Transition state antarafacial

The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

In contrast with the thermal [4 + 2] Diels-Alder reaction, the 2 + 2 cycloaddition of two alkenes to yield a cvclobutane can only be observed photo-chemically. The explanation follows from orbital-symmetry arguments. Looking at the ground-state HOMO of one alkene and the LUMO of the second alkene, it s apparent that a thermal 2 + 2 cycloaddition must take place by an antarafacial pathway (Figure 30.10a). Geometric constraints make the antarafacial transition state difficult, however, and so concerted thermal [2 + 2j cycloadditionsare not observed. 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [1,3] sigmatropic migrations of hydrogen are unknown." On the other hand, the photochemical pathway allows suprafacial [1,3] shifts, and a few such reactions are known, an example being " ... 

The rare [1,4] hydrogen transfer has been observed in radical cyclizations. With respect to [1,7] hydrogen shifts, the rules predict the thermal reaction to be antarafacial. Unlike the case of [1,3] shifts, the transition state is not too greatly strained, and such rearrangements have been reported, for example,... 

Such a transition state is likely to be highly strained, however, and no such 1,3-antarafacial shifts have actually been observed. A 1,7-thermal antarafacial shift in (36, x = 2), where the T.S. is likely to be much less strained (i.e. able to adopt the required helical geometry) has, however, been observed in the vitamin D series. 

The reaction via a planar transition state is n2s + n2s. Here only one of the two new C—C bonds can be formed. This will raise its activation energy impossible to be reached. So there are two (4q + 2) electron suprafacial components and no antarafacial component. Since the total number of counting components is two, an even number, the reaction is thermally disallowed. 

The symbols ji, o and (0 are given respectively to the n systems, o bonds and lone p orbitals which participate in the transition state and the symbols (s) and (a) are indicated for their suprafacial and antarafacial use. The notation is completed by the number of electrons supplied by each component. Thus n2s denotes a two electron n system used in a suprafacial way. woa indicates a vacant p orbital used in an antarafacial way and so on. 

In general four types of transition states can be envisaged for sigmatropic migrations of the order (/, /), two in which the migrating group moves suprafacially (a) and (b) and two in which it means antarafacially (c) and (d)... 

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. 

Cycloadditions of ketenes and alkenes have been shown to have synthetic utility for the preparation of cyclobutanones.101 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.102 To be an allowed process, the [2n + 2n] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the transition state is a [2ns + (2ns + 2ns)] addition.103 Figure 6.6 illustrates these transition states. The ketene, utilizing its low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the... 

Fig. 6.6. HOMO-LUMO interactions in the [2 + 2] cycloaddition of an alkene and a ketene. (a) Frontier orbitals of alkene and ketene. (b) [2ks + 2na Transition state required for suprafacial addition to alkene and antarafacial addition to ketene, leading to R and R in cis orientation in cyclobutanone products, (c) [2ns + (2ns + 271,)] alternative transition state.

Loss of sulfur from these species would give the carbodiimide, whereas addition of alkenes would give the cycloadducts. The authors infer that this intermediate cannot add conceitedly to alkenes in the supra-supra fashion because it would involve a four-electron transition state, but that a stepwise addition would hardly rationalize the stereospecificity observed this suggests that the thiaziridine (or the 1,3-dipole) participates in an antara facial reaction.63 However, according to recent discussions antarafacial addition is seldom observed68 and stereospecificity need not be lost in a stepwise process.69... 

Conformation has a major influence on the chemical reactivity of cycloalkanes. To understand its effect in any one reaction, we first need to know what the conformation is of the transition state, and this requires a knowledge of the reaction mechanism. Next, we have to decide what amount of energy is required for the reactants to achieve transition-state conformations. For example, consider the E2 elimination discussed in Section 8-8D. The preferred transition state requires the leaving groups to be antarafacial and coplanar ... 

To have the antarafacial coplanar mechanism operate with the trans isomer, the transition state would have to have the fert-butyl group in the highly unfavorable axial position. 

A problem with this explanation is that it is a bit more difficult to explain those pericyclic reactions that we shall come to in Chapter 4, which smoothly take place in spite of their having a total of 4n electrons. We shall find that these all show stereochemistry involving an antarafacial component. It is possible to include this feature in the aromatic transition state model—if the... 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

Figure 7 Orbital correlation diagram for the antarafacial [1,3] shift of a proton in propene. The transition-state orbitals are based on the orbitals of Mobius cyclobutadiene (see text)...

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... 

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. 

It should be clear that the selection rules apply to concerted processes between spatially accessible parts of a molecule. Antarafacial [1,3], [1,5], or [3,3] reactions involving u-orbital transfers between carbon atoms are not possible. Antarafacial migrations do become feasible when the transition state forms a cycle of at least 7 or 8 atoms Woodward and Hoffmann (1965b) cite the [1,7] example (76). 

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