Symmetry allowed reactions

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

Symmetry allowed reaction (Section 10 14) Concerted reac tion in which the orbitals involved overlap in phase at all stages of the process The conrotatory ring opening of cy clobutene to 1 3 butadiene is a symmetry allowed reaction... 

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. 

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. 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

By analogy to carbenium ions, it can be expected that alkylnitrenium ions will experience 1,2-alkyl and hydride shifts. These would presumably be orbital symmetry allowed reactions that convert the nitrenium ion into a more stable iminium ion. This idea is supported by several theoretical studies suggesting that simple... 

A symmetry-allowed reaction. The HOMO of butadiene forms a bonding overlap with the LUMO of ethylene because the orbitals have similar symmetry. This reaction is therefore symmetry-allowed. 

This will apply not only to Diels-Alder reactions, but to other Woodward-Hoff-mann symmetry-allowed reactions, as well. For example, the good donor-acceptor pair, cycloheptatriene and tropone, react to give only the allowed ([6 + 4], then [4 + 2]) adduct <517, while cycloheptatriene dimerizes to give not only the analogous adduct, 7, but the forbidden, [6 + 2] adduct, 8, as well as several other unidentified dimers18. ... 

Reactions involving a total of (An) electrons (an even number of curly arrows) are allowed if there is one antarafacial component and all the others are suprafa-cial. In this case, if the dashed, bold or coloured lines include only one antarafacial component, the number of (4q + 2)s and (4r)a components will add up to an odd number, and the drawing will show the geometry of a symmetry-allowed reaction. 

Some other stepwise reactions seeming to disobey the rules, are the 1,2-shifts of ylids, like the Stevens rearrangement 6.107 >6.109. The symmetry-allowed reactions, suprafacial-with-inversion 6.74 or antarafacial-with-retention 6.75, are unreasonable—there is no flexibility for migration across only two atoms, and yet reactions like this take place easily. It is now clear that these reactions are stepwise, taking place by homolytic cleavage 6.107 —> 6.108, followed by rapid radical recombination 6.108 —> 6.109. 

Eq. 5). The essential feature of a symmetry-allowed reaction is the symmetry-match of occupied molecular orbitals in A with those in B and a similar match of unoccupied A and B orbitals. Thus a band of bonds (oc-... 

These rules state which cycloaddition reactions are allowed to proceed in a concerted (one step) fashion and which are not e.g. a thermal cis-cis 2 n-j-2 n cycloaddition reaction (cyclobutane formation) is forbidden as a concerted process. The rules do not state that every symmetry allowed reaction has to proceed in a concerted fashion. If a reaction can occur by two pathways (concerted or disconcerted), then, other factors being equal, the allowed pathway will have the lower activation energy 521>. 

In open-chain and in some cyclic systems, therefore, reactions often take the path which uses the longest part of the conjugated system consistent with a symmetry-allowed reaction, but several other factors, spatial, entropic, steric, and so on, have obviously to be taken into account. Thus various sesquifulvalenes, with HOMO coefficients in the parent system 6.467, react in a variety of ways. The reaction giving the [8 + 2] adduct 6.468 finds the sites with the highest Zc2 value, but the three reactions with tetracyanoethylene giving the adducts 6.469-6.471 show different selectivities, for no obvious reason. These examples serve to emphasise the pitfalls of a too easy application of frontier orbital theory. 

With respect to the stereochemistry, for an orbital symmetry allowed reaction inversion must accompany 1,3-migration (either antarafacial with respect to the allyl unit or inversion of the migrating center), or the reaction must proceed via nonconcerted pathways1181. As the [1,3] reaction of the thermolysis of syn- and ant/-7-(l-propenyl)norbornenol (9 and 12 respectively) produces 14 with high stereoselectivity (at 290 °C a 28 72 mixture of 11 and 14 from 9 and a 94 6 mixture from 12), a stepwise mechanism is probably operative via intermediates 10 and 13 formed by fragmentation and recombination1192. 

Lcl us consider A2II6 as a dimerization product of two AH3 units. 10.11 shows the least-motion approach of two CH3 radicals to form ethane C2H6. The MO correlation-interaction diagram in 10.12 shows that, given conservation of symmetry, the occupied MOs of two -CH3 radicals lead only to those of the ground state of ethane. Hence 10.11 is a symmetry-allowed reaction. If dimerization of... 

There is something more unusual about the electronic structure of the open isomer, redrawn in 11.62. A bond-switdiing process converts 11.62 into 11.63 by way of 11.64. Tliis is a symmetry-allowed reaction. Notice that the symmetry of... 

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