Bonding molecular orbitals 2+4 -cycloaddition

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... 

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic j/)-hybridi2ed carbon atom and alow energy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene iasertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

The above complete regiospecificity of the cycloaddition across only the C=S+ bond was rationalized in terms of frontier molecular orbital coefficients in the salt 95. This cycloaddition was considered to be a LUMOsajt -HOMOdiene reaction. MOP AC 93 PM3 calculation of 95 showed the values of LLJMO coefficients for C(6), S, and N are 0.508, —0.502 and 0.364, respectively, as in Figure 1. These values strongly suggest the preference of the reaction site of the C=S+ bond. 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Theoretical calculations have been an important means of rationalizing the electronic course of hetero-Diels-Alder and related pericylic reactions for the formation of 1,2-thiazines 25 and 26. MOP AC 93 PM3 calculations have been used to deduce the regioselectivity of [4-1-2] cycloaddition reactions involving thiazinylium perchlorate 27 (Scheme 1) <1999TL1505>. Due to the higher lowest unoccupied molecular orbital (LUMO) coefficient at C-6 compared to N-2, the C-6 and S-1 behave preferentially as the dienophile double bond in cycloaddition reactions of this substrate with butadienes 28. 

The mechanism of the reaction has generally been discussed in terms of a thermally allowed concerted 1,3-dipoIar cycloaddition process, in which control is realized by interaction between the highest occupied molecular orbital (HOMO) of the dipole (diazoalkane) and the lowest unoccupied molecular orbital (LUMO) of the dipolarophile (alkyne).76 In some cases unequal bond formation has been indicated in the transition state, giving a degree of charge separation. Compelling evidence has also been presented for a two-step diradical mechanism for the cycloaddition77 but this issue has yet to be resolved. 

Why are [4 + 2] and [2 + 2] cycloadditions different Simple molecular orbital theory provides an elegant explanation of this difference based on the An + 2 rule described in Section 21-9. To understand this, we need to look in more detail at how the p orbitals of the double bonds interact in concerted addition mechanisms by suprafacial overlap, as in 36 and 37 ... 

We will not develop all of the Woodward-Hoffmann rules, but we will show how the molecular orbitals can indicate whether a cycloaddition will take place. The simple Diels-Alder reaction of butadiene with ethylene serves as our first example. The molecular orbitals of butadiene and ethylene are represented in Figure 15-18. Butadiene, with four atomic p orbitals, has four molecular orbitals two bonding MOs (filled) and two antibonding MOs (vacant). Ethylene, with two atomic p orbitals, has two MOs a bonding MO (filled) and an antibonding MO (vacant). 

The easiest explanation is based on the frontier orbitals—the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied orbital (LUMO) of the other. Thus if we compare a [2 + 2] cycloaddition 6.133 with a [4 + 2] cycloaddition 6.134 and 6.135, we see that the former has frontier orbitals that do not match in sign at both ends, whereas the latter do, whichever way round, 6.134 or 6.135, we take the frontier orbitals. In the [2 + 2] reaction 6.133, the lobes on C-2 and C-2 are opposite in sign and represent a repulsion—an antibonding interaction. There is no barrier to formation of the bond between C-l and C-l, making stepwise reactions possible the barrier is only there if both bonds are trying to form at the same time. The [4 + 4] and [6 + 6] cycloadditions have the same problem, but the [4 + 2], [8 + 2] and [6 + 4] do not. Frontier orbitals also explain why the rules change so completely for photochemical reactions, as we shall see in Chapter 8. 

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