Suprafacial overlap

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. 

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. 

Why are [4 + 2] and [2 + 2] cycloadditions different Simple molecular orbital theory provides an elegant explanation of this difference based on the An + 2 rule described in Section 21-9. To understand this, we need to look in more detail at how the p orbitals of the double bonds interact in concerted addition mechanisms by suprafacial overlap, as in 36 and 37 ... 

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. 

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. 

Frontier molecular orbital analysis of a [4 + 2] cycloaddition reaction. The HOMO of either of the reactants can be used with the LUMO of the other. Both situations require suprafacial overlap for bond formation. 

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. 

Suprafacial overlap to form a four-memhered ring can take place only under photochemical conditions, so one of the reactants must he in an excited state. Therefore, one of the two carhon dioxide molecules formed when the four-memhered ring breaks is in an excited state (indicated hy an asterisk). When the electron in the excited state drops down to the ground state, a photon of ultraviolet light is released— which is not visible to the human eye. However, in the presence of a dye, the excited carbon dioxide molecule can transfer some of its energy to the dye molecule, which causes an electron in the dye to be promoted to an excited state. When the electron of the dye... 

Cycloaddition reactions involving 6jc electrons result in the formation of either dihydroaromatic or tetrahydroaromatic derivatives. It depends upon whether the dienophUe is an olefin or an acetylene. Dehydrogenation of the cycloadducts results in the formation of aromatic compounds (Scheme 16.16) [19]. Cycloaddition reactions are thermally allowed under suprafacial-suprafacial overlap mode [4% +2% ] [3]. 

Sigmatropic shifts are not pericyclic because suprafacial overlapping of cyclic p-orbitals is not involved. In these shifts, the orbitals from the breaking bond and the lone pair overlap with the p-orbitals in the TS. These reactions have planar nonaromatic eight-centered cyclic transition states with orthogonal orbital... 

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. 

Bond formation is suprafacial if both a bonds form on the same side of the tt system it is antarafacial if the two a bonds form on opposite sides of the tt system. Formation of rings with fewer than seven ring atoms requires suprafacial overlap. 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

In general, stereochemical predictions based on the Alder rule can be made by aligning the diene and dienophile in such a way that the unsaturated substituent on the dienophile overlaps the diene n system. The stereoselectivity predicted by the Alder rule is independent of the requirement for suprafacial-suprafacial cycloaddition, since both the endo and exo transition states meet this requirement. 

Subscripts s and a are used to indicate a supra and an antara process respectively. Suprafacial, suprafacial (s, s) approach of two polyenes is normally sterically suitable for efficient orbital overlap. The vast majority of concerted additions involves the s, s approach. 

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. 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

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 ... 

Likewise with the dienophile the maleate and fumarate esters 2.90 and 2,92 react with butadiene to give diastereoisomeric adducts 2.91 and 2.93, in which the substituents retain, as a consequence of the suprafacial nature of the developing overlap, the cis and trans relationships they had in the dienophiles. Diels-Alder reactions are much used in organic synthesis, not only because two new C-C bonds are made in one step, but also because the relative stereochemistry of up to four new stereogenic centres is predictable. 

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... 

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