Antarafacial interaction

Any of the combinations s + s, s + a, a + s, a + a is conceivable for a cycloaddition of two components. Comparison of cis-trans stereochemistry of substituents in product to that in reactants establishes which occurred. In additions of relatively short chains, the antarafacial interaction is difficult for the molecule to attain, but when systems with appropriate geometry are contrived, it is found that reactions in which one component acts in the antarafacial manner exhibit an inverted preference with respect to ring size The 2 + 2 additions are now favorable and the 4 + 2 not. 

The 2 + 2 cycloaddition with one suprafacial and one antarafacial interaction is allowed. Show this conclusion via a frontier molecular orbital analysis. Although the reaction is allowed, it is seldom seen. Can you think of a reason not based upon an orbital analysis of why this reaction is difficult ... 

For TT systems and lone pairs the distinction is simple suprafadal interactions involve the same face of the system, while antarafacial interactions are on opposite faces. Although we have only examined tt systems thus far, we will examine pericyclic reactions below that involve a bonds. Therefore, we need a similar definition for these kinds of bonds. For a bonds, the distinction is less obvious, but is consistent with the other systems. With suprafa-cial interactions, the two loops are drawn to either the inner lobes or outer lobes, while with an antarafacial interaction one loop is to an inner and one is to an outer lobe (see Figure 15.9 for this to make sense). Note that a suprafacial interaction at a cr bond involving two sp hy-... 

We have already mentioned the binary nature of the pericyclic reaction rules. If we change the mode of interaction of one of the reactants, we will reverse the allowed / forbidden nature of the reaction. For example, if we change the interaction mode of one of the reaction partners in the cycloaddition from suprafacial to antarafacial, now the [4+2) cycloaddition is forbidden, and the [2 + 2] cycloaddition is actually allowed. The [2+2] cycloaddition is now designated as [ 2 + T 2a]. In Figure 15.10 A we define the [ 2 + 2j] reaction, and in Figure 15.10 B we show a realistic geometry that could achieve the necessary orbital interactions. The two TT systems approach in a perpendicular orientation, and the lines define the suprafacial and antarafacial interactions. 

Figure 15.9 defined suprafacial and antarafacial interactions of a lone pair in a p orbital (called an uj component). Using these definitions, predict if the ring-opening of the cyclopropyl anion shown below will occur in a conrotatory or disrotatory fashion. What will be the stereochemistry of the product ... 

Fig. 3.2 Frontier orbital interactions of thermally allowed antarafacial interaction of a ketene (LUMO) and an olefin (HOMO)...

The conrotatory motion involves an antarafacial interaction between the termini, and disrotation involves a suprafacial interaction between these centres. There are two distinct possibilities for each mode of rotation, and therefore four possible products in aU. In many cases, however, the inherent mmetry of the system may not allow such distinction to be made. Even in cases where the two forms of one mode are distinguishable, it does not necessarily follow that both will occur the geometry of the system and the steric factors can be decisive. For example, the conrotatory ting-opening of trans-3,4-dimethylcyclobut-l-ene should, in principle, yield a 1 1 mixture of trans, /nms-hexa-2,4-diene and as, cis-hexa-2,4-diene (Equation 3.17). However, the inward conrotation of the two methyl groups is unfavourable because of the rapid increase in steric compression between these two substituents, and the 78... 

Clearly, the intramolecular migration of Z from Cl to C must, at the transition state, involve some form of orbital interaction between Z and the termini of the jr-framework system. Sensibly the framework system utilizes its TT-orbitals for this purpose, whereas Z can utilize a rr-type or a o-type orbital, if Z is a hydrogen atom the only available orbital of sufficiently low energy is the Is level (i.e. o-type orbital), upon which an antarafacial interaction has no physical significance because of its spherical symmetry. For an H atom migration, therefore, the cases of Fig. 3.12(c) and (d) are identical respectively with the modes (a) and (b). 

