Orbital overlap antarafacial

In structure (a) the hydrogen orbital overlaps suprafacially with the terminal p orbitals of the n system while in structure (b) the overlap is antarafacially. Therefore the geometry of the two transition systems becomes different. While the suprafacial overlap has a plane of symmetry, the antarafacial migration has two fold axis. 

The rearrangement occurs at least partially with antarafacial allylie participation and was estimated to be concerted in the sense of orbital-symmetry conservation control. Transition-structures appropriate to concerted pathways were judged likely to have exorbitantly high energies as a result of poor orbital overlap and unfavorable steric interactions. 

To achieve this arrangement the ethene molecules approach each other in roughly perpendicular planes so that the p orbitals overlap suprafacially in one ethene and antarafacially in the other, as shown in 38 ... 

There are two modes of orbital overlap for the simultaneous formation of two cr bonds—suprafacial and antarafacial. Bond formation is suprafacial if both cr bonds form on the same side of the tt system. Bond formation is antarafacial if the two cr bonds form on opposite sides of the tt system. Suprafacial bond formation is similar to syn addition, whereas antarafacial bond formation resembles anti addition (Section 5.19). 

As noted earlier, a transition structure in which a ct bond beaks from one face of a 71 system and a new cr bond forms to the opposite face of that n system is termed antarafacial. For example. Figure 11.41 shows a transition structure for the Cope rearrangement that is antarafacial with respect to both of the three-carbon segments. This geometry may be geometrically strained, but it is allowed by the principles of conservation of orbital symmetry because a cr bond with (-)-(-) orbital overlap can form while the original cr bond with (-I-)-(- -) orbital overlap is breaking. A transition structure that is antarafacial with respect to both components is termed the antarafacial-antarafacial (aa) pathway. ... 

There are no surprises The pathway chosen by the correspondence diagram is indeed the one that goes through the antarafacial transition state, as predicted by the WH-Rules. To be sure, there is very little experimental evidence for the occurrence of thermal [l,3]-sigmatropic rearrangements, but their rarity has been reasonably ascribed to the high steric strain [14, p. 1016] and poor orbital overlap [30, p. 99] in the allowed antarafacial transition state. 

Now lets consider the possibility of a thermal [2 + 2] cycloaddition of two alkenes. The HOMO of one alkene is symmetric, and the LUMO of the other alkene is antisymmetric (Figure 25.10). Constructive overlap of one set of 2p orbitals is shown in a suprafacial arrangement. The overlap of the second set of 2p orbitals would be destructive. To achieve a constructive overlap of these 2p orbitals, an antarafacial arrangement would be required. However, geometric constraints prevent an antarafacial addition. 

FIGURE 15.9 Overlap of orbitals in an antarafacial thermal 2+4 cycloaddition. 

It is conceivable that the spherically symmetrical Is hydrogen orbital could, alternatively, overlap across the plane of the polyene s carbon atoms, when the terminal lobes of the latter s HOMO were opposite in phase—antarafacial overlap. The terminal lobes of the HOMO will be opposite in phase for (36, x = 0,2,4...), leading to a... 

When the overlap of the reacting orbitals or product orbitals occurs on the opposite faces of the reacting molecules (called antarafacial way). 

To apply the rule we first draw the orbital picture of the reactants and show a geometrically feasible way to achieve overlap. Then the (4q + 2) suprafacial electrons and 4r antarafacial electrons of the components is counted. If the total is an odd number, the reaction is thermally allowed. Let us take the hypothetical cycloaddition of ethene to give cyclobutane. 

The formation of the new o-bond(s) must occur by an appropriate overlap of the same phases of these orbitals. If only one a-bond is forming, as in electrocyclic reactions, then only the overlap of the HOMO of the open chain reactant is considered. Such an overlap can occur in one of the two fundamental ways suprafacial mode or antarafacial mode (see Fig. 8.14). If two or more a-bonds are formed during the reaction, as in cycloaddition reactions, then the overlap of the HOMO of one reactant with the LUMO of the second reactant must be considered (see section 8.3). 

The easiest way to rationalize the stereospecificity in electrocyclic reactions is by examining the symmetry of the HOMO of the open (non-cyclic) molecule, regardless of whether it is the reactant or the product. For example, the HOMO of hexatriene is 3, in which orbital lobes (terminal) that overlap to make the new a-bond have the same phase (sign of the wave function). Thus, in this case, the new cr-bond between these two terminal orbital lobes can be formed only by the disrotation suprafacial overlap) (Fig. 8.45). If the terminal orbital lobes of HOMO of hexatriene were to close in a conrotatory antarafacial overlap) fashion, an antibonding interaction would result. 

In all of the above discussion we have assumed that a given molecule forms both the new ct bonds from the same face of the n system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [ 2s + 4J-cycloaddition (the subscript 71 indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2 + 4a]-cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photoche-mically allowed. Thus in order for a [fZs + -reaction to proceed, overlap between the highest occupied n orbital of the alkene and the lowest unoccupied 71 orbital of the diene would have to occur as shown in Fig. 15.10, with a + lobe... 

Under thermal conditions, the TS of the [2 + 2] cycloaddition is made up of if/1 (the LUMO) of one component and i[io (the HOMO) of the other. Positive overlap between the orbitals at both termini of the 1t systems can be obtained only if one of the components reacts antarafacially. This orientation is very difficult to achieve geometrically, and hence [2 + 2] cycloadditions do not normally proceed under thermal conditions. However, under photochemical conditions, one of the components has an electron promoted from fo to //1. Now the HOMO-LUMO interaction is between ) / of the photoexcited component and i// of the unexcited component, and thus both components can be suprafacial in the TS. The [2 + 2] cycloaddition of most alkenes and carbonyl compounds do in fact proceed only upon irradiation with light. 

---