Antarafacial overlap

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... 

For transitions involving An + 2 and An electron systems, the pictures will be as fallows. The First leads to Suprafacial overlap while the second to antarafacial overlap. 

Antarafacial overlap on one component in a cycloaddition would need a most unusually long and flexible conjugated system ... 

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. 

On exposure to light, frans,as,trans-2,4,6-octatriene (8.12) gives frans-5,6-dimethyl-l,3-cyclohexadiene (8.15) by conrotatory mode of ring closure antarafacial overlap). 

The frontier molecular orbitals in Figure 29.6 show why this is so. Under thermal conditions, suprafacial overlap is not symmetry-allowed (the overlapping orbitals are out-of-phase). Antarafacial overlap is symmetry-allowed but is not possible because of the small size of the ring. Under photochemical conditions, however, the reaction can take place because the symmetry of the excited-state HOMO is opposite that of the ground-state HOMO. Therefore, overlap of the excited-state HOMO of one alkene with the LUMO of the second alkene involves symmetry-allowed suprafacial bond formation. 

The frontier molecular orbitals in Figure 28.6 show why this is so. Under thermal conditions, suprafacial overlap is not symmetry-allowed (the overlapping orbitals are out-of-phase). Antarafacial overlap is symmetry-allowed but is not possible because of the small size of the ring. 

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

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. 

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 ... 

So far we have only defined what suprafacial and antarafacial mean on rc-systems (Fig. 2.7), but we need to see how o-bonds are treated by the Woodward-Hoffmann rules. Just as a suprafacial event on a rc-bond has overlap developing to the two upper lobes that contribute to the bond, so with o-bonds (Fig. 3.5a), overlap that develops to the two large lobes of the sp3... 

In a [1,7] hydrogen shift, the allowed pathway is an antarafacial shift, in which the hydrogen atom leaves the upper surface at C-l, and arrives on the lower surface at C-7. This can be drawn 5.3 as a [a2s+n6a] process or 5.4 as a [a2a+K6s] process. This time it is structurally an antarafacial shift, but the developing overlap that happens to be illustrated can be described with one suprafacial and one antarafacial component either way round. It is helpful to draw as many suprafacial components as possible, i.e. preferring 5.1 to 5.2, since the structurally suprafacial reaction is then also described with suprafacial overlap developing. Similarly it is helpful to draw 5.3 rather than 5.4, since that makes the antarafacial component the triene system, from one side of which to the other the antarafacial shift of the hydrogen is taking place. 

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