A natural consequence of an antarafacial interaction on the Z component is inversion at the migrating centre, modes (c) and (d). For a carbon atom this is synonymous with a Walden inversion, and therefore such migration must, at the transition state, involve the use of a p-type orbital by the carbon atom concerned. The inversion at the migrating carbon atom can usually only be detected from the stereochemical nature of the product if the migrating centre is chiral. Since divalent or trivalent atoms usually can not support chirahty, it is impossible to detect antarafacial interactions in oxygen or nitrogen atom migrations. 

In [ 1,/]sigmatropic changes the migrating component Z utilizes its own TT-system for the necessary orbital interaction with the framework 7r-orbitals. If antarafacial interactions occur they can be detected, as before, by the transference of chirality from the appropriate centre(s) in the reactant to the expected centre(s) in the product. 

The four possible topological interactions (Fig. 3.12) are not, of course, equally feasible for any particular sigmatropic reaction since the geometrical requirements of the system must also be considered. Thus, if the framework TT-system is constrained in a ring then an antarafacial interaction on that... 

Antarafacial interactions on non-cyclic j components in [I,/] migrations are likewise not possible if / is a small number. Thus, in the case of a [ 1,3] shift the modes (a) and (c), Fig. 3.12, are physically possible whereas modes (b) and (d) are not. When the value of / is increased the possibility of an antarafacial interaction on the framework 7r-system is greatly enhanced. The migration shown in Fig. 3.12(b) is feasible for a [1,7] shift if the carbon chain can adopt a gentle spiral conformation so as to facilitate the transfer of Z from the top face at Cl to the bottom face at C/. These restrictions are less severe for [/,/] shifts, and the interactions (b) - (d) are possible in suitable systems even for relatively small values for i and . 

Greek letter w for this purpose. Such to orbitals can interact in a suprafacial or antarafacial sense, as shown in Fig. 3.17. Thus a suprafacial interaction on the occupied -orbital of Xyz is designated 2 (where 2 indicates the presence of two electrons), whereas an antarafacial interaction on the unoccupied p-orbital is denoted by On this basis the electrocyclic conversion of the cyclopropyl cation into the allyl cation (Equation 3.19) is defined as a [ 2g + ] process if it occurs by the disrotatory mode indi-... 

An antarafacial interaction on one, or both, of the molecules requires that the approach be orthogonal. Because of the high symmetry of the system the supra-antara and antara-supra modes are identical. Correlation diagrams for the [ 2j + 2, ] addition are to be found in the literature (Woodward and Hoffmann, 1965, 1969). We will concentrate here on the other two modes of addition. 

No examples of fourteen electron cyclo-additions have come to light, but one sixteen electron case is known, (Equation 6.76). Of the two predicted pathways, [ 14j + 2s] or [ I4s + 2j], the one involving an antarafacial interaction on the 2 component appears most unlikely from the consideration of molecular models. The structure of the adduct, which was confirmed by single crystal X-ray crystallographic analysis, indicates the assigned [jrl + 2 ] pathway to be correct. The antarafacial interaction on the heptafuivalene molecule is possible because of its twisted shape. 

The [1,6] shifts in pentadienyl anions have rarely been observed. Thermal [1,6] sigmatropic hydrogen shifts occur in the acyclic systems, but not in the analogous cyclic ions. This indicates that an antarafacial interaction on the TT-system is necessary to the migration, as is predicted by the theory. Further confirmation for these results comes from the finding that the cyclic pentadienyl anion is photochemically isomerized by a [1,6] hydrogen shift the favourable [ 2s +, 6J interaction is available to the excited state process. 

Purely geometrical considerations dictate that the bromine atoms must migrate along their initial sides of the ir-system. The antarafacial interaction must therefore involve the utilization of a p-type orbital by one of the bromine atoms, but the inversion at this monovalent atom, of course, is not detectable in the rearranged product. The generality of dyotropic processes remains to be demonstrated. 

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