Suprafacial and Antarafacial Process

The selection rules predict an antarafacial process. In this case, suprafacial and antarafacial processes would lead to the same product. 

Suprafacial and Antarafacial Processes These are two stereochemical modes of cycloaddition. Here two systems containing double bonds add up, therefore it is logical to expect that addition occurs either at the same side of opposite side of the system. As both the Jt-systems are undergoing addition, it is... 

What are suprafacial and antarafacial processes in sigmatropic reactions ... 

The rules based on the Hiickel-Mobius concept have their counterpart among the Woodward-Hoffmann selection rules. There was a marked difference between the suprafacial and antarafacial arrangements in the application of the Woodward-Hoffmann treatment of cycloadditions. The disrotatory and conrotatory processes in elec-trocyclic reactions presented similar differences. The suprafacial arrangement in both of the reacting molecules in the cycloaddition as well as the disrotatory ring closure in Figure 7-25 correspond to... 

These terms resemble the familiar ones syn and anti, but with this difference. Syn and anti describe the net stereochemistry of a reaction. We have seen anti addition, for example, as the overall result of a two>step mechanism. Suprafacial and antarafacial, in contrast, refer to actual processes the simultaneous making (or breaking) of two bonds on the same face or opposite faces of a component. 

Woodward and Hoffman described addition modes based on the facial approach of the reactive termini as suprafacial and antarafacial. "A suprafacial process is one in which bonds made or broken lie on the same face of the system undergoing reaction", as in 11. 2 antarafacial process has the "newly formed or broken bonds on opposite sides of the reaction system," as in 12. 2 Woodward and Hoffman then described several... 

However, the [1,3] thermal suprafacial process that involves the same lobe gives the product with retention of configuration in the migrating group, but the process is not symmetry allowed. On the other hand, symmetry-allowed [1,3] thermal antarafacial process with retention is geometrically forbidden and antarafacial process with inversion is symmetry forbidden (Figure 3.8). 

Let us mention some very basic problems to illustrate this point. Consider two ethylene molecules approaching each other for a [2 -F 2] reaction. The answer to the question of whether that reaction is allowed thermally or photo-chemically, or whether a suprafacial or antarafacial process will take place, or whether the reaction will take place at all, is very much dependent on the symmetry of alignment of the two reacting molecules or moieties. The extremes are D2h for a parallel approach and C2 for an orthogonal approach, and it has been predicted successfully that the former is needed for a suprafacial photochemical formation of cyclobutane. Most of the time, however, the two ethylenes are not in an ideal Djh arrangement. This may be due to an intramolecular frozen conformation of the two double bonds, to non-symmetric sterical hindrance caused by substituents on the double bond, and to the dynamical nature of the system (rotations and translations, especially in viscous media). 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

Cycloadditions of ketenes and alkenes have synthetic utility for the preparation of cyclobutanones.163 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.164 To be an allowed process, the [2ir + 2-tt] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the TS is a 2irs + (2tts + 2tts) addition.165 Figure 6.13 illustrates these combinations. Note that both representations predict formation of the d.v-substituted cyclobutanone. 

The above interaction is suprafacial with respect to one component and antarafacial with respect to the other and is therefore a k2s + n2a] process. A bonding interaction could occur between two pairs of lobes of the same sign, but the nearer alkene molecule would need to twist about its original n bond, and so it will make it geometrically inaccessible. 

Thermal concerted 2 + 2 reactions are predicted to occur between an alkene and a ketene. According to Woodward-Hoffmann rules, addition must be suprafacial to one component and antarafacial to the other if the process is to be concerted. 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

The retention of olefin geometry in the oxidation of cis and trans alkenes408 suggests a one-step concerted [2 + 2]-cycloaddition process, suprafacial in the alkene, and antarafacial in singlet oxygen.363 The strong solvent dependence, however, observed in many cases, points to a stepwise mechanism involving a perepoxide intermediate 403,409... 

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 problem may be restated in terms of the symmetry of all of the orbitals involved. In the transition state the pair of electrons forming the sigma bond to hydrogen may now be considered part of the n system the hydrogen will move suprafacially or antarafacially depending on whether the HOMO is a or b. For (4n + 2)77- electrons a thermal change is allowed, and for (471)77- electrons a first-excited-state process is allowed. 

The following abbreviation is often used in the literature W2S + W2S means that both ethylene molecules are approaching in a suprafacial manner, while W2S + w2a indicates that the same molecules are reacting in a process which is suprafacial for one component and antarafacial for the other. The number w2 indicates that two tt electrons are contributed by each ethylene molecule. 

Moving on to reactions in which antarafacial processes occur, we find that if we incorporate one antarafacial process, and keep the other suprafacial, then those reactions in which a total of 4n electrons are involved become allowed, and those in which a total of (4n + 2) electrons are involved become forbidden. To take just one of the relatively small number of reactions in this class, we can consider the remarkable reaction of heptafulvalene (129) with tetracyano-ethylene (130).128 This reaction is a [14 + 2] cycloaddition in other words,... 

In the above two examples, anti processes are always antarafacial, and syn processes are suprafacial. 

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