Transition state suprafacial

In the case of the [2-f 2]-cycloaddition reaction, the transition state contains four tt-electrons thus, the Hiickel transition state (suprafacial) is anti-aromatic (Fig. 8.36). Therefore, it is thermally forbidden. 

The transition state for such processes is represented as two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic transition state results, and the process is thermally allowed. Usually, a chairlike transition state is involved, but a boatlike conformation is also possible. 

In general, stereochemical predictions based on the Alder rule can be made by aligning the diene and dienophile in such a way that the unsaturated substituent on the dienophile overlaps the diene n system. The stereoselectivity predicted by the Alder rule is independent of the requirement for suprafacial-suprafacial cycloaddition, since both the endo and exo transition states meet this requirement. 

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... 

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 " ... 

Restricted Hartree-Fock calculations of the suprafacial addition of ethylene to l,4-dithioniabicyclo[2.2.0]hexane with the 6-31G basis failed to locate the transition state for this process. Synchronous suprafacial addition should... 

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. 

The transition state in suprafacial attack is designated as of Huckel type in which no sign inversion of the cycle has taken place. The other type of migration involves one sign inversion. This is called mobius type inversion. The Huckel type of inversion occurs when the total number of electrons is 2, 6,. .., (4n + 2). This is also called aromatic transition state. In mobius type the participating electrons is 4, 8,. .. i.e. An. 

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)... 

Molecular orbital calculations have shown that in all these electron-deficient centres suprafacial [1, 2] shift occurs with retention of configuration through a two electron Huekel type transition state ... 

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. 

A suprafacial alkyl [1, 3] shift with retention of configuration and already discussed provides an example. The transition state contains four electrons and is of Huckel type and makes the reaction unfavourable in the ground state but many photo-chemical [1, 3] shifts do occur in the four numbered ring structure. 

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

The authors assume that the halides react with stereoinversion, whereas the tosylates prefer suprafacial attack due to binding interaction with the lithium ion in the transition state . A comparable dependence of the stereochemical course from the leaving group has been observed in other stereodefined benzyllithiums, too . The addition of 268 to A-methyl-benzylideneimine proceeds with only low yield . ... 

The reaction occurs suprafacially across the allyl unit through a five-membered ring envelope-shaped transition state. The five-membered cychc transition state of [2,3]-sigmatropic rearrangement shows greater conformational flexibility than the six-membered transition state of [3,3]-sigmatropic rearrangements and should therefore be far more susceptible to the effects of stereochemical control by substituents. ... 

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.

Alkyl shift is evident in the Cope rearrangement. A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene. This reaction leads to the formation of a six-membered ring transition state. As [3,3] sigmatropic rearrangements involve three pairs of electrons, they take place by a suprafacial pathway under thermal conditions. 

A variety of sigmatropic rearrangements through six-electron transition states are known. In contrast to the rare [l,3]-migrations, [l,5]-shifts of hydrogen in dienes, suprafacially allowed, occur readily. The experiment outlined in Equation 12.88 confirms the predicted stereochemistry.139 The authors estimated the... 

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