Orbital overlap suprafacial

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. 

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 ... 

The orbitals overlap suprafacially on both ir-systems (Fig. 8.35) thus, the [4s4-2s]-cycloaddition reaction is thermally allowed (see the rules summarized in Table 8.1). However, the rules are reversed when the reaction is photochemically induced. 

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. 

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]. 

Because in the ground state the HOMO is 4/3, the hydrogen shift is controlled by the symmetry of the vj 3 of the pentadienyl radical. The v) 3 has similar signs on the terminal lobes i.e. it is symmetrical. Thus, the [ 1,5] -hydrogen shift is thermally allowed and occurs in a suprafacial process. This involves a transition state in which the C-1 and C-5 orbitals overlap with Is hydrogen orbital. This shift is both symmetry allowed and geometrically favourable, as shown in Fig. 8.55. 

The selection rules for [tt4 + tt2 ] and other cycloaddition reactions can also be derived from consideration of the aromaticity of the TS3 In this approach, the basis set p orbitals are aligned to correspond with the orbital overlaps that occur in the TS. The number of nodes in the array of orbitals is counted. If the number is zero or even, the system is classified as a Htickel system. If the number is odd, it is a Mobius system. Just as was the case for ground state molecules (see p. 716), Htickel systems are stabilized with 4 + 2 electrons, whereas Mobius systems are stabilized with 4n electrons. For the [tt4 + tt2] suprafacial-suprafacial cycloaddition the transition state is aromatic. 

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). 

Frontier orbital analysis of a [4 -I- 2] cycloaddition reaction shows that overlap of in-phase orbitals to form the two new a bonds requires suprafacial orbital overlap (Figure 29.5). This is tme whether we use the LUMO of the dienophile (a system with one TT bond Figure 29.1) and the HOMO of the diene (a system with two conjugated rr bonds Figure 29.2) or the HOMO of the dienophile and the LUMO of the diene. Now we can understand why Diels-Alder reactions occur with relative ease (Section 8.8). 

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... 

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. 

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 ... 

Note that this substituent is absent in (71) and hence the radical mechanism would be discouraged in favor of a concerted reverse cycloaddition. The final product (69) could then arise by two-step cycloaddition of (76) to another mole of (67). The above suggested mechanism has the advantage of avoiding symmetry problems associated with the simpler mechanism (73—<-79— 69). If the latter mechanism is concerted according to the FMO theory, the overlapping lobes of the HOMO orbital of one reactant and the LUMO orbital of the other reactant should have the same phase. The coefficients of the HOMOs and LUMOSs of (67) and (78) reveal a mismatch for suprafacial attack and therefore the reaction is disallowed. Similarly, suprafacial extrusion of N2 from (73) is disallowed. 

The C-H bond is parallel with the p orbitals of the ene so that the orbitals that overlap to form the new k bond are already parallel. The two molecules approach one another in parallel planes so that the orbitals that overlap to form the new o bonds are already pointing towards each other. Because the electrons are of two types, n and O, we must divide the ene into two components, one K2 and one a2. We can then have an all-suprafacial reaction with three components. 

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... 

Cycloadditions are controlled by orbital symmetry (Woodward-Hoffman rules) and can take place only if the symmetry of all reactant molecular orbitals is the same as the symmetry of the product molecular orbitals. Thus, an analysis of all reactant and product orbitals is required. A useful simplification is to consider only the frontier molecular orbitals. These orbitals are the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). The orbital symmetry must be such that bonding overlap of the terminal lobes can occur with suprafacial geometry that is, both new bonds are formed using the same face of the diene. 

Some cycloadditions proceed thermally, whereas others require hv. The dependence of certain cycloadditions on the presence of light can be explained by examining interactions between the MOs of the two reacting components. Frontier MO theory suggests that the rate of cycloadditions is determined by the strength of the interaction of the HOMO of one component with the LUMO of the other. In normal electron-demand Diels-Alder reactions, HOMOdiene ( Ai) interacts with LUMOdienophiie (< >i ) There is positive overlap between the orbitals where the two cr bonds form when both components of the reaction react from the same face of the tt system (suprafacially). 

Under inverse electron demand, LUMOdiene 0//2) interacts with HOMOdienophjie (1//0). Again, there is positive overlap between the orbitals at both termini of the two 7r systems when both components of the reaction react suprafacially. 

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. 

Likewise, in the cationic [1,2] alkyl shift, both components must be suprafacial for there to be positive overlap in the TS between orbitals where bondmaking and bond-breaking take place. The migrating group retains its configuration because of the requirement for suprafaciality. 

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