Conrotatory, definition

Fig, 4,18 The stereochemistry of many reactions is easily predicted from the symmetry of molecular orbitals, usually the highest occupied n MO (n HOMO). In the ring closure of 1,3-butadiene to cyclobutene the phase (+ or —) of the HOMO (i//2) at the end carbons (the atoms that bond) is such that closure must occur in a conrotatory sense, giving a definite stereochemical outcome. In the example above there is only one product. The reverse process is actually thermodynamically favored, and the cis dimethyl cyclobutene opens to the cis, trans diene. No attempt is made here to show quantitatively the positions of the energy levels or to size the AOs according to their contributions to the MOs... 

The predicted conrotatory nature of this electrocyclization has been definitively established. The conrotatory mode was found to operate in the individual thermal electrocyclization of the stereoisomeric decatetraenes (430), (431) and (432). The former pair (430 and 431) afforded the rranr-cyclooctatriene (433), while the latter (432) led to the dr-cyclooctatriene (434). Both cyclooctatrienes (433) and (434) were observed to be in equilibrium with the corresponding 6e electrocyclized derivatives (435) and (436), respectively. 

As we shall see. Frontier Configuration (FC) theory tells us that stereoselectivity in 4N + 3 electron systems should be qualitatively the same as stereoselectivity in 4N electon systems. Thus, for example, we expect that 1,3-butadiene, allyl anion, and allyl radical will all undergo electrocyclization in a conrotatory fashion. While there is every indication that this is the case in the first two systems, the ring closure of allyl radical-type systems is predicted by computations to be near disrotatory with indicative (but not definitive) experimental results pointing in the same direction. What is Wrong ... 

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 ... 

---