Antarafacial reaction

Note that both suprafacial and antarafacial cycloadditions are symmetry-allowed. Geometric constraints often make antarafacial reactions difficult, however, because there must be a twisting of the it orbital system in one of the reactants. Thus, suprafacial cycloadditions are the most common for small tt systems. 

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

Figure 12.3. Examples of suprafacial and antarafacial reactions of n systems. Notice that the examples serve to describe the stereochemical course of the reaction only. No mechanism is implied by these examples.

Now it works In fact, extension of this reasoning to other electrocyclic reactions tells you that they are all allowed—provided you choose to make the conjugated system react with itself supra-facially for (An + 2) K systems and antarafaciaWy for (4n) n systems. This may not seem particularly informative, since how you draw the dotted line has no effect on the reaction product in these cases. But it can make a difference. Here is the electrocyclic ring closure of an octatriene, showing the product from (a) suprafacial reaction and ( ) antarafacial reaction. 

There are four possible products two arise from suprafacial reaction and two more from antarafacial reaction. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

Photochemical suprafacial-antarafacial reactions are very rare because the geometrically simpler thermal reaction is likely to occur instead. 

Sigmatropic rearrangements wil be suprafacial for 4n + 2 electrons, but antarafaaal for 4n electrons. Antarafacial reactions are rarer, since antarafacial movement is often impossible geometrically. 

More readily identifiable geometrical factors probably outweigh the contribution of the frontier orbitals in the remarkable reaction 6.47 between tetracyanoethylene and heptafulvalene giving the adduct 6.49 (see p. 261). The HOMO coefficients for heptafulvalene 6.420 (see p. 347) are highest at the central double bond, but a Diels-Alder reaction, with one bond forming at this site is impossible. The best reasonable possibility for a pericyclic cycloaddition, from the frontier orbital point of view, would be a Diels-Alder reaction across the 1,4-positions (HOMO coefficients of -0.199 and 0.253), but this evidently does not occur, probably because the carbon atoms are held too far apart. This is well-known to influence the rates of Diels-Alder reactions cyclopentadiene reacts much faster than cyclohexadiene, which reacts much faster than cycloheptatriene (see p. 302). The only remaining reaction is at the site which actually has the lowest frontier-orbital electron population, the antarafacial reaction across the 1, f-positions, which have HOMO coefficients of —0.199. 

We have drawn little green arrows on the diagrams to show how the methyl groups move as the new a bonds form. For the allowed suprafacial reaction of the 6tc electron system they rotate in opposite directions so the reaction is called disrotatory (yes, they both go up, but one has to rotate clockwise and one anticlockwise) while for the allowed antarafacial reaction of the 4 t electron system they rotate in the same direction so the reaction is called conrotatory (both clockwise as drawn, but they might equally well have both gone anticlockwise). We can sum up the course of all electrocyclic reactions quite simply using these words. 

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