Symmetry-allowed pathway

In the backdrop of the orbital symmetry rules, a need was felt to evaluate the strength of the orbital overlap component of the stereoelectronic effect by designing experiments in which both the competing pathways are orbital symmetry allowed but one pathway is preferred to the other pathway for better orbital overlap. Berson has explored this avenue exhaustively by replacing one double bond of a simple model system by a cyclopropane ring because such a structural change was expected to cause one of the two orbital symmetry-allowed pathways to enjoy better orbital overlap than the other pathway (see below). 

The epimerization at both the ring stereogenic centers causes the diastereomeric interconversion 22< >30. Since the rates of [l,5]-hydride shifts in these two diastereomers are comparable, some of the product formed from the pyrolysis of 22 must actually arise from 30 and vice versa. After making corrections for the concurrent double epimerization by following an established procedure [9, 10], it was shown that the mechanistically significant ratio of the products /i,Z-31 and Z,Z-31 formed directiy from 22 was >220 1. Very clearly, the orbital overlap controlled the reaction of the stereochemically well defined reactant 22 and caused one symmetry-allowed pathway to take prominence over the other symmetry-allowed pathway. 

To sum up the above discussion, we have witnessed that the orbital overlap component of the stereoelectronic effect is indeed a very powerful tool as it controls both the stereochemistry and the rates of a range of pericyclic reactions by allowing exclusively one of the two possible symmetry-allowed pathways for the very simple reason of better overlap of the breaking bonds. 

Notice that a symmetry-allowed pathway is one in which in-phase orbitals overlap a symmetry-forbidden pathway is one in which out-of-phase orbitals would overlap. A synunetry-aUowed reaction can take place under relatively mild conditions. If a reaction is symmetry-forbidden, it caimot take place by a concerted pathway. If a symmetry-forbidden reaction takes place at all, it must do so by a nonconcerted mechanism. 

A symmetry-allowed pathway requires in-phase orbital overlap. 

It is a well-documented yet remarkable tribute to the Conservation of Orbital Symmetry Hypothesis that benzene is not the immediate result of attempts to synthesize any of the other (CH) isomers since their strain energies and the substantial resonance energy of benzene confer enormous exothermicity on the conversion of any of these other isomers to benzene. Indeed, there is no obvious direct symmetry allowed pathway on the thermal energy surface that connects any of the other isomers, except perhaps benzvalene, to benzene. Indeed, heating above 100°C is required to affect the isomerizations. 

The low temperature required suggests that this may be a concerted reaction although a symmetry allowed pathway generating the all cis-COT cannot be written unless there is an allowed conversion to bicyclo[4.2.0]octa-2,4,7-triene (BOT) initially. 

The condition for a symmetry-allowed pathway for a reaction in which a singlet (S) and triplet (T) are interconverted is the retention of overall symmetry along the pathway [2, p. 206] ... 

A symmetry-allowed pathway is one in which in-phase orbitals overlap. 

Problem 4A Suggest a symmetry allowed pathway to account for the following chemical transformation. 

The most obvious pathway, the [ 2 + retro cyclo-addition, is symmetry-forbidden. The symmetry-allowed pathway involves first a [(,2s + 2s + <,2s] retro cyclo-addition, which is followed by a [ 2s + (,2s + 2s] Cope rearrangement, Equation (IV,8). 

Figure 1.2. Endo and exo pathway for the Diels-Alder reaction of cyclopentadiene with methyl vinyl ketone. As was first noticed by Berson, the polarity of the endo activated complex exceeds that of the exo counterpart due to alignment of the dipole moments of the diene and the dienophile K The symmetry-allowed secondary orbital interaction that is only possible in the endo activated complex is usually invoked as an explanation for the preference for endo adduct exhibited by most Diels-Alder reactions.

All-ci5-cyclononatetraene undergoes a spontaneous electrocyclic ring closure at 25°C to afford a single product. Suggest a structure for this product. Also, describe an alternative symmetry-allowed electrocyclic reaction that would lead to an isomeric bicyclononatriene. Explain why the product of this alternative reaction pathway is not formed. 

Symmetry-allowed reactions often occur under relatively mild conditions, but symmetry-disallowed reactions can t occur by conceited paths. Either they take place by nonconcerted, high-energy pathways, or they don t take place at all. 

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

Symmetry-allowed, symmetry-disallowed (Section 30.2) A symmetry-allowed reaction is a pericyclic process that lias a favorable orbital symmetry for reaction through a concerted pathway. A symmetry-disallowed reaction is one that does not have favorable orbital symmetry for reaction through a concerted pathway. 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

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