Pericyclic reactions regioselectivity

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=24) stereo-isomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two... 

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 1,3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (EMO) theory, several accounts of which have been published. 16.317 application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

You may well feel that there is very little to be gained from the Woodward-Hoffrnann treatment of die Diels-Alder reaction. It does not explain the endo selectivity nor the regioselectivity. However, the Woodward-Hoffrnann treatment of other pericyclic reactions (particularly electrocyclic reactions, in the next chapter) is helpful. You need to know about this treatment because the Diels-Alder reaction is often described an an all-suprafacial [4 + 2] cycloaddition. Now you know what that means. 

For each class of pericyclic reactions two or more of the following characteristics will be discussed the typical reactions, regioselectivity, stereoselectivity, and stereospecificity. The discussions of typical reactions and stereospecificity will help you recognize when pericyclic reactions are occurring in a particular chemical reaction. The discussions of regioselectivity, stereoselectivity, and stereospecificity will allow you to predict the structures and stereochemistry of the products obtained from pericyclic reactions. 

Regioselectivity has been previously described in terms of a local hard and soft acid and base (HSAB) principle, and some empirical rules have been proposed to rationalize the experimental regioselectivity pattern observed in some DA reactions.71,72 There is not a single criterion, however, to explain most of the experimental evidence accumulated in cycloaddition processes involving four centre interactions. An excellent source for the discussion of regioselectivity in concerted pericyclic reactions is given in reference.73... 

We have just seen that many photochemical reactions are complementary to the corresponding ground-state reactions, and that the frontier orbitals explain this change. As with thermal pericyclic reactions, discussed in Chapter 4, the frontier orbitals can also explain many of the finer points of these reactions, most notably the regioselectivity of photocycloadditions.118... 

The Alder ene reaction is like a Diels Alder reaction in which one Jt-bond in the diene has been replaced by a C-H bond 121. It does not therefore form a ring and does not fit easily into any of the three classes of pericyclic reaction (cycloaddition, electrocyclic, and sigmatropic). Since a hydrogen atom is transferred from one component to the other it is best described as a group transfer reaction.21 The regioselectivity is determined by the interaction 123 with the Jt-bond of the ene (the HOMO) with the LUMO of the enophile. ... 

Nevertheless, frontier orbital theory, for all that it works, does not explain why the barrier to forbidden reactions is so high. Perturbation theory uses the sum of all filled-with-filled and filled-with-unfilled interactions (Chapter 3), with the frontier orbitals making only one contribution to this sum. Frontier orbital interactions cannot explain why, whenever it has been measured, the transition structure for the forbidden pathway is as much as 40 kJ mol 1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. We shall return later in this chapter to frontier orbital theory to explain the much weaker elements of selectivity, like the effect of substituents on the rates and regioselectivity, and the endo rule, but we must look for something better to explain why pericyclic reactions conform to the Woodward-Hoffmann rules with such dedication. 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

Pericyclic Reactions. Trimethylsilyldiazomethane reacts with chiral acrylates to create optically active A -pyrazolines via regioselective asymmetric [3+2] cycloaddition. Subsequent pro-todesilylation affords A -pyrazolines in good yields (eq 66). This procedure offers a convenient route to azaprolines. ... 

Another problem, specffically appearing in the field of pericyclic reactivity of substituted systems is the question of the so-called regioselectivity [43-48]. Under this name the remarkable tendency of pericyclic reactions to prefer the formation of one specific product from the set of possible isomers differing in the mutual position of substituents is imderstood. An example of such a situation is the competition between the formation of the so-called head to head or head to tail products in the dimerization of substituted ethenes. 

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