Suprafacial, definition

The same definitions apply to longer n systems Suprafacial use of ao bond involves bonding in the same way at both terminals. So there is retention of configuration at both ends or inversion a both ends. 

Recall now our definition (p. 599) of a component as a noncyclic chain of interacting basis orbitals with signs chosen so that no internal phase inversions occur. Components may contain either an even or an odd number of basis orbitals, but we are restricting the discussion to systems with an even number of electrons and may therefore require that the number of electrons contributed by a component be even.51 A component may enter into interaction with other components at its ends in either a suprafacial way, if the signs of the interacting lobes at the two ends are the same, or in an antarafacial way if the signs of the interacting lobes are opposite. There are thus four types of components (4q)s, (4q)a, (4q + 2)j, and (4q + 2)a. 

In general, however, definitive information on the stereochemical course of C —H bond formation in these systems is still lacking. A suprafacial migration, the only feasible mode for steric reasons, is commonly implied in accordance with the requirements of the selection rules... 

For TT systems and lone pairs the distinction is simple suprafadal interactions involve the same face of the system, while antarafacial interactions are on opposite faces. Although we have only examined tt systems thus far, we will examine pericyclic reactions below that involve a bonds. Therefore, we need a similar definition for these kinds of bonds. For a bonds, the distinction is less obvious, but is consistent with the other systems. With suprafa-cial interactions, the two loops are drawn to either the inner lobes or outer lobes, while with an antarafacial interaction one loop is to an inner and one is to an outer lobe (see Figure 15.9 for this to make sense). Note that a suprafacial interaction at a cr bond involving two sp hy-... 

Definitions of suprafacial and antarafacial for various types of orbitals. 

With these descriptors in hand, we can look at the generalized orbital symmetry rule. There is a definite binary nature to the theory of pericyclic reactions. For cycloadditions, [2-f2] is forbidden (all suprafacial), whereas [4-F2] is allowed (all suprafacial). Continuing with the series, [6+2] is forbidden, and [8+2] is allowed. We will also encounter patterns in the other kinds of pericyclic reactions presented electrocyclic reactions, sigmatropic shifts, etc. Based on patterns such as these. Woodward and Hoffmann proposed the following rule for all pericyclic reactions ... 

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 ... 

While photocycloadditions are typically not concerted, pericyclic processes, our analysis of the thermal [2+2] reaction from Chapter 15 is instructive. Recall that suprafacial-suprafacial [2+2] cycloaddition reactions are thermally forbidden. Such reactions typically lead to an avoided crossing in the state correlation diagram, and that presents a perfect situation for funnel formation. This can be seen in Figure 16.17, where a portion of Figure 15.4 is reproduced using the symmetry and state definitions explained in detail in Section 15.2.2. The barrier to the thermal process is substantial, but the first excited state has a surface that comes close to the thermal barrier. At this point a funnel will form allowing the photochemical process to proceed. It is for this reason that reactions that are thermally forbidden are often efficient photochemical processes. It is debatable, however, whether to consider the [2+2] photochemical reactions orbital symmetry "allowed". Rather, the thermal forbiddenness tends to produce energy surface features that are conducive to efficient photochemical processes. As we will see below, even systems that could react via a photochemically "allowed" concerted pathway, often choose a stepwise mechanism instead. 

Suprafacial and antarafacial cycloadditions have their definite stereochemical consequences which can be made clear through the following examples ... 

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