement of the system in the direction defined by the transition vector lowers its potential energy. Figure 1.10 shows as an example the form of the transition vector for the electrocyclic reaction of disrotatory rearrangement of the cis-Dewar benzene into benzene [49]. 

Greek letter w for this purpose. Such to orbitals can interact in a suprafacial or antarafacial sense, as shown in Fig. 3.17. Thus a suprafacial interaction on the occupied -orbital of Xyz is designated 2 (where 2 indicates the presence of two electrons), whereas an antarafacial interaction on the unoccupied p-orbital is denoted by On this basis the electrocyclic conversion of the cyclopropyl cation into the allyl cation (Equation 3.19) is defined as a [ 2g + ] process if it occurs by the disrotatory mode indi-... 

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