Suprafacial approach

Application of CM theory to explain pericyclic reactions was first attempted by Epiotis and coworkers (Epiotis, 1972, 1973, 1974 Epiotis and Shaik, 1978b Epiotis et al 1980). The following analysis is a much-simplified treatment of that approach. Let us compare, therefore, the CM analysis for the [4 + 2] allowed cycloaddition of ethylene to butadiene to give cyclohexene with the [2 + 2] forbidden dimerization of two ethylenes to give cyclobutane. For simplicity only the suprafacial-suprafacial approach is considered, although this simplification in no way weakens the argument. 

In the second example, suprafacial approach of a dienophile to the 2,5 carbons of frans,trarcs-2,4-hexadiene is seen to lead to a product with two methyl groups on the same side of the cyclohexene ring ... 

Figure 11.14 Symmetry elements for the suprafacial approach of two ethylene molecules.

There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

The frontier molecular orbital analysis led to the conclusion that butadiene will react with ethylene to give cyclohexene if both butadiene and ethylene collide in a suprafacial manner. However, ethylene will not react with ethylene in an analogous manner. We can extend these conclusions to the reaction of any conjugated diene with any alkene and to any alkene with another alkene. Presented below are several examples and practical considerations of the allowed 4 + 2 reaction (the Diels-Alder reaction). The suprafacial approach of both reactants has important consequences on the stereochemistry of the Diels-Alder reaction. 

A suprafacial approach of the HOMO of butadiene interacts constructively with a suprafacial approach of the LUMO of ethylene. 

Because suprafacial approach is bond-forming, hence reagent approaches 4grc-system linearly. 

Fig. 5.26. Transition states (a) For linear suprafacial approach (b) Nonlinear suprafacial approach of carbene to a (4q + 2) n-olefinic S3rstem.

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. 

In all of the above discussion, we have assumed that a given molecule forms both the new s bonds from the same face of the 7t 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 [7t2s+,i4s] cycloaddition (the subscript n 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. 

The photocyclization of enamides has been the subject of detailed study and provides a valuable approach to the synthesis of alkaloids. A comprehensive review has been published.31 A variety of reaction types has been reported. The N-benzoylenamine 33, for example, is converted on irradiation to the trans-lactam 34 by a process involving conrotatory photocyclization followed by a thermally allowed [l,5]-suprafacial hydrogen migration.32 The influence of substituents on this transformation has been studied.33 The enacylamine 35 undergoes an analogous cyclization to give a mixture of cis- and trans-lactams 36, the ratio of which is solvent... 

Subscripts s and a are used to indicate a supra and an antara process respectively. Suprafacial, suprafacial (s, s) approach of two polyenes is normally sterically suitable for efficient orbital overlap. The vast majority of concerted additions involves the s, s approach. 

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 ... 

Woodward and Hoffmann refer to this geometry of approach as the nonlineary to distinguish it from the alternative, more symmetrical linear approach (5), in which the sp orbital enters suprafacially. Although Hoffmann has reported calculations by the extended Hiickel method that are in agreement with the mode of addition 4 predicted by the pericyclic theory,6 there is little experimental information. 

Since both the diene and the dienophile in Diels-Alder reactions 2.84 react suprafacially, there are synthetically useful structural consequences, which incidentally prove that the stereochemistry of approach is suprafacial. Thus a diene with substituents at each end forming new bonds to one surface will create an adduct in which the stereochemical relationships are preserved. The trans,trans-diene 2.86 and diethyl acetylenedicarboxylate 2.87 give the... 

The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial with singlet carbenes like dichlorocarbene (p. 28). This is clearly a forbidden pericyclic reaction, if it takes place in the sense 3.49 —> 3.50, 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 reactions above, 3.47 and 3.48, are also linear approaches, but these are both allowed, the former because the total number of electrons (6)... 

The C-H bond is parallel with the p orbitals of the ene so that the orbitals that overlap to form the new k bond are already parallel. The two molecules approach one another in parallel planes so that the orbitals that overlap to form the new o bonds are already pointing towards each other. Because the electrons are of two types, n and O, we must divide the ene into two components, one K2 and one a2. We can then have an all-suprafacial reaction with three components. 

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

On the other hand, in the parallel approach of two ethylenes, both molecules enter the reaction 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. 

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