Antarafacial bond formation

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). 

A cycloaddition reaction that forms a four-, five-, or six-membered ring must involve suprafacial bond formation. The geometric constraints of these small rings make the antarafacial approach highly unlikely even if it is symmetry-allowed. (Remember that symmetry-allowed means the overlapping orbitals are in-phase.) Antarafacial bond formation is more likely in cycloaddition reactions that form larger rings. 

Thermally, it will proceed via a Mobius topology involving one antarafacial component. Both methyl groups will rotate in the same direction conrotation) leaving one endocyclic and one exocyclic. However, under photochemical conditions, a [4n] ir-reaction is predicted to proceed via a Hiickel topology with suprafacial (disrotation) bond formation. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

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... 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

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. 

It is clearly seen that the HOMO-LUMO interaction leads to the formation of one bonding and one antibonding orbital, i.e. this is not a favourable interaction. The FMO approach also suggests that the 2 + 2 reaction may be possible if it could occur with bond formation on opposite sides (Antarafacial) for one of the fragments. 

The cis isomer is not formed at all. To give the trans isomer, the two new C-Br bonds have to be formed on opposite sides of the double bond by antarafacial addition. But this is impossible by a one-step mechanism because the Br-Br bond would have to stretch too far to permit the formation of both C-Br bonds at the same time. 

The simple carbocation intermediate of Equation 10-1 does not account for formation of the antarafacial-addition product. The results with SN1 reactions (Section 8-6) and the atomic-orbital representation (see Section 6-4E) predict that the bonds to the positively charged carbon atom of a carbocation should lie in a plane. Therefore, in the second step of addition of bromine to cyclo-alkenes, bromide ion could attack either side of the planar positive carbon to give a mixture of cis- and trans-1,2-dibromocyclohexanes. Nonetheless, antarafacial addition occurs exclusively ... 

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). 

Houk and co-workers reported a theoretical study of the degenerate rearrangement of bicyclo[3.1.0]hex-2-ene labeled with deuterium in the 4-exo position (113 in Figure 11.113) that offers additional insight into the nonsta-tistical dynamics model of reactivity. Breaking the Cl—C5 bond of 113 leads to a diradical (114) that can reclose to 113 with retention of configuration at both Cl and C5 (the rr path), can close suprafacially to 115 with retention at C5 (the rs path), can close to 116 with inversion at both Cl and C5 (the ii path), or can close antarafacially to 117 with inversion at C5 (the ia path). Baldwin and Keliher had reported that the activation parameters for formation of 115, 116, and 117 are identical and that the ratio of relative rate constants k s ku ki is 48 36 16. ... 

The formation of the above products can be explained by considering the HOMO—LUMO interactions in the linear approach of SO2 antarafacial to the (4n + 2) TT-system (Figure 5.6). As the HOMO—LUMO interactions are bonding, the reaction is thermally allowed in a conrotatory fashion. By the principle of microscopic reversibility, the same conclusion applies to the extrusion of SO2. 

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