Antarafacial approach

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. 

An antarafacial approach of the HOMO of butadiene interacts destructively with a suprafacial approach of the LUMO of ethylene. 

Fig. 5.27. Transition states (a) For linear suprafacial approach (b] For linear antarafacial approach of carbene to 4q n-system.

Linear-antarafacial approach (a) as well as non-linear-suprafacial approach (b) in figure given below for the addition of SO2 to triene, [4qr+2 jz system, both are thermally favourable. 

Conversly linear-suprafacial as well as non-linear-antarafacial approaches both involve antiaromatic transition states with eight electrons and 0 nodes. Hence, both reactions are thermally disallowed. However, there is no way to prove if approach of SO2 is linear or non-linear. 

The reaction mechanism and the stereochemical diversity of the addition of water to disilene has been studied at the MP2/6-311-h+G level. Two pathways are feasible leading to syn and anti-addition. The syn addition proceeds via nucleophilic attack by water oxygen with a barrier of ca. 12 kJ mol k anti-Addition proceeds via intramolecular electrophilic attack by water hydrogen in a weakly bound disilene/water complex with antarafacial approach, in accordance with the Woodward-Hoffmann rules, and leads to an activation barrier of ca. 22 kJ mol ... 

Various geometries are possible for the transition state and they can be classified on whether each of the allyl systems interacts with lobes of the other system on the same side (suprafacially) or on opposite sides (antarafacially). Three transition states have been given. All have been classed on Huckels system, on the basis of aromatic transition state approach and so all are thermally allowed. The following picture gives the allowed transition state for thermal [3, 3] shifts. 

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 ... 

Day21 has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/Hiickel aromaticity, and has defined the terms supra-facial and antarafacial in terms of the nodal structure of the atomic basis functions. His approach makes quite explicit the assumption that the transition state involves a cyclic array of basis functions. Thus the interconversion of prismane (10) and benzene, apparently an allowed (n2s+ 2S+ 2S) process, is in fact forbidden because there are additional unfavourable overlaps across the ring.2... 

Thus, in the orthogonal approach the two molecules enter the reaction differently one of them antarafacially and the other suprafacially. 

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. 

This is known as the linear approach, in which the carbene, with its two substituents already lined up where they will be in the product, comes straight down into the middle of the double bond. The two sulfur dioxide reactions above, 6.127 and 6.128, are also linear approaches, but these are both allowed, the former because the total number of electrons (6) is a (An I 2) number, and the latter because the triene is flexible enough to take up the role of antarafacial component. The alternative for a carbene is a nonlinear approach 6.130, in which the carbene approaches the double bond on its side, and then has the two substituents tilt upwards as the reaction proceeds, in order to arrive in their proper orientation in the product 6.131. The carbene is effectively able to take up the role of the antarafacial component as with ketenes, it is possible to connect up the orthogonal orbitals, as in 6.132 (dashed line), to make the nonlinear approach classifiably pericyclic and allowed. This avoids any problem there might be with reactions like 6.127 and 6.128 being pericyclic and the clearly related reaction 6.130—>6.131 seeming not to be. Similar considerations apply to the insertion of carbenes into cr bonds. 

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... 

The reaction of ketenes with alkenes is assumed to occur via a concerted nonsynchronous mechanism, where the approach of the reacting partners is orthogonal. " As a consequence, the bulkier substituent of the ketene will end up on the sterically more crowded face of the cyclobutanone product. There are two descriptions that explain the experimental results 1) according to the Woodward-Hoffmann rules, the LUMO of the ketene reacts antarafacially with the HOMO of the alkene that reacts suprafacially " 2) the HOMO of the alkene forms a bond with the pz orbital of... 

However, there are alternative processes, called antarafacial processes, in which bonds are formed or broken on opposite faces of the reagent molecule. Let us consider again the addition of two ethylene molecules to form cyclobutane but with a different geometrical approach ... 

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